1.3 ProQ:ram Objectives. The aim of this research program ...... C2F6 in Cl14 are given in Figs. 49a and 49b, respectively. ..... 147 (1982). 15. L. G. Christophorou ...
AD-A 198 641 ORNL/TM-10844
011dC04.1LE
Cr
OAK RIDGE NATIONAL LABORATORY Basic Studies of Gases _for
Fast Switches
Final Report
S. R. Hunter L. G. Christophorou
OTIC S
,,.t,,
AUG 9 1988
ofnent has been approvedi wn
f>r public reiccsa and sale; its ORDctribution is unlimited.
OPERATED DY! MARTIN MARIETTA ENERGY SYSTEMS, INC. FOR THE UNITED STAES DEPARTMENT OF ENERGY
Printed in the United States of America. Available from National Technical Information Service U.S. Department of Commerce 5285 Port Royal Road, Springfield, Virginia 22161 NTIS price codes-Printed Copy: A06- Microfiche A01
This report was prepared as an account of work sponsored by an agency of the United States Government Neither the U nited States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied. or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus. product, or process disclosed, or represents that its use would not infringe privately owned rights Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof
ORNL/TM- 10844
Interagency Agreement DOE No. 1246-1246-Al Navy No. NOO014-82-F-O123
Office of Naval Research Physics Division Arlington, Virginia 22217
BASIC STUDIES OF CASES FOR FAST SWITCHES
Final Report by S. R. Hunter and L. G. Christophorou Health and Safety Research Division Oak Ridge National Laboratory P. 0. Box 2008 Oak Ridge, Tennessee 37831-6123 and Department of Physics The University of Tennessee Knoxville, Tennessee 37996
Date Published - August 1988
OAK RIDGE NATIONAL LABORATORY Oak Ridge, Tennessee 37831 Operated by MARTIN MARIETTA ENERGY SYSTEMS, INC. for the U.S. Department of Energy Under Contract No. DE-AGO5-81OR21400
Contents Page
Section I. 1.1 1,2 1.3 1.4 1.5 II. 2.1 2.2 2.3 2.4 2.5 III. 3.1 3.2 3.3 3.4 3.5 3.6 3.7
3
Background ............................................. Operating Parameters for a Diffuse Discharge Opening Switch ......................................... Program Objectives .................................... Summary of Accomplishments ............................. Publications and Presentations .........................
3
14 17
Experimental Approach ..................................
23
High Pressure Swarm Experiment ......................... High Temperature Swarm Experiment ...................... Pulsed Townsend Experiment ............................. Gas Ionizing W Value Experiment ........................ High Voltage Breakdown Experiment ......................
23 26 26 29 31
Experimental Techniques and Selected Results ...........
33
Room Temperature Electron Attachment Rate Constants and Attachment Cross Sections .......................... Elevated Temperature Electron Attachment Rate Constant Measurements .................................. Room Temperature Electron Drift Velocity Measurements, Room Temperature Electron Attachment and Ionization Coefficien ts ........................................... Elevated Temperature Gas Mixture Studies ............... W-Value Measurements ................................... High Voltage Breakdown Field Strength Studies ..........
5
33 38 42 50 64 75 84
Relevance to Switching Experiments ..................... 91
IV 4.1 4.2 4.3 4.4 4.5 V.
Introduction ...........................................
C 2 F6 /Ar Gas Mixtures ................................... C 2 F6 /CH4 Gas Mixtures .................................. Enhanced Discharge Current Characteristics ............. Operation at Elevated Gas Temperatures ................. Diffuse Discharge Closing Switch Experiments ...........
Conclusions ....................... References ...................... .
.
.. ..
...
.....
......
95 95 99 101 103
.
....
108
...
....
110
3
I.
Introduction This final report covers the contract period October 1, 1982 to March 31,
1988, and includs an outline of the operation of diffuse discharge switches in pulsed power energy storage circuits, the methods by which the operation of these switches may be optimized, and the measurements we have performed on several gas mixtures which exhibit these optimized characteristics.
The goal
of the program has been to identify these optimization procedures, and using basic data on electroratom and electron~molecule collision processes, to find gas mixtures in which the transport properties of the electrons in the gas discharges are "tailored"
to match these optimization requirements.
Using
these criteria, we have found several gas mixtures which have very desirable characteristics
for
use
in
these
switches.
The
potential
of
these
gas
mixtures has been demonstrated in recent switching experiments which have shown considerable operational improvements when these gases were used. )' The
following outline for
this report has
accomplishments of this program are highlighted.. I, a brief background description of switch
in an inductive
transport
and
of
in which
the
For the remainder of Section
the operation of a diffuse discharge
energy storage circuit
other parameters
been adopted
"
the
gas
is outlined.
mixtures
to
be
The electron optimized
are
identified, and the specific objectives of this program are discussed along with a summary of
the accomplishments and publications which have resulted
from this contract.
In Section II, the experimental techniques used to obtain
the electron transport and breakdown measurements are outlined;
the basic
experimental measurements are presented in Section III, while in Section IV, we discuss some of
the implications of these data in the operation of both
diffuse discharge closing and opening switches.
The results of
switching
experiments using our new gas mixtures will also be presented along with a discussion of the strengths and limitations of these switches in pulsed power energy storage circuits.
Finally in Section V some general conclusions are
presented. 1.1
Background
There has been considerable interest in recent years in the possibility of using inductive energy storage devices as a means of storing and rapidly transferring electrical energy in repetitive pulsed power applications.
The
primary advantage to be gained from the use of these energy storage devices is
4
that they have potential energy storage densities 100 to 1000 times
that of
comparable capacitive storage systems and that this energy can be transferred 1-3 to the load on the very short time scale of nanoseconds. One of the major problems
to be faced with this technology before
it can be
introduced
in a
number of applications is that these inductive energy storage systems require a switching device that can repetitively switch (repetition rates > 104 pps and more
than
106
shots)
high currents
confinement) with opening times of withstanding breakdown. 3
high
voltages
(e.g.,
100 kA for
inertial
fusion
the order of a few ns and being capable of
(>100
kV)
during
the
opening
stage
without
One of the most promising contenders for fast repetitive switching is an externally sustained diffuse gas discharge operating at gas pressures of one to several atmospheres.
Two different types of external electron sources have
been proposed for the control of the discharge current. volume
gas
controlled"
ionization
5 '6
)
or
by
by
a
pulsed
resonance
high
ionization
They are by means of
energy
electron
of
gaseous medium
the
beam
("e-beam using
a
pulsed high power laser ("optically controlled" 7'8) The first experiments using e-beam controlled discharges were performed in the early 1970s in an effort to improve the efficiency of high pressure CO 2 discharge lasers, where the efficient operation of these devices requires that they
be
voltage. fields
operated 9
at
Early
electric
researchers
fields in
below
this
field
the
self-sustained
noticed
that at
discharge
high
electric
the
discharge becomes unstable, and oscillations in the discharge 10 current occur. These oscillations have been shown to be due to an electron attachment
instability because
the
electron attachment
coefficient
in
02
increases in magnitude with increasing field strength over a range of E/N 11 values. A small, local reduction in the electron concentration causes a small
increase
diffuse
in
the
discharge,
local
which
electric
results
field
in
in
the
space-charge
dominated
electron attachment and a 11 further reduction in the electron concentration, etc. As a consequence, the
discharge
becomes unstable
electrodes e-beam schemes
in
the
sustained for
applications.
and waves
discharge. lasers,
rapidly Early
this
increased
of
plasma current
Although
this
mechanism
extinguishing
the
experiments using
between
is an undesirable
forms
the
discharge
e-beam
travel
basis in
of
process
most
the in
present
opening
switching
sustained diffuse
discharges
5 12 were mainly conducted in Russia for use in fast closing switch applications.
Hunter 5 and Koval'chuk and Mesyats 13 were the first discharges
sustained diffuse
could also
applications where switching times 10- 1 5 V cm 2 ) while the switch is opening. These
conditions
operating
allow
us
desirable
several
define
to
characteristics of the gaseous medium in the conducting (low E/N) and opening (high E/N)
stages
of
In
switching action.
the
conducting stage,
the
the
requirements are: 1.
Maximum electron drift velocity w - the larger w is, conductivity of the discharge and the greate
-
the higher the
the current density in
the diffuse discharge and the lower the switch resistivity. 2.
Minimum e-beam "ionization energy" W - the smaller W is,
the greater
the current gain in the discharge with a consequent increase in the efficiency
of
the coupling
of
the e-beam
to
the
discharge and
a
greater control of the resultant discharge current. 3.
Minimum electron loss terms k of
the
,
discharge drastically
increase
when
the
highly
kR1, k
and k
decreases and
mobile
relatively immobile negative ions.
- the conductivity
space-charge
electrons
are
problems
converted
into
Similarly, the conductivity will
decrease if the electron and negative ion-positive ion recombination in the discharge is large due to the loss of Another problem
that results
from large
the charge carriers.
recombination coefficients
is that the current gain in the switch will decrease, and the energy released in the recombination process will
result in increased gas
kinetic energy causing heating problems in the gas under repetitive operation. 4.
Minimum ionization rate constant k. required source,
to
be
completely
otherwise
considerably
the
e-beam is switched off.
the conductivity of the gas is
controlled
opening
increased due
-
by
time
the
of
to additional
external
the gas
ionization
switch
ionization
will
be
when
the
6
In the opening stage the requirements of the gas mixture as as follows: 1.
