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PBPL Tech Notes: 2008-00001

TECH NOTES

MICRO PYROELECRIC ELECTRON GUN

Urd Horberg Lacroix

Supervised by Gil Travish and Prof James Rosenzweig

Abstract

This paper describes an ongoing experiment for construction of a ferroelectric micro electron

gun with kinetic energies of 25keV. Described are concepts on electron emission andacceleration with emphasis on using pyroelectric crystals as an electron source. A setup has been

build to test pyroelectric crystals as emitters and as generators of electric fields for electronacceleration. The purpose of constructing a micro electron gun is to use it as a part of a high

gradient laser powered micro accelerating platform capable of accelerating electrons to energies

of ~1 - 2MeV. The construction of this platform is an ongoing project by R. B. Yoder, G.

Travish and J. B. Rosenzweig.[2] The applications of an inexpensive near relativistic microelectron source are manifold and include the possibility of treating cancer with electron or X-ray

radiation directly at the tumor[12] Ferroelectric electron emission have been studied by manyincluding Rosenman[1] and Brownridge[4] all in which the emitting crystals are much larger

than the crystal proposed for the micro gun. So one of the great challenges for this experiment isto measure electron emission from micro crystals. The minimum beam energy for the emitted

electrons is approximately 25keV, which is the threshold energy in which the electrons can be

accelerated in the laser field. Hence another challenge of this project is to provide a field of at

least 25keV.

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Micro pyroelectric electron gun

Urd Hrberg Lacroix

June, 2008

Physics 199 supervised by Gil Travish and Prof James Rosenzweig

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1 Introduction

This paper describes an ongoing experiment for construction of a ferroelectricmicro electron gun with kinetic energies of 25keV. Described are concepts onelectron emission and acceleration with emphasis on using pyroelectric crys-tals as an electron source. A setup has been build to test pyroelectric crystalsas emitters and as generators of electric fields for electron acceleration.

The purpose of constructing a micro electron gun is to use it as a part of ahigh gradient laser powered micro accelerating platform capable of accelerat-ing electrons to energies of 1 2MeV. The construction of this platformis an ongoing project by R. B. Yoder, G. Travish and J. B. Rosenzweig.[2]

The applications of an inexpensive near relativistic micro electron source aremanifold and include the possibility of treating cancer with electron or X-rayradiation directly at the tumor[12]

Ferroelectric electron emission have been studied by many including Rosenman[1]and Brownridge[4] all in which the emitting crystals are much larger than thecrystal proposed for the micro gun. So one of the great challenges for thisexperiment is to measure electron emission from micro crystals. The mini-mum beam energy for the emitted electrons is approximately 25keV whichis the threshold energy in which the electrons can be accelerated in the laserfield. Hence another challenge of this project is to provide a field of at least25keV.

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Contents

1 Introduction 2

2 Background 42.1 Micro Accelerator Platform . . . . . . . . . . . . . . . . . . . 42.2 Injection into accelerating micro structure . . . . . . . . . . . 52.3 Previous experiments on ferroelectric emission and acceleration 6

3 Designing a Micro Gun 73.1 Electron generation . . . . . . . . . . . . . . . . . . . . . . . . 7

3.1.1 Cathodes . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.1.2 Charge output . . . . . . . . . . . . . . . . . . . . . . . 83.2 Electron acceleration . . . . . . . . . . . . . . . . . . . . . . . 83.2.1 Acceleration by field of pyroelectric . . . . . . . . . . . 93.2.2 Acceleration from field of cathode . . . . . . . . . . . . 93.2.3 Using a second pyroelectric as a electric field source . . 93.2.4 Double geometry . . . . . . . . . . . . . . . . . . . . . 10

4 Basic theory on ferroelectric electron emission 114.1 Pyroelectricity . . . . . . . . . . . . . . . . . . . . . . . . . . . 114.2 Conditions for Pyroelectric Electron Emission . . . . . . . . . 114.3 Electron Energies . . . . . . . . . . . . . . . . . . . . . . . . . 12

5 Experimental Electron Emission 145.1 Setup for testing pyroelectric cathode . . . . . . . . . . . . . . 14

5.1.1 Vacuum setup . . . . . . . . . . . . . . . . . . . . . . . 155.1.2 Materials for the emitting crystal . . . . . . . . . . . . 16

6 Ferroelectric field generation 176.1 Experimental testing . . . . . . . . . . . . . . . . . . . . . . . 176.2 Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

7 Conclusion 20

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2 Background

The experimenting with ferroelectric cathode and electron acceleration, serveto understand the particle source for the micro accelerator platform (MAP)a device that generates electrons and accelerates them to near relativisticenergies of 1 2MeV in less than a mm.

