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Proceedings of ERL09, Ithaca, New York, USA

ERL09 WG1 SUMMARY: DC GUN TECHNOLOGICAL CHALLENGES
N. Nishimori1, I. Bazarov2, B. Dunham2, J. Grames3, C. Hernandez-Garcia3,
L. Jones4, B. Militsyn4, M. Poelker3, K. Surles-Law3, M. Yamamoto5
1
2
3
4

JAEA, Tokai-mura, Ibaraki 319-1195, Japan

Cornell University, Ithaca, NY 14853, U.S.A.

Jefferson Lab., Newport News, VA 23606, U.S.A.

STFC Daresbury Lab., Warrington, WA4 4AD, U.K.
5

KEK, Tsukuba, Ibaraki 305-0801, Japan

Abstract
This paper summarizes technological challenges ofphotoemission DC guns being developed for the futureenergy recovery linac (ERL) light sources (LS).
Anticipated new applications of ERL-LS demand anelectron gun capable of producing an extremely low
emittance beam at very high average current. The low
emittance requires unprecedentedly high voltage equal toor greater than 500 kV between cathode/anode electrodes
together with high accelerating gradient on thephotocathode. The technological challenge is to develop ahigh voltage insulator system, which can withstand fieldemission from the electrodes. A high voltage processing
technique and a challenge to suppress field emission are
discussed. The high average current requires prolonged
cathode life time, which is governed by ion backbombardment. Challenges to mitigate the cathode damage
caused by ion back-bompardment are surveyed. We also
discuss high voltage power supply which can afford
sufficient high average current, load-lock system capableof accomodating quick cathode exchange to minimize
accelerator down time, andvacum technology tosuppress both field emission and ion back-bombardment.
A gun geometry satisfying both high gun voltage and highaccelerating gradient is also proposed.

INTRODUCTION
Electron guns capable of providing reliable CW beamwith average current ~100mA and emittance of a few
microns (normalized RMS) are being developed for the
next generation energy recovery linacs (ERL) lightsources in various research laboratories [1,2]. A DC
photoemission electron gun with an activated GaAs
photocathode illuminated with 532 nm laser light isconsidered to be one of most promising candidates of theguns for the ERL light sources, since a 350 kV DC gun
successfully delivered 9.1 mA CW electron beam to theJefferson Lab (JLab) 10 kW IR upgrade Free Electron
Laser (FEL) [3]. In this paper we survey technologicalchallenges and related developments in DC
photoemission electron guns as high current sources for
ERLs.
The high voltage power supply determines limits of the
Injectors, Guns, & Cathodes

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maximum beam energy and current from the guns. The
low emittance necessary for ERLs typically requires a DC
voltage equal to or greater than 500 kV to reduce nonlinear space charge effects in the low energy regime [4].
The fluctuation of beam arrival time at insertion devices
should be suppressed for pump-probe experiments using
fs x-rays from ERLs. A study shows synchronization
stability of ERL systems is governed by injector stability[5]. This sets the requirement on the ripple of DC gun
high voltage to be on the order of 10-3. The next
generation ERL light sources usually require the beam
current from 10 mA to 100 mA. Consequently, a highvoltage power supply with voltage greater than 500 kV,
and stability of 10-3 or better and current greater than 10
mA needs to be developed. Conventional Cockcroft
Walton high voltage power supplies with voltage above
500 kV and currents up to 10 mA are used in JLab,
Daresbury Laboratory (DL), and JAEA/KEK. A highvoltage power supply of 100 mA and 750 kV for Cornell
Univ. is developed using cross transformer technology [6].
The ceramic insulator is a simple structure to support acathode electrode inside the vacuum and is electricallyconnected to a high voltage power supply outside thevacuum. Operation of photoemission guns at voltages
greater than 350 kV is however very difficult, since fieldemission from electrode structures can lead to voltage
breakdown, insulator punch-through, and other problemson the ceramics. Recently three ways to resolve the fieldemission problem have been proposed. The first is the useof a ceramic insulator with a controlled bulk resistivity
utilized at DL. This permits any charge build-up on theceramic surface to be dissipated to ground. Using thisinsulator technology, 485 kV was achieved duringconditioning at DL. The second is an inverted insulator
similar to the metal-ceramic X-ray tubes where a highvoltage feed passing through the insulator center isconnected to a high voltage terminal. The inverted
insulator eliminates the electrode structures typical fornormal insulators, which might be the sources of fieldemission. The third is a segmented insulator, where anumber of ceramics are stacked in series with Kovar ring
electrode sandwiched between adjacent two ceramics.
These insulators are widely used in electrostatic

Proceedings of ERL09, Ithaca, New York, USA

accelerators. The guard rings attached to each electrodeprevents field emitted electrons from reaching the ceramic
surface. Potentials on the guard rings are fixed with an
external resistor divider.
High voltage electrode conditioning up to a voltage
typically 20-30% higher than operational is required toreach high photocathode dark lifetime each time after the
gun chamber is exposed to air. No unique recipe for highvoltage conditioning exists. Gun conditioning using noble
gases can be employed to successfully process fieldemission sites in the gun. Successful experience at JLaband DL using krypton in high voltage processing is
described.
Suppression of field emission from the cathodeelectrodes is essential to protect the ceramic insulator,
avoid pressure rise and other problems such as punchthrough on the insulator surface. Several laboratories havedeveloped test stands for dark current measurements
between large area cathode and anode electrodes todetermine the best materials and surface cleaning
techniques for dark current suppression. A combination ofmolybdenum cathode and titanium anode was reported tobe the best combination a few years ago by a group at
Nagoya Univ. [7]. Employing high pressure rinsing
technique used for SRF cavity cleaning is proved to beeffective to suppress dark current from the cathodeelectrodes [8]. This was one of the highlights of theprevious ERL workshop [9]. Since these separate
measurements were performed with a gap shorter than thereal scale electron gun system at voltage much lower than
500 kV, construction of real scale test stands is planned atseveral laboratories. A new real scale study by Jlab
polarized gun group shows that niobium electrodes
demonstrate smaller dark currents than stainless steel.
Vacuum determined by pump system and outgassing rateof vacuum chamber materials governs the cathodelifetime [10]. Vacuum in the 10-10 Pa range with a partialpressure of oxidant like oxygen of less than 10-12 Pa are
required in the gun chambers as the result. The NEG
pump speed was measured as a function of pressure by
the JLab polarized gun group [10]. A massive pump unitof 22,000 l/s NEG and 400 l/s ion pumps is used for theCornell gun to reach mid 10-10 Pa [8]. Outgassing fromvacuum chamber materials can be suppressed by treating
the material or using different materials from stainlesssteels. A 400 °C/96 hours bakeout for SUS304 and
SUS316L in the air as well as vacuum is found to reduce
the outgassing rate to as low as 2x10-14 l⋅Torr/s-1cm-2 [11].
This technique is used in JLab and Cornell Univ. The
outgassing rate of chemically polished titanium is claimed
to be 4.5x10-16 l⋅Torr/s-1cm-2 [12]. This material is used in
preparation and high voltage chambers for JAEA/KEK

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gun system.
Load-locked preparation systems are used to heat clean
activate and store the photocathodes, then transfer theminto the high voltage gun chambers. Separate preparation
systems from the high voltage chamber are widely used in
GaAs photocathode based polarized and un-polarizedguns inclusive Cornell ERL installation [8], and currently
implemented in the JLab FEL, DL ALICE, and
JAEA/KEK guns. The separate system easily
accommodates several photocathode pucks for reductionof machine down–time, and permit testing of different
cathode materials. A survey of various preparation
systems used in DC photoemission guns is presented.
Photocathode operational lifetime is limited by the ion
back-bombardment, where residual gas between cathodeand anode electrodes is ionized by the electron beam andaccelerated towards the cathode surface. The lifetime canbe improved by increasing the drive laser spot size, since
the ion damage would be distributed over a larger area
while the ion production rate remains the same. Lifetime
enhancement was observed for larger laser spot sizes at
JLab polarized gun [13]. The ion production in a beamtransport line downstream from the anode electrode isanother source of ion back-bombardment. A positive
potential barrier to repel the ions [14] has been
experimentally tested at JLab [15] and University ofMainz in their polarized guns. The results of the test aredescribed in this paper.
The lower limit of achievable beam emitance wasrecently formulated [16], which shows that employing
photocathode material with low thermal emittance andapplying high accelerating field on the photocathode are
the keys for generation of high brightness beam. An
actual gun design requires additional design parameterssuch as optimal gun voltage, transverse focusing, andvoltage breakdown criteria. The gun geometry can be
optimized by a computer simulation which takes into
account of all the gun parameters. A segmented gun
design is proposed to decouple two conflicting
requirements of a higher gun voltage with a large
cathode/anode gap and a higher accelerating field with asmall gap.

