Performance of a second generation X -band rf photoinjector

Abstract

rf photoinjectors produce incredibly bright electron beams enabling advanced photon science applications such as the current generation of free electron lasers and high energy x-rays and gammarays via laser-Compton scattering. A second generation 5.59 cell X-band rf gun has been developed, installed, conditioned, commissioned, tuned, and used to produce laser-Compton x-rays and multiple electron bunches. A charge per bunch from a few pC to 500 pC has been measured, consistent with a quantum efficiency of 5 × 10 −5 using a 263 nm 10 Hz photocathode drive laser. The rf gun has operated close to design performance at high gradient, and more reliably at lower gradient achieving a root mean square normalized emittance of 0.3 mm mrad at both 80 pC at 185 MV=m, and 40 pC at 165 MV=m. Thermal emittance is estimated at 0.55 mm mrad=mm. Energy spread of 0.03% has been achieved. These results agree very well with modeling predictions for the operating conditions under which the measurements were made. Unusually disruptive breakdowns were observed with an applied magnetic field of 0.5T used for emittance compensation.

Ultrafast electron diffraction: Visualizing dynamic states of matter

“…Cryogenic pulsed RF guns are a promising research direction to push the limits of beam brightness (Rosenzweig et al, 2019), as copper at cryogenic temperatures has significantly lower resistivity loss and can withstand much higher surface fields (Cahill et al, 2018a,b). Increasing the frequency to the X-band region has been another main R&D thrust, with the potential to roughly double the acceleration fields of those of S-and L-band guns (Limborg-Deprey et al, 2016;Marsh et al, 2018). Finally, recent implementation of advanced photocatode replacement systems coupled to high frequency RF guns will soon open the doors to testing a much wider range of materials, well beyond what has already been done with Cu, Mg and Cs 2 Te (Filippetto et al, 2015;Qian et al, 2010;Sertore et al, 2000;Terunuma et al, 2010).…”

Demonstration of sub-GV/m Accelerating Field in a Photoemission Electron Gun Powered by Nanosecond $X$-Band Radiofrequency Pulses

“…The highest field an RF resonator can sustain is ultimately limited by the peak surface field E 0 on its walls. The performances of the high-gradient electron sources (E 0 > 200 MV/m) designed in the recent years [8][9][10] have been hindered by RF breakdown and pulse heating -phenomena where the surface of the resonator or emitter deteriorates thus momentarily preventing the storage of electromagnetic energy in the resonator [11]. Besides, these sources often exhibit substantial "dark-current" emissions due to the spurious uncontrolled emission of electrons via quantum tunneling from the surfaces exposed to the high electric fields [12].…”