ERL09 WG1
DC-Gunに関するレビュー
山本 将博
KEKCornell
JLAB CEBAF
JAEAJLAB FEL
Daresbury

Cornell Gun

DC Photocathode Gun-750 kV
Cathode Preparation and Load Lock16.5 inchflangeInsulator
750 kV, 100 mA HVPS
Drive Laser

Focusing electrode,
Cathode support

Laser input
Electronbeam
June 8-12, Ithaca, NY

Energy Recovery Linac Workshop

Cornell University

HV Power Supply

Kaiser-750kV, 100mA

Glassman-500kV, 10mA

•We typically run well below the maximum power, thus thecontrols are not very reliable.
•Added a load resistor in parallel with the gun so that we always
draw current to help with control stability•Processing – another supply would be better•Gas processing
June 8-12, Ithaca, NYEnergy Recovery Linac WorkshopCornell University

DC Gun - Insulator-750 kV•Large size to keep field
gradients low•Field emitted electrons
can build up on theinsulator and punch thru
•External SF6•High mechanicalstresses due to SF6pressure and bakeouts

e-e-

12 MV/m at
750kV•Difficult to find suppliers•Braze difficulties due tolarge size

Braze and punch-thru problems limit us to 250kV for now
June 8-12, Ithaca, NY

Energy Recovery Linac Workshop

Cornell University

DC Gun - Insulators
Multi-segmented(KEK, Cornell)‘New’ Insulator
designcollaboration
Adapting Industrial tubedesigns (‘inverted’)Conductive alumina withimproved braze joint(Jlab, Cornell)(Daresbury, Jlab, Cornell)

June 8-12, Ithaca, NY

Energy Recovery Linac Workshop

Cornell University

Insulator resistanceMorgan AL970CD
sample25I (nA)

20

Rectangular piece
0.12x0.14x2.3 inches

15105Resistivity of 6.5e10 Ohm-cm

00102030405060kVIn line with Daresbury measurement of20 uA at 500kV -> resistivity is roughlylinear with voltage

June 8-12, Ithaca, NYEnergy Recovery Linac WorkshopCornell University

The FEL operates two similar DC
photoemission guns
One gun powers the FEL injector.

60 meters

FEL vault

Gun Test StandThe other gun is a test stand withcharacterization beam line

DISTRIBUTION STATE A

Carlos Hernandez-Garcia, 2009 Energy Recovered Linac Workshop, Ithaca, NY

JLAB FEL Gun

FEL DC Gun status
DateEventMay 2007Conclusion of 3 years of operations at 350 kV with a single GaAs waferdelivering over 7000 Coulombs and 900 of CW beam time . Opened
flange leak at 405kV while testing gun for higher charge studies

Mar. 2008 Punctured insulator at 398kV during conditioning of refurbished gun
Sept. 2008 Rebuilt gun with new insulator, but observed >100 uA of F.E. at 150kV
Oct. 2008

Krypton processed to 375 kV, but too much F. E. from cathodeNov. 2008 Replaced cathode, F. E. reactivated at 320 kV after gun bake, Kr proc.
again
Dec. 2008 No F. E. from cathode, but field emitter re‐activated after cathode heatclean.
Up to date Gun operational at 350kV but with few uA of F. E., lifetime ~10
operational hours. Motorized cathode retraction system cut down re‐cesiation time from 3 to 0.5 hours
DISTRIBUTION STATE A

Carlos Hernandez-Garcia, 2009 Energy Recovered Linac Workshop, Ithaca, NY

Gun Test Stand status
Date

Event

12/06/2007

Started HV conditioning, achieved 85 kV the first day

02/01/2008

Achieved 485 kV after 528 processing‐hours. At 486kV punched‐through ceramic insulator

02/28/2008

Fixed ceramic leak and ensured gun performance at 460 kV. DeclaredHV conditioning done

03/14/2008

First beam at 300kV

Spring 2008

Extracted beam up to 375 kV and observed indications of surface
charge limit

2009

Multi‐slit, second solenoid, kicker cavity ready for installation in
expanded diagnostic beam line

DISTRIBUTION STATE A

Carlos Hernandez-Garcia, 2009 Energy Recovered Linac Workshop, Ithaca, NY

The best FEL 1/e photocathode lifetime
was 550 Coulombs or ~ 30 hr. at 5 mA CW
54.5-0.0018x

y = 4.1348e4QE (%3.532.521.5
10.500100200300400

Extracted charge (Coulombs)

DISTRIBUTION STATE A

Carlos Hernandez-Garcia, 2009 Energy Recovered Linac Workshop, Ithaca, NY500600

JLAB CEBAF Gun

New CEBAF load‐locked gun
Preparation/activation chamber

Loading chamber
HV chamber“suitcase”

Vent/bake gun

Lifetime with Large/Small Laser Spots
Tough to measure >1000 C
lifetimes with 100-200 C runs!

