LCLS Conceptual Design Report - Stanford Synchrotron Radiation ...

6 Injector
TECHNICAL SYNOPSIS
The injector for the LCLS is required to produce a single 150-MeV bunch of charge 1.0 nC
and 100 A peak current at a repetition rate of 120 Hz with a normalized rms transverse emittance
of 1.0 µm. The required emittance is about a factor of 2 lower than has been achieved to date.
The design employs a solenoidal field near the cathode of a specially designed rf photocathode
gun that allows the initial emittance growth due to space charge to be almost completely
compensated by the end of the booster linac. Following the booster linac, the geometric emittance
simply damps linearly with energy. PARMELA simulations show that this design will produce the
desired normalized emittance.
In addition to low emittance, there are two additional electron-beam requirements that pose
a challenge: the timing and intensity jitters must have an rms value of ≤0.9 ps and ≤2%
respectively. For an rf photoinjector, these parameters are determined principally by the laser
system. Commercial laser oscillators are available with a timing stability of 0.5 ps. The laser
system described here uses feedback loops to maintain this stability in the amplification and pulse
shaping stages. The desired laser-pulse energy tolerance is achieved by stabilizing the pumping
laser for the amplifiers and by operating the second amplifier in saturation. RF systems with a
phase stability of 0.5 ps are already routine for the SLAC linac.
Although additional R&D is in progress to ensure the performance of the photoinjector as
planned, confidence in the present design is based on the performance of existing systems and
projected improvement based on multi-particle code simulations. Simulations using these same
codes match the measured performance of rf photoinjectors operating near the emittance level
desired. Laser systems have been employed in high energy physics experiments with timing
stability—with respect to the accelerated electron beam—that is close to the value required.
The injector is divided geographically between the electron source—consisting of an rf gun
and laser system—the booster linac, and the Matching Section. However, to produce the minimal
transverse emittance at high energy, the photoinjector must be treated as one unit.