HIGH VOLTAGE POWER SUPPLY
B. Dunham
The high voltage (HV) power supply for a DC
photocathode gun is one of the most important, but often
overlooked components of the entire system. A wellestablished set of demands is required prior to considering
what power supply to obtain. In this section, a descriptionof these requirements will be covered.

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Proceedings of ERL09, Ithaca, New York, USA

The first item to determine is the highest voltage
needed for routine operation, and what overhead isneeded for conditioning. All HV devices must be
conditioned above the nominal values in order to obtainstable operation, with +20% being a typical number forindustrial devices. Photocathode guns, particularly thoseusing vacuum sensitive photocathodes, often require
additional margin to have good cathode dark
lifetime. Dark currents (from field emission) in the pA
range are sufficient to produce noticeable local heating
and light (x-rays and UV) which contribute to secondaries
and vacuum level increases. For example, if 500 kV is
the desired operating value, 600 kV would provide theminimum acceptable overhead.
For any gun that injects a beam into an RF accelerator,
control of the arrival time, or phase jitter, of the electron
bunch is of critical importance. The phase change at a
distance L away from the gun caused by gun voltage
variation is given by∆ϕ = 2π f

L γ − 1 ∆Vgun,c (γβ )3 Vgun

where ϕ is in radians, f is the RF frequency, c is the speedof light, and ∆Vgun/Vgun is relative ripple of the gun
voltage. In terms of RF phase, variations of the order ± 1
degree are acceptable for low emittance beams. For
example, at 1.3 GHz, ± 1° is approximately ± 2 ps,
corresponding to a shift of ± 450 volts (0.18%) 1 meteraway from a 250 kV gun. The voltage ripple needs to be
specified over the frequency ranges present in the power
supply, typically up to 60 kHz (or more) for modern
switching supplies.
Even monitoring ripple at the levels required at thehigher frequencies for a particular power supply may not
be straightforward. In such a case, we consider using atime-of-flight detector (a beam position monitor, forinstance) downstream from the gun to monitor the arrival
time of the electron bunches. This can easily measure
the arrival time with picosecond accuracy at many tens ofkilohertz, and the resulting signal can be sent back to thepower supply feedback control loop. For known problem
frequencies or instabilities in the HV power supply, one
could also consider feed-forward methods.
The current and current stability are the next mostimportant requirements. The current is determined by the
maximum needs of the accelerator at the nominal voltage,plus some headroom for controls and future
development. Cost of these supplies is typically around$10 per watt, so careful consideration should be given to
the maximum level requested. Additionally, much lessaverage current is needed for processing than for beam

Injectors, Guns, & Cathodes

8

operations, so it is possible to roll-off the current
requirements at higher voltages.
For photocathode guns, the current stability and the
response of the voltage to changes in the current is ofutmost importance. Drive lasers should be stable inpower to < 1%, so the HV should be insensitive tochanges in current of this level over a wide frequency
range. As the cathode efficiency drops over time, it isexpected that the laser power will be increased tocompensate and maintain a constant output.
Another concern for photocathode guns used in ERLs
is how to ramp-up the current to reach the maximum
operating value. Two strategies exist: 1) start in CW
mode at low current and ramp up the bunch charge; or 2)
start in pulsed mode a full bunch charge and increase theduty factor until CW mode is reached. Both methods
have difficulties. For case #1, the focusing changes as thebunch charge is increased, requiring one to either adjust
the optics settings to compensate, or pick a sub-optimal
setting that can work for the full range of bunchcharges. For case #2, one must have a flexible laser pulse
generation system that can handle the full laser power
without damage for duty factors from 0 to 100%. For
existing systems, it is possible to turn on directly to a few
mA without tripping of the RF systems. Beyond that, the
HV power supply must be able to ramp up the current
quickly (50 - 100 ms is desirable) while maintaining a
constant voltage at low ripple.
Accelerator designers often want to modulate the beam
current, or even make gaps in the pulse train, fornumerous reasons. The effects on the power supply
response must be studied carefully for such requests, andincluded in early design of the supply and the control
circuitry. As we have seen already, the voltage must be
kept constant to a few tenths of a percent in order tominimize phase jitter. An instantaneous drop to 0 current
will cause the voltage to rise, and the subsequent turn-on
will cause it to droop, leading to transients in the beamand possible beam loss.
There are a number of mechanical requirements toconsider as well. Many facilities enclose the high voltage
power supply and electron gun in a tank of pressurizedSF6 in order to reduce the size of the device. One
alternative is to enclose the supply in a very large faraday
cage, and connect the gun and power supply using acable, and another is to submerse the gun and power
supply in an oil tank (often used for klystron modulators)[17]. All three have advantages and disadvantages, forexample, using oil is generally not desirable when dealing
with the extreme ultra-high vacuum conditions needed fora photocathode gun. SF6 gas is a greenhouse gas andexpensive, so one must provide a means to recover it
efficiently. For labs that do not cycle the HV pressureProceedings of ERL09, Ithaca, New York, USA

tank very often, commerical SF6 recovery systems areavailable, but tend to take many hours to empty and fill atank. A custom system can be constructed if frequent
empty/fill cycles are needed. There has been somediscussion about finding alternative dielectric gasses incase SF6 is ever banned due to its deleterious effects on
the atmosphere, and our community will have to follow
any legislative changes closely. In addition, as pressuresof 1 to 5 atm are commonly employed, many labs have todeal with recent pressure vessel regulations. The last area
to be concerned about is mechanical vibrations, as mostSF6 systems have a fan (or blower) plus heat exchanger tocool and circulate the gas around the HV
components. The fans should be either isolated or
mounted remotely, and care should be taken to avoid any
mechanical resonances in the pressure vessel that areclose to the frequencies of the fans.
High voltage electrode conditioning techniques will becovered in another section of this paper. To use a DC
power supply for conditioning, a processing resistor must
be inserted between the gun and power supply to limit theamount of current drawn during an arc. Values between
10 and 100 M-Ohm are typically used, but the exact value
will depend on the system. This resistor must be removed
after processing to avoid the voltage drop when drawing
high currents during operation. To do this, the SF6 tankmust be opened, or a method for remotely inserting alower value resistor (or shorting rod) must be included in
the design.
Finally, where does one find the kind of power supplies
that can meet the requirements discussed above? For
voltages up to 225 kV and currents of tens of mA, there
are many products available, as this is in the range ofindustrial X-ray tube manufactures. Beyond this, only a
few companies (in the US) produce the kind of supplies
needed for very high voltage DC photocathode guns. For
example, up to 500-600 kV and currents to ~10 mA,
Glassman High Voltage Inc. and Kaiser Systems Inc.
make good systems. Kaiser Systems has also madecompact supplies to 750 kV/100 mA for the Cornell gun.
Pulse Electronic Engineering Co. Ltd. in Japan has madea power supply to 550 kV/10 mA for the JAEA/KEK gun.

