Graduate School of Science, Nagoya University

PESP2008 Oct.1-3, @JLAB 1

Graduate School of Science, Nagoya UniversityM.Yamamoto, T.Konomi, S.Okumi, Y.Nakagawa, T.Nakanishi

Graduate School of Engineering, Nagoya UniversityN.Yamamoto, M.Tanioku, X.Jin, T.Ujihara, Y.Takeda

High Energy Accelerator Research Organization (KEK)F.Furuta, H.Matsumoto, M.Yoshioka

Graduate School of Advanced Sciences of Matter, Hiroshima UniversityM.Kuriki, C.Shonaka, D.Kubo, H.Okamoto

Status of 200keV beam operationsat Nagoya University


High voltage conditioning of the Mo-Ti electrode

Vacuum improvement (NEG module beam dump)

Dark- and 50A operational Lifetime measurement

Nano-second bunch generation

Pico-second bunch generation

Summary

Outline


200keV Polarized electron source at Nagoya University


200keV gun basic performanceBase pressure: 2x10-9 Pa 200 baking for >100 hours 360 L/s IP, 850 L/s NEGMaximum field gradient (200kV): 7.8MV/m (Cathode) 3.0MV/m (Photocathode)Electrode Cathode: Molybdenum (>99.6%) Anode: Titanium (JIS-grad 2) Finishing: electro-buff polishingCeramic Dividing five segments w/ guard rings. (to avoid field concentration) 500MΩ connection for each <0.3MV/m for each segment at the junctions


Mo(cathode)-Ti(anode) ElectrodeConditioning History

Discharging electrode conditioning was done in UHV condition.

(Last year)200 kV operation after 215kVconditioning. No breakdowns occurred for >300 hours. However, pre-breakdown occurredfor each ~100 hours operation, andIt gave a crucial damage to the photocathode.

(This year)No pre-breakdown occurred morethan 500 hours after 225kV conditioning


Beam transport & Vacuum system

Faraday Cup


NEG-module Beam dump

Beam dump vacuum systemIP: 100 L/sNEG: 1720 L/s(WP950x4:Saes getters)Base pressure: 5x10-9Pa (measured by NIG)

w/o NEG-module dump~25 hours @20A

w/ NEG-module dump>100 hours @20A

Operational Lifetime

(Last year)

(This year)


Dark & Operational Lifetime Measurement

Experimental conditions;Photocathode: Bulk-GaAs (w/o anodization, w/o Cs masking during activation)Dark-Lifetime measurement Base pressure: 2.3x10-9Pa, Bias voltage: -200kV, Dark current: <1nAOperational-Lifetime measurement Laser dia. 1mm, 50A constant output, Beam transport efficiency >96%

dark>200 hrs50A~120 hrs


The under side of the PCwas hidden by Cs dispenser

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Laser Size(4σ ) 0.1mm～

2D-Q.E. Map Measurement in the Act. system

Laser diameter:　 0.1mm @PC

The 2D QE scan was done 1mmstep for each.

・ Supply current of the negative voltage power supply (~-100V) to the photocathode as a photocurrent.・ Laser power was controlled to be ~100nA photocurrent emission due tominimize QE decreasing.

QE scan area


50uA operation

Before operation(immediately after NEA act.)

After 50 A operation

Laser spot area :1mm

Damage area ~10mm

Damage area is larger than the laser spot area. The damage caused by different from ion bombardment that originate from ion generation between the electrode gap.

Result of 2D-QE measurement


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Ion initial energy [eV]

red:30 deg. dispersion beambule:No dispersion beam (parallel beam)

Radius of the photocathode

Backward ion from the transport line

Solenoidposition

Anode holeposition

Photocathodeposition

Photocathode area

GPT simulation: Ion (proton) beam trajectories to the photocathode.(parallel beam with 10eV beam energy)

The main energy of the ion that generated by 100keV electron beam collision to the wall is several tens ~ hundreds eV.

The damage area seems the same range of backward ion spot.

Reducing beam loss in the transport system is the next target.