Minimum mobility mixture.
electron and
drift
hence
velocity
lower
the
w
-
i.e.,
electron
reduce
conductivity
the in
electron the
gas
11
2.
Maximum electron attachment rate constant ka
i.e.,
-
lower the gas
conductivity by converting highly mobile electrons into relatively immobile
negative
ions
and
by
removing
free
electrons
from
the
discharge, reducing the current density due to additional ionization processes as the E/N increases. 3.
High breakdown strength (E/N)e.m (for uniform fields defined as the -15 2 E/N at
> 10- 1 5 V cm
which k i = ka)
(E/N)eim,
the
faster
electron conductivity
the
the higher
-
permissible
rate
of
the
value
decrease
in the discharge, and hence
the
of
in
the
shorter
the
opening time of the switch. 4.
Self-healing gas mixtures
-
for
closed cycle
operation without
time dependent degradation in the performance of
a
the switch, it is
required that the gas mixture composition not change with time.
The
molecular gases in the switch can be fragmented either by collisions with
high
energy
electrons
impact-induced dissociation occurring
during
the
from
the
e-beam
and dissociative
diffuse
discharge,
or
by
electron
attachment
processes
particularly
during
the
opening phase where the E/N quickly rises to very large values (E/N > 10- 1 5 V cm 2). attach
This problem can be reduced by using gases
electrons nondissociatively
at
energies well
in
that
excess
of
thermal energy and also have low electron impact dissociation cross sections
and
large
neutral-neutral
and
negative
ion-positive
ion
recombination coefficients at high E/N values. 5.
In
photoexcited
and
photoionized
laser-controlled discharges) attaching
gas
in
which
it
electron
gas
discharges
is desirable attachment
(required
to have can
an
electron
be
increased 7 8 , radiation. laser the by molecules the of photoexcitation
The desirable characteristics for the E/N dependence of w and ka
for
by
for
the
gas mixture in the diffuse discharge are shown in Fig. 2. 1 4 ,15 When switch S is closed or conducting (i.e., when the electric field across
the the
switch electrodes is small), electron conduction in the discharge is optimized by using gas mixtures which have electron
loss processes
(e.g.,
large electron drift velocities and small
electron attachment and
recombination)
at
the
12
ORNL-DWG 85-9330
CONDUCTING STAGE
L)
I
OPENING STAGE
I
>
,
>
0
/
0
0-%120
3 E/N (10-17 V-cm 2 )
Fig.
2.
Schematic k (E/N)
illustration and
the
of
electron
required of a gas mixture switch.
Approximate
the
electron
drift
of
conducting and opening stages of (from Ref. 14).
velocity
for use
values
attachment w(E/N)
rate
characteristics
in a diffuse discharge E/N
for
the
constant
discharge
opening in
the
the switch are shown in the figure
13
electric field strengths characteristic of this stage of the switching action (indicated by the shaded region in Fig. 2). stage (i.e.,
Conversely, during the opening indicated by the
when the electric field strengths are large,
shaded region in Fig. 2). the electron drift velocity should now be low, and the electron attachment rate constant large in order to reduce the electron conduction in the discharge as quickly as possible. 1.3
ProQ:ram Objectives
The aim of this research program has been to find gas mixtures which would optimize
the gas discharge requirements outlined above
conduction and
opening
stages
of
the
switching action.
in both
This
has
the been
accomplished by performing the following measurements on potential gases and gas mixtures. 1.
Measurements constant k
a
of
the
room
temperature
electron
attachment
rate
were performed on selected electronegative gases in both
N 2 and Ar buffer gases as a function of E/N (or mean electron energy
()) and gas pressure in an effort to find the energy dependence and electron attachment mechanisms of these candidate gases.
If any of
these gases possessed the required characteristics (i.e.,
low ka at
thermal and near thermal electron energies,
and high ka at (E) > 2
eV) then the following measurements were performed. 2.
Room
temperature
electron
drift
velocity
w
measurements
were
performed on the candidate gases in gas mixtures with both Ar and CH, over a wide range of concentrations (typically varying from 0.1% to 100% of the attaching gas)
to observe any enhancement
may occur at low E/N values,
and to give additional
in w that
insights into
the electron scattering processes which may be occurring in these electronegative gases. 3.
Electron
attachment
coefficient
T1/N
and
electron
ionization
coefficient a/N measurements were also performed on a few selected gas
mixtures
with
Ar
and
concentrations in an effort
CH4
over
a
similarly
wide
range
of
to understand how these gases would
perform in switching gas mixtures.
14
4.
High voltage
breakdown
field
(EIN)tim measurements were
strength
performed on selected gas mixtures in order
to determine if these
gases were able to withstand the transient high voltages that occur when
the
switch
determine
is
opened.
These
measurements
the minimum concentration of
enabled
the attaching gas
us
to
that was
required to sustain these voltage transients without breakdown. 5.
For repetitively operated switches, the gas temperature within the switch is expected to rise several hundred degrees centigrade, and under
these
circumstances,
the
electron
transport
and
rate
coefficients are expected to be functions of the gas temperature T. Knowledge of
these parameters as a function of T is desirable for
modeling the operation of the diffuse discharge switch in practical applications. w (E/N).
Consequently, the above measurements (namely k a(()),
a/N (E/N),
r1 N (E/N)
and
(E/N)tim)
were
performed
as a
function of T on selected gas mixtures to simulate the environment found in repetitively operated switches. 6.
Finally,
the
W value
positive ion pair particle) effort
of
to
current minimize
average energy
to produce an electron-
from the energy decay of a high energy ionizing
several
find
gas
density
in
the
(the
gases
and gas
mixtures the
which
switch
"resistivity"
of
mixtures would
during the
was
measured
maximize
switch
the
electron
conduction
discharge
and
in an
(i.e.,
consequently
maximize the current switching efficiency). 1.4
Summary of Accomplishments
The following is a list of the most significant accomplishments achieved during the course of this program. 1.
Measurement
of
the
constant
as
a
ka
(Z 0.04 eV)
to
room
function
(C) Z 5 eV
temperature of
(E)
electron
from
thermal
attachment electron
in N 2 and Ar buffer gases
rate energy
for several
molecules (many of which have been found to possess very desirable electron attachment opening switches). 1-CF
6
characteristics
for
use
in
diffuse
discharge
These molecules include CF4, C 2 F6, CF,. c-C3 F 6 ,
, c-C 4F8 . n-C 4 Fo, n-C 5 F1 2 , i-C 5 F1 2, n-C 6 F,4 , GCIF 3 , (CF3 )2 0,
(CF,) 2 S. NF3 , N2 0 and S02.
15
2.
Measurement of the room temperature electron drift velocity over a wide E/N range (0.002 < E/N < 400 x 10- 17 V cm 2 ) in the following CF 4. C 2 F 6 , C 3 F8,
pure gases;
n-C 4 Fo, (CF3 )2 0. SiF 4 . BF3 . Ar. CH 4
and N 2 . 3.
Measurement of the room temperature w in the following gas mixtures which were proposed for use in diffuse discharge opening and closing Ar/CH 4 , CF 4 /Ar.
switches;
CF 4 /CH 4 ,
C 2 F6 /Ar,
C 2 F6 /Cli 4,
CaF,/Ar,
C3 F9 /CH 4. (CF3 )2 0/Ar. (CF3 ) 2S/CH4, SiF 4 /He. 4.
Developed a new procedure for the measurment of electron attachment and ionization
coefficients
in a pulsed Townsend experiment where
both electron ionization and attachment processes are present. 5.
Measurement of the electron attachment coefficient rI/N as a function of E/N and gas pressure CaF.,
n-C 4 F1 0,
was measured all
these
SiF
02.
to test
molecules
in
the
following pure gases;
and BF3 .
4
With the exception of 02
the accuracy of only
attach
CF 4 , C 2 F6,
the experimental
electrons
(which
technique),
efficiently
at
high
electric field strengths. 6.
Measurement of the electron ionization coefficient a/N as a function of E/N in the following gases; CF 4 , C 2 F6 , C3 F9 , n-C 4 F,
SiF
4
, BF,
and CH 4 . 7.
Room temperature il/N and a/N measurements were also
performed
in
C 2 F 6 /Ar and C 2 F 6 /CH 4 gas mixtures, as these mixtures were chosen to represent
the expected
coefficients
in
E/N and
typical
concentration dependence
diffuse
discharge
of
opening
these switch
applications. 8.
Room temperature a/N and 7/N measurements were also performed in the following mixtures; mixtures
were
SiF 4/He. SiF 4 /He/TEA and SiF 4 /He/TMAE.
chosen
for
study
as
they
possessed
almost
These ideal
characteristics for self-sustained diffuse discharge closing switch applications. 9.
The
high
voltage
performed at room
temperature
field in
the
strength
measurements
following mixtures;
The average energy to produce an electron-positive measured for CH 4 , Ar,
were
C 2 F6 /Ar,
C3 F9/Ar, CaFa/CH 4 and c-C 4 F,/Ae.
C 2 F 6 /CH4,
10.
breakdown
N
2
the
, C 2H 2
following pure gases; and 2-C 4 H8 .
ion pair W was
CF 4 , C 2 F 6 , CaFa.
n-C 4 F1 0 ,
16
11.
