Report from WG1DC guns

John Lewellen

Nobuyuki Nishimori

DC t t t lkDC gun status talks

Di i t h l i l h llDiscussions on technological challenges

Apology for missing “electrode shape for small emittance”.

At least 30 minutes would be necessary for a subjectAt least 30 minutes would be necessary for a subject.

O tli f j i t itiOutline for a joint paper writing

• Title “DC gun technological challenges”• Abstract Introduction [N Nishimori (JAEA)]• Abstract , Introduction [N. Nishimori (JAEA)]

– A comprehensive overview of current status and challenges for > 500 kV DC guns for ERLschallenges for > 500 kV DC guns for ERLs

• HV power supplies and SF6 [B. Dunham (Corenll)]list of current status– list of current status 

– ripple (

O tliOutline• Insulator [L Jones (Daresbury) B Dunham(Cornell) one of• Insulator [L. Jones (Daresbury), B. Dunham(Cornell), one of 

JAEA/KEK]– List of current status of conical, inverted, segmented , , ginsulators

– Braze joint on large, (14 and 16) inch CF flangesd d l i i f i l d– X‐rays and secondary electron emission for conical and 

inverted insulator– Cleaning will be challenging for segmented– Cleaning will be challenging for segmented

• HV processing [C. Hernandez‐Garcia (JLab)]– Krypton processing 10‐3 Pa (JLab Daresbury Cornell)– Krypton processing, 10 Pa (JLab, Daresbury, Cornell)– Monitor (vacuum, current, radiation by PMT)– With small gaps (movable anode) by M PoelkerWith small gaps (movable anode) by M. Poelker

O tliOutline• Vacuum [M Yamamoto (KEK)]Vacuum [M. Yamamoto (KEK)]

– List of vacuum chambersOutgassing of the material (SUS316 SUS304 Ti )– Outgassing of the material (SUS316,SUS304, Ti …)

• Heat treatment of stainless steels [C. D. Park et al.]• Chemical treatment of titanium [H Kurisu et al ]Chemical treatment of titanium [H. Kurisu et al.]

– Ion pump as source of outgassing ? By M. Poelker• Field Emission from electrodes [M Poelker (JLab)]• Field Emission from electrodes [M. Poelker (JLab)]

– SUS304, SUS316, Nb, Mo, TiLi t f l t d fi ti– List of electrode configuration

– Small electrodes (inverted guns, JLab)Cl i (HPR d i l i ?)– Cleaning (HPR, dry‐ice cleaning?)

Basic information of Nagoya 200kV gun

1. Chamber area (vacuum side):M t l t ( 1 7 2) C i t ( 0 6 2)

M. Yamamoto, KEK

Metal part ( ~1.7 m2) , Ceramic part ( ~0.6 m2)2. Chamber material:

SUS Titanium Copper AluminumSUS, Titanium, Copper, Aluminum, …

3 Surface finishing & condition:3. Surface finishing & condition: CP/EP (standard EP), Vacuum/air fire ( nothing )Coating ( nothing )Coating ( nothing )

4. Main vacuum pump & pumping speed:Ion pump ( 400 L/s) NEG ( 850 L/s) other pump (nothing)Ion pump ( 400 L/s), NEG ( 850 L/s), other pump (nothing)NEG pump activating condition (400C,~3hours,~10-3 Pa)

5. In situ baking temperature and duration:5. In situ baking temperature and duration:200 C, 100~150 hours, (~10 times baking up to now)

6. Ultimate vacuum:2x10-9 Pa, (5.6x10-10Pa with additional ~3800 L/s NEG)

O tliOutline

• Load‐locked design [B. Militsyn (Daresbury)]– List of load locks

• Ion back‐bombardment and cathode degradationIon back bombardment and cathode degradation [J. Grames (JLab)]

A d bi i (JL b)– Anode biasing exp. (JLab)• New ideas and research development [I. BazarovNew ideas and research development [I. Bazarov

(Cornell)]

Preparations facilities for the III-V photocathodes B. Militsyn, Daresbury

Installation/Institution

Design Preloading treatment

Preactivation treatment

Activation procedure

Results with bulk GaAs

Rejuvenation procedure

Vacuum conditions

Remarks

Cornell University

Two chambers

chemical etching (H2SO4)and anodizing

Heating 550 °C for 2 hrs

Cs-NF3 “Yo-Yo” 10-15% at 532 nm Heat cleaning PC 5.0·10-12 mbar LC 5.0·10-11 mbar

/ h l h l / “ 11 bSTFC/DL Three chambers (6 samples in carousel)

HCl etching (not implemented yet)

Heat cleaning at 450 °C

Cs-O2/NF3 “Yo-Yo”

15% at 635 nm Atomic hydrogen cleaning

PC 5.0·10-11 mbarLC 5.0·10-10 mbarHCC 4.0·10-11 mbar

JAEA Tow HCl etching Heat cleanin at Cs-O “Yo-Yo” 7-10 % at 780 nm Atomic AC 1 0 · 10-11 PaJAEA Tow chambers

HCl etching Heat cleanin at 500 °C 1 hr

Cs-O Yo-Yo 7-10 % at 780 nm (?)

