A Laser Triggered Synchronizable, Sub-Nanosecond Pulsed Electron Source Kenneth Batchelor, J. Paul Farrell, R. Conde Brookhaven Technology Group, Inc., 120 Lake Ave. So, Nesconset, NY 11767 T. Srinivasan-Rao, J. Smedley Brookhaven National Laboratory, Upton, NY 11973 A laser excited, sub-nanosecond, pulsed electron beam system is described. The system consists of a high voltage pulser and a coaxial triggered spark gap. The liquid spark gap discharges into a pulse forming line designed to produce and maintain a flat voltage pulse of 1ns duration on the cathode of a photodiode. A synchronized laser is used to illuminate the photo-cathode to produce an electron beam with very high brightness, a short duration and with peak currents up to the space charge limit. The system can be configured to operate at energies from 1 MeV to 5 MeV and beam pulse widths from 10ps to 200ps with at least 10nC charge in the electron beam. Initial operating experience with a prototype system is discussed. PACS Codes: 29.27 Background Among the many important parameters that characterize an electron beam are energy, brightness, emittance and energy distribution. Current efforts in accelerator research and research in new beam applications put very strict demands on these parameters. RF photocathode electron guns are currently the preferred choice for the production of high brightness electron beams. These guns satisfy many of the requirements for advanced accelerator investigations and they are currently used extensively for that purpose. Typically, even with the use of linear emittance compensation schemes, photo-excited rf guns provide a normalized beam emittance of ~1.0 to 1.5π mm-mrdn emittance at a total charge of < 0.5 nC and brightness of ~ 10 13 A/m 2 -rdn 2 at energies up to 4.5 MeV, an average accelerating gradient of ~ 60 MV/m at an operating frequency of 3 GHz leads to a physical length of ~ 10 cm. An alternative approach to generating high brightness beams is acceleration of electrons in a pulsed, constant voltage, high ( 1GV/m) electric field. In this approach, a ~ 1 ns, 5 MV maximum voltage pulse is applied to a simple diode configuration. During this short time interval, the photo-cathode is excited by a laser pulse of < 100 ps duration. Since the voltage remains constant during the voltage pulse, the electron gun essentially operates in a dc mode with minimal change in voltage. Technical Approach The pulsed, high gradient, laser excited photo-cathode electron gun is based on the studies of voltage breakdown conducted by Juttner et. al 1 . and Mesyats et 2 . al. Their studies indicate that metals can withstand voltage gradients of a few GV/m if the duration of the field is ~ few ns. Voltage pulses in the range of 800 kV to 750 kV (1.6 GV/m to 1.5 GV/m), applied across a 0.5 mm gap in the BNL vacuum diode 3 confirmed the above results. The dark current measured using a Faraday cup, was very sensitive to the field at gradients of ~ 1.5 GV/m. There was no measured dark current at a field of ~ 1 GV/m.