Polarized Electrons Using the PWT RF Gun

1. Polarized ElectronsUsing the PWT RF Gun J. Clendenin, R. Kirby  Stanford Linear Accelerator Center, Menlo Park, CA 94025 Y. Luo, D. Newsham, D. Yu  DULY Rearch Inc., Rancho Palos Verdes, CA 90275 Future colliders that require low-emittance highly-polarized electron beams are the main motivation for developing a polarized rf gun. However there are daunting technical problems and some physics issues in generating highly polarized electron beams using rf guns. The PWT design offers promising features that may facilitate solutions to technical problems such as field emission and poor vacuum. Physics issues such as emission time now seem to be satisfactorily resolved. Other issues, such as the effect of magnetic fields at the cathode—both those associated with the rf field and those imposed by schemes to produce flat beams—are still open questions. Potential solution of remaining problems will be discussed in the context of the PWT design.

2. MOTIVATION Colliders require ~1 nC per microbunch charge and emittance of e ~ 10-8 m in vertical plane. Primarily because of longitudinal space charge, dc guns for high-charge use long pulses (~1 ns) followed by rf bunching, resulting in e ~10-4 m. RF guns now e ~ 10-6 m for 1-nC bunches [J. Yang et al., EPAC2002]. Low emittance beam easier to inject into a damping ring and/or the damping ring can be simpler or less expensive. Eliminate rf bunching system (or magnetic compressor). Potential to lower transverse emittance to ~10-8 m in one plane using optical transformation [Brinkmann et al., 2001].

3. Requirements for a Polarized Source • Cathode: GaAs photocathode activated with Cs and oxide to form NEA surface • Cathode radius must be small, ~ 1 mm. Cathode must not exhibit surface charge limit as modified by high extraction field (Schottky effect). • Laser: Wavelength tuned to emission threshold; pulse length ≤ 20º rf phase • Measured response for low-charge pulses a couple picoseconds [J. Schuler et al., SPIN00]. For high charge, response depends on extraction field and particle energy. • Vacuum: <10-11 Torr best, <10-10 Torr may be ok (depends on residual gas species). • Dark current: <<1 nC per ms of rf • RF breakdown: no rf breakdowns permitted: gun must be initially rf processed using a dummy cathode • Magnetic field: • Initial spin vector is axial, i.e., parallel to the propagation direction of the excitation light. There is an extremely small azimuthal rf magnetic field at the photocathode during extraction. The spin vector will precess in this field resulting in a small, probably negligible depolarization effect. • An axial magnetic field (dc or rf) will have no effect on the spin.

4. The Integrated PWT Photoinjector A p-mode, standing-wave, linac structure. Disks suspended and cooled by by 4 water-carrying rods inside large cylindrical tank. RF power coupled first into the annular region of the tank in TEM-like mode, then coupled to the accelerating cells in TM-like mode on axis.

5. Parameter Value Frequency 2856 MHz Energy 20 MeV Charge per bunch 1 nC Normalized emittance 1 mm Energy spread <0.1% Bunch length 2 ps rms Rep. rate 10 Hz Peak current 100 A Linac length 58 cm Beam radius <1 mm Peak B field 1.8 kG Peak gradient 60 MV/m Peak brightness 21014 A/(m-rad)2 S-Band PWT Design Parmeters

6. Comparison of PWT Integrated Injector to 1.6-Cell Disk-and-Washer Gun-Plus-Booster Emittance of 10-6 m for 1 nC achieved with lower peak field (60 vs. 120 MV/m). Avoids any field emission due to an inversion layer at high fields. Separation of tank from disks improves conductance for vacuum pumping. Large tank provides greater stored rf-energy for muplti-bunch operation. Long filling times may reduce rf breakdown. Separation of tank and disks allows wide range of Q values and thus filling times.

7. Modifications to Enhance Suitability as Polarized Source • Improve material and production quality. • Class 1 OFHC Cu forged using hot isostatic pressure (HIP) method. • Single-point diamond maching to roughness 0.05 mm or better. • Simple Rinsing in ultra-pure water [C. Suzuki et al., NIM A 2001]. • Improve vacuum pumping. • Coat tank wall with thin film TiZr or TiZrV [C. Benvenuti et al., JVST A 1998]. • Increase tank diameter for better pumping conductance. • Improve multi-bunch operation. • Increase tank diameter to increase stored energy. • Water cooling can be increased for 25 MW peak power at 180 Hz.

8. Load-Lock Load-lock system, depicting cathode plug exchange in progress: 1) photocathode plug, 2) gun cathode rack, 3) cathode exchange rack, 4) cathode exchange support anvil, 5) rack and pinion gun loader, 6) motorized drive, 7) to pumping system, 8) gun isolation valve, 9) load-lock isolation valve, 10) process/exchange unit isolation valve, 11) quick-disconnect flange, 12) to process/activation stations, 13) to photoinjector, 14) cesiation unit (channel cesiators, laser window and laser, photocurrent detector). In this design, the activated cathode is introduced through the valve 10. Simple re-cesiations are performed in the load-lock chamber as shown with the valve to the gun 8 closed.

9. Future Plans and Conclusions • SBIR-I awarded to DULY Research Inc. - Engineering design started. • If SBIR-II awarded, DULY Research Inc. build gun, test at SLAC: • Test for QE and lifetime of activated GaAs photocathode at full energy (20 MeV). • Monitor QE during test using green laser (532 nm), 2-10 ps pulse length. • Polarization and emittance of beam to be measured during subsequent testing. • A PWT integrated rf photoinjector built specifically for polarized electrons has many features that will be important for successful operation of polarized electron beams. Such a gun can be built and tested in the near future.