HIGH VOLTAGE INSULATORS
L. Jones
The use of a DC photocathode electron gun confers
many design and operational advantages compared to RF
guns. The flexibility to design a vacuum chamber withany number of ports permits extremely high vacuum
levels (XHV) to be achieved, allowing the use of high

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quantum efficiency cathodes such as Gallium Arsenide,
coupled with longer-wavelength drive laser withsignificantly reduced power compared to RF guns.
However, the complication of applying the DC HV bias
needed to accelerate the photoemitted electron beam is an
issue which is pushing the current boundaries ofengineering technology. The insulator must fulfil anumber of key operational criteria, specifically:
separating the gun XHV vacuum from the pressurised HV
insulating gas; providing electrical insulation to the levelof hundreds of kilovolts; withstanding field emission anddissipating charge.
Field emission is the primary limiting factor in theperformance of a photoinjector gun. The presence of afield emitter can severely degrade the electron beam
quality delivered by the gun, or can cause the charging of
ceramic insulators. In extreme cases, this can cause
failure of the ceramic due to punch-through or tracking,or damage/failure of the vacuum vessel due to localised
heating through electron-stimulated desorption. Field
emission also affects the vacuum, so severely degrades
photocathode lifetime.
Early DC guns such as the IR-FEL gun at JLab [18]used an impregnated surface coating to dissipate any
charge accumulated on the ceramic surface through fieldemission. However, the first embodiment of this coating
was not successful as the gun failed to operate at theintended design voltage, though the upgraded gun did
perform at its design voltage.
Another solution is the use of a ceramic with bulkdoped controlled resistivity. This approach has beensuccessfully implemented at Daresbury Laboratory using
the proprietary WESGO 970CD material. This insulator
proved highly effective during HV conditioning to 485
kV, and in beam operations at 350 kV, though problems
have been encountered with the long-term reliability ofthe vacuum joints under load due to thermal cycling
during baking. The favourable electrical performance ofthe material prompted a 3-way collaboration between
Daresbury, Jefferson and Cornell, with the aim of finding
a workable solution using a bulk-doped ceramic withreliable vacuum joints. Presently, CPI have delivered a
14” unit with a ‘book-end’ style vacuum joint toDaresbury, and Kyocera have delivered a 16” version toCornell, both using the WESGO material. Neither ofthese units have yet been tested under electrical ormechanical load.
SLAC proposed the use of an inverted ceramic, using
standard components developed by X-ray tube
manufacturers [19]. This had the significant advantage ofusing off-the-shelf parts, so was cheap, but the draw-backwas that the power supplies and ceramics are only rated to
225 kV, so limiting the operational voltage of a gun based

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Proceedings of ERL09, Ithaca, New York, USA

on this technology to perhaps 200 kV. At Jefferson
Laboratory, the CEBAF source group have recently
installed a polarised gun based on this ceramic
technology, the characteristics of which match well with
their 100 kV gun operating voltage. The IR-FEL group
have designed a gun using two inverted ceramics
mounted in opposition. These will use the WESGO bulkdoped material, and will be rated for operation at 500 kV.
The use of two ceramics in opposition serves to balancemechanical loads within the gun, and to provide
geometric symmetry. The shape of the inverted ceramic
conveys an additional advantage in that the surface planeof the ceramic is almost perpendicular to the HV cathodeball, so significantly reducing the likelihood of fieldemitted electrons impacting directly on the ceramics
themselves, and increasing reliability.
The use of segmented ceramics is common in DC
electrostatic accelerators.
The complete ceramiccomprises a series of small hoops stacked alternately withoverlapping ‘chevrons’ which act as shields for the
insulating material. The chevrons are highly effective atshielding the ceramic from field-emitted electrons, though
clearly there is a large number of ceramic-metal joints
which have to be made in the manufacture of such a
device. The chevrons also have to be connected via aladder of resistors to grade their potential, with one end
tied to earth. This design has been used successfully forthe 100 kV gun at NIKHEF [20], the 200 kV gun at
Nagoya University [21], and the 230 kV gun at JAERIFEL [22]. The segmented ceramics employed in the
JAEA/KEK 500 kV gun is made by Hitacahi-Haramachi[23, 24]. The high voltage conditioning is under progress.
Cornell plan to use a similar design in their next ceramic
to be made by Kyocera.
The use is growing of external load-lock cathodepreparation systems, and this itself is an important step
forward. The use of Caesium in the cathode activation
process invariably leads to contamination of the insulator
over time, ultimately resulting in its electrical failure.
This has been experienced most recently at Daresbury
where insulator failure resulted in the failure of theconditioning resistor.
The focus for development should be to design an
insulator with appropriate levels of vacuum performance,
operating at perhaps 600 kV, and conditioned to 750 kV.
Technology and economics may dictate that such an
insulator will be in multiple segments, so failure of asingle segment will not then necessitate complete
replacement of the insulator, though it does mean there
are additional vacuum joints which must then withstandrepeated cycles of baking under high mechanical load.

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HIGH VOLTAGE PROCESSING
C. Hernandez-Garcia

Introduction
The surface chemistry on the Cs:GaAs photocathodeimposes extreme requirements for the vacuum in the gunchamber, while the need to extract and quickly accelerate
the electron beam demands hundreds of kV with gradientsaround 10 MV/m. It is not sufficient to polish and clean
the electrodes to minimize field emission. Afterassembling the gun and establishing ultra high vacuum
conditions, the electrodes need to be high voltage
conditioned. At an average rate of 5 kV per hour, this is atime consuming but essential process before the gun canbe operated at the desired voltage.
The JLab FEL team has successfully conditioned two
generations of DC photoemission guns to 450 kV for
operation at 350 kV [3,25]. However, field emission has
caused numerous problems puncturing insulators, opening
vacuum leaks and damaging electrodes. These problems
are common to all DC photoemission guns.

High Voltage Processing
High voltage conditioning in DC guns is nominallyperformed under vacuum conditions. Basic requirements
include a current-limiting (conditioning) resistor in series
with the high voltage power supply (HVPS), the ability to
immediately shut-off the voltage at a desired current set
point, plus radiation monitors and vacuum gauges in theform of ion pump read-back. It is important to shut thevoltage off instead of lowering the voltage when thecurrent reaches the desired limit set point, this allows forany charge accumulation in the insulator to drain whilethe voltage is ramped back up.
There are commonly three cases of field emission for aparticular voltage set point: a) erratic current, b) current
increasing with time, and c) self-sustained current. In casea), the usual procedure is to maintain the voltage for a few
minutes until the field emission current self-extinguishes,
although in some occasions a sudden current burst
precedes the emitter burn-off. For case b), while thevoltage is held constant, the current slowly increases,
eventually reaching the trip limit and turning-off the
HVPS. In some occasions, the trip limit is reachedsuddenly with a current burst, and when the voltage isrecovered the field emission current has extinguished; inother occasions, the field emitter can sharpen by surface
migration, leading to higher current at the original onset
voltage. A sharp emitter is relatively easy to burn off byadjusting the voltage to limit the field emission current at~10 µA until it self-extinguishes. Case c) is probably theProceedings of ERL09, Ithaca, New York, USA

most difficult to process since the field emission current
can be self-sustained at levels beyond 100 µA. At
hundreds of kV, there is enough power to cause damage.
Pulsing the voltage for a few µs would be ideal, as is donein RF cavity processing. However, the response time of
the HVPS is in the order of tens of ms. Typically this typeof field emitter burns off in tens of minutes if the voltage
is held constant until the field emission current
extinguishes.
Beyond ~150 kV, voltage-induced gas desorption
contributes to the complexity of the process. In theabsence of field emission, the pressure in the gun vacuum
chamber rises from 10-10 Torr to 10-8 Torr with every kV
increment, and the Residual Gas Analyzer (RGA)
indicates increases in H2, CH4, CO and CO2. The time
taken for the vacuum to recover is voltage dependent.
Below 200 kV, it takes around 5 minutes; near 400 kV it
can take up to 60 minutes. However, field-emitted
electrons striking the chamber walls and desorbing gas
dominate the vacuum behaviour. This voltage-inducedgas desorption phase has a very sharp onset. If the gun isfully-conditioned to 350 kV, it can operate for years atthat voltage, but if it is increased by 1 kV, gas desorption
is observed again.

