Polarized Electron PWT Photoinjectors

David Yu, Marty Lundquist, Yan Luo, Alexei Smirnov

DULY Research Inc.

California, USA

PESP 2008 at Jefferson Lab

Work Supported by DOE SBIR

PWT Features and Benefits

Feature Methodology Benefit

Low emittance, achieved with a low

peak rf field

Emittance compensation, with solenoids close to

the cathode

Simplify beam steering and focusing;

flat beam possible

Mitigation of back- scattered electrons on

photocathode; no backstreaming ions

Low rf peak field; diamond machining; surface cleaning and

coating

Suppress dark current; help survivability of

semiconductor photocathode

Ultra high vacuum

Large vacuum conductance; massive

NEG pumping; SS tank with low outgassing rate

GaAs cathode has long life with good quantum efficiency

Parameters for Polarized Electron Pulsed and CW PWT Guns

ILC ILCTA CEBAF

Frequency 1300 MHz 1497 MHz

Energy 12 MeV 3 MeV 500 keV 280 keV

Bunch Charge 3.2 nC 1 nC 0.4 pC

RF Power 20 MW 2.5 MW 240 kW <100 kW

Heat Load 136 kW 34 kW 240 kW <100 kW

Average Current 48 A 15 A 200 A

Peak Electric Field 24 MV/m 24 MV/m 7 MV/m 6 MV/m

Linac Length 84 cm 23 cm 20 cm 10 cm

Number of cells 8 2 2 1

Vacuum 10-10 ~ 10-11 Torr

1.6-cell L-band gun at Fermilab A0 Laboratory

CEBAF old 100 kV DC guns

CEBAF new 100 kV load-locked DC gun

Schematic of a 2-cell, L-band, PWT polarized electron gun

Schematic of a 1-cell, L-band, PWT polarized electron gun

2-Cell, L-Band, PWT Structure

Coaxial coupler

Design goals PWT advantages

• Excellent vacuum conductance due to open PWT cells and the SS sieve

• Low desorption of stainless steel wall• High shunt impedance (14 Mohms/m for a

6-rod design): enough to provide 12 MeV in an 8-cell cavity (Ep=24 MV/m); or 280 keV in a 1-cell cavity (Ep=6 MV/m)

• Robust design with very strong cell-to-cell coupling (large modal separation)

• Very low emittance

Operating the L-band PWT at a low peak field helps prevent backstreaming electrons emitted from the first PWT iris from

reaching the photocathode.

Dark Current Suppression at Low Peak Field

0

20

40

60

80

100

120

140

160

180

200

220

0.11 0.22 0.33 0.44 0.55 0.66 0.77 0.88 0.99 1.1 1.21 1.32

Axial Distance from Iris Center (cm)

Thre

shold

Peak F

ield

on A

xis (M

V/m

)

i

0° rf phase at emission

90° rf phase at emission

FNAL/TESLA 1.6 cell gun

DULY L-band PWT

1.6 cell L-band gun scaled from S-band

Thr

esho

ld P

eak

Fie

ld (

MV

/m)

Axial Distance from Iris Center (cm)

Secondary Electrons Backstreamingfrom first disk iris to cathode

rf peak field=35 MV/m, 0 deg

Energy Gain

from first disk iris to cathoderf peak field=20 MV/m, 90 deg

Cathode

1st Iris

1st Iris

1st Iris

1st Iris

Cathode

• 2D (Superfish, Poisson) and 3D (GdfidL and Gd1) EM codes used to optimize cavity parameters

• External Q-values computed with absorber in the coax waveguide, or by KY method with shorted waveguide (N. Kroll and D. Yu, Particle Accelerators, 1990)

• During cold test, adjust the coax inner conductor length for critical coupling, i.e. Q_ext=Q_unloaded

PWT RF and Magnet Design

Pulsed 2-cell PWT gun plus

4 TESLA 9-cell cavities

CW 2-cell PWT gun

at 2m from the cathode

CW 1-cell PWT gun

at 2m from the cathode

Charge per bunch 3.2 nC 0.4 pC

Frequency 1300 MHz 1497 MHz

Energy 51 MeV 500 keV 280 keV

Normalized emittance

(incl. thermal emit.)

