Linac Coherent Light Source Project Research Articles

RobustCsBr/Cuphotocathodes for the linac coherent light source

The linac coherent light source (LCLS), an x-ray free-electron laser project presently under construction at SLAC, uses a 2.856 GHz rf photocathode gun with a copper cathode for its electron source. While the copper cathode is performing well for the LCLS project, a cathode material with higher quantum efficiency would reduce the drive laser requirements and allow a greater range of operating conditions. Therefore a robust CsBr/Cu photocathode with greater than 50 times the quantum yield at 257 nm relative to the present LCLS copper cathode has been investigated. Preliminary experiments using a dedicated electron source development test stand at SLAC/SSRL are encouraging and are presented in this paper.

LCLS undulator—recent developments: Undulator tapering to compensate for particle energy loss (simulations, continuous case). First article measurements and tuning

Computer simulations using a continuous change in particle energy are an important tool in improving the performance at the final stage of the Linac Coherent Light Source (LCLS) undulator line. Changes in the particle energy cause the break length between undulator segments to no longer be optimized. Radiation losses less than 0.1%/undulator segment can be easily compensated by adjustment of an undulator deflection parameter K. For higher losses, up to 0.4%, adjustment of K can still be compensated, but careful optimization is required. Compensating for losses greater than 0.4% requires tapering of the field within the undulator segment. The LCLS undulator segments are in mass production at this time. The first two undulators for the LCLS project were delivered to the APS magnetic measurement facility. They have already been tuned, and meet all requirements for the LCLS free-electron laser (FEL). The final results of this tuning will be presented, with emphasis on the FEL-specific requirements. A systematic zero shift in the Sentron [ 〈http://www.sentron.ch/〉. [1]] 2-axis Hall probe was discovered, which required correction in order to obtain the proper field integrals.

Undulator for the LCLS project—from the prototype to the full-scale manufacturing

The design of a new hybrid-type undulator with a fixed gap of 6.4 mm, a period of 30 mm and a length of 3.4 m is presented. The fractional variations in deflection parameter K between segments of 130.4-m-long undulator line for the Linac Coherent Light Source (LCLS) must be ⩽1.5×10 −4. Major design features and their choices are discussed. Lessons learned while working with this prototype are critical for successful project execution. Although the prototype undulator met all of the LCLS specifications, development continued in order to simplify the system. A canted-pole geometry was adopted, which allows the K value to be changed by lateral translation of the entire undulator segment. The prototype undulator was subsequently modified to test the canted-pole concept. Magnetic measurements demonstrated that the undulator with canted poles meets all LCLS specifications and is more cost effective to implement.

Longitudinal phase space tomography at the SLAC gun test facility and the BNL DUV-FEL

The Gun Test Facility and the accelerator at the DUV-FEL facility are operated as sources for high brightness electron beams; the first for the Linac Coherent Light Source project and the latter driving an FEL using High Gain Harmonic Generation in the UV. For both accelerators, projections of the longitudinal phase space on the energy coordinate were obtained by varying the phase of an accelerating structure after the gun and measured with a downstream spectrometer dipole. Using an algebraic reconstruction technique, the longitudinal phase space at the entrance to the varied accelerating structure could be reconstructed over a large range of charge from 15 to 600 pC .

Longitudinal phase space tomography at the SLAC gun test facility and the BNL DUV-FEL

The Gun Test Facility and the accelerator at the DUV-FEL facility are operated as sources for high brightness electron beams ; the first for the Linac Coherent Light Source project and the latter driving an FEL using High Gain Harmonic Generation in the UV. For both accelerators, projections of the longitudinal phase space on the energy coordinate were obtained by varying the phase of an accelerating structure after the gun and measured with a downstream spectrometer dipole. Using an algebraic reconstruction technique, the longitudinal phase space at the entrance to the varied accelerating structure could be reconstructed over a large range of charge from 15 to 600 pC .