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Polarized Electron PWT Photoinjectors

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. Parameters for Polarized Electron Pulsed and CW PWT Guns.

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Polarized Electron PWT Photoinjectors

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  1. 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

  2. PWT Features and Benefits

  3. Parameters for Polarized Electron Pulsed and CW PWT Guns

  4. 1.6-cell L-band gun at Fermilab A0 Laboratory

  5. CEBAF old 100 kV DC guns

  6. CEBAF new 100 kV load-locked DC gun

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

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

  9. Coaxial coupler 2-Cell, L-Band, PWT Structure • 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

  10. Dark Current Suppression at Low Peak Field Threshold Peak Field (MV/m) Axial Distance from Iris Center (cm) Operating the L-band PWT at a low peak field helps prevent backstreaming electrons emitted from the first PWT iris from reaching the photocathode.

  11. Secondary Electrons Backstreaming from first disk iris to cathode rf peak field=35 MV/m, 0 deg from first disk iris to cathode rf peak field=20 MV/m, 90 deg 1st Iris 1st Iris 1st Iris 1st Iris Cathode Cathode Energy Gain

  12. PWT RF and Magnet Design • 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

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

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

  15. 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)

  16. 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)

  17. 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

  18. How to achieve UHV better than 10-11 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: 6.3x10-12 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(?)

  19. Vacuum Pumping Paths 1.6-cell Gun (left) and PWT Gun (right)

  20. 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)

  21. PWT Load Lock Design Transverse Motion Device Longitudinal Motion Device Heater Port 2-1/2” ID VAT Valve View Port NF3 Bleeder Valve Port Port Aligner (Optional) Spare Port IR Thermometer Window Heater Chamber Cesiator Port Hydrogen Gas Cracker Port Activation Chamber To PWT Bellows RGA Port Turbo Pump Port Laser Port Ion Pump Spool Activation and cleaning of GaAs photocathode require an ultra high vacuum < 10-10 Torr

  22. 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. a) sieve with slots b) sieve with holes

  23. Properties of SAES NEG strips

  24. L-band PWT Thermal Hydraulic Design • ILCTA parameters: 5-Hz, 1370 microsecond-long rf pulses, 5 MW peak power at L band; 34.25kW 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 64.5L/m. • A pressure head of 86 psi is sufficient to provide the needed flow through the sieve circuit and the disk/endplate cooling circuits.

  25. Endplate Disk 1  Circuit 1 4 2 3 ω= ω1+ ω2 ω4 = ω2+ ω3 2ω1 + ω0 = 2ω3+ ω5  1 Circuit 2 4 3 2 0 0 Circuit 3 5 5 Endplate Disk 1 Disk 2 1  1 ω= ω1+ ω2 ω10 = ω3+ ω4 ω8 = ω2+ ω5 ω9 = ω4+ ω6 ω1 + ω3 + ω0 = ω5+ ω6 + ω7 5 8 Circuit 1 5 2 10 10 3 9 4 9 6 Circuit 2 0 0 0 7 7 Circuit 3 7 Schematics of the PWT cooling circuits: 3 parallel circuits cool the back endplate and 2 disks (upper) or 1 disk (lower).

  26. 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.

  27. 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.

  28. 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 = Cpω T2 ,T2= temp difference between inlet and outlet water

  29. 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

  30. Heat transfer calculations for disk/endplate cooling circuits for a CW, 1-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

  31. 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

  32. PWT Disk Thermal Hydraulic Study • Microcomputer measurements • Cooling fluid flow control • Temperature monitoring • Closed loop temperature feedback control

  33. Thermal Hydraulics: Comparison of Measurements with Calculations Disk flow rate vs pressure drop, measurements vs calculations Disk channel film coefficients, measurements vs calculations

  34. 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

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