Low emittance electron beam production and characterization with a 17 GHz photocathode RF gun

Abstract

This thesis presents theoretical and experimental results of the production of very high brightness electron beams using a 17 GHz photocathode RF gun. A beam brightness of up to 8x 1013 A/([pi]m rad)2 was measured for a 1 MeV, 1 ps, 50 pC electron bunch. Such high brightness beams are critically important as injectors for linear accelerators and free electron lasers. The present experimental work represents the first complete characterization of an electron beam produced by an RF photo-injector operating above 3 GHz. Previously published theoretical results show that beam brightness should scale very favorably with increasing RF frequency. Both a 1.5 cell RF gun and a 2.4 cell RF gun were designed, fabricated, and tested. Expected performance was determined through numerical simulations. Cold tests were performed by a novel "bead hang" technique. A beamline, including an emittance compensating solenoid, beam imaging system, and energy spectrometer, was designed and built for testing these guns. The RF guns were powered with up to 4 MW of microwave power from a 25 MW, 17 GHz relativistic klystron amplifier built in collaboration with Haimson Research Corporation. For the 1.5 cell gun, 1 ps bunches with charge up to 0.1 nC at up to 1 MeV were obtained at a peak accelerating gradient of 200 MV/m. The normalized emittance at the exit of the gun was determined to be 1 rmm mrad for a 50 pC bunch, corresponding to a normalized brightness of 8x 1013 A/([pi]m rad)2. This high value of beam brightness is very useful for applications.

(cont.) Even higher brightness could be achieved with a higher electron beam energy of 2 MeV; however, RF breakdown in the tested structures limited the beam energy to about 1 MeV. Results were also obtained for the 2.4 cell RF gun. While this gun provided a modest increase in beam energy to 1.25 MeV, the presence of undesirable quadrupole modes in the structure led to a lower beam brightness. Techniques for further improvement of the RF gun are described.