2.1 Micro Accelerator Platform

The micro accelerator platform is a monolithic particle source, a micro devicethat generates electrons and accelerates them to near relativistic energies of 1 2MeV. The electrons are generated by the cathode and injected into

a dielectric micro structure where they are accelerated by a resonant laserfield.The resonant field is constructed by coupling a 1m laser into a sandwichof two dielectric layers separated by a 1m vacuum gap for particle accelera-tion(Fig. 1). Periodic slots in a reflective layer above and below the dielectriccreates an electric field where the phase velocity along the beam trajectorymatches the phase velocity of the particles. This structure results in a highgradient( 1GeV/m) for the particle acceleration.

Accelerators cover a wide range of industrial and health care application,but the applications and accessibility of conventional accelerators are limited

by size and cost. Since fabrication of the MAP makes use of standard semi-conductor techniques and can be powered by a cheap conventional laser thecost of fabrication will be less than one tenth of current radiotherapy linacs.While radiation therapy has proven very successful to treating some typesof cancer, the sideeffects of radiating the body internally by radioactive sub-stances or externally by gamma-radiation are serious and many cancer typesstill have a very low survival rate. Using MAP as a low cost radiotherapylinac will save lives of patients who do not have accessibility to conventionalradiation therapy. The size of the MAP will make it fit on an endoscope forinternal radiation therapy and might provide a better alternative than the

existing linacs.

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Figure 1: A diagram showing the micro accelerator platform from [12].Shown are the electron source the coupling slot and the relevant dimensions.a 0.1m,b 0.3m and total length is 1mm or 1600 structure periods

2.2 Injection into accelerating micro structure

Injection electron beams into micro structures requires a narrow beam sourceto be directed into a narrow a 1m slit and the time scale of the accel-erating fields requires the electron beam to be pulsed in femtosecond timescale.Previous solutions to overcome these problems include modulating thebeam from an RF gun by optical bunching[13].The MAP would avoid these injection problems by having the electron gen-eration and injection being an integral part of the accelerating device.Still the size of the micro structure would provide a challenge by setting alimit to the size of the emitter.The condition for electron acceleration w/kz = c sets a minimum beam

energy of 23.keV for the wave-equation to be solvable. Where is the fre-quency of the structure, is the normalized particle velocity and c is thespeed of light. Producing this field will provide a great challenge for themicro gun since to be an integral part of the MAP this requires the gun toaccelerate electron from rest to this energy within a millimeter.

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The cathode represented in this project suggests that this is solved by ferro-

electric field acceleration.

2.3 Previous experiments on ferroelectric emission andacceleration

Electron emission from ferroelectrics has been a research topic for many years.In 1974 Rosenblum published a paper on the study of pyroelectric electronemission from the LiNbO3. Later several other people have been study-ing ferroelectric electron emission including Rosenman[1], and Brownridge[4]who also studied the effects of dilute gases on electron emission and electronenergies.

In 2004 Geuther and Danon published a paper experimentally showing thatthe field between two LiNb03 facing each other was enough to accelerateelectron to energies higher than 215keV[3].

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3 Designing a Micro Gun

3.1 Electron generation

3.1.1 Cathodes

While this project focuses on ferroelectric electron emission other types ofcathodes are possible.We consider three possibilities (See Table 1.):

1. Photo emission

2. Thermionic emission

3. Field emission

With photo emission, the electrons are excited by a laser to escape thesurface potential, which gives large electron current. The disadvantages arethat this would be a much more impractical construction since it would re-quire ether a second external laser or a BBO for frequency doubling of thelaser for the MAP.An other option is thermionic emission where heating to high temperatureswill release electrons from material such as LiB6. The problem with this isthat the temperatures should be in the order of 1000K which will heat thewhole accelerator. While a extremely hot electron radiation source is notdesirable high temperatures might also course structural damage.Field emission is a process where a strong applied electric field removes elec-trons from surface of a material. Emission occurs by lowering the surfacebarrier through the Schottky effect. In practice shaping the surface allowsfor higher peak fields at desired emission point.For this project we focus on field emission and use ferroelectric crystals togenerate the required fields. A ferroelectric is a crystal which can be spon-taneously polarized by either heating(pyroelectricity) or by a stress on thecrystal(piezoelectricity). The polarization gradient at the surface will giverise to two charged surfaces. The negative surface can be used as a cathode

when the electrons can escape the surface barrier, ether because of a largesurface field(field emission) or by photoelectric emission.

Several experiments have already been done with pyroelectric electronemission[1][6][4], where small temperature changes are enough to produce

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electron currents by field emission alone.