1.6 (mm-mrad) 0.14 (mm-mrad) 0.10 (mm-mrad)

Initial peak current 160 A 40 mA 40 mA

Bunch length (rms) 13 ps 50 ps 40 ps

Energy spread 0.7 % 4.5 % 2.5 %

Beam size (rms) 1.6 mm 0.6 mm 0.8 mm

Peak magnetic field 1234 Gauss 110 Gauss 180 Gauss

Peak PWT electric field 23.4 MV/m 7 MV/m 6 MV/m

Peak TESLA electric field 22.2 MV/m N/A N/A

Peak brightness 5.6 x 1013 A/m2-rad2

2.3 x 1011 A/m2-rad2 6.0 x 1011 A/m2-rad2

Beam Dynamics (ASTRA) Simulations of Pulsed and CW PWT Guns

Peak PWT Electric Field = 23.4 MV/m, Peak TESLA Electric Field = 22.2 MV/m, Bunch Charge = 3.2 nC, Initial Beam Size = 3.9 mm, Magnetic Field = 1234 Gauss.

Pulsed 1300 MHz, 2-cell PWT plus 4 TESLA 9-cell cavities

CW 1497 MHz, 2-cell PWT gun 200 A, 10 ps bunch, peak field = 7 MV/mNormalized Transverse Emittance (mm-mrad) Energy Gain (MeV)

Transverse Beam Size (mm) Energy Spread (keV)

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.00 0.50 1.00 1.50 2.00

z (m)

rms

Em

ittan

ce (

mm

-mra

d)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.00 0.50 1.00 1.50 2.00

z (m)

rms

Bea

m S

ize

(mm

)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.00 0.50 1.00 1.50 2.00

z (m)

KE

(M

eV)

0.00

5.00

10.00

15.00

20.00

25.00

0.00 0.50 1.00 1.50 2.00

z (m)

Del

_kE

(ke

V)

CW 1497 MHz, 1-cell PWT gun 200 A, 10 ps bunch, peak field = 6 MV/m

Normalized Transverse Emittance (mm-mrad) Energy Gain (MeV)

Transverse Beam Size (mm) Energy Spread (keV)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

0.00 0.50 1.00 1.50 2.00

z (m)

rms

Bea

m S

ize

(mm

)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.00 0.50 1.00 1.50 2.00

z (m)

KE

(M

eV)

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

0.00 0.50 1.00 1.50 2.00

z (m)

Del

_kE

(ke

V)

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.00 0.50 1.00 1.50 2.00

z (m)

rms

Em

ittan

ce (

mm

-mra

d)

Peak Field

(MV/m)

Normalized RMS Emittance (including thermal emittance), initial RMS pulse length = 10 ps

(mm-mrad)

6 0.107

7 0.087

Laser RMS pulse length (ps)

RMS Bunch length (ps)

Particle loss

10 20 None

15 70 Some

30 160 Many

Comparisons of peak fields and pulse lengths for a 1-cell, L-band PWT for CW operation (at 2m from cathode)

Peak field = 7 MV/m

How to achieve UHV better than 10-11 Torrrequired by the NEA GaAs photocathode?

• Open PWT structure (large vacuum conductance)• Material (SS tank, Class 1 OFHC copper disks)• Cleaning (diamond machining + high pressure rinsing for Cu parts; electropolishing for SS) • Bake out (250°C for 20 hrs: 6.3x10-12 Torr l/s cm2 for SS 500°C for 40 hrs: 8.0x10-16 Torr l/s cm2 outgassing)• Coating inner surface of pressure vessel with TiZrV• Removable NEG strips or SNEG film + ion pump• Load lock for activated GaAs cathode • Cooling the pressure vessel to further reduce outgassing(?)

Vacuum Pumping Paths

1.6-cell Gun (left) and PWT Gun (right)

Conductance(l/s) Outgassing Rate

(10-9Torr-l/s)

Q

Pump Speed (l/s, at

10-10 Torr)S1

Pumping Speed at cathode

(l/s)S2

Pressure at Cathode

(10-11 Torr)

Q/S2

Sieve or

Slot F1

Cathode to pump except

F1

TotalCond

F

FNAL (A0) 1.6-cell gun

90 8.7 40 (Ion & TSP

pumps)

<28 >31 (no rf)

>110 (w/ rf)

L-band

PWT

(2-cell)

7600

(48 sets of slots)

3900 2200 9 30000

(SNEG film at 10-11 Torr)

2050 0.4 (no rf)

L-band

PWT

(1-cell)

3800

(48 sets of slots)

3900 1925 7 20000

(SNEG film at 10-11 Torr)

1756 0.4 (no rf)

Vacuum Conductance, Outgassing Rate and Pressure at Cathode for A0 1.6-cell Gun and PWT Guns

Outgassing rate and cathode pressure can be further reduced by more than two orders with high-temperature bake out (400-500°C for 40 hrs)