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procedure did not work for the FEL at 270 kV, and Kr gas
processing continued for tens of hours. At 315 kV apattern was observed in both the current and radiation
traces with every kV increment. The pattern resembled
the gas desorption phase observed under nominal vacuum
conditions, only that the signal from the radiation
monitors behaved as the ion pump pressure. The radiation
tracked the high voltage current with every voltage
increment, showing a sharp rise followed by an
exponential decay. High voltage conditioning with Kr
was successful in eliminating emitters and at the sametime the process had evolved into the gas desorption
phase, which continued to 415 kV at a rate of 1 kV/hour,
until both the current and the radiation were at baseline.
Progress was monitored every ten hours with the gun
under nominal vacuum conditions, by verifying that theon-set voltage for observing radiation increased by about
10 kV. Finally, the voltage was ramped to 365 kV andmaintained for several hours while both current andradiation remained at background levels [28]. The FEL
gun is currently operational at 350 kV.

High Voltage processing with inert gases
Gas processing is very effective in burning fieldemitters with self-sustained current. Field emission
current ionizes the inert gas atoms that are accelerated
towards the negatively biased electrodes, effectively
back-ion bombarding the field emitter until the geometryor the work function is altered.
Helium is commonly used for processing
superconducting RF cavities and has also shown goodresults for the Cornell gun [26]. However, there is always
a risk to develop a leak especially in the ceramic insulator
due to the lack of vacuum diagnostics at the 10-5 Torrlevel where the gas processing takes place. It must be
ensured that the pressure is set at the vacuum chamberand not by ion gauges near the turbo pump, where thepressure will be lower. The NEGs do not react with inert
gasses and continue pumping other gasses. It should be
noted that this process is not a DC glow discharge since
the pressure is too low to ignite plasma.
In the JLab FEL gun helium processing was lesseffective, but krypton quickly burned emitters off below
250 kV, as shown in Fig. 1. A detailed description of the
setup is given in [27], later this procedure has also beenhighly successful used at DL.
After processing a field emitter, the Kr gas can bepumped–out, and the ion pumps turned back on to resume
normal high voltage conditioning. However, thisFigure 1: Emitter burning off while ramping up to 250
kV. The horizontal scale is in minutes. The red trace is thecurrent (0-0.5 mA), purple and green traces are theradiation monitors signals (0-100 mR/h), and yellow traceis the voltage (0-400 kV).

FIELD EMISSION MEASUREMENS AT
JEFFERSON LAB
M. Poelker and K. Surles-Law

Intdoduction
As mentioned numerous times above, field emission
inside DC high voltage photoguns can lead to big
problems at accelerator facilities.
Constant low-level
field emission degrades vacuum within the gun, reducing
gun operational lifetime via electron stimulated
desorption of gas and subsequent photocathode QE loss

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Proceedings of ERL09, Ithaca, New York, USAfrom ion back-bombardment. Large bursts of fieldemission can be catastrophic, leading to damage of thephotocathode and other gun components, particularly thehigh voltage insulator, which sometimes results in acomplete loss of vacuum.
These problems wereparticularly difficult to overcome in older “vent/bake”
style photoguns, where the photocathode was activated tonegative electron affinity within the high voltage
chamber, with cesium serving to reduce the work functionof the GaAs photocathode surface, but also inadvertently
reducing the work function of the metallic cathodeelectrode structure as well. Often, a vent/bake gun could
support just a few photocathode activations before
cesium-enhanced field emission made the gun inoperable.
Today, gun groups adopt a load lock-style design, withcesium applied to the photocathode in a separate vacuum
chamber isolated from the high voltage region of the gun.
The key factors that influence field emission insidemodern load-lock style photoguns are the desired
operating voltage of the gun, the gun geometry which
determines field gradient, and the choice of electrodematerials and polishing techniques. Vacuum may notplay a role in the onset of field emission, but cancontribute to enhancement of field emission via ionizationof residual gas and ion back-bombardment. Other factors
are frequently discussed, for example, surface cleanlinessand contamination, and more academic topics such as therole of hydrogen diffusing from the electrode material andgrain boundaries.
Extremely demanding emittance requirements ofproposed ERLs necessitate very high bias voltages: ~ 350
kV or more. The field gradient within the gun can be
adjusted to some extent, for example by choosing anappropriate cathode/anode gap, and by prudently
choosing large distances to other grounded gun
components such as the vacuum chamber - but just as
ERL emittance requirements dictate high bias voltage, ahigh gradient within the photogun is unavoidable because
the beam must be quickly accelerated to relativistic speed
to overcome deleterious effects of space charge. It seems
certain that DC high voltage guns for ERLs must operatewith gradients of 10 MV/m or more, roughly a factor of
two higher than gradients inside the original DC highvoltage photoguns used for decades to generate polarized
electron beams at nuclear and high energy physics
accelerator facilities.
Traditionally, photogun electrodes have been
manufactured from vacuum-arc remelt stainless steel,polished by hand to sub-micron finish with diamond grit[29]. More recently, groups have begun to explore inearnest different electrode materials and polishingtechniques, recognizing the need to reliably manufacture“quiet” electrodes that can operate at very high bias
Injectors, Guns, & Cathodes

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voltage and gradient without field emission [7,30]. Of
note is the extremely thorough study of ref. [7] that
explored primary field emission from the cathodeelectrodes and subsequent field emission enhancement
due to ionization of residual gas and ion-backbombardment of the cathode electrode, including theeffect of stimulated desorption of gas from the anode.
Their work indicated molybdenum performed very wellas cathode material, and titanium serving best for theanode.

Figure 2: The Jefferson Lab High Voltage Test Stand.
Bias voltages up to 250 kV can be applied to cathodeelectrodes attached to the inverted insulator that extends
into the UHV chamber, visible at top of photograph.
Vacuum translation stages, bottom of photograph, provide
a means to reduce the cathode/anode gap to a few
millimeters, to reach gradients > 30 MV/m, but larger
gaps more closely explore the field emission properties ofactual gun designs.
At Jefferson Lab, a high voltage test stand wasconstructed (Fig. 2) with an inverted insulator that allows
high voltage processing of full-size photogun electrodes
up to 250 kV, with a GaAs photocathode installed (butnot activated) and with anode/cathode gap that can be
adjusted from 4 to 50 mm, to vary the gradient over alarge range. Besides providing a means to study different
electrode materials and polishing/processing techniques,
the test stand provides a means to operate cathodeelectrodes at actual CEBAF voltage and gradient before
installation inside a photogun - electrodes can be reworked or discarded if found unacceptable without
wasting accelerator time. In addition, the test standprovides a means for more “aggressive” high voltage
processing techniques without fear of damaging the actual
photogun. More recently, the test stand was used toProceedings of ERL09, Ithaca, New York, USA

quantify benefits of krypton-ion processing (krypton-ion
back bombardment of field emitters) [28].

