energy spread) and electron beam facility optimization to make it valid for many (direct ..... Electron beam curing is defined as: "the use of electron beams as an.
Regulating the Performance Parameters of Accelerated Particles for Industrial Applications BY
ASHRAF ALI MOHAMMED EL-SAFTAWY
CONTENTS Abstract …………………………………………………….....
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Introduction …………………………………………………..
2
1.1. Electron beams …………………………………………...
2
1.2. Historical review …………………………………………
3
Electron emission sources …………………………………….
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2.1. Thermionic emission …………………………………….
4
2.2. Field emission ……………………………………………
5
2.3. Photo emission …………………………………………...
5
3
Thermionic electron guns …………………………………….
6
4
Beam quality ………………………………………………….
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4.1. Beam perveance ………………………………………….
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4.2. Beam emittance ………………………………………….
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4.3. Beam brightness …………………………………………
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4.4. Energy spread ……………………………………………
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4.5. Beam energy……………………………………………...
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4.6. Spot size…………………………………………………..
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4.7. Beam uniformity………………………………………….
10
5
Pierce gun……………….…………………………………….
10
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Electron beam applications …………………………………...
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6.1. Electron beam processing advantages ………...................
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6.2. Industrial electron beam applications ……………………
12
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Proposed work.………………………………………………..
15
8
References …………………………………………………….
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1
2
1
ABSTRACT The first step in the electron acceleration process is to extract low energy electrons from a source and to form them into a beam. The electron source and the initial acceleration gaps constitute the injector (electron gun). This injector is the main part in any electron accelerator and often presents the most difficult physical and technological problems. The limitations of the injector are often the main constraint on the performance of a large accelerator. In this review we shall make an overview on the performance parameters of the electron beam (perveance, emittance, brightness, and energy spread) and electron beam facility optimization to make it valid for many (direct and indirect) applications. 1. INTRODUCTION
1.1. Electron Beams A charged particle beam is a group of particles that have about the same kinetic energy and move in about the same direction. Usually the kinetic energies are much higher than the thermal energies of particles at ordinary temperatures. The high kinetic energies and good directionality of charged particles in beams make them useful for applications such as, beam lithography for micro circuits, thin film technology, radiation processing of food, and free electron lasers(1). Electron beam technology has a wide application in various fields including high energy Physics, industrial and material research. Electron beams were used in such devices as cathode ray tubes, X-ray tubes, electron microscopes, and charged particle accelerators. Low-current 2
beams with value of currents within the range microamperesmilliamperes are typical for these devices. The space charge density in such devices is extremely small and its action on the charged particle motion is negligible(2). High current electron beams are used in various microwave tubes: O-type tubes (klystron, traveling wave tubes, etc.), M-type tubes and in the installation of the electron beam technology for electron beam welding, melting and refining of metals. High current accelerators were developed for industrial applications(2). Charged particle beams with high values of currents of order of amperes and tens of amperes are in these beam applications. The space charge density of such beams is rather high and the motion of charged particle is considerably affected by the electric self-field created by the space charge(2).
1.2. HISTOTICAL REVIEW In this section we will briefly include a survey of the historical events that brought up the electron beam technology where it is today. In 1881 T. Edison discovered thermionic emission, J. J. Thompson discovered electrons in 1897, and J. A. Fleming invented the thermionic valve, s diode, in 1905. Soon in 1907, de Forst invented the triode. In the year after 1915, vacuum tubes were further developed (tetrode and pentode) which enters in many applications such as radio, wireless communications, and the beginnings of TV. The development towards higher energies, first driven by the needs of X-ray. Around 1913 Coolidge built X-ray tubes with 100kV which
3
increased to 200 kV in 1922 and to 1 MV in 1931. In 1931 Van der Graaf built 1.5 MV electrostatic generator. In 1939, Pierce developed conical electrode geometries that allowed the electrons to be emitted in a well defined way and open the way to start beam metallurgy. In 1948, results of experiments on 22 species of bacteria (and X-ray) prompted interested from medical devices manufactures. This led to the first commercial sterilizer. Subsequently electron beams were used in medicine for diagnostics and therapeutic purposes since 1970, and for waste water treatment since 1990 (3,4).