Table 1. A comparison of three forms of particle emissionEmission Advantages Disadvantages

Field Emission Simple Low emitting currentElectrons accelerated

by the pyroelectric fieldLow temperatures needed

Photo Emission High peak current Need a second external laserEasy to control or a BBO for frequencyCan be Pulsed doubling the MAP laser

Thermionic Emission High average current Operate at high temperature

( 1000C)Simple How to heat the crystal

Continuous emission

3.1.2 Charge output

The output requirement MAP is a pulsed current with 104 105e in 100fswith a repetition rate of 107Hz. This gives a desired beam output for theelectron gun on the order of 107A average current.While Rosenmann[1] suggests that pyroelectric field emission will give cur-rents in the order 1012109A/cm2 dependent on the pyroelectric material,

experiments reported there are with large crystals and long time scales com-pared to that of the MAP, and we might see different effects when emittingfrom micro crystals in very short timescales. Another distinction betweenour experiment and their work is an external field applied to the emittingcrystal which should enhance the current. Also etching structures into theemitting surface might give a higher emission rate.

3.2 Electron acceleration

The minimum trapping energy of 25keVrequires a voltage difference of

25kV between the cathode and the slit of the micro structure. If the distanceis 1mm this will require a strong electric field in the order of 10 5 106V/cmA simplified calculation on the energy of a electron accelerated in the field of aLiNbO3 crystal can be made by assuming that the field from the pyroelectricwill be that of a circular disc of charge = T. A temperature change of

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10K will generate a surface charge of 107C/cm2 with a surface field in

the order of 10MV/m.If the radius of the disc is R = 0.5mm the electrons will gain an energyU eR/20 = 28.4keV.

3.2.1 Acceleration by field of pyroelectric

The screening charges of the electrons on emitting crystal will create highelectric field which in will result in acceleration of the emitted electrons.A simplified calculation on the energy of a electron accelerated in the field of aLiNbO3 crystal can be made by assuming that the field from the pyroelectricwill be that of a circular disc of charge = T. A temperature change of

10K will generate a surface charge of 107

C/cm2

with a surface field inthe order of 10MV/m.If the radius of the disc is R = 0.5mm the electrons will gain an energyU eR/20 = 28.4keV.

3.2.2 Acceleration from field of cathode

The emitted electrons will be accelerated in the field produced by the emittingcrystal itself and in the case of a large crystal this can be enough to accelerateelectrons to above 25keV. Brownridge has done experiments with LiNbO3and LiTaO3 with electron energy beams up to 170kEV[4].

3.2.3 Using a second pyroelectric as a electric field source

All these experiments are done with large crystals and in the case of a crystalwith an emitting area of 1m 20m this might not be the case and anexternal field will be needed. Still considering the pyroelectric ability tocreate high field the external field could be provided by a larger pyroelectriccrystal with a high pyroelectric coefficient(Fig.2 (a))Also when needing an external electron source pyroelectric crystal could stillprovide the high electric field for electron acceleration.

For instance TGS crystals are not a very good electron emitters but canproduce a very high electric fields and has experimentally proven to accelerate

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electrons to 130keV1

(a) (b)

Figure 2: Emission with pyroelectric field enhancement. (a) The electronemitting crystal is mounted to a larger pyroelectric crystal in order to createa large field for acceleration. (b) A pyroelectric with a geometry allowing forelectrons to pass through it is placed with its z+ surface facing the electronemitting crystal.

3.2.4 Double geometry

When the field of the pyroelectric crystals is used to accelerate the emittedelectrons double geometry can enhance the electron energy. Fig.2 is a setupfrom an experiment with double geometry where two heated LiNbO3 crystals

are placed with the opposite charged surfaces pointing towards each other[3].If this is to be used for an electron gun, one could imagine the non-emittingcrystal being a coil.

1Rosenmann[1] has reference to an experiment by Sujak and Syslo with TGS in whichelectrons are accelerated to 130keV.

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4 Basic theory on ferroelectric electron emis-

sionIn the following chapter we will provide some basic theory on ferroelectricelectron emission. Most of this work is from Rosenmans paper Electronemission from ferroelectrics[1]

4.1 Pyroelectricity

In 314BC the Greeks discovered that the pyroelectric material turmuline be-came attractive when heated, while in 1756 this was proven to be related toelectricity2.A pyroelectric is a crystal that without an external electric field become po-larized when heated. The polarization is homogenous throughout the crystaland the polarization difference at the surface gives rise to a depolarizatingfield. To compensate for the field a layer of localization charges of non-ferroelectric origin acts as screening charges(Fig. 3). The polarization of thepyroelectrics when heated or cooled is given by P = T where is thepyroelectric coefficient which depend on the material.

