LCLS Emittance Goals Achieved at Full Bunch Charge

The transverse beam emittance measured in the LCLS injector has reached its design goal of 1.2 microns with a 1-nC bunch charge. Many measurements have been made using both OTR screens and wire-scanners, which confirm this normalized rms emittance level in both transverse planes at an electron energy of 135 MeV. The various measurements at 1 nC extend from 1 micron and on occasion up to 1.5 microns, with an undetermined source of variation. The bunch length is also at its design level of 10 ps FWHM, which produces a peak current of 100 Amperes.

This rms emittance value is measured over the full bunch length (time-projected), but the rms analysis includes only 95% of the core particles. Measurements which use the full transverse distribution are much more sensitive to baseline noise and weak charge tails, and typically report emittance levels closer to 1.5 microns.

The 'time-sliced' emittance has also been measured using an S-band transverse RF deflecting cavity. This method commonly reveals a 0.9-micron horizontal emittance in the 1-picosecond long slice at the center of the bunch. The vertical 'time-sliced' emittance is rarely measured.

The LCLS electron beam has been transported through the full SLAC linac with an energy of up to 16 GeV and has also been time-compressed by a factor of 4-5 in the first stage compressor. The next challenge will be to preserve this low emittance beam through the 1-km linac, both bunch compressors, and into the LCLS undulator, eventually producing 1.5-Angstrom FEL x-ray radiation. Some of these challenges will be the focus of the next phases of LCLS commissioning starting again in December 2007.