polarized electron pwt photoinjectors david yu, marty lundquist, yan luo, alexei smirnov duly...
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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