DEVELOPMENT OF 200 KeV POLARIZED ELECTRON SOURCE
K . Wada, M. Yamamoto, T. Nakanishi, S. Okumi, T. Gotoh, K. Togawa, C. Suzuki, F. Furuta,
T. Nishitani and N. Miyamoto
Department of Physics, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan
H . Matsumoto, Y. Takeuchi, M. Yoshioka
High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801, JapanAbstract
A 200 keV polarized electron source with a load locksystem has been developed to produce high peak current
and low emittance beam to satisfy various requirements offuture e+- e- linear colliders. The components of thissystem were almost constructed and the performance tests
are in progress. The dark current between the accelerating
electrodes of the gun must be reduced to a level of lessthan 20 nA to assure a long lifetime of the Negative
Electron Affinity (NEA) surface of the photocathode. The
successful result was obtained for this issue, which
achieved an extremely low dark current of < 0.1 nA at aDC high voltage of 200 kV.

1 INTRODUCTION
Nowadays, polarized electron beam has been used forthe spin physics programs at Bonn, Mainz, MIT/Bates,
SLAC and TJNAF [1]. It is also expected that a polarized
electron beam will take an essential part for an electron
injector of e+- e- linear collider, such as Japan LinearCollider (JLC).
The polarized electron beam is produced by photoemission from the GaAs-type semiconductor photocathodewith an NEA surface. The NEA surface is made by bothof the band bending at the heavily p-doped surface and theelectric dipole field of a Ga--Cs+ monolayer formed bydeposition of cesium and oxygen atoms [2][3].
A high field gradient gun is indispensable for linear
colliders. It makes possible to produce the beam thatsatisfies two conflicting performances, high-intensity andlow-emittance, since the current density limited by spacecharge effect becomes higher and the beam divergencecaused by space charge force is suppressed by relativistic
effect.
The electron source for C-band linac scheme of JLC
must produce 72 micro-bunchs with a bunch separation
time of 2.8 ns and each micro-bunch must contain2 × 1010 electrons in ~ 700 ps bunch width, whichcorresponds to about 3 A peak current [4]. The required
normalized emittance at the gun exit is ~ 10 π mm mrad(r.m.s.).
We have demonstrated that the DC gun with asuperlattice photocathode can produce such a beam
overcoming the NEA surface-charge-limit phenomenon[5][6]. However, in this experiment the gun bias voltage
was limited below 70 kV and thus the peak current waslimited to be 1.6 A by space charge effect. The gradualincrease of the dark currents, which was caused by cesium
atoms accumulated on the electrodes during the repetitive

200 kV Gun Chamber
Ceramic Insulators

Insulator Tank

Activation
Chamber

Photocathode
Transpoter

Electrodes

LoadingChamber

Ion Pump

200 kVPower Supply

Photocathode
Transpoter

50 cm

Figure 1: Schematic view of the 200 kV polarized electron gun system.

NEA activation process, did not allow the operation of thegun at higher voltage. The desorbed gas from theelectrodes due to the dark current will be ionized by theelectron beam itself and the back-bombardment of theseions cause the degradation of the NEA surface.
In order to overcome this problem, a new DC highvoltage (HV) gun with a load-lock system has been
constructed.

2 THE POLARIZED ELECTRONSOURCES
2.1 Load-Lock System
The schematic view of the new polarized electron
source is shown in Figure 1. It consists of three vacuum
isolated chambers: a photocathode loading chamber, aphotocathode activation chamber and a HV gun chamber.
The photocathode can be transferred between the chambers
by using two transporters without breaking the ultra-highvacuum (UHV). A photocathode can be installed to theloading chamber from atmosphere and cleaned by atomic
hydrogen method [7]. Heat cleaning by RF induction
heating method and NEA activation of the photocathodes
are done at the activation chamber. Finally, thephotocathode is installed to a center of the cathodeelectrode in the gun chamber, which has axial symmetricgeometry to the beam line. To avoid the increase of darkcurrent, the cesiation is done only in the activationchamber.

To reduce the dark current of accelerating electrodes, thecurvature of the cathode electrode was enlarged (~ 30 mm)
so that the field gradient become smaller. The field
gradient on the photocathode and the cathode electrodesurface at 200 kV were estimated to be 3.0 MV/m and 7.8
MV/m at maximum using the simulation code ofPOISSON.
It is well known that the dark current level must be
kept below 20 nA to preserve high quantum efficiency of
the NEA surface. Non-metallic impurities on the electrodesurface would become the emitting sites of the darkcurrent. Therefore, the fabrication procedure of theelectrodes is so important to reduce the dark current. A
basic research on this issue was made using a testapparatus build at KEK [8]. To reduce the non-metallic
impurities, the super-clean stainless steel made byremelting method was used as an electrode material.
Surface polishing by electro-chemical buffing method andthe subsequent rinsing with ultra-pure water were also
employed to remove the contamination or dust on theelectrode surface. As a result, the dark current from thetest electrode with an area of ~ 7 mm2 could be reduced to~ 90 pA under a high field gradient of 34 MV/m. This
technique has been applied to fabricate the accelerating
electrodes of the present 200 kV gun.

°
30.0φ 62

Pack Holder

Cathode Electrode(-200 kV)

φ 60

Pack

2.2 High Voltage Insulation

3522.0°°A cross section and an equipotential of the accelerating
electrodes are shown in Figure 2. The cathode-anode gap
distance and the photocathode diameter were chosen to be
35 mm and 18 mm φ respectively. The space-chargelimited current at 200 kV in the case of full laser
illumination was estimated to be about 30 A using thesimulation code of EGUN. This value is enough high tosatisfy the charge requirement of JLC.