Table 1. Physical parameters for pyroelectrics 3

Sample Pyroelectric coefficient,C/(cm2K) relative dielectric permitivity

LiNbO3 0.82 108

31LiTaO3 2.3 108 47

TGS 3 108 49

4.2 Conditions for Pyroelectric Electron Emission

The first time pyroelectric was observed experimentally to emit electronswas by by Rosenblum who studied LiNbO3 in 10

6Torr vacuum[1]. HeatingLiNbO3 to 400K and letting it cool to room temperature, he observed fer-roelectric electron emission without any photostimulation.Electron emission occurs from the polar surfaces of pyroelectric due to un-

compensated charges, by tunneling or overbarrier emission. In air a changeof polarization due to change in temperature will be compensated by free

2[11]3data is found in [1] and [10]

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charges in the air, but if the crystal is placed in vacuum the change in po-

larization will give rise to an electric field which lowers the surface barrier.If this field is large, a process of electron emission will occur neutralizinguncompensated charges at the polar surface.

(a) (b)

Figure 3: (a) Equilibrium state for ferroelectric bulk. (b)Ferroelectric emis-sion. The surface field is outside the crystal bulk is Ed,out. The layer oflocalized screening charges can be considered a dielectric, with a dielectricconstant cr. Both figures are from [1]

4.3 Electron Energies

Both the electron emission and the energy of the emitted electrons dependon the field outside the crystal bulk. For a flat planer crystal placed parallelto a detector(Fig. 3) this field will be given by[1].

Ed,out =P

0

1

1 + dgabdcr

cr. (1)

Where 0 is the permativity of free space and cr is the permativity of the

crystal and the distances are given on Fig. 3. The electron energy will dependon the ferroelectric surface potential cr. With two charged capacitors, thecrystal Ccr, and the capacitor formed by the outer surface of the crystal andthe surface of the electron detector Cgap, connected in parallel the potential

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becomes(Fig. 3).

cr = PCcr + Cgap(2)

Hence the electron energy will increase when the crystal thickness or the gapbetween the crystal and the detector is increased.For a small emitting crystal the electron energies should be optimized by Cgapand equation 1 suggest that this could be done by using a second pyroelectriccrystal as field generator.

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Figure 4: Setup for pyroelectric electron emission. The breadboard in thepicture will be mounted in the vacuum chamber (Fig. 5). The crystal ismounted on a peltier junction which is mounted to a temperature sink. Thepeltier junction is controlled by a temperature controller. In front of theemitting edge of the crystal is a scintillator screen on a motor allowing forthe distance between the crystal and the screen to be varied. A camerameasures emitted light from the scintillor

5 Experimental Electron Emission

A setup has been build to test electron emission from LiNbO3 and is almostready for testing.

5.1 Setup for testing pyroelectric cathode

The heater used to heat the crystal is a small peltier junction which is con-trolled by a temperature controller providing the option for both heating andcooling. The crystal is mounted to the peltier with its z- surface placed infront of a piece of plastic scintillator. When electrons hit the scintillator itfluoresces photons which can be measured with a camera in order to deter-

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Figure 5: Vacuum chamber and ion pump. The empty chamber is pumpedto a pressure of 107Torr. The breadboard with the setup will be mountedvertically inside the chamber.

mine the electron count. Since the electron to photon ratio is ?(have to lookthat up) even very small current will be measurable.The scintillator is attached to a motor such that the distance between thescreen and the crystal can be varied in order to see how this effect the current.

5.1.1 Vacuum setup

The whole setup is mounted on a board placed in a vacuum chamber. Thechamber is first pumped by an turbo molecular pump and then by an ion-

pump. The pressure is measured by an ion gauge an by the pump currentand the data is collected by a computer. A pressure at low 107Torr hasalready been achieved with an empty chamber.In future a controlled leak of gas (e.g. Argon) will be connected to allow forstudy of the effects of background ions on the electron emission. Picture of

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vacuum chamber

5.1.2 Materials for the emitting crystal

LiNbO3 is the material of choice since it has a large pyroelectric coefficientand in many previous experiments has proven to produce a large emittingcurrent.The first experiments are to be done with an x-cut 500m thick LiNbO3wafer which has been diced into 1cmx1cm crystals. The plan is to have thez-surfaces polished with a focused ion beam which can also be used to createpatterns in the surface to enhance the emission.

For use in the micro accelerator platform the emitting area of the crys-tal is limited by the gap in which the electron are accelerated which is 1m 50m. The actual crystal could be bigger. Going from severalmm to microns will have an effect on field and an effect on the charge outputbecause the emitting area is smaller.

