High-brightness electron beams from a DC, high-voltage, GaAs photoemission gun

We have built a high-DC-voltage GaAs photoemission gun and used it to measure the rms transverse emittance and rms momentum spread of the highly-charged, short-duration electron bunches it generates. We report these measurements as functions of microbunch charge for different beam radii, pulse lengths, and voltages/field gradients at the cathode, and compare them with the theoretical predictions of the computer program scPARMELA.; The measurements were performed at the Thomas Jefferson National Accelerator Facility (Jefferson Lab) in Newport News, Virginia, using the photoemission gun, a drive laser, and a short diagnostic beamline. While the electron gun was designed to operate at 500kV, field-emission-induced ceramic punch-throughs forced us to limit beam energy to 250 and 300 kV for this experiment. A modelocked, frequency-doubled, Nd:YLF laser served as the photo-cathode driver, producing beams with rms lengths between 20 and 25 ps. The beamline used a scanning slit, wire scanners, and a dipole magnet to measure the transverse emittance and momentum spread for beam conditions ranging from the emittance-dominated to the space-charge-dominated regimes.; As the microbunch charge was varied from 0.5 to 125 pC, the transverse emittance increased from 0.2 to 2.3 {dollar}pi{dollar} mm mr. Using scPARMELA estimates of the bunch lengths at the measurement plane, we calculated that the normalized peak brightness of these bunches varied from {dollar}10sp9{dollar} to {dollar}10sp{lcub}10{rcub}{dollar} A {dollar}rm msp{lcub}-2{rcub}radsp{lcub}-2{rcub}{dollar} over the charge range investigated. The momentum spread increased from 0.1 to 1.3% over the same range of bunch charge. Comparison of these measured values with scPARMELA simulations indicated that the program predicted the transverse emittance to within 70% and the momentum spread to within 50%. This level of agreement establishes the utility of scPARMELA for the design of high-brightness electron guns in a new regime. Finally, by assuming a Maxwellian velocity distribution, we inferred the average transverse energy {dollar}(ksb{lcub}B{rcub}T){dollar} of the electrons emitted by our GaAs wafer to be 0.20 {dollar}pm{dollar} 0.03 eV at a laser wavelength of 527 nm.