CEBAF/ILC Inverted GunM. Poelker, P. Adderley, J. Clark, J. Grames, J. Hansknecht,

M. Stutzman, R. Suleiman, K. Surles-Law

CEBAF 100kV polarized electron source

• Two-Gun Photoinjector - One gun

providing beam, one “hot” spare

• vent/bake guns – 4 days to replace

photocathode (can’t run beam from

one gun while other is baking)

• Activate photocathode inside gun – no

HV breakdown after 7 full activations HV breakdown after 7 full activations

(re-bake gun after 7th full activation)

• HV breakdown after just 4 activations

when Ti-alloy electrodes are used

• Infrared drive laser light: operate at

bandgap

• Extract ~ 2000 Coulombs per year

• Beam current ~ 100uA, laser 0.5mm

dia., lifetime: ~ 100C, 1x105 C/cm2

Preparing for Demanding New Experiments

Vent/Bake Guns: need improvement

– Difficult to meet demands of approved high

current/high polarization experiments like PRex

(100uA) and Qweak (180uA and 1-year duration).

– Our vent/bake guns can provide only ~ 1 week

operation at 180uA

– 12 hours to heat/reactivate, four days downtime to

replace photocathode

Design Goal for New Gun: One Month Uninterrupted Operation at 250uA, One Shift to

Replace Photocathode

Solution:

(1) LLGun for quick photocathode swap, (2) better vacuum and, (3) higher bias voltage

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Ele

ctr

on

Bu

nch

len

gth

(ps)

Ave. Gun Current (µA)

Bunchlength Vs Gun Voltage 200KeV

115KeV

100KeV

85KeV

70KeV

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Ave. Gun Current (uA)

Electron Bunchlength vs Gun Voltage

115kV

100kV

85kV

70kV

Measurements at CEBAF/JLab PARMELA Simulation Results

Benchmarking PARMELA Simulation Results Against Beam-Based

Measurements at CEBAF/Jefferson Lab – work of Ashwini Jayaprakash, JLab

Similar Ave. Gun Current (µA)Ave. Gun Current (uA)

Message: Beam quality, including transmission, improves at higher gun voltage

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Tra

nsm

issi

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(%

)

Ave. Gun Current (uA)

Transmission vs Gun Voltage115kV

100kV

85kV

70kV

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Tra

nsm

issi

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Ave. Gun Current (µA)

Transmission Vs Gun Voltage

200KeV

115KeV

100KeV

85KeV

70KeV

Similar

Trends

Key Features:

•Smaller surface area

•Electropolished and

vacuum fired to limit

outgassing

•NEG-coated

•Never vented

•Multiple pucks (8 hours

to heat/activate new to heat/activate new

sample)

•Suitcase for installing

new photocathodes

(one day to replace all

pucks)

•Mask to limit active

area, no more

anodizing

New CEBAF load-locked gun

Loading chamber

Preparation/activation chamber

HV chamber

“suitcase”

HV chamber

Vent/bake gun

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

1500350

2≈ 18

Expectation:

5

15

“Further Measurements of Photocathode Operational Lifetime at Beam Current > 1mA using an Improved 100 kV DC High Voltage GaAs Photogun,” J. Grames, et

al., 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 nm

Laser Spot Size 450 mm

Current 1 mA

Duration 8.25 hr

High Initial QE

* Note: did not actually

measure polarization

Charge 30.3 C

Lifetime 210 C

#How long at 1mA? 10.5 days

Vacuum signalsLaser PowerBeam Current

# prediction with 10W laser

LL Gun at CEBAF, Installed Summer 2007

LLGun Lifetime at CEBAF

100

150

200

Ch

arg

e L

ife

tim

e (

C)

Hall A

Hall B

Hall CLLGun2

HGun2

HGun3

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Ch

arg

e L

ife

tim

e (

C)

Activation/Spot

Why only 30C lifetime? And unusually “constant”?

“Inverted” Gun

e-

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

Medical x-ray technology

New Ceramic• Compact• ~$5k

New design

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

neg modules

How to Improve the CEBAF LLGun?

• Keep the preparation chamber and suitcase: very satisfied with these

• Side-ceramic with “inverted” insulator

• Push to higher voltage

• No field emission: identify materials, processes, minimum gap (i.e. max gradient)

• Greater appreciation for cathode/anode design: “grab” and deliver all beam over larger portion of the photocathode surface (present design, we use only 5mm out of 13mm)

• Vacuum: lots of opportunity• Vacuum: lots of opportunity

• Reduce outgassing

• Ion pumps?

• NEGs

• cryopumping

• Gauges

Field Emission – Most Important Issue

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50mm

40mm

30mm

20mm

10mm

4mm

Stainless Steel and Diamond-Paste Polishing Good to ~ 5MV/m and 100kV.