The W values were measured in the following binary gas mixtures: CF./Ar, C 2 F 6 /Ar, C 3 Fs/Ar. CF4 /I
4
, CF 4 /C 2 H 2 , C 2 F 6 /C 2 H 2 , C 3 F,/C 2 H 2 ,
C 2 F6 /2-C4 H,, CF,/C2H 2. Ar/C 2 H 2 and Ar/2-C4 Hs.
12.
The
W
were
values
also
the
C 2 F6 /Ar/C 2 H 2 ,
CF 4 /Ar/C 2 H 2 ,
mixtures;
in
measured
gas
ternary
following
and
C 2 F 6 /Ar/2-C 4HO
These mixtures have not only been found to possess
C 3 Fq/Ar/C 2 H2 .
excellent w, a/N and rjN characteristics, but also for specific
C 2 H2
or 2-C4 He concentrations, can also considerably enhance the electron production in the discharge by the external electron beam, thereby maximizing the current switching efficiency. 13.
Electron attachment rate constant k
a
measurements were performed
over the gas temperature range 300 < T < 700 K in OCIF 3 . C2 F6 . CFs, n-C 4 F1 o, c-C4 Fs, c-C 4 F 6 , and c-C6 F 6 .
These measurements helped to
elucidate the electron attachment mechanisms in these molecules. 14.
Electron
drift
performed
in
C 2 F 6 /CH 4
at
measurements opening
velocity,
a/N
and
pure C2 F6 and CH4 . the gas
switches will
not be
also
were
the gas mixtures C2 F6 /Ar
and
500 K and 700 K.
These
the operation of repetitively
pulsed
temperatures
have shown that
and
7/INmeasurements
300 K,
seriously degraded by operation at
these gas temperatures. 15.
High voltage breakdown
field strength measurements
(E/N)Rim were
performed in the following gases over the temperature range 300 K < T < Z 600 K at atmospheric gas pressures; CF 3 CI, C 2 F 6 , C 3 F8 , I-C3 F6,.
c-C4 Fs and n-C4 F1 o.
These measurements have shown that for C2 F,
(which is considered a prime candidate for use in diffuse discharge opening switches) operation of the switch at these temperatures does not degrade the dielectric properties of this gas, and in fact, the dielectric strength increases slightly over this temperature range.
17
1.5 A.
Publications and Presentations
Journal Publications
The following journal papers were published under full or partial support of this contract. 1.
S. M. Spyrou, S. R. Hunter. and L. G. Christophorou, "Studies of Negative Ion Formation in Fluoroethers and Fluorosulphides Using Low-Energy ((10 eV) Electron Beam and Electron Swarm Techniques." J. Chem. Phys. 81,
4481-4493 (19S4). 2.
K. Nakanishi. L. G. Christophorou, J. G. Carter, and S. R. Hunter, "Perning Ionization Ternary Gas Mixtures for Diffuse Discharge Switching Applications," J. Appl. Phys. 58. 633-641 (1985).
3.
S. R. Hunter, J. G. Carter, and L. G. Christophorou. "Electron Transport Studies of Gas Mixtures for Use in e-Beam Controlled Diffuse Discharge Switches," J. Appl. Phys. 58. 3001-3015 (1985).
4.
S. M. Spyrou and L. G. Christophorou, "Effect of Temperature on the Dissociative Electron Attachment to OClF 3 and C 2 F 6." J. Chem. Phys. 82, 2620-2629 (1985).
5.
S. R. Hunter. J. G. Carter, and L. G. Christophorou, "Electron Transport Measurements in Methane Using Improved Pulsed Townsend Experimental Techniques," J. App[. Phys. 60, 24-35 (1986).
6.
G. F. Reinking. L. G. Christophorou, and S. R. Hunter. "Studies of Total Ionization in Gases/Mixtures of Interest to Pulsed Power Applications," J. Appl. Phys. 60, 499-508 (1986).
7.
P. G. ]htskos and L. G. Christophorou, "Variation of the Electron Attachment to n-C 4 F 1 o with Temperature," J. Chem. Phys. 86, 1982-1990 (1987).
S.
S. R. Hunter. "Comment on Shortening of Electron Conduction Pulses by Electron Attachers 02, N 2 0, and CF4," J. Appl. Phys. 60, 4335-4337 (1986).
9.
S. R. Hunter. J. G. Carter, and L. G. Christophorou. "Electron Attachment and Ionization Processes in CF4, C 2 F6 , CaFe, and n-C4 F 1 o," J. Chem. Phys. 86, 693-703 (1987).
10.
S. R. Hunter, J. G. Carter, and L. G. Christophorou, "Electron Motion in CF 4 , C 2 F 6 . C 3 F9, and n-C 4 F1 o," Phys. Rev. A, in press.
11.
L. G. Christophorou, R. A. Mathis, S. R. Hunter, and J. G. Carter, "Effect of Temperature on the Uniform Field Breakdown Strength of Electronegative Gases," 1. AppI. Phys. 63, 52-59 (1988).
12.
S. R. Hunter, J. G. Carter, Attachment. Ionization and Drift preparation.
and L. G. Christophorou, in SiF 4 and BF3 ," J. AppI.
"Electron Phys., in
18
"Electron
Attachment
to
NF3 ."
J.
Appl.
Phys.,
in
13.
S. R. Hunter, preparation.
14.
S. R. Hunter, J. C. Carter, and L. G. Christophorou. "Electron Transport in C 2 F 6 /Ar and C 2 F/M4 Gas Mixtures at Elevated Temperatures," 1. Appl. Phys., in preparation.
B.
Conference Proceedinqs
The following papers were published in conference proceedings under full partial support of this contract.
or
1.
L. G. Christophorou, S. R. Hunter, J. C. Carter, and S. M. Spyrou. "Gases for Possible Use in Diffuse-Discharge Switches," In Proceedings of the Workshop on "Diffuse Discharge Opening Switches" (M. Kristiansen and K. M. Schoenbach, Eds.), Texas Tech University, Lubbock. Texas, 1982, pp. 236-251.
2.
J. C. Carter, S. R. Hunter, L. C. Christophorou, and V. K. Lakdawala, "Electron Drift Velocity and Ionization and Attachment Coefficients in Gases/Mixtures for Diffuse-Discharge Opening Switches," In Proceedings of the 3rd International Swarm Seminar (W. Lindinger, H. Villinger, and W. Federer. Eds.), Innsbruck, Austria. 1983, pp. 30-36.
3.
L. G. Christophorou. S. R. Hunter, J. V. K. Lakdawala. "Basic Studies of Gases Applications," In Proceedings of the Externally Controlled Diffuse Discharges, Texas, 1983, pp. 104-133.
4.
L. G. Christophorou, S. R. Hunter, J. G. Carter, S. M. Spyrou, and V. K. Lakdawala. "Basic Studies of Gases for Diffuse-Discharge Switching Applications." In Proceedings of the 4th IEEE Pulsed Power Conference (T. H. Martin and M. F. Rose, Eds.), The Texas Tech Press, Lubbock, Texas, 1983, pp. 702-708.
5.
S. R. Hunter. J. G. Carter, L. G. Christophorou, and V. K. Lakdawala, "Transport Properties and Dielectric Strengths of Gas Mixtures for Use in Diffuse Discharge Opening Switches," In Gaseous Dielectrics IV (L. G. Christophorou and M. 0. Pace, Eds.), Pergamon Press, New York, 1984, pp. 224-237.
6.
K. Nakanishi, L. G. Christophorou, J. G. Carter, and S. R. Hunter, "Penning Ionization Ternary Gas Mixtures for Diffuse Discharge Switching Applications," In Proceedings of the 5th IEEE P'ilsed Power Conference (M. F. Rose and P. J. Turchi, Eds.), IEEE, New York, 1986, pp. 40-43.
7.
S. R. Hunter, J. G. Carter, L. C. Christophorou, and S. M. Spyrou, "Temperature Dependent Electron Transport Studies for Diffuse Discharge Switching Applications," In Proceedings of the 5th IEEE Pulsed Power Conference (M. F. Rose and p. J. Turchi, Eds.), IEEE, New York, 1986, pp. 402-409.
C. Carter, S. M. Spyrou, and for Diffuse-Discharge Switching U.S.-F.R.G. Joint Seminar on Texas Tech University, Lubbock,
19
8.
L. G. Christophorou, S. R. Hunter. J. G. Carter, and S. M. Spyrou, "Effects of Temperature on Dissociative and Nondissociative Electron Attachment," In Swarm Studies and Inelastic itectron-Molecule Collisions (L. C. Pitchford, V. McKoy. A. Chutjian, and S. Trajmar. Eds.), Springer-Verlag. New York. 1986, pp. 303-308.
9.
S. R. Hunter, J. G. Carter, and L. G. Christophorou. "Electron Drift Velocity and Attachment and Ionization Coefficients in CH4, CF 4 , C 2 F6 , CF,. and n-C 4 F," In Swarm Studies and Inelastic Electron-Molecule Collisions (L. C. Pitchford, V. McKoy, A. ChuLjian. and S. Trajmar, Eds.), Springer-Verlag, New York, 1986, pp. 93-94.
10.
S. R. Hunter. "Gas Engineering for Pulsed Power and Switching," In Gaseous Dielectrics V (L. G. Christophorou and D. W. Bouldin. Eds.), Pergamon Press. New York. 1987, pp. 363-372.