Figure 3: “Benchmark” results from diamond-paste
polished 304 stainless steel. The cathode electrode had
been used inside a CEBAF 100 kV photogun for many
years. Same field emission data, but plotted versus
gradient (top) and bias voltage (bottom).

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ON” at disappointingly low values of ~ 5 MV/m and ~100 kV.
Everyone knows diamond-paste polishing (DPP) is alabor-intensive process – it can take weeks to polish acomplicated electrode structure. And because results
often vary sample-to-sample and across laboratories, there
are nagging fears that results depend on subtle variationsin polishing technique: for example, diamond particles
can become embedded beneath “peaks’ that get rolled
over due to excessive pressure applied to the sample
during polishing. For these reasons, groups have revisited electropolishing as alternative to DPP. Full detailsof experimental results must wait for another publication,
but preliminary results from Jefferson Lab indicate
electropolishing provides comparable results as shown in
Fig. 3, but requiring significantly less time and effort.
Two single-crystal niobium electrodes were
manufactured and evaluated inside the high voltage test
stand. Both electrodes were polished using the standard
SRF practice known as BCP (buffer chemical polish).
Another SRF-practice was employed: high pressurerinsing as a means to remove contaminants. Niobium
electrode #1 performed very well, exceeding theperformance of DPP stainless steel (Fig. 4), “quiet” to ~150 kV with 50 mm gap.
Niobium electrode #2,
however, performed poorly initially, but improved after
several iterations of krypton processing. Regrettably,both electrodes suffered high voltage breakdowns andcould not sustain subsequent application of comparable
high voltage. This work will continue at Jefferson Lab,
includingstudying
otherniobium
electrodes:
polycrystalline material referred to as fine grain and large
grain niobium, and using the electropolishing technique.

Results
Tests using stainless steel electrodes provide a benchmark
against which other electrode materials and polishingtechniques are compared. The data in Fig. 3 wereobtained using electrodes made of vacuum-arc remelt 304
stainless steel (SS), first polished with silicon carbide
paper and then with diamond grit of successively finer
grit size. The cathode electrode had been used inside a
CEBAF photogun for years, and is considered thoroughly“processed”. One obvious feature of these plots is that
results obtained with small gaps are not particularly
useful – i.e., one cannot assume that since this cathodeelectrode exhibited no measureable field emission at 25
MV/m with a 4 mm gap, it would perform well at 50 mm
gap. Rather, these plots suggest field emission fromdiamond-paste polished stainless steel electrodes, when
configured with typical gun anode/cathode gaps, “turns

Figure 4: Field emission measurements of two single
crystal niobium electrodes, polished with BCP. See text
for details.

Injectors, Guns, & Cathodes

13

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Proceedings of ERL09, Ithaca, New York, USA

Table 1: Basic information of gun vacuum systems
InstituteCornellChambermaterial
SUS316L

Chambertreatment
400°C air-bake,100 hours

Vacuumpump
system
NEG: 20000 L/s
IP: 400 L/s

Bake
condition
150 °C,
24 hours

JLAB FELSUS316 LN

400°C air-bake,360 hours

NEG: 3000L/s
IP: 80L/s

CEBAF

SUS316L &SUS316LN

Daresbury

SS304L &SS316LN

EP-ed,
400°C
vacuum-bake, 200
hours
--

JAEA(250kV)

TitaniumCP

KEK/Nagoya(200kV)

SUS316L &SUS304L

EP

NEG
activation
400 °C,
45 minUltimate Vacuum

250°C
160hours

400°C,
60min

5E-10 Pa

NEG: 8800 L/s(ten WP1250s)
IP: 30 L/s
NEG:3,900 l/s
IP: 150 L/s

250°C
30 hours

400°C,
60 min4 E-10 Pa

200 - 220 °C~ 2 weeks~500 °C,
60 min2 to 4 E-9 Pa

NEG: 2000 L/s
IP: 500 L/s
NEG: 850 L/s
IP: 400 L/s

200 °C
20 hours
200 °C,~100 hours

450 °C,60 min400 °C,~ 3 hours

5 E-9 Pa

Conclusion
For stainless steel electrodes, keeping gradient below ~5MV/m seems prudent, although understandably, thismight be impossible for very high voltage ERL
photoguns. So the search for materials and polishingtechniques that provide quiet electrodes to 10 MV/m at
actual gun voltage must remain a critical R&D focus forthe DC high voltage photogun community.

VACUUM
M.Yamamoto
The ultra-high vacuum system is indispensable forsuppressing ion back-bombardment in photocathode DC
guns. This is because the residual gases in the gun
vacuum chamber are ionized by the extracted electron
beams and accelerated back into the photocathode,resulting in damage of the cathode crystal structure ordegradation of the negative electron affinity of thecathode surface. Improving the ultimate vacuum isstraightforward way to solve the ion back-bombardment
problem.
The ultimate pressure p [Pa] is denoted by p=qA/S,
where q [Pa m/s] is the outgassing rate of the vacuum
chamber material per unit area and unit time, A [m2] theinternal vacuum chamber area and S [m3/s] the pumpspeed. Use of a massive pump system and a chambermaterial with low outgassing rate is essential forachieving the extremely high vacuum (XHV).
The basic information on vacuum system of electronguns at various laboratories are summarized in Table 1.
For reduction of outgassing, components installed in the
Injectors, Guns, & Cathodes

14

out

4 E-10 Pa

2 E-9 Pa

vacuum chamber and chamber itself are rinsed inultrasonically cleaned acetone, ethanol, or deionizedwater solutions. They are polished electrolytically or
chemically, and degassed by vacuum firing before
assembling. The gun chamber is then baked to 150~250°C for about a day to a week to eliminate hydro-carbons,carbon dioxide, carbon monoxide, nitrogen and water.
The partial pressure of water and carbon dioxide should
be sufficiently low 10-11 Pa or less for long cathode darklifetime [2,31]. The most significant source of outgassing
in UHV/XHV is mainly hydrogen dissolved in materials.
There are two ways to reduce the outgassing rate. One isformation of a passive layer, which acts as a barrier forbulk hydrogen diffusion or inhibits surface processes ofadsorption and recombination. The other is formation of asurface layer with low hydrogen content.
Air-baked stainless steel is employed in Cornell
University and Jefferson FEL [8,11]. The thick oxide
layer formed on the stainless surface after the air-baking
reduces the subsequent baking temperature to ~150°C for
shorter time duration around a day. The CEBAF injector
group employs vacuum baked stainless steel (400°C for
200 hours), which was electro-polished and high pressure
rinsed before vacuum baking. A chemically polishedtitanium used for JAEA/KEK gun has very low
outgassing rate of 6x10-13 Pam/s [12]. Use of other low
outgassing materials such as BeCu and SUS316L with
TiN coat may help improve vacuum of gun vacuum
chambers [32-34].
A non-evaporable getter (NEG) pump and an ion pump(IP) are employed in gun vacuum chambers. The NEG
pump provides extensive pumping of the dominant
residual gas of hydrogen under UHV/XHV condition,
while the IP pumps noble gasses and methane that areProceedings of ERL09, Ithaca, New York, USA

poorly pumped by the NEG pump. The IP pump speed
decreases as the operating pressure decreases under UHV
condition less than 10-6 Pa, since ionization rate ofresidual gases inside IP becomes low. Recently, M.
Poelker et al. performed pump speed measurements ofsome commercially available IPs under XHV condition.
The preliminary data indicates the effective pump speed
decreases down to almost zero in the range of 10-10 Pa. A
cryopump designed carefully to fulfill the XHV
specifications may be a candidate as an alternative of IP[35,36].
A limit of ultimate pressure of NEG pumps is estimatedfrom Sievert’s law. This gives extremely low equilibriumpressure of hydrogen at the room temperature. However,there are few experimental data of the NEG pump speed
in the XHV environment except for ref. [37].
In order to achieve ultimate vacuum of the order 10-10
Pa or less with several m2 vacuum area of a gun chamber,
one should use low-outgassing chamber materials of ≤10-10 [Pa m/s] and a vacuum pump with large effective
pump speed of >5 m3/s under XHV condition.