To PWT

Transverse Motion Device

Heater Chamber

Longitudinal Motion Device

Cesiator Port

Laser Port

RGA Port

Ion Pump Spool

NF3 Bleeder Valve Port

2-1/2” ID VAT Valve

Heater Port

Activation Chamber

Turbo Pump Port

Hydrogen Gas Cracker Port

Spare Port

View Port

Bellows

Port Aligner (Optional)

IR Thermometer Window

PWT Load Lock DesignActivation and cleaning of GaAs photocathode require

an ultra high vacuum < 10-10 Torr

a) sieve with slots b) sieve with holes

• SNEG coating of the pressure vessel or an array of replaceable NEG strips, plus ion pump and turbo pump, provides effective pumping.

• PWT open cells and the perforated cylindrical wall (sieve) provide a large vacuum conductance.

WP 750 WP 950 WP 1250 Surface of the strip (cm2) 870 1100 1750 Substrate thickness (mm) 0.2 0.4 0.2 Power Coating thickness on each side (microns) 70 70 70 Total mass of St 101 alloy (g) 29 37 58 Electrical resistance () 0.16 0.10 0.2 Approximate total weight (g) 290 435 450 Overall dimensions (mm): total length active length width height

207 145 50 30

252 190 50 30

312 250 50 30

510 640 170 310

580 65

670 840 230 410

740 80

880 1100 300 540

1160 130

Pumping Speed (L/s) H2 Room Temperature H2 400C CO Room Temperature CO 400C Sorption Capacity (Torr-L) H2 Room Temperature CO 400C H2 400C Depends on equilibrium pressure

Properties of SAES NEG strips

L-band PWT Thermal Hydraulic Design

• ILCTA parameters: 5-Hz, 1370 microsecond-long rf pulses, 5 MW peak power at L band; 34.25 kW average power.

• In a PWT structure with 2 copper disks, 6 rods, a SS tank and two endplates, 6.03 kW goes into the 2 disks (3.015 kW each disk), 7.61 kW into the 6 rods (1.27 kW each rod), 16.06 kW into tank and 4.55 kW into endplates.

• A flow rate of 25 L/m inside the disk cooling channel 0.1” wide would keep the average disk temperature rise less than 10.3°C.

• Using 3 parallel cooling circuits, the required flow rate through each of the 3 inlet pipes is 64.5 L/m.

• A pressure head of 86 psi is sufficient to provide the needed flow through the sieve circuit and the disk/endplate cooling circuits.

Circuit 1

Circuit 2

Circuit 3

Disk 1 Disk 2

Endplate

Schematics of the PWT cooling circuits: 3 parallel circuits cool the back endplate and 2 disks (upper) or 1 disk (lower).

1 1

25 5

8

10 10 3

49 69

0 0 0

7 77

ω = ω1+ ω2

ω10 = ω3+ ω4

ω8 = ω2+ ω5

ω9 = ω4+ ω6

ω1 + ω3 + ω0 = ω5+ ω6 + ω7

Circuit 1

Circuit 2

Circuit 3

Disk

Endplate

2

2

1

1

3

3

4

4

0 0

55

ω = ω1+ ω2

ω4 = ω2+ ω3

2ω1 + ω0 = 2ω3+ ω5

CW L-band, 2-cell, PWT Thermal Hydraulic Design

• CEBAF parameters: 200 A current, 1497 MHz, CW power of 240 kW.

• In a PWT structure with 2 copper disks, 6 rods, a SS tank and two endplates, 42.3 kW goes into the 2 disks (21.15 kW each disk), 53.3 kW into the 6 rods (8.8 kW each rod), 112.5 kW into tank and 31.9 kW into endplates.

• A flow rate of 70 L/m and a water temperature of 15°C inside the disk cooling channel 0.1” wide would keep the average disk temperature rise at 13°C.

• Using 3 parallel cooling circuits, the required flow rate through each of the 3 inlet pipes is 50-155 L/m.

• A pump head of 240 psi is sufficient to provide the needed flow through the sieve circuit and the disk/endplate cooling circuits.

CW L-band, 1-cell, PWT Thermal Hydraulic Design

• CEBAF parameters: 200 A current, 1497 MHz, CW power of 100 kW.

• In a PWT structure with 2 copper disks, 6 rods, a SS tank and two endplates, 15.5 kW goes into the disk, 19.6 kW into the 6 rods (3.3 kW each rod), 41.4 kW into tank and 23.5 kW into endplates.

