Vacuum System of ERL High Voltage DC Gun at Cornell

Xianghong Liu

Cornell Laboratory for Accelerator-based ScienceS and Education (CLASSE)

Cornell University, Ithaca, NY 14853, USA

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Outline

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Introduction First Gun Second Gun Photocathode Pursuing extreme high vacuum General procedure of

preparation and assembly Summary

The Energy Recovery Linac Project at Cornell

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Cornell

Electron

Storage Ring

Tunnel

1. Injector

2. Linac 1

3. 2.5 GeV turn-around

4. Linac 2

5. X-ray beamlines

6. 5 GeV turn-around (CESR)

7. X-ray beamlines

8. Beam dump

Energy Recovery Linac

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Accelerating bunch

Returning bunch

Without Energy Recovery:

5 GeV * 100 mA = 500 MW of

power!

With Energy Recovery:

15 MeV * 100 mA = 1.5 MW of

power

Total 64 cryomodules, each:

• six packages of 7-cell cavity/Coupler/tuner

• a SC magnets/BPMs package

• five regular HOMs/two taper HOMs

Main Linac cryomodule for the ERL (MLC)

Linac A 344 m with 35 cryomodules

Linac B 285 m with 29 cryomodules

nominal length: 9.8 m

Beamline HOM absorber 7-cell cavity SC magnets & BPMs Intermodule unit

Slides from Ralf Eichhorn 4/4/2014 5

4/4/2014 6

4/4/2014 7

How it looks today

Ralf Eichhorn | CLASSE | Cornell University LHeC Workshop 2014, 21.1.2014 8 4/4/2014

1.8K meas.

Design specs

16MV/m, 2.0e10 @1.8K

ERL 7-2 to 7-5 Vertical Test Results

Ralf Eichhorn | CLASSE | Cornell University LHeC Workshop 2014, 21.1.2014 9 4/4/2014

Photo-injector prototype (5 - 15 MeV)

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1

2 3

4

5

1 2

3

4

5

DC Photo-cathode gun keV Beamline w/ Laser ports, buncher, etc.

10-cell SC RF cavity Cryo-module 600 kW Beam Dump

MeV Beamlines w/ full suite of beam instrumentation

1st Gun The ultimate emittance of the ERL is determined by the injector!

Photoinjector performance

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750 kV is not really necessary; 500 kV is enough according to our simulation!

We are now moving on to use the injector for LCLS II development

Photo-cathode DC gun #1

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GaAs Cleaning Chamber

GaAs Activation Chamber

Load-lock

HV Supply

SF6 Tank

Laser input

Electron

beam GaAs

Cathode

-750 kV

Insulator

16.5 inch

flange

Vacuum suitcase to transport cathode

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Vacuum system

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• 400 l/s Perkin-Elmer ion pump

• Massive NEG pumping for H2 - using 20 modules of WP1650 – ST 707. (740 l/s x 20) ~ 15,000 l/s

• 400°C air firing of vessel and internal components, followed by 160°C bakeout*

• 5 x 10-12 Torr typical static pressure

* Reduction in hydrogen outgassing from stainless steels by a

medium-temperature heat treatment

J. Vac. Sci. Technol. A 26 (5), 1166 (2008)

Insulator

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e- e-

12 MV/m at

750kV

-750 kV

It turned out • Field emitted electrons can

build up on the insulator and punch through

• Obvious damage sites can be seen on inside surface

• Found massive amount of powder on bottom of insulator

• It has conductive coating on inner surface to “prevent” charging-up

Bulk conductive insulator

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• The problematic insulator was then replaced with a bulk conductive insulator

• worked well since at up to 400 kV • (may work for higher voltage)

Gun #2

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Segmented insulator (with guard rings) • Field emitted electrons

are stopped by the guard rings

• Series of HV resistors to assure uniform grading of the HV

Larger size flange with wire seal

Larger diameter for gun chamber

Can install all the pumping elements as Gun #1

First installed an electrically floating anode for diagnosis and mitigating ion back bombardment

Replaced with a translatable anode

Segmented insulator

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Vacuum system at present

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• Two CapaciTorr D3500 NEG pumps and two VacIon 55 ion pumps at present; base pressure 4x10-11 Torr

• “Relaxed” on vacuum requirement since using multi-alkali antimonide photocathode

Translatable anode

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• Cathode field is a crucial figure of merit.

• Translatable anode allows us to tailor the field.

2-5cm adjustable gap

Anode electrode

2 welded bellows

Gate valve

No vacuum force involved in translation

Gun #2 assembly (first time)

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Problems

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• During NEG activation, one vacuum window cracked. • Processing was rocky:

• Excess current from power supply (steady state 10s of uA, spikes up to 100uA and beyond),

• Excess current on resistors (spikes of ~10uA) • Current on floating anode (<1uA, excess by definition) • Radiation inside lead (up to 10 R/hour) • Vacuum (base 1e-10 torr) spikes of 1e-7…or worse.

