have been performed to study magnatlc turning of laser-guided electron beams. This technique. 1s of Interest for beam transport. In circular high-current electron.
© 1985 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE. IEEE Transactions on Nucluar Science. Vol. NS-32. No. 5, Oct&er
MAGNETIC C.
BENDING A G.
OF LASER
Frost, S L. T. Lelfeste,
GUIDED
ELECTRON
1985
BEAM2
Shope, R. 8. Miller, and C. E Crtst
Sandla Natlonal Laboratories Albuquerque, New Mexico 87185 W Science
W
Appllcatlons Albuquerque,
Abstract Experiments have been performed to study magnatlc This turning of laser-guided electron beams for beam transport In technique 1s of Interest circular high-current electron beam accelerators such A l-Me!‘, Z-kA, 50as Sandla’s reclrculatlng llnac electron beam was turned through a 45’ angle wth ns The 45O bend was \i 90% current transport efficiency.
Relnstra InternatIonal New Mexico
Corporation 87109
conventional solenoIda beam transport using high applied magnetic guide fields The magnetic field energy, however, 1s quite large. and the use of electrostatic guldlng would have a slgniflcant advantage. For scaling to higher energies. the advantage would increase Figure 1 shows conceptually the use of dipole turning sectlons to interconnect four straight laser-guided sectlons for reclrculatlon.
accomplished by switching the beam between two laser lonlzed guide channels which Intersected In the Center The beam radlus was of a 680 Gauss turning magnet. observed to Increase as a result of turning by the uniform field in agreement with single particle These slmulatlons predict much smaller slmulatlons emlttence growth for optlmlzed sector magnet bending ‘This work supported under Contract #DE-ACO+76-DP00789 Research Laboratory
by
the
U
S.
Department
and
the
U
S
Army
of
Energy
Bal llstlc
Introduction
Figure
Reclrculatlng
1
sections Laser guldlng’ls a new technique that uses a laser lonlzed channel In a very low pressure background gas to guide a high current relatlvlstlc electron beam by electrostatic attractlon to an ion core which 1s formed when the beam space charge blows Laser guldlng 1s out the less rlgld plasma electrons 2 In that It can prevent the similar to wire guldlng growth of coherent beam motion by phase mls damping In For high current the anharmonic channel potential beams. the electrostatic attractlon to the Ion core 1s and guldlng strengths equivalent to a very powerful, Most lOO-kC solenoIda field can be obtained laser guldlng has been used to transport the recently, 3 lo-kA beam through the ATA with good results This technology promises a revolution In high to fully ut111ze However, current accelerator design the electrostatic guldlng technique, a method is needed to bend the beam so that circular machInes, with multiple passes through the same accelerating If a way could be found to bend gap 1 cure possible the lonlzed channel Into a closed path or to interconnect multiple straight sectlons so as to form a closed path, then betatrons, cyclotrons, and other conventIona accelerator designs could be extended to the high current regime using the powerful electrostatic forces to prevent beam self-expansion Our experiments and damp transverse osclllatlons have been directed at developing such a technique A particular appllcatlon is the reclrculatlng 4 which IS based on the lnductlvely Isolated llnac, CARP Sandla’s test-bed faclllty. accelerating gap ~111 use a 2.5 MV Isolated Blumlen Injector with four passes through the 1 5 MV accelerating gap that 1s energized with a bipolar waveform from a mismatched 4 This scheme transmlsslon line driver tET-2! The requires three passes around a 30-m delay path beam transport requirements for CARP can be met with
linac connected
with by
laser
dipole
guldlng turning.