III-V PHOTOCATHODE PREPARATIONSYSTEMS
B.Militsyn
Originally III-V family photocathodes such as GaAs,
GaAsP, InGaAsP and similar were mainly used in DC
guns for production of polarised electrons. As grown,these materials have a positive electron affinity, which for
GaAs is 4 eV. In order to make GaAs photocathodes able
to emit electrons when illuminated by 532 nm light,
typical for ERL DC guns, its surface should be brought to
Negative (NEA) or small, less than 1 eV, Positive Electron Affinity (PEA) state. This process basically comprises deposition on the atomically-clean photocathodesurface of a thin layer of Cs and an oxidant, typically O2
or NF3, and is called activation. Before the activation, thesurface of the photocathode is heat cleaned in order toremove As and Ga oxides.
At earlier stages of the photocathode gun development
and at certain installations operating currently, heatcleaning and activation of the photocathodes were
performed directly in the gun [21,38-40]. Eventually it
was recognised that activation in the gun had serious
disadvantages: activation process control was poor, it wasdifficult to provide extra high vacuum conditions forphotocathode operation, products of the photocathodeheat cleaning and vapour of caesium could contaminate
the gun ceramic which limited the maximum high voltage
achievable in the gun and, finally, replacement of the

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photocathode required several weeks which was not
acceptable for practical installations.
In modern photoinjectors, activation takes place in a
dedicated Photocathode Preparation System (PPS). The
first PPS was developed at SLAC [41] and operated withpolarized electron source of the Stanford Linear Collider.
It was a dedicated vacuum system consisting of twochambers - loading and preparation. The photocathodewas brought into the loading chamber, and thentransferred to the preparation chamber, whose vacuum ismaintained at extreme high vacuum (XHV) conditions,with a vacuum manipulator. For replacement of thephotocathode, the PPS was temporally attached to the gun
forming united vacuum system; the photocathodeactivated in the preparation chamber was then transferredto the gun with a manipulator. The SLAC preparation
system was a great step forward for improving quality ofphotocathode preparation, although the downtime
required for photocathode exchange was still high – aseveral hour period.
The next step in PPS development was made at the
University of Mainz in the framework of the developmentof a polarized electron source for the MAMI project [42].
In the MAMI design, a side-loading mechanism for thephotocathode was implemented which allowed the PPS tobe permanently connected to the gun. This dramaticallyreduced the downtime required for photocathode exchange to the order of one hour. Another solution, which
permits the PPS to be permanently connected to the gun
and allows the more preferable back-loading ofphotocathodes, was proposed at SLAC in their so-called“Inverted gun” [17]. Permanent PPS connection was also
used in traditional guns with a double insulator scheme[19,43] where the PPS was connected to the gun from thehigh voltage side with an additional full voltage insulator.
Recent gun designs are based on vertical orientation of
the insulator and horizontal orientation of the electrodesystem [8,44] also allowing back-loading of thephotocathode.
Modern PPS consist typically of two chambers: aloading chamber (LC) and an activation chamber (AC).
Recently, some PPS have also been equipped with aHydrogen Cleaning Chamber (HCC). Figure 5 shows anengineer’s view of the three chambers PPS which has
been designed for operation with ALICE ERL [45].
As activated photocathodes are very sensitive to thepresence of oxidants in the residual atmosphere, forexample the 1/e lifetime of GaAs does not exceed 2⋅10-8
mbar⋅s of oxygen exposition [46], XHV conditions aremaintained in the AC. The typical pressure in an AC isless than 10-11 mbar, with partial pressures of oxygen,water vapours and CO2 of less than 10-14 mbar. In order toInjectors, Guns, & Cathodes

15

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Proceedings of ERL09, Ithaca, New York, USA

Table 2: Basic parameters of the preparation photo cathode systems designed for operation with high average current
photoinjectors.
Institution/
Installation

DesignPreloading
Preactivation
Activatio
Results
Rejuvenation
VacuumRema
treatment
treatmentn procedure with bulk procedure
conditions
rks
GaAs
CornellTwo chambers
ChemicalHeat cleaning
Cs-NF3
10-15%Heat cleaning
PC 5.0·10-12
University
etching
at 550°C for 2“Yo-Yo”
at 532 nm
mbar(H2SO4) and hours
LC 5.0·10-11
anodizingmbar
STFC
Three chambers
HCl etching
Heat cleaning
Cs-O2/NF3 15% at
AtomicPC 1.4·10-11
Daresbury(up to 6 samples in (not yet
at 450°C“Yo-Yo”
635 nmhydrogenmbar
Laboratory
carousel)
implemented)
cleaning
LC 5.0·10-10
mbar
HCC 4.0·10-11
mbar
JAEA
Two chambers
HCl etching
Heat cleaning
Cs-O
7-10%
AtomicAC 2.5 · 10-9 Pa
at 500°C for“Yo-Yo”
at 633 nm hydrogen
LC 5.0·10-8 Pa
1 hourcleaning(optional)
KEK (Nagoya)Two chambers
HCl etching
Heat cleaning
Cs-O
7-10 %
AtomicAC 1.0·10-8 Pa,
at 500°C for“Yo-Yo”
at 780 nm hydrogen
LC 1.0·10-7 Pa
1 hourcleaning(optional)
TJNAF/CEBAF
Four chambers
Heat cleaning
Cs-NF320% atHeat cleaning
PC 7.0·10-12
Mask
inclusive suitcase
at 550°C for“Yo-Yo”
532 nmmbar
activation
and bakable
2 hours
LC high 10-11
adaptermbar

routinely maintain such extreme vacuum conditions, AC
is usually equipped with a high performance Ion Pumps(IP) and Non-Evaporable Getters (NEG), and it is neverventilated to atmosphere. Vacuum in the AC is measured
with an extractor gauge and a RGA.

Figure 5: Engineer’s view of the PhotocathodePreparation System designed for ALICE ERL.
Loading of the photocathode is performed via the LC,
which is preferably vented to dry nitrogen gas only duringloading. The pumping system of the LC includes an IP
and an oil-free preliminary pumping station. Afterthe photocathode is placed into the LC, it is pumped down
to a pressure of 10-9 mbar and eventually baked out at atemperature of 120-150 °C in order to remove water fromthe samples. Once acceptable vacuum in the LC is established, the photocathode is heat cleaned at a temperatureof typically 600 °C for 1-2 hours. This procedure, depending on PPS design, may take place either in the LC,
HCC or AC. Temperature of the samples is measured with

Injectors, Guns, & Cathodes

16

a pyrometer. In some installations the surface is etched
before loading in a pure nitrogen atmosphere using hydrochloric or sulphuric acid to remove oxides. Thereafter thephotocathode is transferred into the LC in a transport
vessel in pure nitrogen atmosphere to prevent appearanceof new oxides. The heat cleaning temperature of theetched photocathodes may be reduced to 450 °C. Some
vendors cover photocathodes with a thin arsenic layer inorder to prevent its oxidation. This “arsenic cap” isevaporated before activation. A heat cleaned photocathodeis then activated by means of a Cs-O2 or Cs-NF3 “Yo-Yo”
procedure.
Caesium is evaporated from a Cs dispenser while thehigh purity oxidant gas is delivered from a cylinder via aleak valve. Sometimes for better control of the gas stream,
a computer-controllable piezoelectric leak valve is used.
During activation the photocathode is illuminated with alamp or laser and the photocurrent is monitored with apico-ammeter. As at high temperature the Cs source may
emit ions which mask photocurrent, a modulated laser ispreferable. A synchronous detector is then used for
current detection.
Activated photocathodes are transferred to the gun
using a vacuum manipulator. For a short time, the vacuum
valve between PPS and gun is opened, and the depleted
photocathode is retracted back to the AC. The freshly
activated photocathode is transferred to the gun and thevalve is closed. The depleted photocathode may becleaned and reactivated again. For rejuvenation of adepleted photocathode, atomic hydrogen cleaning iscarried out in the HCC [47].
As the lifetime of photocathodes expressed by total
extracted charge when operating in a DC gun is restricted
to only a few hundred Coulombs [48], corresponding tooperational life-time of a few hours at ERL operational