11
S. R. Hunter. L. G. Christophorou. and J. G. Carter, "Gas Engineering Studies for High Pressure Self-Sustained Diffuse Discharge Closing Switches," In Gaseous Dielectrics V (L. G. Christophorou and D. W. Bouldin, Eds.), Pergamon Press, New York, 1987, pp. 404-411.
12.
J. G. Carter, S. R. Hunter, and L. G. Christophorou, "Temperature Dependent Electron Transport and Rate Coefficient Studies for e-BeamSustained Diffuse Gas Discharge Switching," In Gaseous Dielectrics V (L. G. Christophorou and D. W. Bouldin, Eds.), Pergamon Press, New York, 1987. pp. 47-54.
13.
L. G. Christophorou, R. A. Mathis. S. R. Hunter, and J. G. Carter, "Effect of Temperature on the Uniform Field Breakdown Strength ot Electronegative Gases," In Gaseous Dielectrics V (L. G. Christophorou and D. W. Bouldin, Eds.). Pergamon Press. New York. 1987, pp. 88-95.
14.
S. R. Hunter, J. G. Carter, and L. G. Christophorou, "Electron Transport Studies in Gaseous Media for Diffuse Discharge Closing Switches," In Proceedings of the International Conference on Phenomena in Ionized Gases (ICPIG XVIII), Swansea, United Kingdom, July 13-17, 1987.
15.
S. R. Hunter, L. G. Christophorou, J. G. Carter, and P. G. Datskos, "New Concepts in High Current Self-Sustained Diffuse Discharge Closing Switches," In Proceedings of the 6th IEEE Pulsed Power Conference, Arlington, Virginia, June 29-July 1, 1987. pp. 1-8.
C.
Book Chapter
The following book chapter was published under contract.
the partial
support
of
this
L. G. Christophorou and S. R. Hunter, "Electrons in Dense Gases." In Swarms of Ions and Electrons in Gases (W. Lindinger, T. D. Mark. and F. Howorka, Eds.). Springer-Verlag/Wien, Austria, 1984, pp. 241-264.
20 D.
Patents and Patent Applications
The following patents and patent disclosures were developed under partial support of this contract. 1. L. G. Christophorou, S. R. Hunter. and J. G. Carter. Gas Mixtures for Diffuse-Discharge Switches, Patent No. 4,490,650 (December 25, 1984). 2.
L. G. Christophorou and S. R. Hunter, Penning Ionization Ternary Gas Mixtures for Diffuse-Discharge Switch (granted).
3.
S. R. Hunter and L. G. Christophorou, Binary and Ternary Gas Mixtures for Diffuse Glow Discharme Closing Switches.
4. L. G. Christophorou and S. R. Hunter, Binary and Ternary Gas Mixtures with Tempe:ature Enhanced Characteristics for Use in Diffuse Glow Discharge Closing Switches. E.
Presentations
The following invited and contributed talks and lectures were presented on the work developed during this contract. 1. L. G. Christophorou, S. R. Hunter, J. G. Carter, and S. M. Spyrou (invited), "Gases for Possible Use in Diffuse-Discharge Switches," Workshop on "Diffuse Discharge Opening Switches," Tamarron, Colorado. January 13-15, 1982. 2.
L. G. Christophorou (invited), "Gases for High Voltage Insulation and Pulsed-Power Technologies." Sandia National Laboratories, Albuquerque, New Mexico, January 21, 1983.
3.
S. R. Hunter (invited), "Electron Transport Studies for Diffuse Discharge Switching Applications," Chemical Physics Seminar. Oak Ridge National Laboratory, Oak Ridge, Tennessee, May 9, 1983.
4.
L. G. Christophorou. S. R. Hunter, J. G. Carter, S. M. Spyrou, and V. K. Lakdawala (invited), "Basic Studies of Gases for Diffuse-Discharge Switching Applications," 4th IEEE Pulsed Power Conference, Albuquerque, New Mexico, June 6-8, 1983.
5.
J. G. Carter, S. R. Hunter, L. G. Christophorou, and V. K. Lakdawala (contributed), "Electron Drift Velocity and Ionization and Attachment Coefficients in Gases/Mixtures for Diffuse-Discharge Opening Switches," 3rd International Swarm Seminar, Innsbruck, Austria, August 3-5, 1983.
6.
L. G. Christophorou (invited), "Development of New Gases for Diffuse Discharge Opening Switches," Workshop on "Externally-Controlled Diffuse Discharges," Bad Honnef, West Germany, August 15-19, 1983.
21
7.
L.
G.
Christophorou,
S.
R.
Hunter,
J.
G.
Carter,
S.
M.
Spyrou.
and
V. K. Lakdawala (invited), "Basic Studies of Gases for Diffuse-Discharge Seminar on Externally Joint U.S.-F.R.G. Switching Applications," Controlled Diffuse Discharges. Bad Honnef, West Germany, August 15-19. 1983. 8.
L. G. Christophorou (invited). "Electron-Molecule Interactions and the Development of Gases for High Voltage Insulation and Pulsed Power Department of Electrical Engineering. Old Dominion Technologies," University, Norfolk. Virginia, March 23. 1984.
9.
S. R. Hunter, J. G. Carter. L. G. Christophorou, and V. K. Lakdawala (contributed), "Transport Properties and Dielectric Strengths of Gas Fourth Mixtures for Use in Diffuse-Discharge Opening Switches," International Symposium on Gaseous Dielectrics, Knoxville. Tennessee, April 29-May 3. 1984.
10.
S. R. Hunter (invited), "Electron Transport Studies of Gas Mixtures for Pulsed Power Diffuse Discharge Switching Applications," Health and Safety Research Division Information Meeting, Oak Ridge National Laboratory. Oak Ridge, Tennessee. October 3-5, 1984.
11.
L. G. Christophorou (invited). "Basic Physics of Gases for Pulsed Power." Ecole Superieure d'Electricite de France, Gif-sur-Yvette, France, July 5, 1984.
12.
L. G. Christophorou (invited), "Basic Physics of Gases for Pulsed Power Technologies," Naval Research Laboratory, Washington. D.C., October 24. 1984.
13.
S. R. Hunter. J. G. Carter, L. G. Christophorou, and S. M. Spyrou (invited), "Temperature Dependent Electron Transport Studies for Diffuse Discharge Switching Applications," 5th IEEE Pulsed Power Conference, Arlington, Virginia, June 10-12, 1985.
14.
R. Nakanishi, L. G. Christophorou, J. G. Carter, and S. R. Hunter (contributed), "Penning Ionization Ternary Gas Mixtures for Diffuse Discharge Switching Applications," 5th IEEE Pulsed Power Conference, Arlington, Virginia, June 10-12, 1985.
15.
L. G. Christophorou, S. R. Hunter. J. G. Carter, and S. M. Spyrou (invited), "Effects of Temperature on Dissociative and Nondissociative Electron Attachment," Joint Symposium on Swarm Studies and Inelastic Electron-Molecule Collisions, Tahoe City, California, July 19-23, 1985.
16.
S. R. Hunter, J. C. Carter, and L. G. Christophorou (contributed), "Electron Drift Velocity and Attachment and Ionization Coefficients in CH4 , CF4 . CF 6,. CaFa, and n-C4 F1 0 ," Joint Symposium on Swarm Studies and Inelastic Electron-Molecule Collisions, Tahoe City, California, July 19-23, 1986.
22
17.
J. G. Carter, S. R. Hunter, and L. G. Christophorou (contributed), "Electron Drift Velocity and Attachment and Ionization Coefficients in C2 F6 /Ar and C 2 F6 /CH4 Gas Mixtures at Elevated Gas Temperatures," 38th Annual Gaseous Electronics Conference, Monterey, California, October 15-18. 1985.
18.
L. G. Christophorou (invited), "From Basic Research to Application: Gas Engineering for High Voltage Insulation and Pulsed Power Technologies," Department of Electrical Engineering, Mississippi State University, Mississippi State, Mississippi, November 21, 1985.
19.
S. R. Hunter (invited). "Electron Transport Studies in Vibrationally and Electronically Excited Gases/Mixtures for Use in e-Beam Controlled Diffuse Gas Discharge Switches," Department of Electrical Engineering, Old Dominion University, Norfolk. Virginia, March 21, 1986.
20.
S. R. Hunter, L. G. Christophorou, and J. G. Carter (contributed), "Electron Drift Velocity and Attachment and Ionization Coefficients for Gases/Mixtures for Use in Diffuse Discharge Switching Applications," 1986 IEEE International Conference on Plasma Science, Saskatoon, Saskatchewan, Canada, May 19-21, 1986; abstract published in IEEE Conference Record Abstracts, IEEE Catalog No. 86CH2317-6, IEEE. New York, 1986, pp. 7-8.
21.
S. R. Hunter (contributed), "Electron Attachment to NF3 Revisited," Thirty-ninth Annual Gaseous Electronics Conference, Madison, Wisconsin, October 7-10. 1986.
22.
S. R. Hunter and L. G. Christophorou (contributed), "Gas Engineering Studies for High Pressure Self-Sustained Diffuse Discharge Closing Switches," Fifth International Symposium on Gaseous Dielectrics, Knoxville, Tennessee, May 3-7, 1987.
23.
L. G. Christophorou, D. L. Mcorkle, and S. R. Hunter (contributed), "Gas Mixtures for Spark Gap Closing Switches with Emphasis on Efficiency of Operations," Fifth International Symposium on Gaseous Dielectrics, Knoxville, Tennessee, May 3-7, 1987.