Atomic hydrogen cleaning (optional)

AC 1.0 · 10 11 PaLC

KEK (Nagoya) Tow HCl etching Heat cleanin at Cs-O “Yo-Yo” 7-10 % at 780 nm Atomic AC 1.0·10-8 Pa, LC ( g y )chambers

g500 °C 1 hr (?) hydrogen

cleaning (optional)

,1.0·10-7 Pa

TJNAF/CEBAF Four h b

Heating 550 °C f 2 h

Cs-NF3 “Yo-Yo” 20% at 532 nm Heat cleaning PC 7.0·10-12 mbar Mask chambers inclusive suitcase and bakable adapter

for 2 hrs LC high 10-11 mbar activation

B.L. Militsyn, ERL’09 Workshop, Cornel University, Ithaca, USA, 8-12 June 2009 10/

Photocathode Lifetime at electrostatic center at 2mA

M. Poelker, JLab

S lid f h t lkSummary slide for each talk

CeB6 Thermionic Electron Gun for Single‐pass Free electron Lasers (K T /RIKEN)Free‐electron Lasers (K. Togawa/RIKEN)

500‐kV Pulsed Electron GunC B C h d

SpecificationsCeB6 Cathode

CathodeSingle‐crystal CeB6(φ3 mm, ~1500 deg C)

Beam Voltage 500 kV (ripple < 10‐4)Beam Voltage  500 kV, (ripple 

Cornell DC GunB. Dunham, Cornell

-750 kVCathode Preparation and Load Lock

16.5 inch flange

InsulatorInsulator

Drive Laser

750 kV, 100 mA HVPS•In operations for the past several years for

th d / itt t di d Drive Lasercathode/emittance studies and as a source for the ERL injector

•HV operation limited to 250kV pdue to insulator problems – new, improved insulator just received and ready to try

Laser inputElectron beam

June 8-12, Ithaca, NY Energy Recovery Linac Workshop Cornell University

B. Dunham, Cornell

•The gun system was used initially with a heavily populated diagnostic beamline to allow g y y y p p gcomplete phase space characterization at 77pC/bunch.

•Many publications, in particular, excellent agreement between simulations and measurementsmeasurements

•DC currents up to 20mA

•The gun was moved and jointed to the injector cryomodule, and has performed reliably.The gun was moved and jointed to the injector cryomodule, and has performed reliably.

•4mA (CW@1300MHz)

•A new insulator material that has a bulk conductivity, which is first tried at Daresbury

•Alternative design options are still being pursued as backupsg p g p p

June 8-12, Ithaca, NY Energy Recovery Linac Workshop Cornell University

CEBAF/ILC “Inverted” GunPresent Ceramic• Exposed to field emission

L

Medical x-ray technology• Large area

• Expensive (~$50k)

technology

e‐ New Ceramic• Compact• ~$5k$5k

New designneg modules

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

Single Crystal Niobium:• Capable of operation at higher voltage 180pA

) BCP Niobium vs Stainless Steel• Capable of operation at higher voltage and gradient

• Buffer chemical polish (BCP) much i th di d t li h

120

140

160

180

Current (p

niobium

304 SS

304 SS #2

easier than diamond-paste-polish

40

60

80

100

Emission

 C

0

20

40

0 50 100 150 200

Field E

Voltage (kV)

Replace conventional ceramic insulator with

“Inverted” insulator: no

Conventional geometry: cathode electrode mounted

SF6 and no HV breakdown outside

chamber

on metal support structure Work of Ken Surles-Law, Jefferson Lab

S Jl b DCSummary Jlab DC gunC. Hernandez-Garcia, JLab

• Delivered record 7000 Coulombs from single GaAs wafer and 900 hours of CW beam at over 5 mA

• Photocathode lifetime 550 Coulombs or 50 hours at 5 mA CW• Photocathode suffered surface damage multiple times, but recovered 

each time with heat‐clean and NEA reactivationeach time with heat clean and NEA reactivation • Achieved 485kV after high voltage conditioning, but operating at 350kV 

after ceramic punch‐through at 485 kV• Very successful high voltage conditioning using Kr gas at 1E‐5 Torr, 

conditioned up to 375 kV with Kr. • Implemented motorized cathode retractor decreasing FEL down time for• Implemented motorized cathode retractor, decreasing FEL down time for 

QE replenishing from 3.5 hours to 0.5 hours.• Need to further study surface charge limit from GaAs• Jlab FEL developing inverted insulator gun concept with load‐locked 

system, expect high voltage testing by January 2010

ERL Photoinjector status summaryL Jones Daresbury

Photoinjector output characterised using dedicated diagnostic beamline:- Described in Y.M. Saveliev et al.; Proc. EPAC ’08, 208-210; MOPC062 + 210-212; MOPC063

Problems encountered with field emission beam halo HV breakdown poor quantum

L. Jones, Daresbury

Problems encountered with field emission, beam halo, HV breakdown, poor quantum efficiency and lifetime during commissioning. Much experience gained ☺.

Successful use of Kr processing in HV conditioning to counter field emission. Application of modulated lightsource plus improved vacuum techniquesyielded greatly increased QE and dark lifetime approaching 900 hours.

Repeated failure of photoinjector ceramic has delayedRepeated failure of photoinjector ceramic has delayedprogramme and limited operating voltage. Now havea butt-joint unit from CPI. Investigating other options.

Issues with laser cooling and corrosion > Power dropIssues with laser cooling and corrosion => Power drop.

Plan to re-install diagnostic beamline in parallel with an external cathode preperation system (collaboration with A.S.Terekhov, Instiute of Semiconductor Physics).

4

5

2

3

Am

plitu

de [a

rb]

0

1 PhotocurrentPressure15% Q.E. at 635 nm

JAEA/KEK gun status N. Nishimori, JAEASegmented insulator with guard rings500kV-10mA HV power supply and SF6 tank

Preparation chamber

1HV test will be performed soon.