• Flat electrodes and small gaps not very useful

• Want to keep gun dimensions about the same – suggests our 200kV gun needs “quiet” electrodes to 10MV/m

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A)

Gradient (MV/m)

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Fie

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Voltage (kV)

50mm

40mm

30mm

20mm

10mm

4mm

Work of Ken Surles-Law, Jefferson Lab

5MV/m

100kV

Electropolished Stainless Steel

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Gradient [MV/m]

50mm gap

30mm gap

10mm gap

8mm gap

6mm gap

4mm gap

• Results similar to diamond-

paste polishing: limiting

gradient 5MV/m

• Considerable time saving

• Perhaps better results if we

start with smoother surface

Work of Ken Surles-Law, Jefferson Lab

Gradient [MV/m]

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25000

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A]

Voltage [kV]

50mm gap

30mm gap

10mm gap

8mm gap

6mm gap

4mm gap

EP(1) 500X

DP 500X

EP (2) 1300X

DP 1200X

Single Crystal Niobium:• Capable of operation at higher voltage and gradient

• Buffer chemical polish (BCP) much easier than diamond-paste-polish

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Voltage (kV)

BCP Niobium vs Stainless Steel

niobium

304 SS

304 SS #2

Replace conventional ceramic insulator with

“Inverted” insulator: no SF6 and no HV

breakdown outside chamber

Conventional geometry: cathode electrode mounted on metal support

structure Work of Ken Surles-Law, Jefferson Lab

Electrostatic Field Mapping: Asymmetric Gun

POISSON: only for symmetric

guns with input “by hand”

Field Precision Software:

• Takes a CAD file…

• Mesh it

• Apply Field Solver

• Output compatible with

Parmela and GPT

100kV

bias

200kV

bias

High Temperature Bake to Reduce Outgassing Rate

• As much “thin-wall” material as possible

• 316LN (hard knife edges)

• Manufactured and electropolished by

NorCal

• 400C bakeout for 9 days, under vacuum

• Pumped by oil-free turbo, then added

ion pump, while monitoring “effluent”

with RGA

• At 9th day, vacuum still improving by

~15% per 24 hours~15% per 24 hours

• RGA shows H2, methane, CO and HCl

(from electropolishing)

• Accumulation technique suggests

outgassing rate 10-14TL/scm2, or better

• Have not vented yet, and remeasured

• Now working to de-gas internal

components…

Pump Current (nA) = 8x1013Pressure1.300

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10000

Ion

Pu

mp

Cu

rre

nt

(nA

)Do Ion Pumps Limit Ultimate Pressure?

Bleed-in data from vent-bake guns

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1

1E-12 1E-11 1E-10 1E-09 1E-08

Ion

Pu

mp

Cu

rre

nt

(nA

)

Pressure (Torr)

• We always measure same extractor gauge value on our guns and test stands, ~7e-12Torr

• This pressure is larger than expected, using simple formula for Pressure (outgassing rate,

surface area and pump speed). Why?

• Pinched-off and baked ion pumps have current 0.1nA or lower. Does this mean ion pump

is in -12Torr range, or OFF?

• Mount different Ion Pumps on BeCu chamber with Leybold Extractor gauge and leak valve

• Bakeout, then measure “Ultimate Pressure 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

0.00E+00

5.00E-11

1.00E-10

1.50E-10

2.00E-10

2.50E-10

3.00E-10

3.50E-10

4.00E-10

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Pre

ssu

re (

Torr

)

Pump Voltage (au: 500 to 7000V)

brown 11L/s PE pump

brown 30L/s 032105

black Gamma 40L/s pump

brown 60L/s PE pump

BeCu chamber disappointing

Ion Pumps and 10-12 Torr Vacuum

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Pu

mp

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Nominal Vendor-Stated Pump Speed (L/s)

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2.00E-11

4.00E-11

6.00E-11

8.00E-11

1.00E-10

1.20E-10

1.40E-10

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Ult

ima

te P

ress

ure

(To

rr)

Nominal Vendor-Stated Pump Speed (L/s)

Nominal Vendor-Stated Pump Speed (L/s)

• Would pressure extend to –12 Torr on chamber with smaller load?

• Bigger Ion Pump not necessarily better

Vacuum Studies: Ion Pump Limitation?

9.20E-12

Valve to Ion Pump Open/Closed

Used old gun as test bed: “flapper” valve

installed between gun and ion pump

8.00E-12

8.20E-12

8.40E-12

8.60E-12

8.80E-12

9.00E-12

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Pre

ssu

re (

Torr

)

Measurement Number

OPEN

CLOSED

Conventional wisdom says:

Ion pumps required for gasses not

pumped by NEGs – but might be

limiting our ultimate pressure. Our

next test at Test Cave…