2. ELECTRON EMISSION SOURCES 2.1. Thermionic Emission Thermionic emission is the escape of electrons from a heated surface. Electrons are effectively evaporated from the material. To escape from the metal, electrons must have a component of velocity at right angles to the surface and their corresponding kinetic energy must be at least equal to the work done in passing through the surface. This minimum energy is known as the work function. If the heated surface forms a cathode, then at a given temperature T ( oK) the maximum current density emitted is given by: J AT e 2
E W kT
(1)
Where EW is the work function, A is a constant depending on the cathode material. It can be seen that the most important parameter for thermionic emission is the work function. The work function must be as low as possible to use a cathode at an acceptable temperature.
4
Thermionic emitters are used in electron tubes and in specialist electron guns, as for example in klystrons, welding, industrial materials processing and in accelerators for lepton production (5).
2.2. Field Emission The application of a high voltage between a fine point cathode and an anode give a sufficient energy to an electron so that it escapes from the cathode surface. This phenomenon is known as high field emission. It should not be forgotten that the electric field around a point is greatly enhanced relative to the apparent average electric field between the electrodes. The current density emitted by such a point is given by: J eE 1.54 X 10
6
3 2 E2 7 Exp 6.83 X 10 K E
(2)
Where E is the electric field at the emitter, Φ is the work function and K is constant approximately equal to 1. The major disadvantage of this type of sources is that an excessive current density can destroy the points either by erosion or self-heating(5).
2.3. Photo Emission Photons illuminating a metal surface may also liberate electrons. If the photon has energy at least equal to the work function, then the electrons will be emitted. To obtain reasonable emission with normal wavelengths, a low work function material is needed. Intense electron beams require intense light sources, and lasers have been used to obtain very short high intensity electron pulse trains intended for the generation of microwave power of future linear colliders(5).
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3. THERMIONIC ELECTRONIC GUNS Electron guns may have different forms depending on the application. All electron guns utilize an electron source of some kind with the majority using a thermionic source as shown in fig. (1). Wehnelt
Filament current supply
Bias resistor high voltage supply Filament anode
Cross-over
Fig 1. Thermionic Electron gun.
A thermionic electron Gun(6) function is in the following manner. A positive electrical potential is applied to the anode. The filament (cathode) is heated until a stream of electrons is produced. The electrons are then accelerated by the positive potential. A negative electrical potential is applied to the whenelt cap. As the electrons move toward the anode any ones emitted from the filaments side are repelled by the whenelt cap toward the optic axis. A collection of electrons occurs in the space between the filament tip and the whenelt cap. This collection is called the space charge. Those electrons at the bottom of the space charge (nearest to the anode) can exit the gun area through the small hole in the anode. The voltage potential between the cathode and the anode plate accelerate the electrons down the column and is known as the accelerating voltage. Principal parameters on which a gun is based are perveance, working pressure, acceleration voltage, beam current, focusing field, electrode's shape and spacing and minimum spot size. 6
4. BEAM QUALITY 4.1. Beam Perveance An important parameter of the beams which characterized the beam intensity is beam perveance that is determined by: P
I
(3)
3
Ua2
Where I is the beam current expressed in amperes, U a is the accelerating voltage expressed in volts.
If the beam current is expressed in microamperes, the above parameter sometimes is called as microperveance. It is known from experiments and computations that in electron beams the space charge effects become appreciable at perveance values P ≥ 0.1 μA/V3/2
(7)
.
4.2. Beam Emittance Beams extracted from different types of electron sources have to be transmitted without any loss. It is very important that the particles strike the target should have the parameter required for the application. For a beam transmission without loss of the particles, the cross section of the beam must not exceed a given maximum value of a well defined point. To achieve this we introduce an important quantity known as emittance which one wants to minimize for any given current (1, 8). Any electron beam is described in a sex dimensional phase space (x, y, z, px, py, pz). Where (x, y, z) represents the position of the electrons and (px, py, pz) is the components of momentum of the electrons. The emittance of the beam must be invariant and independent of any electrostatic or magnetic fields through which the beam passes, relates to Lioville's theorem (9, 10).
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Lioville's theorem states that the motion of charged particles under the action of conservative fields is such that the local number density in the six dimensional phase space is a conservative quantity. If a group of ions or electrons accelerated through the same potential difference and traveling with small angles relative to the z-direction, we can say that they have a very small spread in z directed velocities and that this spread is an invariant. Then the phase space (x, y, Px,Py) should be an invariant of the beam. For a beam with cylindrical symmetry this implies that the two dimensional area with coordinates (r, Pr) should be an invariant. The .
electrons make a small angle r with the z direction where (11). pr pz
.
r
(4)
And then; r
1
.
