Results of the SLAC LCLS gun high-power RF tests

Abstract

The beam quality and operational requirements for the Linac Coherent Light Source (LCLS) currently being constructed at SLAC are exceptional, requiring the design of a new RF photocathode gun for the electron source. Based on operational experience at SLAC's GTF and SDL and ATF at BNL as well as other laboratories, the 1.6cell s-band (2856MHz) gun was chosen to be the best electron source for the LCLS, however a significant redesign was necessary to achieve the challenging parameters. Detailed 3-D analysis and design was used to produce near-perfect rotationally symmetric rf fields to achieve the emittance requirement. In addition, the thermo-mechanical design allows the gun to operate at 120Hz and a 140MV/m cathode field, or to an average power dissipation of 4kW. Both average and pulsed heating issues are addressed in the LCLS gun design. The first LCLS gun is now fabricated and has been operated with high-power RF. The results of these high-power tests are presented and discussed.

Mega-electron-volt ultrafast electron diffraction at SLAC National Accelerator Laboratory

“…The LCLS-type photocathode rf gun is identical to the one used for the LCLS injector. 39,40 The rf gun is powered by a pulse-forming-network-(PFN-) based modulator and a 50-MW S-band klystron. The modulator was upgraded by adding a top-off charging power supply to the PFN that stabilizes the output voltage to be better than 50 ppm.…”

RF design of normal conducting deflecting structures for the advanced photon source

“…For 2.83 MW of power lost in the structure, a 1 kHz repetition rate, and a pulse width of 2 μs, the heat deposited into the structure will be about 5.7 kW. This heat prompts careful design of cooling system, similar to one done by E. Jongewaard of SLAC for LCLS rf gun [16]. The heat load projected for this two cell gun is 4 kW at 120 Hz repetition rate.…”

Emittance growth due to multipole transverse magnetic modes in an rf gun

“…Nonaxisymmetric structure excites multipole field components, such as the dipole, quadrupole, and octopole, which kick the beam time dependently, leading to the emittance growth [1]. While removal of the kick from the fundamental mode is difficult [3], minimization of the multipole rf field has been investigated and implemented in the design of the state-ofthe-art photocathode rf gun [4,5], which aims to produce high-brightness electron beams.…”

“…The third-generation BNL-type rf gun was designed to feed rf power into the rf gun by a single coupling hole [5,6]. However, the one-sided coupling scheme causes asymmetry in the electromagnetic field inside the cavity.…”

“…To reduce these multipoles, many innovative ideas were introduced. The Linac Coherent Light Source (LCLS) gun is one of the rf guns which many of these ideas were applied to [5,8,9] and it showed a successful result [5,10].…”