Experimental and theoretical investigations of a 17GHz RF gun

Abstract

We report on experimental and theoretical investigations of a 17 GHz RF photocathode electron gun. This is the first photocathode electron gun to operate at a frequency above 2.856 GHz. The 1.5 cell, π−mode, copper cavity was tested with 50 ns pulses from a 17.150 GHz klystron amplifier built by Haimson Research Corp. A Bragg filter was used at the RF gun to reduce the reflection of parasitic modes back into the klystron. Coupling hole theory in conjunction with cold test measurements was used to determine the field profile in the RF gun. The particle in cell code MAGIC as well as coupled envelope equations were used to simulate the beam dynamics in the RF gun. With power levels of 4 MW, the on axis electric field at the cathode exceeds 300 MV/m, corresponding to an average accelerating gradient of 200 MV/m over the first half cell of the gun. Breakdown was observed at power levels above 5 MW. Electron bunches were produced by 20 µJ, 1 ps UV laser pulses impinging on the RF gun copper photocathode and were measured with a Faraday cup to have up to 0.1 nC of charge. This corresponds to a peak current of about 100 A, and a density at the cathode of 8.8 kA/cm 2 . Multiple output electron bunches were obtained for multiple laser pulses incident on the cathode. Phase scans of laser induced electron emission reveal an overall phase stability of better than ±20°, corresponding to ±3 ps synchronization of the laser pulses to the phase of the microwave field. A Browne-Buechner magnetic spectrometer indicated that the RF gun generated 1MeV electrons with a single shot rms energy spread of less than 2.5%, in good agreement with theoretical predictions.3

A high gradient 17 GHz RF gun for the production of high brightness electron beams

“…These results have previously been reported in greater detail [3]. The bunch charge measurements (yielding values up to 0.1 nC) were made with a high speed Faraday cup placed downstream of the gun.…”

A High-Energy, Ultrashort-Pulse X-Ray System for the Dynamic Study of Heavy, Dense Materials

“…(1 nC), and subpicosecond duration, are currently being developed [79][80][81][82][83] for a variety of applications ranging from the Next Linear Collider (NLC) to compact, pre-bunched free-electron masers 84 .…”

Low emittance electron beam formation with a 17 GHz RF gun

“…Initial experiments demonstrating beam production from a previous version of the 17 GHz RF gun were completed at MIT and were reported [7]. The results of beam measurements are listed in Table I.…”

“…The results presented in this paper are for a new RF gun experiment. This experiment has a number of improvements over the previous experiment [7], including an improved beam line that features the addition of an emittance compensating solenoid and emittance diagnostics, as well as a new RF gun with improved longitudinal field balance. Field profile measurements of the original gun revealed the field strength in the half cell to be about 20% stronger than that in the full cell.…”