24.
S. R. Hunter (invited), "Gas Engineering for Pulsed Power Switching," Fifth International Symposium on Gaseous Dielectrics, Knoxville, Tennessee, May 3-7, 1987.
25.
J. G. Carter, S. R. Hunter, and L. G. Christophorou (contributed), "Electron Transport and Rate Coefficient Studies for e-Beam-Sustained Diffuse Gas Discharge Switching." Fifth International Symposium on Gaseous Dielectrics, Knoxville. Tennessee, May 3-7, 1987.
26.
S. R. Hunter, L. G. Christophorou, J. G. Carter, and P. G. Datskos (invited), "New Concepts in High Current Self-Sustained Diffuse Discharge Closing Switches," 6th IEEE Pulsed Power Conference, Arlington. Virginia, June 29-July 1, 1987.
23
27.
S. R. Hunter, J. G. Carter, and L. G. Christophorou (contributed), "Electron Transport Studies of Gaseous Media for Diffuse Discharge Closing Switches," International Conference on Phenomena in Ionized Gases (ICPIG XVIII), Swansea, Wales, United Kingdom, July 13-17, 1987.
28.
L. G. Christophorou, S. R. Hunter, L. A. Pinnaduwage, J. G. Carter, and P. G. Latskos (contributed), "Electron Attachment Properties of Excited for Pulsed Power Dielectric-Gas Molecules and their Possible Use IX International Conference on Gas Discharges and their Switching," Applications, Venice, Italy, September 19-23, 1988.
II.
Experimental Techniques Several
different
experimental
data discussed in this report. commencement of
Most of
to obtain
were employed
the
these experiments existed before the
this program, but some were modified during the course of the
research program.
present
techniques
experiments are described briefly
These
in
this
section. High Pressure Swarm Experiment
2.1
rate constants in 16 1 and Hurst. ' 7 Bortner by the present swarm studies was originally devised The technique used
the electron attachment
to obtain
In Fig. 3 is shown the schematic diagram of the present apparatus.
Electrons
are produced in a plane at the source electrode, perpendicular to the applied electric
field,
particles.
E,
by
the
passage
through
the
gas
of
high
energy
alpha
The alpha particle trajectories are well collimated such that the
electron swarms produced by the energy decay of the alpha particles lie in a Each alpha particle (produced by the decay
well-defined plane at the source. of Cf
2
6
2
,
energy Z 6.1 MeV) produces a swarm of Z 2.3 x 105 electrons in N 2
within Z 5 x 10- 9 s at electrons then drift
133 kPa to 2 x 10- '° s at 3.2 MPa.
to the anode under
These swarms of
the influence of a uniform electric
field established between the cathode and anode. The change in the number n(t) of electrons in the swarm during the time interval dt is dn(t) dt
where
T1
wn(t)
_ -
(11)
-
is the average number of attaching collisions per centimeter of drift,
and w is the drift velocity of the swarm.
The measurements were performed at
high total gas pressures (133 kPa < PT < 3.9 MPa) such that the influence of
24
PROGRAMMABLE SUPPLY(O-3OkV) S
VOLTAGE DIVIDER AND FILTER
HIGH PRESSURE CERAMIC FEEDTHROUGHS
PRESSURE GAGES, GAS HANDLING AND
PURIFICATION SYSTEMS
CATHODE D
VIDEO TO UHV PUMPING
SOURCE
GOLD PLATED ELECTRODES
STAINLESS STEEL HIGH PRESSURE
VESSEL
x Av1
i
PRINTE-R
!,,CROCOMPUTER MULTICHANNEL ANALYZER
PREAMPLIFIER LINEAR AMPLIFIER
Fig. 3.
Schematic diagram of conditioning
the electron attachment apparatus and signal
electronics
used
in
the
room
temperature
attachment rate constant measurements (Ref. IS).
electron
25
diffusion upon the drift of
the swarm was negligibie.
Equation (11)
has the
following solution
(12)
n(t) = n o exp[-(/N a)Na wt]
where 77/Na
is defined as the attachment coefficient with the units of cm2 , Na
is the partial attaching gas number density, and n0 is the initial number of electrons produced by each a-particle at the beginning of the drift space. To follows:
obtain
the attachment coefficients,
the
experiment
A pure buffer gas sample is admitted
into
is performed
as
the vessel at a given
pressure, and a distribution of voltage pulse heights are obtained at a given E/N, which are accumulated in a multichannel analyzer (MCA) operating in the pulse height analysis mode. E/N
The most probable pulse height is recorded.
is changed and a series of most probable pulse heights
function of E/N.
The
is obtained as a
If the pressure dependence of the attachment process is to
be studied, then the above set of measurements are performed over a range of buffer gas pressures as well. to the drift vessel in the present
A small quantity of attaching gas is then added
(usually one part in 105 to 108 of the total gas pressure
study),
the chamber
refilled with
the buffer gas, and a new
series of most probable puise heights obtained as a function of E/N and PT. The ratio of the pulse heights with and without the attaching gas is obtained. The electron attachment coefficient 9/N is then obtained from this ratio. a The drift region electrodes of the experimental apparatus shown in Fig. 3 are housed in a stainless steel high pressure vessel, capable of operating at total
gas pressures up
to 10 MPa.
The chamber
system with a base pressure of a few parts -
of
10 3 Pa per hour.
enameled
insulators
All are
used
reactive attaching gases. Cf
2 52
electrodes
reduce
in 10 6 Pa and an outgassing rate the
chamber are gold plated, and
possible
surface
reactions
with
The alpha paticle source used in these studies was
which was electroplated
the source electrode.
to
in
is pumpcd by a UHV pumping -
onto a thin platinum ribbon and housed within
This source produced
Z104
detectable swarms per second.
The induced anode voltages, produced by the motion of the swarm, are detected and
amplified
microprocessor
by
a
based,
preamplifier 2048
channel
and
linear
amplifier
multichannel
and
analyzer
passed
(MCA),
into
where
a the
26
The pulse height
distribution of the resultant peak pulse heights are stored.
histogram is analyzed by the microprocessor to find the most probable pulse is then used
which
height,
programmable voltage supply and hence obtained,
the
and
procedure
whole
the
resets
then
microprocessor
The
procedure.
iterative
numerical
using a simple
thus ka
i/Na , and
to obtain
the E/N value, and a new histogram is of
details
Further
repeated.
the
experimental technique are given in Ref. 18. High Temperature Swarm Experiment
2.2
The high temperature swarm apparatus is shown schematically in Fig. 4. of
The operation except
that
number
of
were
modifications
the measurements at high gas
facilitate other
a
swarm
distances of
the
apparatus,
similar
is very
this experiment
swarm
that described
to
made
to
chamber
made
was
-100
at a
much
design
cm
long,
to
to our
contrast
the chamber
T7 avoid leaks at the flanges when heating or cooling, and was kept
basic
In
temperatures.
the two end flanges from the middle of
flanges was water cooled
the
above,
and
the
are --50 cm.
the region around the
lower
temperature
than
collision region (the region between the anode and cathode: see Fig. 4).
the This
arrangement also allowed the high voltage and signal feedthroughs to be kept low T.
at a
Special
care was also
taken
in
the construction of
supporting stands holding the anode and the cathode (the Cf
2s
2
the
long
alpha particle
source used to produce the electron swarms by the energy decay of the alpha particles was mounted on a plate supported on the cathode), which consist of a stainless steel rod with insulating rods at its two ends. A
furnace
and
Applied Test System, Inc.) The
temperature
(chromel-alumel,
in
control
temperature
the
system
(0-10000 C,
resolution
1C,
was used to heat the central region of the chamber.
collision region was
type KX) located as close
measured
by
six
thermocouples
to the drift region as possible.
With the aid of two independently controlled heating elements embedded in the insulating walls of
the
furnace, 0
was controlled to within 1-2 C.
the gas
temperature between the electrodes
Further details of the experimental technique
are given in Ref. 19. 2.3
Pulsed Townsend Experiment
The electron drift apparatus used to obtain the room temperature and high temperature electron drift velocity and attachment and ionization coefficient data is shown schematically in Fig. 5.
The high pressure chamber was designed
27
C)
c-
-------
-
----
L.
biD
;S15 i
C)
n
28
0
00
I-
1 atm) electron attachment
High pressure alkanes
and several fluorinated ethers
molecules possess electron attachment
26
the perfluoro-
of
studies
have shown that several of these
rate constants which have
desirable
energy dependences for diffuse discharge switching applications (i.e.,
they
attach electrons efficiently at high energies and have much reduced electron These measurements are
attachment rate constants at near-thermal energies).
summarized in Fig. 8a and have been obtained using a high pressure swarm technique. 18 '26 -2 7
The
molecules
C2 F6
and
interest
for opening switch applications due
thermal
electron
attachment
and
that
the
CF3 OCF 3 to peak
are
their in
comparatively high electron energies for both molecules.
considerable
of
very
low rates of
ka((E))
occurs
at
These two molecules,
along with C4 and CF3 SCF 3 attach electrons dissociatively.