45.0

2.3 Accelerating Electrodes

22.8°

6.55

2.54
4.06
7.04

6.65
7.82 7.34 6.92

φ 12Anode Electrode(0 V)

12472818In order to operate the load-lock system at grand level,
the HV is supplied to a central flange of two ceramic
insulator-columns (see Figure 1) and the cathode electrodesupport tube is fixed to this flange. Each ceramic
insulator is divided into five segments and five 500 MΩ
divider resistors are used for HV distribution. Themaximum voltage of a HV power supply is 250 kV,
which is used for aging the electrode surface to assure thestable operation at 200 kV.
All HV components are enclosed in the insulation gastank. Dry nitrogen gas is flowed into the tank to remove
the humidity in air to decrease the leakage currents along
the ceramic surface, and then it is pressurized to suppress
the corona discharge.

2.97 2.87

6.52 MV/m

Figure 2: Accelerating electrodes (Unit: MV/m).

3 ULTRA-HIGH VACUUM
At the gun chamber, UHV is especially important tomaintain the good NEA state. The gun chamber has twomain pumps: 360 l/s ion pump and 850 l/s NEG pump.
During the baking process, the temperature was ramped
up or down at the rate of ± 4 °C/ hour to avoid themechanical breaking of ceramic insulators due to thedifferent thermal expansions between metal and ceramics.
After baking at 200 °C for 100 hours, the total pressure atroom temperature decreased to 3.5 × 10-11 torr which wasmonitored by an extractor gage.

Voltage (-kV)
Dark Current (nA)

2000.11.14 NPES3125600

10040075

200500246Time (hours)

8Vacuum

2001.3.18 NPES-34200

3.91503.81003.750

3.608000Dark Current (nA)

Voltage (-kV)

150

Voltage

250010Figure 3: Reduction of the dark current by the HVProcessing.
A dummy photocathode of aluminum whose flat
surface was made by the diamond turning was installed at
the first high voltage test. The dark current could be kept
at low level of < 0.1 nA up to 138 kV and then a sudden
HV breakdown occurred. For this damaged electrode, thedark current was stepped up to 140 nA even at 65 kV. To
improve this situation, the HV processing method wasemployed by introducing 99.9999 % pure nitrogen gas
into the gun chamber until the pressure become to ~ 1 ×
10-6 torr [9]. In order to avoid giving the permanent
damage to the electrode surface, the dark current level waskept < 1 µA during the processing. The HV wasmaintained at the constant value until the dark current wasdecreased. This process is shown in Figure 3. As a result,following a sudden HV breakdown at 125 kV the darkcurrent dropped to a level of < 0.1 nA.
The dark current test in UHV condition at higher
voltage was again continued and the extremely low darkcurrent of < 0.1 nA was kept up to 200 kV. The gaspressure of dry nitrogen in the insulation tank was 3.6
atm for this experiment. Figure 4 show the increase ofUHV pressure together with the increase of the bias

102030Time (min.)40

Vacuum (10-11 torr)

In order to measure the field emission dark current (I )produced by accelerating electrodes, this current must be
distinguished from the other discharge currents caused byvarious HV elements. For this purpose, both of the HV
power supply and the insulation tank are electricallyisolated from ground. The discharge current generated at
the corona rings is collected in the insulation tank andgoes to ground through a current meter (I ). The loop
current that flows the divider resistors returns to the HV
power supply without passing through ground. I and I ,which are isolated from this loop, go to ground and thencome back to the HV power supply through anothercurrent meter (I ). Therefore, I can be estimated simply asI =I -I.

voltage. For this clean surface, it took only 40 minutes toreach at 200 kV. At no bias voltage, the total pressure
was 3.54 × 10-11 torr. The initial pressure at 200 kV was
3.80 × 10-11 torr but it dropped to 3.74 × 10-11 torr after
10 minutes. Therefore, the absolute pressure rise was lessthan 2.0 × 10-12 torr, which will be enough low tomaintain the high quantum efficiency of the NEA
photocathode.

Voltage (-kV)

4 HIGH VOLTAGE TEST

3.550Figure 4: The increase of pressure with supplying a highvoltage of 200 kV.

5 SUMMARY
The HV test of the new polarized DC-gun gave a
successful result, which achieved an extremely low darkcurrent of < 0.1 nA at a DC high voltage of 200 kV. The
extraction of the 200 keV polarized electron beam fromthe NEA photocathode will be done soon.

REFERENCES[1] Proceedings of Polarized Electron Source (PES2000)
Workshop (SPIN2000 Pre-symposium Workshop),
Nagoya, Japan, 2000
[2] T. Mizoguchi et al., SPIN2000 proceedings[3] A. S. Terekhov et al., SPIN2000 proceedings[4] T. Nakanishi et al., “Polarized Electron Source” in
JLC Design Study, KEK Report 97-01, 1997, pp. 3648.
[5] K. Togawa et al., Nucl. Inst. and M eth. in Phys. Res.
A 4 1 4, 431-445 (1998)[6] K. Togawa et al., Nucl. Inst. and M eth. in Phys. Res.
A 4 5 5, 118-122 (2000)[7] C. K. Sinclair, “Polarized Electron at Jefferson
Laboratory” in 7th International Workshop on
Polarized Gas Target and Polarized Beams, AIP
Conference Proceedings 421, Urbana-Champain,USA, 1998, pp. 218-228[8] C. Suzuki, Ph. D. Thesis, Nagoya University, 2001
[9] R. Alley et al., Nucl. Inst. and M eth. in Phys. Res.
A 3 6 5, 1-27 (1995)