(5)
A (r , r )
.
where r is the transverse emittance or simply emittance in the ( r - r ) .
plane. The value A (r , r ) represents two dimensional phase space areas and remains constant along drift region.
4.3. Beam Brightness Another figure of merit used for characterizing the electron beam quality is the beam brightness which is defined in terms of emittance as(11), B
2I
2 r2
(6)
It should be noted that if the emittance is constant, the beam brightness is also a conserved quantity(1).
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4.4. Energy Spread The energy spread of the beam is an important parameter, that is, the width of the energy distribution of the electrons extracted from an electron gun. The distribution of the initial energies of the electrons created in the source should be a minimum(12). There are two factors causes the energy spread: i- the thermal energy that is depends on the electron temperature, ii- the variation of the potential of the point in the source where an electron is created(13). The energy spread depends on the fluctuations in the discharge current and the amplitude of the oscillations in the discharge. The contribution from these effects to the energy spread depends on the gas pressure, the high frequency power and the extraction voltage(14).
4.5. Beam Energy One of the most important characteristics of the beam is the kinetic energy. The beam energy is determined by overall potential difference that the beam as they travel from the source to the final aperture of the gun. This energy, measured in electron volts, is controlled by the energy power supplies and can rage from 5 eV to 100 keV depending on the gun(15).
4.6. Spot Size The spot size is the diameter of the electron / ion beam at a given distance from the gun. The spot is measured visually on a phosphor screen, or by a Faraday cup array; as the edge of the circle may not be clear cut, the full width at half maximum measurement (FWHM) is defined as the width that includes all beam current densities greater than half the maximum densities(16). 9
4.7. Beam uniformity Beam uniformity describes the beam current within the spot. The beam current can be further characterized by measuring the beam current density. The variation of beam current densities at different positions in the spot is called beam current distribution. In most guns this distribution is similar to a normal curve called Gaussian curve. The spot is brightest in the central area and fades off at the edges(16, 17).
5. PIERCE GUN From fig (2), we can discuss the principle of the Pierce gun design and the way in which a properly shaped focusing electrode can result in an electric field that causes the ion and electron flow to be rectilinear (nearly parallel). The Pierce gun design follow from the analysis of one dimension electron or ion flow between plane parallel electrodes
(18)
.
Poisson equation can be written as(8): d 2V J 2 dx 4 0
(7)
Fig. (2) The electrode shapes for a Pierce gun
The primary conditions are dV dV 0 , this means that, no surface dx
dy
charge exist at the beam edge which can give rise to electric fields normal to the beam boundary. 11
The potential distribution inside the beam flow was found by solving Poisson equation to be as: 1
2
3 m 3 J 3x V e 2e 0 2
4
3
(8)
Pierce showed that in this planar problem, an analytical solution to the Laplace equation gives the potential distribution outside the beam as(12): 1
m 3 9J V 2e 4 0
2
3
Z
2
y2
2
3
y 4 cos tan 1 Z 3
(9)
Equation (9) shows that, the zero potential occurs along a line passing through Z = 0 and angle equal to 3 4 2 67.50 with the beam edge. An electrode, which has the source potential and with the shape of this zero equipotential, form one boundary to the fields external to the beam. This electrode known as focusing or Pierce electrode bends electric fields to generate focusing forces near the source. The electric force counteracts the defocusing beam-generated forces on the edge of the beam(1).
6. ELECTRON BEAM APPLICATIONS Applications of electron beams are characterized by their large number and diversity. In high energy physics, the electron guns are widely used to improve the quality of the beam to utilize it for various accelerators applications and research.
6.1. Electron Beam Processing Advantages. Radiation processing particularly with electron beam offers some distinct advantages, some of which are(19,20):
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1. The process is very fast, clean and can be controlled precisely. 2. There is no permanent radioactivity since the machine can be switched off. 3. The electron beam can be steered very easily to meet the requirements of various geometrical shapes of products to be irradiated. This cannot be achieved by X-rays and γ-rays. 4. The high penetrating power of the electron beam allows the efficient curing of thick polymeric articles, highly pigmented inks
and
coatings. Pigmented inks and coatings cannot be
otherwise cured by UV radiation due to low penetrating power. 5. The electron beam radiation process is practically free of waste products and hence there is no serious environmental hazard.