Because laser guldlng has been shown to work well through accelerating gaps. the new feature 1s beam turning The capabIlIty to turn electrostatically guided beams would have appllcatlons beyond circular accelerators It could be used for a bean-switch yard to direct beams to multiple experlmental areas, or to guide high current electron beams to x-ray converter targets This would allow the comblnatlon of multiple beams. We have developed two different methods for turning electrostatically guided beams One method uses a low energy i1 keV). low current (c 1 A), electron beam to lonlze a curved path in the lowpressure (1 mTorr) argon gas The low energy beam can easily be bent Into a circular path using small magnetic fields The high-energy beam follows this curved path with very little emlttance gain. This method, which may be preferred for the recirculating 5 llnac, IS reported in companion papers. In this experiments that use a dipole paper > we describe magnet to switch the electron beam from one laser guide channel to another. As shown in Fig 2, the 5 x s force 1s used to direct the particles into the
OolS-9499/85ilOoO-2754$01.000
Flgure
2
Magnetic
1985 IEEE
bendIng
of
laser
guided
e-beam
2755 the To accomolish this fB. dl second channel It 1-t also IS matched to the required turning angle necessary to provide sufficient field lntenslty to overcome the electrostatic channel force, 1 e , cB > the beam propagation vector 1s In this manner, E r The accurately locked to the laser defined dIrectIon dipole magnet provides coarse steering while the laser guide channel gives vernier steering and prevents beam expansion. Beparatus The A l-MeV, laser-based
experiment 1s shown schemetlcally in Fig. 3. 2-kA, 50-ns. electron beam 1s generated III a follless diode which IS described ln a 5 and transported 0 6 m on a lasercompanton paper. lonlzed guide channel to a dipole turning magnet It 1s deflected to a second laser lonlzed Here, Intersects the first at a 45O angle In channe 1 which the center of the 7.5 cm square magnet pole piece. Current monitors 11, 12, and 13, respectively, measure the InJeCted current, current exltlng at O’, and current exlt1n.g at 45’. Quartz windows at the O” and admit the w-laser beams, and also serve 45O locatlons as Cherenkov and x-ray converters for beam proflle The Cherenkov light and x-rays are characterlzatlon. diagnosed by open shutter photographs and pin diodes.
ExperImental First, laser guided was characterized wlthout monitors I and I showed 1 2 ln]ected and transported
Results transport through the the bendlng magnet. ldentlcel waveshapes (0’)
beam
chamber Current for
Monitor
I3
showed
that there was no slgnlflcant beam current transmltted on the second laser guide channel Next, the 680 Gauss x 7.5 cm dipole magnet was Installed, and the beam was efflclently turned through the 45’ bend. This can be seen In Fig 5. which compares the transported current waveform at 45’ (from 13) with the ln]eCted waveform (from I,) The pulse width of the deflected beam pulse is narrowed wth respect to the Injected pulse width We believe this pulse narrowing 1s due to klnetlc energy sweep of the ln]ected beam during the rlslng and falling portIons of the beam pulse The flat top portion of the inJected beam pulse, which is centered in the energy acceptance of the transport, 1s totally transmitted. The slight peak on the front of the deflected waveform 1s Hall current from redlally elected plasma electrons,whlch are dlrected in the forward direction by the ; x s8 force. This Hall current has been seen r in simulations of the electrostatically guided 7 transport
--I
’
fi ’
6.
’
‘-___ ,N,E&O CURRENT ED r
m Figure
3
45O
beam
bendlng
experiment
The lonlzed channels are prod”ced.by Z-step photolonlzetlon of the 0 l-O.5 mTorr background of dlethylanlllne gas using “Y (X = 266 nm) laser light linear lonlzatlon density 1s calculated The channel 6 The laser from 2-step photoionlzatlon coefflclents which 1s measured with dylux film and a fluence, and the dlethylanlllne pyroelectrlc energy meter, which 1s measured with an ionlzatlon gauge pressure cross calibrated against a baratron capacltlve The laser are inputs to this calculation manometer, fluence was adjusted to obtain a charge neutrallzatlon fraction fe of l/2
40
60
80
100
1 140
120
Current waveforms Figure 5 Figure 6 shows plots of the beam proflles from digitization of time Integrated x-ray pinhole These plots display the total current photographs. The contalned inside a radius r as a function of r. radll conteinlng half of the total current are 0.47 cm and 0 73 cm for the Injected and turned beams The beam radius thus increases by a respectively. factor of 1.5 as a result of turning through 45’. Analysis of these plots Indicates an on-axls current 2 for the two beams w1 th dens] ty of 1,7 and 0 7 ka/cm both dlsplaylng a Bennett-like proflle 2.2 ,
Figure 4 1s a photograph of the apparatus vlewed exit port with the quartz wlndow and through the 45’ The permanent magnet, which current monitor removed is vlslb1.e In the center of the 40-cm radius chamber. IS adJustable ln strength by moving the yoke