Proceedings of ERL09, Ithaca, New York, USA

conditions with average extracted current of 100 mA, the
PPS should be ready to deliver an activated photocathodeevery few hours. For this, the PPS will normally containseveral photocathodes, one of which is activated.
Presently, PPS design may be considered to be wellestablished. The typical initial quantum efficiency of theactivated photocathode can reach 20% at a wavelength of
532 nm making it sufficient for operation in ERL highcurrent guns.

ION BACK-BOMBARDMENT
J. Grames and M. Poelker
Ion back-bombardment is the key factor limiting
photogun operating lifetime. Residual gas inside the gun
vacuum chamber and nearby beam line can be ionized by
the extracted electron beam or field emission from thehigh voltage electrodes. Ions produced within or reaching
the cathode/anode gap are accelerated toward thephotocathode by the gun’s static electric field. Ions with
sufficient kinetic energy can strike the photocathodesurface and sputter away the chemicals used to create thenegative electron affinity condition necessary forphotoemission. Energetic ions can also penetrate thephotocathode surface, damaging the GaAs crystal
structure or serving as unwanted dopant species that alter
the photocathode band structure, reducing quantum
efficiency (QE). This process is illustrated in Fig. 6, witha characteristic photocathode “QE scan” exhibiting theeffect of ion back-bombardment.
At CEBAF and other accelerators, production
photoguns exhibit charge lifetime of a few hundred
Coulombs (i.e., before QE falls to 1/e of initial value).
Some ERLs however must deliver thousands of
Coulombs per day.
To put high current ERL
requirements into perspective, consider that a photogun
with just 100C charge lifetime could satisfy accelerator
requirements for only minutes before some sort of action
would be required, for example, move the drive laser spot
to a fresh photocathode location, heat/reactivate thephotocathode, or replace the photocathode. Each of theseactions represents downtime for the accelerator.
Therefore, improving vacuum inside the gun is critical forhigh average current, milliampere-class ERLs: both static
vacuum without beam, and during gun operation withbeam.
Improving static vacuum inside DC high voltage
photoguns has been a central R&D focus for years, withall photoguns today relying on non-evaporable getter(NEG) pumps and ion pumps (to pump inert gasses not
pumped by NEGs). It is now typical that vacuum inside

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NEG/ion-pumped photoguns is in the upper-10-12 to low10-11 Torr range but accurate pressure measurement in thisrange is difficult.
On the static vacuum front, the Cornell group recently
verified the efficacy of reducing the outgassing rate of
vacuum chamber materials via the LIGO “hightemperature” 400C bake process [11], with more than anorder of magnitude reduction in outgassing rate compared
to “typical” stainless steel baked at 250C. The technique
is relatively easy to implement and should provide
significant base pressure improvement - provided there
are no fundamental limitations of NEGs and ion pumps.
On the “dynamic” vacuum front (i.e., vacuum while
operating the gun), it is extremely important to eliminate
field emission from the cathode electrode, which candegrade vacuum via electron stimulated desorption. In
addition, it is extremely important to effectively manage
all of the extracted beam leaving the photocathode,including beam not intentionally produced, for example,
from extraneous laser reflections or background lightilluminating the activated surface of the photocathode.
Anodizing the edge of the photocathode [2], or limiting
the active area with a mask [49], are helpful steps towardeliminating this unwanted electron beam. In addition,
cathode/anode designers must consider beam transport
from the entire photocathode surface, not just from thedesired location of the beam. The gun electrodes must be
designed to capture the “extra” beam and deliver it far
from the gun.
Short of improving vacuum, there are several
techniques that can be employed to prolong photocathodelifetime. Hydrogen is the dominant gas species inside a
UHV/XHV chamber and the hydrogen ionization crosssection peaks at ~ 30 V, falling sharply at higher voltages[50]. One technique – employed “for free” by the veryhigh-voltage ERL gun community - is to operate at very
high bias voltage. At very high bias voltage (assuming
there is no field emission), there should be considerably
fewer hydrogen ions created by the extracted beam,
although this claim awaits experimental verification.
It has been known for years that ions created near theanode are preferentially directed toward the electrostatic
center of the photocathode [51]. Another technique toprolong photocathode lifetime is to operate with the laser
beam positioned away from the electrostatic center of thephotocathode. Unfortunately, modeling predictions
suggest this leads to emittance degradation of the beam
[52].
More recently, Grames et al., determined that ionsproduced downstream of the anode contribute tophotocathode QE decay [15], and these ions are alsodelivered to the electrostatic center of the photocathode.
It is relatively easy to eliminate these ions by simply

Injectors, Guns, & Cathodes

17

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Proceedings of ERL09, Ithaca, New York, USA

applying a small positive bias (~ few hundred volts) to anelectrically isolated anode.
Finally, Grames et al., determined that operating thephotogun with a larger laser spot size can improve
lifetime [13], by effectively distributing ion backbombardment over a larger area of the photocathode.
However, this technique (like off-axis drive laser
operation) leads to emittance degradation.

vacuum inside DC high voltage photoguns remains an
extremely important task. In the realm of improving
static vacuum, there are a number of topics that needR&D attention: cryogenic-pumping as an alternative to
NEGs and improved vacuum gauging, to accurately
measure pressure in the 10-12 Torr range and lower.
Complimentary studies to identify limitations of NEGs
and ion pumps also seem warranted. Finally, there needs
to be greater appreciation for the role of cathode electrodedesign, in terms of transporting all of the extracted beamfrom the photocathode – both wanted and unwantedbeam.

NEW IDEAS AND DESIGNCONSIDERATIONS
I. Bazarov
The photoemission DC guns have been reliably
delivering up to about 10 mA of average currents withnormalized RMS emittances of ~5-8 mm⋅mrad. A numberofemerging
applications
neverthelessrequiresubstantially improved emittances (on the order of
0.1 mm⋅mrad) at comparable or higher beam currents.
R&D programs are underway in several laboratories toaddress the outstanding issues for very low emittance DC
photoemission guns capable of delivering beam currentsof 10-100 mA. Below we survey some new directions
under exploration to achieve the improved performance.

Cathode field and thermal emittance

Figure 6:
Top: Illustration showing cathode/anodestructure, photoemitted electrons and ion backbombardment for off-axis illumination of photocathode.
Bottom: plot of QE across the surface of the photocathodedamaged by ions. The electron beam was extracted fromthree different radial locations. Note QE “trenches” thatterminate at a common “electrostatic center”.

