A laser-heterodyne bunch length monitor for the SLC interaction point

Since 1996, the transverse beam sizes at the SLC interaction point (IP) can be determined with a `laser wire`, by detecting the rate of Compton-scattered photons as a function of the beam-laser separation in space. Nominal laser parameters are: 350 nm wavelength, 2 mJ energy per pulse, 40 Hz repetition rate, and 150 ps FWHM pulse length. The laser system is presently being modified to enable measurements of the longitudinal beam profile. For this purpose, two laser pulses of slightly different frequency are superimposed, which creates a travelling fringe pattern and, thereby, introduces a bunch-to-bunch variation of the Compton rate. The magnitude of this variation depends on the beat wavelength and on the Fourier transform of the longitudinal distribution. This laser heterodyne technique is implemented by adding a 1-km long optical fibre at the laser oscillator output, which produces a linearly chirped laser pulse with 4.5-A linewidth and 60-ps FWHM pulse length. Also, the pulse is amplified in a regenerative amplifier and tripled with two nonlinear crystals. Then a Michelson interferometer spatially overlaps two split chirped pulses, which are temporally shifted with respect to each other, generating a quasi-sinusoidal adjustable fringe pattern. This laser pulse is then transported to the Interaction Point

Results on Plasma Focusing of High Energy Density Electron and Positron Beams

We present results from the SLAC E-150 experiment on plasma focusing of high energy density electron and, for the first time, positron beams. We also discuss measurements on plasma lens-induced synchrotron radiation, longitudinal dynamics of plasma focusing, and laser- and beam-plasma interactions.Comment: LINAC 2000 paper No. THC13, Monterey, CA. Aug.21-25,2000, 3 pages, 2 figure

PHOTOCATHODES IN AN RF GUN *

Normalized rms emittances well below 10-6 m (with thermal emittance ignored) are now predicted for a 1-nC, 10-ps, flattop beam using an S-band rf gun. The expected thermal emittance of a Cu cathode excited at 263 nm is shown to be ~0.3 x 10-6 m, which is potentially a serious limit on the overall minimum emittance. For GaAs, the photoelectron energy parallel to the emitting surface is now known as a function of the perpendicular energy. By adjusting the vacuum level for the semiconductor, it appears that the thermal emittance can be reduced (compared to Cu) by a factor of 2Ñeven more if the cathode is cooled. The prospects for operating an rf gun with a III-V semiconductor photocathode such as GaAs are summarized

Presented at the International Symposium on New Visions in Laser-Beam Interactions

The transverse emittance from optimized rf photoinjectors is limited by the thermal emittance. The thermal emittance can be lowered by a factor>2 by using a semiconductor photocathode