On
the other
hand, molecule C3 Fg (and n-C4 Fo) is particularly noteworthy in that electron attachment to this molecule at atmospheric pressures and ambient temperatures 18
is predominantly
by
parent
negative
ion
stabilization,
and
thus
this
molecule could possibly be used in closed-cycle switches. The electron attachment rate constant for NF3 is plotted in Fig. 9 as a of (C) function measurements.28,29 comparatively 10-
°
-
10- 9
high cm3 S-I]
switch applications.
electron previous several along with This molecule was previously thought to thermal
electron
attachment
rate
constant
attachment possess a [(ka)th
and consequently to be unsuitable for use in opening The present measurements show that the thermal electron
attachment in NF3 is up to two orders of magnitude lower than these estimates [(ka)th
3 x 10- 1 1
cm3 S-1], making this molecule a suitable candidate for
study in switching applications.
34
000 co
0
00 0 0~
0
o
0.
U, WJ
0
(A
>
U.
-- "I
/-
a:
11
x
4--1
'
wr
L.J
z
(.)
so
0 C,,
0~~V
0
o~
wi
00
0
(U~
V
Ln
u
>0
0
09
0)
*
>4
C
LD
0
0
V
mo i
v
Sc
U
~~~~~
SLI.
0
(s
LuD)
-
4
f
CDV
V
o00
0 r4
0
zS.4
U*Q
4
00
W)
"
.
-
-
0C.
SA4
V
I
*-
'M 1NVSNOO ]1V8 IN3Y4HOViIV IVIOI
0
0
0~'S-
35
QSNFiNF3 r
~1o'°
0
N
N
NF3 in Ar
10-
f
1
0
2
0-
and - N " a LAKDAWALA MORUZZI (1980) 2 nLAKDAWALA and MQRUZZ 1 (1950)
a
E-
CHANTRY (1979)
0 SPRESENT
Z
He
-
*
MEASUREMENTS
A
PRESENT MEASUREMENTS
rU
-
N2
-
Ar
£-2
10
Fig. 9.
id
100 10(E) (eV) ENERGY ELECTRON MEAN
The electron attachment
rate constant k
as a function of
mean
a
electron
energy
(C)
for
NF3
in
comparison
with
the
previous
measurements of Chantry (Ref. 28) and Lakdawala and Moruzzi 29).
(Ref.
36
The
total
electron attachment
section a (E) for
cross
of
each
these
as a function of (E)
molecules is related to the attachment rate constant k a (or
E/N)
and
distribution
energy
the electron
of
f(E,E/N)
function
the gas
mixture by
ka(E/N) = rI(E/N)aw(E/N)
where
r1/N a
is
electron mass.
the
normalized
0f
E Y2 cra(E) f(E,E/N) dE
electron attachment
coefficient
,
(13)
and m is
the
The electron energy distribution function is normalized by
.f
(14)
f(E.E/N) dC
0 If both k a(E/N) and f(E,E/N) are known over a wide range of E/N values, then
a(E) can be determined over a wide range of electron energies using a swarm 30 ,iven in The attachment rate constpnrt meg'rc~ment unfolding technique. Figs.
8a
and
9
have
been
obtained
for
these
electronegative
performing the measurements in gas mixtures composed of 106
to
108) of
traces
gases
by
(one part in
the attaching gas in the high pressure buffer gas of argon.
The electron energy distribution functions f(EE/N) and w(E/N) values used in 18 as we have made the justifiable assumption Eq. (13) are those for pure Ar, are unaffected by 18,26 (Any the addition of minute quantities of the attaching gas to the Ar. that
the electron
transport
parameters
and hence
f(EE/N)
observed dependence of k a on the attaching gas partial pressure can be removed 18,26 by extrapolating the measurements to zero attaching gas pressure.) The electron attachment cross sections a (E) obtained from these analyses are plotted in
Figs.
8b and
10 and
values given in Figs. Sa and 9, electronegative electron
gases
energies
which
well
in
indicate
of
indeed have
that we
attach electrons excess
along with the ka((E))
these measurments,
over
thermal
a
wide
energies
found several
energy (E
)
range 0.3
at
eV).
Consequently, the electron attachment processes in a potential switching gas
37
22q
o PNF
3
oS1.8
0
MEASUREMENTS
A,
b 1.6
zo
........ PRESENT
A
1.4
HARLAND and
A
U
A
V)
CHANTRY (1979)
__
,,
iFRANKLI
N (1974)
12
and
.HARLAND
FRANKLIN (19-4) RENORMALIZED
1.0 A
z
~
0.8
A
0.6 0.4
Z&-
0A
0
0
Ae
i0.
E-4
L) f
0~
0
s.
0.0 0.0
1.0
2.0
3.0
4.0
ELECTRON ENERGY
Fig.
10.
The electron attachment
cross
section
5.0
6.0
7.0
8.0
E (eV)
a (E)
for
NF,
in
comparison
with the previous measurements of Chantry (Ref. 28) and Harland and Franklin (Ref. 31).
38
mixture using these gases can be positioned at appropriate (E) or E/N values by choosing a gas which efficiently attaches electrons in that E/N range. Elevated Temperature Electron Attachment Rate Constant Measurements
3.2
energy
storage
circuit is fired, the gas temperature within the switch is expected
to rise
several
hundred
Whenever
several
the
diffuse
discharge
centigrade, and
degrees
in an
switch
inductive
temperatures
operating
An understanding of
likely for repetitively operated switches.
degrees are
of
the factors that affect the operation of the switch under these conditions can be
obtained
by
the
performing
electron
transport
and
rate
coefficient
measurements at gas temperatures above room temperature. has been measured for C 2 F 6
The electron attachment rate constant k ((E)) a over
the
temperature
< T
300
range
< 750 K
in
order
to
investigate
the
influence of gas heating on the electron attaching properties of this molecule (Fig.
11).
the gas
As
temperature
increases,
k a((E))
increases, and
this
increase is progressively larger at lower energies such that the threshold and the k a((E))
the peak in
shift to lower energies at higher gas temperatures.
increases by Z30% over this temperature range near its peak at (E)
The ka((E))
3 eV (Fig. 11). Measurements of ka((E)) have also been performed in CF, and n-C 4 F1 o as a function of
gas
temperature
to 750 K
up
in Ar
electron energy range 0.76 < (E) < 4.8 eV). Figs.
12 and
slightly up
13 and show T - 400 K,
to
to
be
of
(E),
rapidly decreases with
then
gas
the
(over
mean
These measurements are given in
that at a given value
T - 450 K, and finally significantly temperature.
buffer
increases with
k a
decreases only
increasing
T up
increasing T above
to
this
The lower temperature measurements (T < 450 K) have been found
strongly
dependent
on
total
gas
pressure.
indicating
that
parent
negative ion formation processes are significant electron attachment processes at
these
dependent
temperatures. electron
At
higher
attachment
gas
temperatures
processes
are
(T > 450 K)
negligible
pressure
indicating
that
electron attachment to CFa and n-C 4 F1 o at these temperatures is predominantly dissociative. the
gas
These measurements
kinetic
energy
indicate
(and hence
in
the
that
relatively small
vibrational
changes in
populations
of
the
attaching gas) can have a large influence on the electron attaching properties of CFg and n-C 4 Fo which could, in turn, significantly affect the performance
39
ORNL-DWG 84-15088R ,
/
,
/750
K
Z5
400 K
300 K
04 30
E-3 2 F6
zC
IN ARGON
120 0.5
1
1.5
2
Z5
3
3.5
4.5
4
5
5.5
MEAN ELECTRON ENERGY, (E) (eV)
Fig. 11.
The total electron attachment rate constant k
mean
electron
as a function of the
a
•
energy
(E) for
C2 F6
between 300 K and 750 K (Ref. 19).
at
several
gas
temperatures
40
0tL-DWG
S5--94,24R
t0
300 K .
-..-.
.
0'.0" 425 K
.... . -.
l .a
S
S
5.'
-
z
2
U
12
0-
$w
,-..
--
450 K /
**
,'
wC3F8
,
i
-
,.".T
IN Ar i
i
,
0
E
750 K 0
8
.
"
2 75 K
3.
3
4..
3.0
3.5
4.0
E-
0D
'A".
10~
di ssion su
0.5
at th4a ofeeto
Fig.
12.
1.0
1.5
2.0
2.5
4.5
5.0
eprtrsgvnintefgr. MEAN ELECTRON ENERGY, (E) 300se Tecrea K ef.•1.orfl (eV) atcmn atti temertue
Electron attachment
rate constant
kI 1 for NT -
(and N
i
- 0)
for
CF, as functions of mean electron energy ()
in a buffer gas of Ar
at the gas temperatures given in the figure.
The curve at 300 K in
Fig. 12b is the dissociative attachment component to the total rate of
electron attachment
discussion).
at
this
temperature
(see
Ref.
19 for
full
41
ORNL-DWG 86-12903 30
n-C 4 F 10 IN Ar
(a) -
25
300 K 350
V)
S 0
15
1
450
&H 5
z Z 0
0 14
(b)
H L2
z
"10 w 10
H
~
6
606
400
0.5
1.0
1.5
2.0
as5
3.0
as5
40
4.5
S
MEAN ELECTRON ENERGY, (z~) (eV)
Fig.
13.
Electron attachment rate constant kIfor n-C.F10 obtained under the same conditions as those for GaF8 in Fig. 12.
42
of repetitively operated switches operating at elevated gas temperatures using C3F. or n-C 4 Fjo. Room Temperature Electron Drift Velocity Measurements
3.3
Pure Cases
A.
in several pure gases
The electron drift velocity w has been measured using our pulsed Townsend experimental apparatus.