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Figure 6: Setup for pyroelectric electron acceleration. Electron are emittedby a DC electron gun and accelerated by a pyroelectric crystal. The electronbeam passes through an anode and a pair of deflectors and the deflectedangle i measured by scintillator and a camera. The position of the anodeand deflector plates are controlled by a motor and the pyroelectric crystal ismounted on a peltier junction.

6 Ferroelectric field generation

We can separately test the field generating crystal by using an external elec-tron beam to probe the field near the surface. Fig. 6 shows the apparatuswe plan to use where an electron beam of low energy ( 2kV) is propagatedperpendicularly to the field generator crystal.

6.1 Experimental testing

In order to experimentally test if small TGScrystals could be used as a fieldgenerator a proposed setup is use an emitting cathode and in vacuum let

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a beam pass by a small TGS crystal parallel to the charged surface. The

electron will then be accelerated by the crystal orthogonal to the chargedsurface. The anode is for controlling the separation distance and angle inwhich the electrons are deflected by the deflecting plates can be used todetermine the kinetic energy of the electrons. field strength of the crystal.The deflection angle is measured with plastic schintillator and a camera.Both the anode and the deflector is mounted on a motor in order to adjustfor the beam trajectory.

6.2 Simulation

(a) (b)

Figure 7: (a) The figure shows a simulation of electron beam deflection by aferroelectric crystal. (b) The electric field Ex due to the ferroelectric crystalat the center x-axis which is orthogonal to the charged crystal surface.

We performed simulations to study the correlation between the angle ofthe electron beam and the field strength of the crystal. Fig. 7 is a simula-tion in Oopec assuming that the ferroelectric can be simplified as a positivelyand a negatively charged surface when the electrically charged surfaces. Thisassumption is valid in perfect vacuum and with no conducting boundaries.

This is of course a simplification when looking at 1, so we can not regard thecharge that the actual screen charges, but since we are just interested in theelectric field strength this does not matterFig. 7 (a) is a simulation of a electron beam passing a TGS crystal withsurface area 0.5mm 0.05mm and thickness 1mm when the temper-

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ature change is T = 10K4. Fig. 7 (b) shows the Ex field of the ferroelectric.

4In this case the pyroelectric has been simplified as two surfaces with a surface chargeofT

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7 Conclusion

We have begun an investigation of pyroelectric electron emission and fieldgeneration. A test stand has been fabricated and various subcomponenthave been build. We have an initial theoretical and numerical model of hourgeometry . IN the coming month we intend to refine our models and beginmeasurements on the crystal emitters and field generators.

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References

[1] Electron emission from ferroelectricsG. Rosenman, D. Shur, Ya. E. Krasik and A. Dunaevsky Journal ofapplied physics 88, 2000

[2] Laser-powered dielectric structure as a micron-scale electron sourceR. B. Yoder, G. Travish, J. B Rosenzweig

[3] Electron and positive ion acceleration with pyroelectric crystalsJeffrey A. Geuther and Yaron Danon Journal of applied physics 97, 2005

[4] Electron and Positive Ion Beams and X-rays Produces by Heated and

Cooled Pyroelectric Crystals such as LiNbO3 and LiTaO3 in DiluteGases: Phenomenology and Applications James D. Brownridge andStephen M Shafroth ? From internet

[5] http://www.cathode.com/pdf/TB-211.pdf

[6] Pyroelectric electron emission from -Z face polar surface of lithium nio-bate monodomain single crystal

El Mostafa Bourim, Chang-Wook Moon, Seung-Woon Lee, VadimSidorki, In Kyeong YooJ Electroceran, 2006

[7] Kinetics of Electron Emission from the TGS Ferroelectric CrystalA. A. Sidorkin, S. D. Milovidova, O. V. Rogazinskaya, and A. S.Sidorkin,Physics of the Solid State, Vol. 42, 2000

[8] Pulsed photoelectric field emission from needle cathodesC. Hernandez Garcia and C. A. Brau

[9] Advanced photocathode simulation and theoryK.L Jensen, D.W. Feldman, P.G. OShea

[10] http://ieeexplore.ieee.org/iel2/3931/11380/00522500.pdf?arnumber=522500

[11] http://en.wikipedia.org/wiki/Pyroelectricityhttp://en.wikipedia.org/wiki/Ferroelectric

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[12] The Micro Accelerator Platform

Gil Travish Particle Beam Physics Laboratory. UCLA Department ofPhysics May 2007 unpublished

[13] Laser-Driven Dielectric-Structure AcceleratorsEric R. Colby

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