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 FeatureMethodologyBenefit 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 ILCILCTACEBAF Frequency1300 MHz1497 MHz Energy12 MeV3 MeV500 keV280 keV Bunch Charge3.2 nC1 nC0.4 pC RF Power20 MW2.5 MW240 kW<100 kW Heat Load136 kW34 kW240 kW<100 kW Average Current 48  A15  A200  A Peak Electric Field24 MV/m 7 MV/m6 MV/m Linac Length84 cm23 cm20 cm10 cm Number of cells8221 Vacuum ~ 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 (E p =24 MV/m); or 280 keV in a 1-cell cavity (E p =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 Threshold Peak Field (MV/m) Axial Distance from Iris Center (cm)

Secondary Electrons Backstreaming from first disk iris to cathode rf peak field=35 MV/m, 0 deg Energy Gain from first disk iris to cathode rf peak field=20 MV/m, 90 deg  Cathode 1 st 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 bunch3.2 nC0.4 pC Frequency1300 MHz1497 MHz Energy51 MeV500 keV280 keV Normalized emittance (incl. thermal emit.) 1.6 (mm-mrad) 0.14 (mm-mrad)0.10 (mm-mrad) Initial peak current160 A40 mA Bunch length (rms)13 ps50 ps40 ps Energy spread0.7 %4.5 %2.5 % Beam size (rms)1.6 mm0.6 mm0.8 mm Peak magnetic field1234 Gauss110 Gauss180 Gauss Peak PWT electric field23.4 MV/m7 MV/m6 MV/m Peak TESLA electric field22.2 MV/mN/A Peak brightness5.6 x A/m 2 - rad x A/m 2 - rad x A/m 2 -rad 2 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/m Normalized Transverse Emittance (mm-mrad)Energy Gain (MeV) Transverse Beam Size (mm) Energy Spread (keV)

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)

Peak Field (MV/m) Normalized RMS Emittance (including thermal emittance), initial RMS pulse length = 10 ps (mm-mrad) Laser RMS pulse length (ps) RMS Bunch length (ps) Particle loss 1020None 1570Some 30160Many 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 Torr required 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: 6.3x Torr l/s cm 2 for SS 500°C for 40 hrs: 8.0x Torr l/s cm 2 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 -9 Torr-l/s) Q Pump Speed (l/s, at Torr) S 1 Pumping Speed at cathode (l/s) S 2 Pressure at Cathode ( Torr) Q/S 2 Sieve or Slot F 1 Cathode to pump except F 1 Total Cond F FNAL (A0) 1.6-cell gun (Ion & TSP pumps) <28>31 (no rf) >110 (w/ rf) L-band PWT (2-cell) 7600 (48 sets of slots) (SNEG film at Torr) (no rf) L-band PWT (1-cell) 3800 (48 sets of slots) (SNEG film at Torr) (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 ( °C for 40 hrs)

To PWT Transverse Motion Device Heater Chamber Longitudinal Motion Device Cesiator Port Laser Port RGA Port Ion Pump Spool NF 3 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 Design Activation and cleaning of GaAs photocathode require an ultra high vacuum < Torr

a) sieve with slotsb) 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.

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; 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), 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 1Disk 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 11 22 55 55 88  10 1010 33 44 99 66 99 00 00 00 77 77 77 ω = ω 1 + ω 2 ω 10 = ω 3 + ω 4 ω 8 = ω 2 + ω 5 ω 9 = ω 4 + ω 6 ω 1 + ω 3 + ω 0 = ω 5 + ω 6 + ω 7 Circuit 1 Circuit 2 Circuit 3 Disk Endplate 22 22   11 11 33 33 44 44 00 00 55 55 ω = ω 1 + ω 2 ω 4 = ω 2 + ω 3 2ω 1 + ω 0 = 2ω 3 + ω 5

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), 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 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 L/m (or 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  T 1,  T 1 = temp difference between water and wetted metal surface Q = C p ω  T 2,  T 2 = 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. = 40  C)

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. = 40  C)

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 ~ MHz /  C ) Purple: disk flow rate= 70 LPM Blue: disk flow rate = 156 LPM

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 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