6.2. Industrial Electron Beam Applications There are numerous research and industrial applications of stationary and pulsed electron beams which are related to present or future applications of the electron beam technology(21). In fig (3) an attempt is made to schematically summarize the most important industrial applications of electron accelerators. Electron beam applications are based on different physical action principles (21): 1- Generation of strong local heating (Electron beam treatment). 2- Coulomb interaction with electrons in inorganic materials generating
molecular
excitation
and
ionization
(chemical
processing) or defects (semiconductor treatments). 3- Bremsstralung generation (materials inspection). When electrons with typical energies in the keV and MeV range are absorbed in matter, secondary electrons are produced as a result of the
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energy degradation process. By coulomb interaction of these electrons with the atoms or molecules of the absorber, finally ions, thermalized electrons, excited states and radicals are formed(22).
Fig. (3). Industrial applications of electron accelerators.
Radiation chemical processing of monomers and polymers such as polymerization of monomer / oligomer systems, crosslinking, grafting, and degradation of polymers is based on the generation of chemically reactive species. Crosslinking occurs when two radicals produced on neighboring polymer units recombine. The reactive molecular mass increase and the melting point rises. Crosslinking of polyethylene cable insulators is most widely applied. But also the production of polymer foams, shrink tubes and foils is industrialized since many years (23). However, the most important applications of fast electrons with typical energies between 120 and 300 keV is the curing of solvent free monomers / oligomer coatings, varnishes, paints and printing inks. Electron beam curing is defined as: "the use of electron beams as an energy source to induce a rapid conversion of especially formulated 100% reactive liquids to solids". Irradiation by fast electrons leads to radical or cationic polymerization followed by intensive crosslinking. 13
Dense polymer networks exhibiting excellent abrasive, scratch and chordical resistance are produced in such a way(24). In a similar way, degradation of pollutants in air or water can be initiated by electrons. Electron energies between 150 and 500 keV are sufficient to ensure electron penetration between 0.1 and 1.5 m in air. For technical applications electron-induced chain reactions play an important role. Mineralization of chlorinated hydrocarbons in air is one example of such an energy efficient electron beam process. A chlorine radical stimulated oxidation chain is the main mechanism leading to the mineralization products HCl, Cl2, CO and CO2. It was shown that no hazardous radiolysis products are formed during the irradiation process. Irradiation with fast electrons offers a universal method to degrade also many other air pollutants such as aromatics, ethers and even dioxins and furans(21). Electron-induced defect formation in doped silicon semiconductors is an industrial application which is based on both the formation of atomic displacements in the lattice and the electrostatic interaction of fast electrons. Homogenously distributed defects are generated, e.g., in silicon power switching devices allowing the fine-tuning of switching times(25). During stopping of fast electrons in matter a few percent of their energy is transformed to bremsstrahlung. The total mass stopping power of electrons consists of the mass collision and the relative stopping power. The latter determines the amount of bremsstrahlung which is generated within the absorber. Bremsstrahlung conversion of electrons is effective for absorbers with large atomic numbers and electron energies above 5 MeV. It is often used for materials inspection, but also for sterialization and food irradiation(26).
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From the point of view of radiation damage, ionization and excitation effects are important for gases, liquids, organic solids, polymers, semiconductors, ionic crystals etc. but not for metals (27). Low energy electrons with typical energies of a few ten to 150 keV are not able to induce atomic displacements within the crystallites. The electron energy is transformed to heat. Electron beam power densities up to 10 6 W cm-2 can be obtained from windowless electron sources, leading to immediate melting of the metal. The large field of electron beam welding, melting hardening, film deposition and defect annealing make use of controlled heat generation by electron beams(21). Nanosecond electron pulses withbeam currents of 10 2 to 104 A are used for laser pumping, microwave generation, and plasma heating. Picosecond electron pulses from high quality electron beams are excellent for free electron laser excitation(21).