Conclusion
The “tricks” described above to prolong photogun
operating lifetime certainly help enhance ourunderstanding of these complicated devices, but are
unlikely to provide sufficient means to meet therequirements of high current ERLs. Therefore, improving
Injectors, Guns, & Cathodes

18

Even for the DC guns operating currently at voltages of
up to 350 kV, a significant improvement in beam
brightness will be made possible through emittancecompensation processes, and a better control of the initial
electron bunch 3D distribution via laser shaping. A lower
limit to the achievable emittance has been recently
formulated for photoemission guns in terms of thecathode field and intrinsic (thermal) emittance of thephotocathode material [16]ε nx (mm ⋅ mr) = α × 0.015 q (pC)kT⊥ (meV) , (1)Ecath (MV/m)with q being the charge per bunch, Ecath the accelerating
gradient at the photocathode and kT⊥ the effective
transverse energy (temperature) of the photoemitted
electrons. The parameter α depends on additional details
such as the 3D laser pulse distribution, the degree of
emittance compensation, etc. For a well-designed injector
system, α ≈ 0.3 − 0.9 . For example, using typical DC gun
parameters: Ecath = 3.5 MV/m (the gun voltage 350 kV

Proceedings of ERL09, Ithaca, New York, USA

and the cathode-anode gap of 10 cm), kT ⊥ = 120 meVcorresponding to GaAs at 520 nm wavelength
illumination [53], and the bunch charge of 80 pC, one
concludes from Eq. (1) that emittances of no larger than
0.8 mm⋅mrad should be achievable. Similar conclusion
follows from computer optimizations of the beam
dynamics with experimentally benchmarked space charge
codes [54]. Therefore, proper realization of space charge
emittance compensation is the primary route towards
significant reduction in emittance beyond that which has
already been demonstrated from DC guns.
Additional improvements in achievable beam
brightness will become possible when employing
photocathodes with lower transverse energy spread andby increasing the cathode electric field. Without further
discussing the important subject of photocathodes for low
emittance beam production, we point out several
additional considerations to low emittance beam
production arising from the gun electrode design.

Optimal Gun Voltage
While the gun voltage is not a parameter that directly
defines the beam brightness at the photocathode, 1/γ 2
scaling of space charge forces in the gun vicinity as well
as operational experience make it a key design objective
for DC guns. Previous simulation studies suggest that aproperly designed 400-600 kV DC gun can allow low
emittances (0.2 mm⋅mrad at 80 pC), and that theemittance improvements are modest for gun voltages
above 750 kV at these charges [4]. Besides, this gun
voltage level is well matched to the use of an RF buncher
downstream of the gun for velocity compression.

Transverse focusing
Ideally, external focusing in the gun, either due toelectric or magnetic fields should counteract thedefocusing due to the space charge, which at the gun exit
can be estimated using

1
I d mc 2 1
2d ,
≈−
ln
2
f s .c .
I 0 r eVgun βγ (1 + γ ) zi

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2

1 1 + eVgun mc
f anode4d 1 + 12 eVgun mc 2
The required focusing can be achieved throughappropriate electrode shaping (Pierce-like electrodegeometry) and magnetic solenoidal fields. The former
allows focusing in the vicinity of the photocathode but
does so at the expense of a somewhat reduced Ecathpossible otherwise for a flat cathode with the same
cathode-anode gap. Magnetic focusing, on the other hand,requires a vanishingly small field at the photocathode to
avoid emittance increase due to canonical momentum
conservation, and is most effective when the solenoid isplaced some distance from the cathode unless a bucking
solenoid can be employed. The DC gun geometry
typically prevents bucking solenoid coil placement in theimmediate vicinity of the photocathode, thus, acombination of both cathode Pierce-like angle and an
external solenoid placed right after the gun are needed toachieve the desired focusing effect.
1≈−

Voltage breakdown criteria
The main technological challenge to the DC guns is due
to problems of field emission and the voltage breakdown,
which limit the maximum available gradient and voltage.
The best practices to the material selection, surface
preparation and cleanliness are required in order toachieve the best performance. A body of experimental
work on performance of large area electrodes has been
accrued over the years, which should serve as a guidewhen arriving at the actual gun geometry. Most notably,
the trade-off of the highest achieved voltage data versus
the gap for large area parallel electrodes is summarized inFig. 7. As a rule of thumb, the electric field should notexceed the breakdown condition anywhere on the surfaceof the cathode electrode. The actual electric field on thephotocathode itself for a given voltage and gap can benoticeably smaller than what’s suggested in Fig. 7 whenelectrode shaping for transverse focusing is employed.

for a cylindrical beam with radius r, (peak) current I,
cathode-anode gap d, and normalized momentum βγ
corresponding to kinetic energy eVgun , and zi being on
the order of the bunch dimension ( zi ~ r ). I 0 = 17 kA.
Additional defocusing due to anode is always present in
DC guns and contributes further to beam divergence. The
effective defocusing is largely independent from theanode geometry and is given by:

Injectors, Guns, & Cathodes

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Proceedings of ERL09, Ithaca, New York, USA

consisting of the gun and a favourably placed solenoid.
Each point in Fig. 8 corresponds to an optimized gun
geometry for the given voltage. The cathode angle and thegap in the plot vary from 20 to 30° and 32 to 42 mm
respectively across the gun voltages span.

Figure 8: Beam emittance performance for optimized gun
geometry at different gun voltages. The bunch charge is
80 pC, and the rms laser pulse duration is 12 ps.

Segmented gun
Figure 7: Adopted from [55]: (top) voltage breakdown vs.
gap; (bottom) maximum field vs. voltage.

Parameterized gun geometry
Overall, the optimal gun geometry is subject to several
potentially conflicting requirements: maximize Ecath ,increase the gun voltage, and provide a stronger
electrostatic focusing at the photocathode. The last twogenerally lead to a reduced maximum Ecath when
operating at the voltage breakdown limit. The problem ofchoosing optimal gun geometry in the cathode-anoderegion can be most efficiently addressed via computer
optimizations:
field
maps
correspondingtoa parameterized gun geometry are calculated and thesmallest emittance possible with this geometry is
numerically determined via space charge code
simulations. A scan of the gun geometry parameters (e.g.
the gap and the cathode angle) and the gun voltage is thenperformed subject to a number of realistic constraints toarrive at the optimum geometry for best emittance
performance [56]. Figure 8 shows results of such study
with optimizations performed for 80 pC/bunch withemittance minimized after a short beamline (1.3 m)

Injectors, Guns, & Cathodes

20

To a certain extent, it is possible to decouple the two
conflicting requirements of a higher gun voltage (a largegap) and a higher Ecath (a small gap) by considering a
two gap DC gun. Figure 9 illustrates the concept. Such a
gun will feature a small (1.5-2 cm) gap with a modest 250
kV voltage followed by a larger gap with uncriticaldimension of 5-10 cm with a larger 350-500 kV voltage.
This would allow the creation of a high field at thephotocathode (>10 MV/m) together with the more
optimal overall gun voltage of 600-750 kV. Further
investigations about the practicality of this approach arerequired.
Cathode field ~ 15 MV/m
Total voltage ~ 700 kV(e.g. 250+450kV with
15 and 70 mm gaps)
cathodepuck

Figure 9: A double-gap DC gun.

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CONCLUSIONACKNOWLEDGMENTS

This paper summarizes the presentations and discussionof DC gun sessions of Working Group 1 at ERL09
Workshop. The superconducting and normal conducting
RF guns, drive laser and cathode sessions are summarizedin an accompanying paper [57]. The technologicalchallenges presented and discussed at ERL09 will be
addressed worldwide to promise a brighter future of ERL
light sources. The conveners of WG1 express theirappreciation to all the participants and organizers of
ERL09 for a fruitful workshop.

This work is supported by the Office of Naval
Research, the Joint Technology Office, the Air Force
Research Laboratory, USA, the U.S. Night Vision Lab,
the Commonwealth of Virginia, by DOE Contract DEAC05-060R23177, both USA, and Science andTechnology Facilities Council, UK, MEXT Quantum
Beam Technology Program, JSPS Grants-in-Aid for
Scientific Research in Japan (20360424).

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