P. Emma

The optical design of the soft x-ray self-seeding at LCLS

After the successful demonstration of the hard X-ray self-seeding at LCLS, an effort to build a system for working in the soft X-ray region is ongoing. The idea for self-seeding in the soft X-ray region by using a grating monochromator was first proposed by Feldhauset. al. The concept places a grating monochromator in middle of the undulators and selects a narrow bandwidth “seed” from the SASE beam produced by the upstream section of undulators, which is then amplified to saturation in the downstream section of the undulators. The seeded FEL beam will have a narrower bandwidth approaching the transform limit. The challenge is to accommodate a monochromator and refocusing system as well as the electron beam magnetic chicane into a very limited space. The Soft X-raySelf Seeding system replaces only a single undulator section of ~ 4 m. Theoverall project and the expected FEL performances are described elsewhere. Here we present the detailed optical design solution, consisting of a fixed incidence angle toroidal blazed grating with variable groove density, a rotating plane mirror (the only required motion for tuning the energy) to redirect the selected monochromatic beam onto an exit slit, and two more mirrors, one sphere and one flat, to focus and overlap the ‘seed’ onto the electron beam in the downstream undulators.

First Results of the LCLS Laser-Heater System

The Linac Coherent Light Source (LCLS) is an x-ray Free-Electron Laser (FEL) project that has just achieved its first lasing at 1.5 {angstrom} radiation wavelength. The very bright electron beam required to drive this FEL is susceptible to a microbunching instability in the magnetic bunch compressors that may increase the slice energy spread beyond the FEL tolerance. To control the slice energy spread and to suppress the microbunching instability, a laser heater (LH) system is installed in the LCLS injector area at 135 MeV, right before the RF deflector that is used for the time-resolved electron diagnostics. This unique component is used to add a small level of intrinsic energy spread to the electron beam in order to Landau damp the microbunching instability before it potentially breaks up the high brightness electron beam. The system was fully installed and tested in the fall of 2008, and effects of heating on the electron beam and the x-ray FEL were studied during the 2009 commissioning period. The laser heater system is composed of a 4-dipole chicane; a 9-period, planar, permanent-magnet, adjustable-gap undulator at the center of the chicane; one OTR screen on each side of the undulator for electron/laser spatial alignment; and morexa0» an IR laser (up to 15-MW power) which co-propagates with the electron beam inside the undulator generating a 758-nm energy modulation along the bunch. The final two dipoles of the 4-dipole chicane time-smear this modulation leaving only a thermal-like intrinsic energy spread within the bunch. Table 1 lists the main parameters for this system. The very bright electron beam required for an x-ray free-electron laser (FEL), such as the LCLS, is susceptible to a microbunching instability in the magnetic bunch compressors, prior to the FEL undulator. The uncorrelated electron energy spread in the LCLS can be increased by an order of magnitude to provide strong Landau damping against the instability without degrading the FEL performance. To this end, a laser-heater system has been installed in the LCLS injector, which modulates the energy of a 135-MeV electron bunch with an IR laser beam in a short undulator, enclosed within a four-dipole chicane. The last half of the chicane time-smears the energy modulation leaving an effective thermal energy spread increase. We present the first commissioning results of this system, its operational issues, its impact on the microbunching instability, and finally its effect on the FEL performance. «xa0less

Radiological Protection Studies For NGLS XTOD

R ADIOLOGICAL P ROTECTION S TUDIES FOR NGLS XTOD S HANJIE X IAO , M ARIO S ANTANA -L EITNER AND S AYED R OKNI SLAC N ATIONAL A CCELERATOR L ABORATORY R ICK D ONAHUE , P AUL E MMA , J AMES F LOYD AND T ONY W ARWICK L AWRENCE B ERKELEY N ATIONAL L ABORATORY SLAC-TN-13-003 LBNL-DOC-### December, 2013 P REPARED FOR THE D EPARTMENT OF E NERGY U NDER C ONTRACT N UMBER DE-AC02-76SF00515 & DE-AC02-05CH11231

Report on Modifications to the BX12 and BX13 BC1 Dipoles

Emittance growth seen during the last commissioning run in the bunch compressor optics section, BC1, was blamed on inadequate dipole field quality. The significant linear and nonlinear field non-uniformities generated large horizontal dispersion errors beyond BC1. The linear dispersion after BC1 was corrected using two small corrector quadrupoles placed in BC1 for this purpose, but the remaining nonlinear field caused growth of the normalized horizontal emittance of 40% or more. At best {gamma}{epsilon}{sub x} went from 1.2 {micro}m before BC1 up to 1.7 {micro}m after BC1. The problem was magnified by the larger-than-design energy spread in BC1 due to a long initial bunch length. To improve the field quality we decided to modify the two inner dipoles, BX12 and BX13, of the four magnet chicane during the four month down time in the Fall of 2007. Only the two inner dipoles were chosen because of the limited time available and the fact that the beam is particularly sensitive to field quality of the inner dipoles due to its very large transverse size when going through them. The modifications were completed in November and included new poles and a new pinning scheme. The outer dipoles were left unchanged.