The w measurements in gases
which are potentially useful in diffuse discharge opening and closing switches are
in
given
Figs.
14 and
15,
(with
these gases
All
respectively.
the
exception of n-C 4 Fo) possess significantly enhanced w values (i.e. regions of conductivity
negative differential
at
[NDC])
low E/N
comparatively
values,
which as indicated in Fig. 2 is a very desirable characteristic of gases for use in diffuse discharge opening switches. Gas Mixtures
B.
The electron drift velocity w has been measured in several gas mixtures composed of gases
buffer
the attaching gases shown in
an
attempt
to
in Fig. 8a using either Ar or CH 4 as
combine
the
electron
mobility
enhancement
afforded by
the NUC effect at low E/N observed in gas mixtures using Ar and
CH 4 ,
the
with
electronegative
very gases
desirable given
in
electron Sa
Fig.
attaching at
E/N
high
of
properties (or(C)).
the These
measurements are plotted in Figs. 16 to 19 over the concentration range of 0.1 to 100% of the attaching gas in the buffer gas. accuracy of
the
The measurement technique and
resultant data have been discussed
in Ref.
32.
These gas
mixtures all possess pronounced regions of NDC over a range of E/N values. The
NDC
effects
observed
in
several
of
these gas
mixtures
are among
the
largest that have been observed in any gas mixture and are the result of large vibrational inelastic energy loss processes in these electronegative gases at comparatively low electron energies [0.1 < C < 1.0 eV]. Electron drift velocity measurements in several SiF 4/He gas mixtures are plotted in Fig. 20.
These gas mixtures have been found to possess the very
desirable characteristics outlined discharge closing switches.
later in
this section for use in diffuse
They have the added advantage of possessing high
w values at E/N values near the operating E/N value of the discharge when the switch is closed.
43
ORNL-DWG 86-13258
11
U
i
I
Z
10
I
1
OI
1
10
.E-.
a
C3F 8 n-C4Fo
,-4 .&.%J10-"~
t
10 0
E/N (V cm
Fig. 14.
2
Electron drift velocity w versus E/N for n-C4+F 1 o.
-I
CH4, CF4,
C2F6,
CF, and
44
ORNL-DWG 87-10824
K
>--
/
XN
//,' m, SiF4
E/6
id'
4
4
o ,
/CF CH4
?t
10-
i0
1
-16
C
0-
E/N (Vom2)
Fig. 15.
Electron drift velocity w versus EI
for CH4, CF4, SiF4 and BF,.
45
AA U
0.
0
Sk. 0.
0
o Zo
a
.
00
10
00
p.
C
(ISUJ
0L
iO03
0
-4
!
i1C]NK31
00
V46
0-0
CA) 1
C4 (N
(N
Wo
(vL-s
10
0
g900 M
kil:O13A
N
ildO NOdO313W
LAJJ
I.--
00
(f L.JN
(D 1 U
I 'oW
0c
L
I to~ 10 10 C*4 C
C>
(N
L c,, C4
L. C-4C14CL)
0
P
0
0 -
010(
LWC- go90)M Xi 10013A .LJI80 NO LLOJ13
0
47
cv,
C)
0.
U')'
Cza 00
4.
It.*
oO
L
*00 cN
c0
(N
00
W))
C
co
I-S
U-1
900
M
pn
to
iO-1Ai-G
V
r4
O
3
0c
48
V0
af)
-
Lz'K
Z:k U)IOU
0.A
"I pe) PO pn
L
LL
t t
oo 10o3
~
.
~
(I-S
*-
L
)
.'0Ui
9W
.....
~
0
cm.
uG
'010
.-)N
~
N0
(/)
1-
w
000
1*
L
4
idl
....
O
37
biOO3
49
ORNL-DWG 87-6072 |*
I I
|
I
I
I II
II|
I
1
l
0
7
~10 SiF4 /He rin
S
GAS MIXTURES
A"I
0i > 4PERCENTAGE
OF SiF4 £
7~
10 %
0507%
N.100%
d
E/N (V cm 2 )
Fig. 20.
Electron mixtures.
drift
velocity
w
versus
E/N
for
several
SiF 4 /He
gas
50
Room [emperature Electron Attachment and Ionization Coefficients
3.4
The electron attachment and ionization coefficients can be obtained from the pulsed Townsend (PT) experiment using the following analysis. A schematic 20 . diagram showing the operating principle of the PT technique is given in Fig. 21.
In
the
PT method, a
photoelectrically
from
a
small
group
source
on
or
the
swarm
of
cathode
electrons
of
a
drift
is
produced
chamber
by
irradiating the cathode with a short duration ultraviolet (UV) laser radiation light
pulse.
influence
of
The electrons diffuse as the
electronegative
applied
gas
or
at
uniform high
they drift
electric
fields,
to
field.
the
the
In
swarm
anode under
the
may
the
presence of
experience
an
electron
attachment and/or ionization collision processes with the gas. The
present
experiments
were
performed
operating in the voltage integrating mode.
with
the
The motion of
detection
circuit
the electrons and
ions in the drift gap induces a charge on the anode, and hence across C, and thereby establishing an Lime
constant
of
the
increasing potential V(t) preamplifier
input
(T
=
across R (Fig. 21).
RC Z
1 s)
experiments was much greater than those of the electron (Te positive and negative T e (< T++
T,-'
negative
ions
ion
transit
times
(T+ Z T_
10- 4
in 10
Z
the
The
present
-10
-
_l0 - 2 s).
s) or
5
Since
the voltage drop across R due to the drift of the positive and is
negligible
during
the
electron
swarm
transit
time
T
e
Consequently, a break will cccur in the voltage transient as shown in Fig. 22 for w
CF 4
,
allowing
Te
,
and
hence,
the
measured
electron
drift
velocity
= md/Te e , to be obtained from the discontinuity in the waveform. The absolute values of the attachment and ionization coefficients may be
obtained from the ratio of the voltage transients shown in Fig. 22. time
constant
of
the detection circuit
T = RC >> T- Z T+
(i.e.,
When the the
charge
depletion from C is negligible during the positive and negative ion transit times),
then the potential drop across R will rise
to a constant value when
all of the ions have drifted to the anode of the drift gap, i.e.,
VT(t > T_ and T+)
ne -C T
,when
RC >> T_ or T +
(15)
51
ORNL-DWG 81-17850 NEGATIVE HIGH
VOLTAGE SUPPLY
I
CATHODE
PULSED LIGHT SOURCE
'
ANODE
IMPEDANCE PREAMPLIFIER [/ HIGH
ANODE i""O
.......
.... .....
Fig. 21.
I TRANSIENT
>
[>I
RECORDER
I
Schematic diagram showing the principle of the pulsed Townsend (PT)
technique for measuring
the electron drift velocity and electron
attachment and ionization coefficients.
52
ORNL-DWG 85-15787 1.2
I
(a)
1.0 0.8
2 E/N=25.0x10'17 V cm
0.6
0.4 -CF
4 Nd=5.45xl1
0.2 C/)
E-
0.0
_
_
_
_
_
8
_
cm 2 _
_
_
_
_
_
_
_
_
0.8 0.6
17
V cm 2
17
V c2
,E/N=30.0x10-
< 0.4 O
0.2
1.0
(C)
0.8
/N=50.0xl10
0.6 0.4 0.2
0.0
2.0
4.0
50.0
100.0
150.0
200.0 250.0 300.0
TI ME ()us) Fig. 22.
Digitized voltage waveforms obtained in CF, for three values of E/N. corresponding
to
(a)
electron attachment.
low, The
(b) firs-
moderate, segments
and
(c)
high
rates
of
the
waveforms
of were
digitized at 10 ns/channel. and Lhe second segments (occurring after the vertical dajhed lines) were digitized at 50 ns/channel.
53
We define the voltage ratio as eV(Te) + V_(t) + V+(t) V(T)
R
In the presence of voltage ratio R
significant
when t > T
(16)
orT
electron attachment and
ionization,
the
is given by
- r1d
R
- ad exp[ad]
(17)
(1 - exp[ad])
v
ild exp[r7d] - ad exp[ad] -
(lS)
(exp[rtd] - exp[ad])
When electron ionization is negligibly small, Eq. (17)
R
pd
v
1-
(19)
exp[-d](
or when electron attachment is negligible, Eq. (17)
R
R-
v
A.
further reduces to
reduces to
ad ad
(20)
1 - exp[-ad]
Pure Cases
We have performed an extensive series of measurements of ri/N and a/N 02,
kPa.
CF4, C 2 F6 , CF
8
and n-C 4 Fjo over the gas pressure range 0.03 < P < 100
The present measurements in 3
the previous
in
literature data.i '
34
02
are shown in Fig. 23 in comparison with Low energy electron attachment to
02
is
due to three body parent anion formation, which is very strongly dependent on gas
pressure.
changes
in
present
and
the
The measured
ri/N values are
consequently very
experimental
parameters and
the good
previous
measurements
in
this
gas
agreement
indicates
that
sensitive between the
to the
present
technique is capable of obtaining reliable measurements with good accuracy.