7. PROPOSED WORK The electron beam Play an important role in material research and materials technology. There are different kinds of electron beam processes that play a role in surface modifications such as: Deposition of thin films, Etching, Surface activation and fictionalization of polymers and Surface cleaning and hardening. The propsed work includes the following steps: 1. Adjusting the electron beam optics and transport parameters by theoretical and experimental methods. The electron beam parameters such as: Flux, Energy, and the divergence will be quantified and controlled to match the required application.
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2. Designing an appropriate focusing system (using electric and / or magnetic field) and controlling its effect on the electron beam line. This will be done by theoretical and / or experimental methods. 3. Application of direct and indirect electron beams. 4. Designing an appropriate system to raise the electron beam energy (with conventional accelerating system and / or plasma accelerating system). 5. Regulating and optimizing the performance parameters of the accelerating system.
8. REFERENCES 1. S. Humphries, Jr. "Charged Particle Beams", John Wiley &sons, NY, 2002. 2. A. Zhigarev, "Electron Optics and Electron Beam Devices", Mir Publishers, Moscow, 1975. 3. R. Bakish, "Electron Beam Technology", John Wiley & Sons Inc., 1962. 4. S. W. Shultz, Proc. Of ebeam2002. Int.conf. on High Power Electron Beam Technology, USA, 2002. 5. M. Iqbal and F. e-Aleem, Modern Trends in Physics Research, edited by Lotfia El Nadi, IOP, 2005. 6. S. Shillar, "Electron Beam Technology", John Wiley & Sons Inc., 1982. 7. S. I. Molokovsky and A. D. Sushkov, "Intense Electron and Ion Beams", Spriger-Verlag Berlin Heidelberg, 2005. 8. A. A. El-Saftawy, M.Sc Thesis, Faculty of Science, Zagazig University, 2007.
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9. A. T. Forrester., "Large Ion Beams Fundamental of Generation and Propagation" , Wiley Interscience Publications, New York 1988. 10.M. M Abdelrahman,., Ph.D. Thesis, Institute fǜr Allgemeine Physik, Vienna University of Technology, Vienna, 2004. 11.Winter H.P., Production of Multiply Charged Particles, Vol. II, Academic Press, NY. 1967. 12.R. G. Wilson, G. R. Brewer., Ion Beams with Application to Ion Implantation, Wiley Interscience Publications, New York 1973. 13.L. Vali, Atom and Ion Sources. A, Wiley Interscience Publication, New York (1977). 14.Zakhary S. G., Rev. Sci. Instrum. 66, 12, 1995. 15. C. J. Karzmark, C. S. Nunan and E. Tanabe, "Medical Electron Accelerators", Mc Graw Hill Inc. 1993. 16.S. K. Mahapatra, S. D. Dhole, and V. N. Bhoraskar; Nucl. Instrum. Methods in Phy. Research A 536, 222, 2005. 17.S. Tsimring, "Electron Beams and Microwave Vacuum Electronics", Wiley & Sons Inc., 2007. 18.Child, C. D., Phys. Rev., 32, 492, 1911. 19. H. F. Mark, N. M. Bikales, C. G. Menges, J. LKroschwitz. eds. Encyclopedia of Polymer science and engineering, Vol.4 John Wiley & sons. New York, 1988. 20.N. P. Cheremisinoff, "Advanced Polymer Processing Operations", Noys Publications, NJ, 1998. 21.R. Mehnert, Nucl. Instrum. Methods. Phy, Research B, 113, 81, 1996. 22.R. Mehnert, in: Ulmann's Encyclopedia of industrial chemistry, Vol A22 (VEB Chemie, Weinheim, 1993. 23.R. Mehnert, in: Application of Particle and Laser Beams in materials Technology, ed. P. Misaelides, nato ASI Series E: Applied Sciences, Vol 283 (Kluwer, Dortrecht, 1995. 17
24.P. K. T. Oldring, ed., Chemistry and Technology of UV and EB Formulation for Coatings, Inks and Paints (Sita Technology Ltd., London, 1991. 25.G. Arcoria, A. Patti and P. G. Fuochi, Radiat. Phys. Chem. 42, 1015, 1993. 26.M. R. Cleleand, C. C. Thomson and M. Strelczyk, Radiat. Phys. Chem. 35, 632, 1990. 27.A.Sosin and W. Bauer, in Studies in Radiation Effects in Solids, ed, E. I. Dienes, Vol. 3 (Gordon and Breach, NY), 1969.
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