Expectation:
21500 ≈ 18350
515“Further Measurements of Photocathode Operational Lifetime at Beam Current > 1mA using an Improved 100 kV DC High Voltage GaAs Photogun,” J. Grames, etal., Proceedings Polarized Electron Source Workshop, SPIN06, Tokyo, Japan

1mA at High Polarization*
Parameter

Value

Laser Rep Rate

499 MHz

Laser Pulselength

30 ps

Wavelength

780 nmLaser Spot Size

450 mmCurrent

1 mADuration

8.25 hrCharge30.3 C

Lifetime

210 C#How long at 1mA?

10.5 days#prediction with 10W laser

Vacuum signals
Laser PowerBeam Current* Note: did not actually
measure polarization

High Initial QE

“Inverted” Gun
Present Ceramic• Exposed to field emission• Large area
• Expensive (~$50k)

e‐Medical x-raytechnology

New Ceramic• Compact• ~$5kNew designneg modules

Move away from “conventional” insulator used on most GaAs photoguns today– expensive, months to build, prone to damage from field emission.

Field Emission – Most Important Issue
Stainless Steel and Diamond-Paste Polishing
Good to ~ 5MV/m and 100kV.

5MV/m• Flat electrodes and small gaps notvery useful• Want to keep gun dimensions
about the same – suggests our
200kV gun needs “quiet”
electrodes to 10MV/m

100kV

Work of Ken Surles-Law, Jefferson Lab

Electropolished Stainless Steel• Results similar to diamond‐
paste polishing: limiting
gradient 5MV/m• Considerable time saving• Perhaps better results if westart with smoother surface

DP 1200X

DP 500X

EP(1) 500X

EP (2) 1300X

Work of Ken Surles-Law, Jefferson Lab

Single Crystal Niobium:•Capable of operation at higher voltage
and gradient•Buffer chemical polish (BCP) mucheasier than diamond-paste-polish

Conventional
Replace conventional
geometry: cathode ceramic insulator withelectrode mounted “Inverted” insulator: noon metal support
SF6 and no HV
structure
breakdown outsidechamberWork of Ken Surles-Law, Jefferson Lab

Ion Pumps and 10‐12 Torr Vacuum• Mount different Ion Pumps on
BeCu chamber with Leybold
Extractor gauge and leak valve• Bakeout, then measure “UltimatePressure vs Bias Voltage” and“Ion Pump Current vs Pressure”• Known Outgassing Rate provides
pump speed at Pult
• Admittedly, outgassing rate of
BeCu chamber disappointing

Ion Pumps and 10‐12 Torr Vacuum• Would pressure extend
to –12 Torr on chamberwith smaller load?
• Bigger Ion Pump notnecessarily better

Vacuum Studies: Ion Pump Limitation?
Used old gun as test bed: “flapper” valve
installed between gun and ion pump

Valve to Ion Pump Open/Closed
Conventional wisdom says:
Ion pumps required for gasses not
pumped by NEGs – but might belimiting our ultimate pressure. Our
next test at Test Cave…

Daresbury Gun

ALICE ERL: Gun assembly
Cathode ball

• Based on the JLab IR-FEL gun
using a Cs:GaAs cathode• 500 kV DC supply• Single bulk-doped ceramic

CathodeElectrons

SF6Vessel removed

XHV

Laser
Support stem
Anode plate

Ceramic
L.B. Jones, ERL ’09 Workshop, June 8th – 12th 2009,Cornell University, Ithaca, NY, USA.

Gun 500 kV power supply

L.B. Jones, ERL ’09 Workshop, June 8th – 12th 2009,Cornell University, Ithaca, NY, USA.

Gun 500 kV power supply

L.B. Jones, ERL ’09 Workshop, June 8th – 12th 2009,
Cornell University, Ithaca, NY, USA.

Cathode ball, insulating ceramic & vacuum vessel

L.B. Jones, ERL ’09 Workshop, June 8th – 12th 2009,Cornell University, Ithaca, NY, USA.

Drive laser: Summary
Diode-pumped Nd:YVO4
Wavelength: 1064 nm, doubled to 532 nm
Pulse repetition rate: 81.25 MHz
Pulse duration: 7, 13, 28 ps FWHM
Pulse energy: up to 45 nJ (at cathode)
Macropulse duration: 100 µs @ 20 Hz
Duty cycle: 0.2% (maximum)
Timing jitter:< 1 ps (specified)< 650 fs (measured)

Spatial profile: Circular top-hat on
photocathodeLaser system commissioned at Rutherford
Laboratory in 2005, then moved to
Daresbury Laboratory in 2006
L.B. Jones, Status of the ERLP Photoinjector
driver laser, ERL ’07 proceedings
L.B. Jones, ERL ’09 Workshop, June 8th – 12th 2009,Cornell University, Ithaca, NY, USA.