• Couldn’t go beyond 350kV. • Decided to open the gun to investigate…

• Opened bottom flange, purged with N2 gas, counted particles 0.3 um and larger. Saw a few bursts of large counts. Visual inspection didn’t find anything suspicious.

• Quickly sealed and tried to reprocess without success.

• Made hard decision to reclean and rebuild. • Found powder on bottom flange of NEG module assembly • Began rebuild in SRF cleanroom facility (always class 10). • Stalk showed definite signs of field emission

A side note: Hollow cathode discharge?

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• Interesting to notice areas of discoloration / bluish coloration

• Believe being resulted from hollow cathode discharge during gas processing at 1e-5 to 1e-4 Torr

• Don’t think it does any harm to the electrodes

… and the rebuild

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• Hand polished problematic areas of stalk • Electropolished all cathodes • Did 400C air bake • High pressure water rinse • Assembled in class 10 clean room and

transport back to Wilson lab

• Removed all NEG modules • Replaced the anode with the

translatable one

High voltage processing

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It works much better after the rebuild! Reached 470 kV and continuing.

(R128) New experimental beamline

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Gun Anode

Sol Corr. Mag.

EMS Slits

Def Cav

Coll. Slit

F.cup Spec. Dipole

Why pushing hard on vacuum?

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GaAs photocathode • High QE, fast response, low MTE • Negative Electron Affinity (NEA) • Requires atomically clean substrate • High QE relies on activation by ~monolayer of CsF • Sensitive to residual gases, especially oxidant (dark

lifetime)

Better vacuum = better lifetime • Ions generated by the electron beam are accelerated

and bombard the cathode; causes decay of QE

Picture of centered GaAs photocathode

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To reduce beam halo, we define a much smaller active area (instead of whole wafer).

Ion back bombardment

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Cathode Center Damages -- Ion back bombardment (ions generated down-stream of the gun)

Off-center cathode

Multialkali photocathode: robust!

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Before use After use

• 10s of nm grown on a substrate: bulk! • Also suffer from ion back bombardment • Therefore most are off-center

• 60mA run with CsK2Sb had ~30 h 1/e lifetime

• 65mA run with NaKSb had ~66 h 1/e lifetime

Ref: Dunham et. al. APL 102, 034105 (2013)

Pursuing extreme high vacuum

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S

QP

• Minimize gas load (outgassing) • Maximize pumping speed

Minimizing outgassing

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• Did 400C 100 hour air bakeout for all stainless steel parts (which reduces outgassing rate to 2e-14 Torr l s-1 cm-2 for 1.65 mm wall thickness)

• Reduce area of thick material: total gas load is dominated by thicker parts.

• Thoughts only: • Thin wall (double wall) chamber to take full advantage of

400C bake • Chilled chamber to reduce outgassing rate

𝐷(𝑇) ∝ 𝑒−𝐸𝑑𝑘𝑇

Temperature (C) Reduction factor

25 1

0 9.4

-25 140

Take Ed=14.5 kcal/mol (H in 304 SST)

Diffusion constant

Maximize pumping

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• NEG modules: 15000 l/s for H2

• Large ion pump: 400 l/s

• Looked into cryo-pumping, but gave up due to particulate concern and cost (of bakable cryo-pump and a large all-metal gate valve).

Parts preparation process (cleaness and particle control)

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1. UHV parts standard cleaning 2. Hand polishing (electrodes only); cleaning again 3. Electropolishing (electrodes only), won’t touch the polished surface from

now on; transported and stored in DI water 4. 400C-100hr air bakeout (all stainless steel parts: chamber and internal

parts) 5. Rinse, ultrasound bath 6. Move to class 10 or 100 clean room 7. High Pressure Rinse all parts with DI water 8. For components that cannot go through HPR, blow with filtered high

pressure dry nitrogen and check with particle counter 9. Assembly and leak check

Pumping down and venting very slowly to prevent particulate moving around!

Summary

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Gun #1 • Has been in operation for many years • HV processed to over 400 kV; normally operated at 350 kV • Base vacuum: 5e-12 Torr (measured by extractor gauge); NEGs haven’t been

reactivated for many years • Delivered up to 75 mA (limited by RF cavities) • 1/e lifetime of >60 hours at 65 mA run with alkali cathode • Currently being processed for higher voltage

Gun #2 • First assembled in summer 2012; rebuilt October 2013 • HV processed to over 470 kV and continuing • Segmented insulator has worked well • Base vacuum: 4e-11 Torr (w/o NEG modules or large ion pump)

Acknowledgements

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Thanks to The ERL team Especially,

Bruce Dunham Karl Smolenski Jared Maxson

Picture of field emitters (on p22)

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