Jefferson Lab, Newport News, Virginia
Operation of the JLab IR Upgrade FEL at CW powers in excess of 10 kW requires sustained production of high electron beam powers by the driver ERL. This in turn demands attention to numerous issues and effects, including: cathode lifetime; control of beamline and RF system vacuum during high current operation; longitudinal space charge; longitudinal and transverse matching of irregular/large volume phase space distributions; halo management; management of remnant dispersive effects; resistive wall, wake-field, and RF heating of beam vacuum chambers; the beam break up instability; the impact of coherent synchrotron radiation (both on beam quality and the performance of laser optics); magnetic component stability and reproducibility; and RF stability and reproducibility. We discuss our experience with these issues and describe the modus vivendi that has evolved during prolonged high current, high power beam and laser operation.
Jefferson Lab, Newport News, Virginia
Recent upgrades to the CEBAF Polarized Source include a fiber-based seed laser, capable of producing pulses with frequency centered at 499 MHz. Combined with the existing three-beam Chopper, an aliasing, or beat frequency technique is used 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 RF Laser modulator uses a divider and heterodyne scheme to maintain coherence with the accelerator Master Oscillator, while providing delay resolution in increments of 2ns. Laser repetition frequencies producing bunch repetition rates between 20 MHz and 100 MHz are demonstrated, resulting in time delays between 50 and 10 ns, respectively. Also, possible uses for such a beam are discussed as well as intended development. Authored by Jefferson Science Associates, LLC under U. S. DOE Contract No. DE-AC05-06OR23177
Jefferson Lab, Newport News, Virginia
IUCF, Bloomington, Indiana
BTG, New York
BNL, Upton, Long Island, New York
ANL, Argonne, Illinois
Experiments on the study of fundamental quark-gluon structure of nucleons require an electron-light ion collider of a center of mass energy from 20 to 65 GeV at luminosity level of 1035 cm-2s-1 with both beams polarized. A CEBAF accelerator based ring-ring collider of 7 GeV electrons/positrons and 150 GeV light ions is envisioned as a possible next step after the 12 GeV CEBAF Upgrade. The developed ring-ring scheme takes advantage of the existing polarized continuous electron beam and SRF linac, the green-field design of the collider rings and the ion accelerator complex with electron cooling. We report results of our design studies of the ring-ring version of an electron-light ion collider of the required luminosity.
Jefferson Lab, Newport News, Virginia
A new 100 kV GaAs DC Load Lock Photogun has been constructed at Jefferson Laboratory, with improvements for photocathode preparation and for operation in a high voltage, ultra-high vacuum environment. Although difficult to gauge directly, we believe that the new gun design has better vacuum conditions compared to the previous gun design, as evidenced by longer photocathode lifetime, that is, the amount of charge extracted before the quantum efficiency of the photocathode drops by 1/e of the initial value via the ion back-bombardment mechanism. Photocathode lifetime measurements at DC beam intensity of up to 10 mA have been performed to benchmark operation of the new gun and for fundamental studies of the use of GaAs photocathodes at high average current*. These measurements demonstrate photocathode lifetime longer than one million Coulombs per square centimeter at a beam intensity higher than 1 mA. The photogun has been reconfigured with a high polarization strained superlattice photocathode (GaAs/GaAsP) and a mode-locked Ti:Sapphire laser operating near band-gap. Photocathode lifetime measurements at beam intensity greater than 1 mA are measured and presented for comparison.
"Further Measurements of Photocathode Operational Lifetime at Beam Intensity >1mA using the CEBAF 100 kV DC GaAs Photogun", J. Grames et al., Proc. of the 17th Inter. Spin Symposium, Japan (2006).
Jefferson Lab, Newport News, Virginia