RMS Geometric emittance (function of bunch charge)GEOMETRIC EMITTANCE (RMS), π mm-mrad

5RMS geometric emittanceas a function of bunchcharge:4- Horizontal ( )3- Vertical ( )ALICE ERL target wasspecified as 1·π mm-mradby ASTRA for Q = 80 pC

21Some factors are missing
from the ASTRA model 1,200

10

2030

40

50607080

BUNCH CHARGE, pC
1 I.V. Bazarov et al., Proceedings of PAC’07, Albuquerque, 2007, pp. 1221-1223.
2 F. Zhou et al., Phys. Rev. ST - AB 5, 094203, 2003.

L.B. Jones, ERL ’09 Workshop, June 8th – 12th 2009,Cornell University, Ithaca, NY, USA.

Design criteria demonstrated so far ……
Y.M. Saveliev et al.; Proc. EPAC ’08, 208-210; MOPC062
Y.M. Saveliev et al.; Proc. EPAC ’08, 210-212; MOPC063• Beam energy: 350 keV• Bunch charge: > 80 pC• Quantum Efficiency (Q.E.):
4.0% with 1/e lifetime nearing 900 hours• Bunch train length: Single 7 ps pulse to 100 µs• Train repetition rate: Operated up to 20 Hz
L.B. Jones, ERL ’09 Workshop, June 8th – 12th 2009,Cornell University, Ithaca, NY, USA.

3-Stage photocathode preparation system
Courtesy B.L. Militsyn, DL

L.B. Jones, ERL ’09 Workshop, June 8th – 12th 2009,Cornell University, Ithaca, NY, USA.

3-Stage photocathode preparation system
Courtesy B.L. Militsyn, DL

1. Hermetically-sealedtransfer vessel andphotocathode loading
chamber

2. Atomic hydrogencleaning chamber
3. Heat-cleaning
and activation
Chamber

L.B. Jones, ERL ’09 Workshop, June 8th – 12th 2009,
Cornell University, Ithaca, NY, USA.

Photocathode preparation system: 1st Result ☺
515% Q.E. at 635 nm

Amplitude [arb]432

Photocurrent
Pressure

1000:00

00:1400:28

00:43

00:57

01:12

01:26

01:40

01:55

Elapsed Time [hr:min]Courtesy B.L. Militsyn, DL
L.B. Jones, ERL ’09 Workshop, June 8th – 12th 2009,Cornell University, Ithaca, NY, USA.

JAEA Gun

High voltage power supply
value

Cockcroft Walton
power supply

C

2.4nF

N

12I

10mA

f

40kHz

Cf

0.2nF

Lf

2 0H
2.0H

output registerripple = NI/(8π2f3C CfLf)= 25 V
6High voltage power supply

A high voltage test to -550 kV was done.
Frequency of transformer of power supply is lowered from 40 kHz to 25 kHz because oftemperature increase of IGBT.
Th voltage
Theltripplei l iis 8 titimes greatert ththan ddesign.
iThe IGBT (SKM 300GB125D) will be replaced with a different one (SKM 300GB063D).
7

Insulator
Φ106
Φ200730

Φ400
515

10 segments spaced by 65 mm
for a 500 kV gun

Φ435

8 segmentst spaceddbby 45 mm
for a therminoic gun for JAEA FEL

8Plan for HV test

1. Assembly of guard rings and HV chamber.
2. HV test of the insulator with a gguard ringg but without a support rod.
3. HV test of the insulator with a support rod.
4. If we face any problem, we can reverse the guard ring.
z A 1000 l/s turbo pump will be used for the HV tests.
z A NEG pump unit for the HV chamber will be installed after August.
12

Cathode preparation system
preparationgate valve

loadingvacuum gauge

50 l/s ion pump
Cs dispenser

leak valve
transfer rod

vacuum gauge

tungsten heater

800 l/s NEG pump
300 l/s turbo pump

z JLab polarized gun is the model of our chamber system.
z Vacuum: 4.5 x 10-9 Pa at preparation, <5.0x10-8 Pa at loading.

14

QE measurement
Bias 40V
chargeg collector ((Cu))

GaAs
He-Ne laser
13 μW

z Thee base ppressureessu e iss6
6.5
5 x 100-9 Pa.
az The life time needs to be improved.
16

WG1全体に関するまとめは、最終日のレポート
Report From Working Group 1 - Injectors, Guns, & Cathodes

DC-gunに関しては、西森氏
SRF-gunにかんしては、John Lewellen氏のスライドをご参照ください。