LCLS Conceptual Design Report - Stanford Synchrotron Radiation ...

6.4.7.2 Pulse Duration
L C L S C O N C E P T U A L D E S I G N R E P O R T
Like timing jitter, the stability of the pulse duration is important for LCLS performance. The
pulse duration should fluctuate by no more than the allowable timing jitter. A stable oscillator is
again essential. If the bandwidth of the oscillator pulse is wider than that transmitted by the phase
and amplitude masks used to shape the pulse in time, so that the masks are illuminated almost
uniformly, then fluctuations in the oscillator width have little effect on the final pulse length or
shape. The latter are determined only by the masks and the pulse compressor following the
amplifiers.
6.4.7.3 Optical Energy
If the optical energy and thus the charge at the photocathode varies with an rms value of 6%
over a period of a few seconds at 120 Hz, the contribution to the peak charge jitter in the
undulator will be at the LCLS limit of 12%. See Table 7.4. However, when all sources of charge
jitter are taken into account, the rms optical energy at the cathode is required to be ≤2% (see
Table 7.5). Thus a criterion of ≤2% rms (in the UV) has been adopted as indicated in Table 6.3.
Harmonic generation compounds the difficulty of this criterion, since 2% jitter in the third
harmonic requires 0.7% stability in the fundamental. It is difficult for a Pockels cell to trim the
amplitude of a broadband pulse without affecting its temporal shape. Thus the UV energy at the
gun is stabilized by using laser-diode pumping in the oscillator, by carefully controlling the beam
mode and its pointing through the amplifiers through Fourier relay imaging, and by stabilizing the
amplifier pumping with feedback.
The older generation of Ti:sapphire oscillators, both CW and mode locked, were pumped by
green light from argon-ion lasers. In the newer generation, these have been replaced by diode-
pumped, frequency-doubled Nd:YVO4 lasers, which have far lower noise. With 10 W of green,
the pumps from both Spectra-Physics ( ® Millennia Xs) and Coherent ( ® Verdi-V10) have a rated
noise (above 10 Hz) below 0.04% rms. Most of this performance carries forward to the
Ti:sapphire output, although there are some differences in how the manufacturers have tightened
their specifications since moving to the new pumps. At the LCLS wavelength, an output power of
at least 1.2 W with rms noise of 0.1% or less is expected.
To control the amplifier’s pumping, the relatively long upper-state lifetime of Ti:sapphire
(3.2 µs, long compared to the few-nanosecond duration of the pump pulse) will be used to hold
the total pump energy constant on every pulse. The pump beam has an rms jitter of about 2.2%
(for the ® Infinity delivering 200 mJ of green at 60 Hz) to 3% (for the ® Quanta-Ray, which can
deliver more than 300 mJ and so provides some “headroom”). These pump lasers have maximum
repetition rates of 100 Hz. To obtain 120-Hz pumping for LCLS, we use a polarizer to merge the
beams from two frequency-doubled Nd:YAG lasers operating in alternation at 60 Hz (Figure 6.4-
1). One beam enters with vertical polarization, the other horizontal. After the polarizer, a Pockels
cell pulsed at 60 Hz rotates the vertically polarized beam to create a 120-Hz train with horizontal
polarization. (This scheme has the additional feature that a failure of one pump cuts the repetition
rate in half, rather than stopping LCLS completely.) The jitter will be corrected by picking off 10-
6-36 ♦ I NJECTOR