• A flow rate of 156 L/m (or 78 L/m) and a water temperature of 25°C (or 18 °C) inside the disk cooling channel 0.1” wide would keep the average disk temperature rise at 13°C.

• Using 3 parallel cooling circuits, the required flow rate through each of the 3 inlet pipes is 80-160 L/m (or 40-80 L/m).

• A pump head of 260 psi (or 65 psi) is sufficient to provide the needed flow through the sieve circuit and the disk/endplate cooling circuits.

PWT Disk Temperature Distribution (F) from a COSMOS/M 2D Model

Q = h A T1 , T1= temp difference between water and wetted metal surface

Q = Cp ω T2 , T2= temp difference between inlet and outlet water

Water temp. inside disk cooling channel (purple), temp. diff. between metal surface

and water (red), temp. diff. between inlet and outlet pipes (blue) vs flow rate

Pump head vs flow rate inside disk cooling channel; flow rates through

other parts are adjusted by the orifice size to remove heat load

Heat transfer calculations for disk/endplate cooling circuits for a CW, 2-cell PWT gun

(average disk temp. = 40C)

Temperatures vs Flow Rate

0

5

10

15

20

25

50 70 90 110 130 150

Flow Rate (LPM)

Tem

pera

ture

s (°C

)

Water temp. diff. betw een inlet and outlet pipesWater temp. inside disk cooling channelTemp. diff. betw een metal and w ater

Head Loss vs Flow Rate

0

100

200

300

400

500

600

700

800

900

1000

50 70 90 110 130 150

Flow Rate (LPM)

Hea

d Lo

ss (p

si)

Water temp. inside disk cooling channel (purple), temp. diff. between metal surface

and water (red), temp. diff. between inlet and outlet pipes (blue) vs flow rate

Pump head vs flow rate inside disk cooling channel; flow rates through

other parts are adjusted by the orifice size to remove heat load

Heat transfer calculations for disk/endplate cooling circuits for a CW, 1-cell PWT gun

(average disk temp. = 40C)

Head Loss vs Flow Rate

0

50

100

150

200

250

300

50 70 90 110 130 150

Flow Rate (LPM)

Hea

d Lo

ss (p

si)

Temperatures vs Flow Rate

0

5

10

15

20

25

30

50 70 90 110 130 150

Flow Rate (LPM)

Tem

pera

ture

s (°

C)

Water temp. diff. betw een inlet and outlet pipesWater temp. inside disk cooling channelTemp. diff. betw een metal and w ater

PWT Frequency Tuning

• During cold test: Change last cell length by cutting sieve length (f ~ - 2 MHz/ mm)

• During operation: Adjust water temperature inside disk cooling channel (f ~ - 0.04 MHz / C )

Purple: disk flow rate= 70 LPM Blue: disk flow rate = 156 LPM

0

20

40

60

80

100

120

0 20 40 60 80 100

Water Temperature (°C)

Ave

. met

al te

mpe

ratu

re (°

C)

PWT Disk Thermal Hydraulic Study

• Microcomputer measurements• Cooling fluid flow control• Temperature monitoring • Closed loop temperature feedback

control

Thermal Hydraulics: Comparison of Measurements with Calculations

Flow Rate vs Pressure Drop through PWT Disk

0

0.5

1

1.5

2

2.5

0 10 20 30 40 50 60 70

Pressure Drop (psi)

Flow

Rat

e (li

ter/m

)

Calculated flow rates are represented by solid lines

do = 0.02 inch, measured

do = 0.04 inch, measureddo = 0.06 inch, measured

Film Coefficient vs Flow Rate for PWT disk

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

1300

1400

0 0.125 0.25 0.375 0.5 0.625 0.75

Flow Rate (liter/m)

Film

Coe

ffici

ent (

B/h

r ft²

°F)

Film coefficient obtained from experimental dataFilm coefficient calculated using equationLinear fit of experimental data

Disk flow rate vs pressure drop, measurements vs calculations

Disk channel film coefficients, measurements vs calculations

PWT R&D Status and Goals

• Feasibility of UHV, cooling and beam dynamics for a warm, polarized electron RF gun demonstrated by simulations and limited tests

• Mechanical drawings for a 1300 MHz, 2-cell PWT completed

• Design of a 1497 MHz, 1-cell PWT in progress

• Test UHV with a SNEG pressure vessel of PWT at JLab

• Use JLab loadlock and GaAs photocathode with PWT

• Demonstrate GaAs survivability in PWT with RF power 2.5 MW, 1300 MHz klystron and modulator at Fermilab, 100 kW, 1497 MHz klystron (CPI VKL-7966A ) at JLab