Laser condition　 Spot size: 13mm (estimated SCL~20A) Wavelength： 780nm　 Pulse width: 20ns (FWHM) 　 Pulse repetition： 10Hz

Photocathode 23mm, Bulk-GaAs (Zn dope density: 3.2x1019 /cm3)

Current (Bunch charge) monitor

Nano-second high bunch charge generation

To measure the performance limit (space-charge-limit) of the 200 keV gun

Purpose

The measurement of direct or attenuated output from Faraday cup.

Extracted charge was estimated from the integrated voltage between GND and 2.7kOhm resistor.

Beam shape monitor

OSC(1MΩ)

OSC(50Ω)F.C.

Power Supply

Laser

2.7kΩ

Experimental conditions;

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#03 7.6661nC#04 29.7025nC#05 61.212nC#06 83.0148nC#07 111.617nC#08 153.626nC#09 178.632nC#10 188.399nC#11 213.202nC

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#07 249.6nC#06 182.4nC#05 134.4nC#04 100.8nC#03 50.4nC#02 18.24nC

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Laser Energy[uJ/Pulse]

21.9nC/uJ(Q.E.~3.5%)

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34.4nC/uJ(Q.E.~5.5%)

QE~3.5% QE~5.5%e-

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•The response of the beam current increasing was unable to follow and saturated against the laser pulse increasein the initial region.

•Peak beam current is different depend-ing on its quantum efficiency.

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Surface charge limit

In order to compare with the actual beam current and laser shape, the shape of laser pulse is scaling at no charge limit region (initial and end low current region).

Surface charge limit

For researching the gun performance limit, high QE superlattice with high surface doping is necessary.


Emittance measurement by Pepper Pot method

Bulk-GaAs(blue)GaAs-GaAsP SL(red)

N.Yamamoto et al., J.Appl.Phys. 102,024904 (2007)


Emittance measurement for pico-second beam

• Average current must be kept < 1A due to suppress charge-up and protect from discharg-ing damage of the scintillator film.

• NEA surface is easily damaged by the outgas from the target in high average current.

• Operation current is limited ~600A due to the high voltage power supply. In the CW (81.25MHz) mode, the bunch charge is limited ~7pC/bunch in this system.

Restriction of Pepper-pot and our electron source systems

Duty factor control of the laser can avoid these problems

Plastic scintillator:BC-422 (produced by Bicron)Thickness: 10mLifetime of fluorescence: 1.6ns


CW pulse slicing by BBO pockels cell

Laser powermeasurement

Net Incidence laser power to the gun can be estimated fromLaser Osc. output x P.C. Duty factor x Transport efficiency (430 mW) x (1.26x10-4) x (0.472) = 25.6 W Max.~2.5nJ/pulse

Ti:Sap Laser modelock frequency: 81.25 MHz BBO pockels cell: PBC06C-DC04/800Produced by Inrad


6.3s<30ns

Laser monitor system


Pico-second bunch generation from bulk-GaAs

* Extracted bunch shape will be tailed for ~100ps due to drift electrons in the bulk region. (P.Hartmann JLAB-ACP-98-04)

No charge limit was detected until 28pC/bunch

Low extinction ratio (1:1500) of the pockels cell is problem. It allows passing through background light and generates 1A CW beam emission. Additional chopper will be needed.

The data was averaging of ~500 pulses. Containing < 5 imperfect pulses during switching the pockels cell are negligible.


Summary High voltage conditioning of the Mo-Ti electrode

Long 200kV operation (>500 hrs) with dark current of <1nA is assured by the discharge conditioning up to 225kV.

Vacuum improvement (NEG module beam dump) Vacuum increase due to outgas at the beam dump is suppressed less than

1 order by NEG-modules.

Dark- and 50A operational Lifetime measurement Dark-lifetime >200hr, 50A operational-lifetime~120hr were observed. As a result of 2D QE scan, ion back from beam transport line is a major

part of damaging NEA surface in present.

Nano-second bunch generation Bunch charge of 250nC/pulse can be generated by illuminating 50J,

20ns(FWHM) laser pulse. The space charge limit cannot be detected due to the surface charge limit.

Pico-second bunch generation Bunch charge of 28pC/pulse was generated from bulk-GaAs photocathode

by a 807nm, 30ps laser shot.

Graduate School of Science, Nagoya University

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