Figure
6
Measured current as a function of
through r
rad:us
r
Bending through a smaller angle was studled by reducing the lntersectian angle of the laser-Ionized guide channels to 18” ann reducing the magnetic field to 280 Gauss For this case, the radius increase was We also demonstrated that the beam not as great without using the could be bent through a small angle dipole bender by eliminating the ftrst guide channel
radtus of curvature closely geometrically for this ConfiguratIon
of
the particles could be more matched The emlttance growth was about 12%
a
and picktng the beam up at a To angle from a foil. the beam propagation vector was In all cases studled. This was accurately locked to the laser dlrectlon. observed by exact overlap of the x-ray image on the with the laser fluorescence quartz converter by reducing the magnetic field to 80 Finally. Gauss and blocklng the second laser beam, It was guiding can be used to cross demonstrated that laser Full transport was observed magnetic field lines across the field Slmulatlon TRACKER, ~8s developed to solve A computer code, the three dimensronal, relatlvlstlc single partfcle equations of motion for a large (- 200) number of representative electron beam particles subject to the external fields produced by multiple charged channels of lnfinlte length and spectfted external static The beam self fields are ignored magnetic fields Since the self fields cancel to order 1,“~~. thts Also. for at higher energtes approximation 1s better a curved beam the self fields fall to cancel due to This 1s not signlflcant If fintte curvature effects the beam radtus is small compared with the radius of curvature The channel forces are modeled as infinitely long linear channels with an essentially The beam 1s Bennett proftle of charge density assumed to be Initially propagating in equlllbrlum The particles are loaded tnttlally on a phase space ellipse. The emittance of t.his tnltial load 1s taken to be the area of this elltpse 7 shows the tralectory plots for the 45” Figure 2-kA beam of 0 5 cm bending experrment with a I-MeV, radius propagating from left to right along an tonlzatlon channel of the same radius with f = l/2.
cl
1 0.2
0.
c4*
11llll, 083
I I
8
Stmulatton result as a functton of
1.4
I 18
I
I Id
I
CY
R Figure
12
current
through
radtus
r
r
Conclusion Experiments and slmulatlons have shown that laser guided beams may be effictrntly deflected through large angles by ustng turning magnets to switch the beam from one iontzed gutde channel to another The demonstrated level of performance should be adequate for many appllcatlons such as beam swatch-yard steering. or directing multiple high current electron beams to x-ray targets The observed emittance gain, however, is cumulative and could lead to severe current loss rn the multtpie bends required for beam transport In the recirculating llnac We have developed another method for generattng curved electrostatic guide channels, which should be superior for the recirculating linac This method. whtch uses a low energy electron beam to ionize a channel In low pressure gas, has already been used to transport an 18 kA beam around a 90’ bend and is described in a 5 companion paper fieferences
[II [21
-II.
Figure
7
Parttcle
trajectory
plot
45’
bend
A second identical The dtpole field is 680 Gauss. tonizatlon channel Intersects the first at an angle of It is seen that the beam is well trapped on the 45” second channel Figure B gives the total current contatned InsIde a radius r as a function of r at the exit point. The slmulatlon predicts efficient Comparing the radius transport of the beam current contatnlng one-half of the total current for inlected and transported beams tmplles a radius increase of The which is in agreement with the experiment two, space plot gtves an emfttance growth by a phase where the emlttance IS taken to be factor of three, the product of the average radius and the average In order to better match the perpendicular momentum beam onto the second channel, a sector magnet desrgn was simulated. A sector magnet confrguratton was chosen wth the dipole field on the inside of the turn was stronger than that on the outside so that the
[31 [41
[51
[61 [71
181
W E MartIn, G. J Caporaso. W M Fawley, D Prostnltz, and A G. Cole, Phys Rev Lett. 2, 685 (1985) J. C Clark, E. D. S Prono. G. J Caporaso, J. Lauer. and K W Struve. IEEE Trans. Nut 1 SCl, NS-30, No 4. 2510 (1983) D. S Prono Proc. of the 1985 Part. Accel in IEEE Conf Paper N3, to be published Trans on Nut Scl (Oct. 1985) Smith Rev W. K. Tucker, to be publlshed I Instrm 50 714 (1979). and D. SC 1 and J. K Temperley, JAP &2 3649 Eccleshall, (1978) S. L. Shope C A. Frost, G. T. Lelfeste, C P D. Klekel. J W Poukey, and B E Crist, Paper X49 and R B Miller, B Godfrey, of the 1985 Part ACC-21 Paper N4, Proc to be published in IEEE Trans Conf Paper on Nut SCI (Oct. 1985) J R Woodworth, T A Green, and C A Frost 5;, 1648 (1985) J Appi Phys “Magrc Slmulatton.” J W Poukey, unpublished G. T. Letfeste, C A Frost, C A Ekdahl. “Laser Guidtng of a Low and R B Miller. 1292 Gamma Beam,” Bull Am Phys SOC (1984)
