[en] An accelerator-based experiment was performed using the CEBAF accelerator of the Thomas Jefferson National Accelerator Facility to investigate a predicted sensitivity of the beam polarization to the vertical betatron orbit in the recirculation arcs. This is the first measurement of any such effect at CEBAF, and provides information about the polarized beam delivery performance of the accelerator. A brief description of the accelerator is given, followed by the experimental methods used and the relevant issues involved in measuring a small (approximately 10^-2) change in the beam polarization. Results of measurements of the polarization sensitivity parameters and the machine energy by polarization transport techniques are presented. The parameters were obtained by measurement of the strength of the effect as a function of orbit amplitude and spin orientation, to confirm the predicted coupling between the spin orientation and the quadrupole fields in the beam transport system. This experiment included characterizing the injector spin manipulation system and 5 MeV Mott polarimeter, modeling of the polarization transport of the accelerator, installation of magnets to create a modulated orbit perturbation in a single recirculation arc, and detailed studies of the Hall C Moeller polarimeter

[en] Fiber-based drive lasers now produce all of the spin-polarized electron beams at CEBAF/Jefferson Lab. The flexibility of these drive lasers, combined with the existing three-beam CEBAF photoinjector Chopper, provides a means to implement a beat frequency technique to produce long time intervals between individual electron microbunches (tens of nanoseconds) by merely varying the nominal 499 MHz drive laser frequency by < 20%. This submission describes the RF Laser modulator that uses a divider and heterodyne scheme to maintain coherence with the accelerator Master Oscillator (MO), while providing delay resolution in increments of 2ns. Some possible uses for such a beam are discussed as well as intended future development

[en] Even at a continuous wave facility such as CEBAF at Jefferson Lab, an electron beam with long time intervals (tens of ns) between individual bunches can be useful, for example to isolate sources of background via time of flight detection or to measure the energy of neutral particles that cannot be separated with a magnetic field. This paper describes a demonstrated method to quickly and easily deliver bunches with repetition rates of 20 to 100 MHz corresponding to time intervals between 10 to 50 ns (respectively). This is accomplished by changing the ON/OFF frequency of the RF-pulsed drive laser by a small amount (?f/f < 20%), resulting in a bunch frequency equal to the beat frequency between the radio frequencies of the drive laser and the photoinjector chopper system

[en] Excellent vacuum is essential for long photocathode lifetimes in DC high voltage photoelectron guns. Vacuum Research at Thomas Jefferson National Accelerator Facility has focused on characterizing the existing vacuum systems at the CEBAF polarized photoinjector and on quantifying improvements for new systems. Vacuum chamber preprocessing, full activation of NEG pumps and NEG coating the chamber walls should improve the vacuum within the electron gun, however, pressure measurement is difficult at pressures approaching the extreme-high-vacuum (XHV) region and extractor gauge readings are not significantly different between the improved and original systems. The ultimate test of vacuum in a DC high voltage photogun is the photocathode lifetime, which is limited by the ionization and back-bombardment of residual gasses. Discussion will include our new load-locked gun design as well as lifetime measurements in both our operational and new photo-guns, and the correlations between measured vacuum and lifetimes will be investigated

[en] We report on recent experience with a 5 MeV Mott scattering polarimeter to monitor electron beam polarization in the CEBAF injector. At 5 MeV, the effective analyzing power of a 100 nm gold foil is close to the calculated single-scatter analyzing power -0.522 (at null=172.6-). Statistical uncertainties of order 1% are achieved quickly since elastic signal rates at 2 muA CW are high. Recent chamber modifications and signal electronics changes have improved signal-to-noise. Calibration work continues. A variety of measurements are feasible which affect machine operations, polarized source development and serve the facility user community

[en] Polarized beam commissioning has been initiated at TJNAF. Measuring the polarization properties of the accelerator, which consists of two superconducting linacs and more than 2200 magnetic elements at its maximum energy, provide useful information about the beam energy and beam transport through the accelerator. Stability of the output beam polarization depends upon time-dependent orbit and energy oscillations. Modeling indicates that the spin transport dynamics are sensitive (∼ 10^-2) to vertical betatron oscillations in the recirculation arcs. This sensitivity is calculated to be enhanced in the arcs where the spin tune is most nearly matched to the vertical betatron tune. Modeling also suggests that depolarization phenomena associated with the particle trajectory differences for electrons within the accelerator's nominal transverse phase volume are too small to be measured between the injector and an experimental hall. Preliminary results from measurements obtained during polarization development runs will be presented

[en] Jefferson Lab is planning a major upgrade of CEBAF accelerator from 6 to 12 GeV. The injection energy needs to be increased accordingly from 67 MeV to 123 MeV. While the present 100 keV electron gun and beam formation up to 5 MeV would remain unchanged, the accelerating SRF modules in the current injector cannot provide the desired energy increase. Two options for attaining the energy increase have been considered: (1) replacing the present injector SRF modules with new, higher gradient modules, or (2) re-circulating the electron beam through the existing cryomodules to achieve the necessary energy gain in two passes. In this paper we present computer simulation studies for these two options of the injector upgrade and list their advantages and disadvantages

[en] DC high voltage GaAs photoguns are key components at accelerator facilities worldwide. New experiments and new accelerator facilities demand improved performance from these guns, in particular higher current operation and longer photocathode operating lifetime. This conference submission explores bulk GaAs photocathode lifetime as a function of beam current, active photocathode area, laser spot size and the vacuum of the gun and beam line. Lifetime measurements were made at 100 microamps, a beam current relevant for accelerators like CEBAF, and at beam currents of 1 milliamps and 5 milliamps, a regime that is interesting for high current Free Electron Laser (FEL) and Energy Recovery Linac (ERL) operation

[en] A new GaAs DC high voltage load lock photogun has been constructed at Jefferson Laboratory (JLab), with improved vacuum and photocathode preparation capabilities. As reported previously, this gun was used to study photocathode lifetime with bulk GaAs at DC beam currents between 1 and 10 mA. In this submission, lifetime measurements were performed using high polarization strained-superlattice GaAs photocathode material at beam currents up to 1 mA, with near bandgap light from a fiber based drive laser having picosecond optical pulses and RF time structure

[en] The polarized photoinjector at Jefferson Lab continues to provide high average current, high polarization, high quality beam to nuclear physics Users in as many as three endstations simultaneously. Long lifetime operation has been obtained from two identical polarized guns. A new high power mode locked Ti-sapphire laser has been constructed to enhance the effective operating lifetime of the photoinjector. Efforts to enhance beam polarization and reduced helicity correlated beam systematic effects are underway