54
G 85-18989
-17ORNL-DW
02
z V
I-A
Y(0
DENS
7
n3
A
AIL-
GA0 NUBERG(9)
A
*.3 PRESENT 267 CHANINEta (196 )
LOU
o
2.7t 1.98
z•z-
CCIF 3
~ 1.96
U
L94 1.92
I
I
250
300
350
400
450
500
GAS TEMPERATURE T (K)
Fig. 48.
(E/N)e m versus T for C 2 F6 and OC1F,.
550
600
91
each value of T, the measurements in Figs. 45 and 46 (and similar measurements on the rest of the gases investigated) were fitted to the expression
(E/N)m = (E/N)'im + A/Nd
(24)
(where A is a constant) and the limiting value, (E/N)eim, of E/N was obtained. The values of (E/N)eim at various T are plotted in Figs. 47 and 48 for all six gases studied. These measurements indicate that the effect of increasing gas temperature on the hold-off voltage in opening switch applications will be minimal for gas temperatures T < 600 K for most of these gases. C2 F6 , the gas dielectric properties
increase slightly
respectively) with increasing T, while for slightly
1-C 3 F6
there are
larger decreases
n-C4 Fo (Z 8%) (and c-CF
and C3 F8 , (E/N)em decreases
6
in (E/N)Rm for
be
used
to
c-C4 F8
[?]) over this temperature range.
reductions in (E/N)pim are not beneficial can
(Z 3.6% and Z 1.0%,
(Z 2.6% and 3.0%. respectively) over this temperature range.
other hand,
they
Specifically, for OCIF 3 and
enhance
the
On the
(Z25%) and
Although these
for opening switch applicaitons,
conductivity
in
self-sustained
closing
switches, and thereby increasing the current switching efficiency of this type of switch.4 IV.
Relevance to Switching Experiments From
developed
the measurements given in Section III, several
gas
mixtures
characteristics shown in Fig. 2.
with
the
it is clear
optimized
w(E/N)
that we have and
ka(E/N)
These gas mixtures contain a low percentage
perfluorocarbon (0.1% to 30% in the total gas mixture) and either Ar14 .15 cr 20
CH,4
as a buffer gas.
The E/N dependence of w and the density normalized
electron attachment coefficient q/Na of gas mixtures containing C 2 F6 in Ar and C 2 F6 in Cl14 are given in Figs. 49a and 49b, respectively. (e.g.
the 2% C2 F 6 mixture
in
Fig.
49a and
the 10% C 2 F6
These measurements
mixture
in
Fig.
49b)
indicate that these mixtures have the near ideal characteristics required to optimize Jsw(E/N)
[and hence
Jsw(t)]
shown in
Fig.
2.
Small
scale
e-beam
sustained switching studies have been performed using these gas mixtures and 43 4 6 have shown that switch opening times > 50 ns are achievable.
92
ELECTRON ATTACHMENT COEFFICIENT 2 77/Na (10-17 cm ) 00 0
W
:
0
~Au, E
(3
cc,~ 00 00c
-
0
uj
ELECTRON ATTACHMENT COEFFICIENT 2 7 'Na (10-17 cm ) a,
(
(U
0
0D
Li
-D
0 Go
(i-s wo 9 01.)
m
,kIIDO13A
0
v
Iildal
NOK3313
0
N
4
93
i
0
0
@0
0-0
Ln
LL
bc
'u-
u
i
C4
C
U
m
(-WO V)IM*e A.LISN30 ±N3uuIfo
94
In
z o
w
a~ ~
u %%MOO-
C;.
Cm.L
U))
0 13.
w 00 en
(WO WLqO)/d A.LIALLSISMU
c
ILl
>b
95
4.1
CF/Ar Gas Mixtures
The current density JSW and discharge resistivity p measurements obtained as a
function of
colleagues several
E/N
in the e-beam switching experiments of
(Ref. 44, 45) are given
concentrations
of
C2 F6
in Figs. 50
in
Ar.
A
and 51,
strong
Schaeffer and
respectively.
negative
for
differential
conductivity is seen in the measurements given in Fig. 50, with the peaks in
JSW occurring at E/N z 2-3 x 10- 11 V cm2 (1 Td = 10-11 resistivity measurements given in Fig. 51 2.5 < E/N ( 5 x over
an
order
-
10 £7
of
indicate
2
V cm . the resistivity of
magnitude.
These
characteristics at decrease
low E/N
in conductivity
2
Similarly, the
).
that over
the E/N
indicate
that
increase
which occur during switch opening.
a
mixture
the best electrical conductivity
(i.e. during switch conduction), and
(i.e.
range
the discharge increases by
measurements
composed of approximately 2% C 2 F 6 in Ar has
V cm
the
in resistivity) at high E/N
largest values
The reduction in conductivity was found to
be primarily due to the onset of electron attachment at E/N = 2-3 x 10cm
2
(Fig. 49a) rather than due to the reduction in w at high E/N.
17
V
At low E/N,
the discharge is recombination dominated and the magnitude of the peak ii-jSW was found to be controlled by the maximum in the electron mobility 4(= w/E) at these E/N values (Fig. 5, Ref. 44).
Studies of the switching characteristics
of opening switches using a UV light source to provide a source of electrons in the discharge by photoionization of 45 results. 4.2
the
--As mixture have obtained similar
CF&/CH, Gas Mixtures
The transient electron beam current Ib' the switch discharge current ISW and the switch voltage VSW obtained in the e-beam switching measurements of Commisso and colleagues (Ref. 43) using a mixture composed of 1% C 2 F 6 in C- 4 are given in Fig. 52. Fig.
49b
give
rapid
These measurements indicate that the mixtures shown in switch
opening
upon
termination
of
the
discharge
sustaining source, with switch opening times an order of magnitude faster than those
for pure CH 4 .
This can be seen in Fig. 53 where
the switch voltage
obtained in pure N 2 , CH 4 and a 1% C 2 F 6 in CH4 gas mixture are compared (Ref. 43).
Even at the lower total gas pressure, the C 2 F6 /CH 4 gas mixture exhibits
a larger voltage transient with a much faster decay than pure CH 4. opening
time
in
porc
N,
is
also
orders
of
magnitude
slower
The switch
than
for
the
96
ORNL 87-7883
15
'
'
'
300
'
5otm 99% CH4 -1I% C2 F6 L= 1.5,H A = 500 cm2 R=2cm 10
-200 v
is
w
// I
I-
zwJ
/ /
>.
_
/>
W
5-
-00
// / D/
/
-
/
V(0) VC
b 00
0
Fig. 52.
0.5 TIME (O=sec)
The diffuse
discharge
current
and
Ib
,
switch
the voltage
current
1.0
ISW,
the
sustaining
transient developed across
e-beam
the switch
VSW during switch opening using a gas mixture of CH, containing 1% of C 2 F6 (Ref. 43).
97
300
i
i
'
,
1% C2 F6 -CH 4 (5 atM)
200
-
00% CH4
Cl
(10 otto)
.
I I
(/1
>I too-
1100%
-
-
Fig. 53.
-
N2
.
-
-
--
-(6otm}
40C 300 200 00 0 I00 CLOSURE (ns) TIME RELATIVE TO DIVERT SWITCH
Comparison of
switch voltage VSW for the switch opening times and
pure N2 . pure CH4 and a
(Ref. 43).
500
gP
mixture of CH4
containing
1% of C2 F6
98
0
b
LU
C
Le
0
Z
Li)
ul/ (z
j-0 -:
)'N
-..-
)
d
3'(
M
S
-(JO
99
C 2 F 6 /Ca 4 gas mixture.
The calculated discharge parameters (density normalized and E/P) in the C 2 F 6 /CH 4 gas mixture are shown in
electron mobility pN, TI/Na
These calculations show
Fig. 54 in comparison with the discharge current ISW.
(i.e. rapid increase in the
that the rapid decrease in the discharge current
discharge resistivity) in this mixture is due initially to the reduction in siN.
the
times,
later
and at
increase
rapid
at
in rj/t a
switch
the higher
voltages (i.e. high E/P values) causes a further rapid reduction in ISW* 4.3
Enhanced Discharge Current Characteristics
Although
the discussion given above has gas
diffuse
sustained
externally
using
the
indicated
an
as
discharges
feasibility of opening
switch
concept, several problems have been identified which may potentially limit the usefulness of these switches. e-beam
electrons
efficiency) and switch.
in the
the
Of primary concern are the energy losses by the gas
switching
lack of
(i.e.
timing
precise
reduced
control
of
current the
switching
opening of
the
Possible gas engineering solutions to these problems are outlined in
the following discussion. The
electron
source
function
S
in
an
sustained
e-beam
diffuse
gas
discharge is
S= where
(dE/dx) is
the
)JBW
(25)
average energy deposited
electron in the e-beam, JB
in
the gas
primary
is the flux of the e-beam, and W is the average
energy required to produce an electron-positive ion pair by electrons in the e-beam.
by each
the high energy
The W value of a gas is usually approximately twice
the value of the ionization onset energy, with the remaining electron energy being
used
processes
in in
vibrational the
gas.
(for molecular
The
excitation
gases) energy
and is
a
electronic excitation loss
process
in
the
discharge, but a sizeable fraction of the electronic excitation energy can be recovered (i.e., mixture with a
J
increased) d,,ring sw~itclh 1nduction by
low ionization onset gas additive.
If
the
sceding the gas ionization onset
energy is less than the energy of any metastable or resonant electronic states of the predominant gas then extra electrons can be generated in the discharge
100
1.00
c'J %C
0
E
2
0