LCLS Conceptual Design Report - Stanford Synchrotron Radiation ...

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
ε n,rms,th = r c
2
kTe 2 . (6.6)
moc Using the data shown in Figure 6.7, the effective temperature is estimated to be 0.14 eV resulting
in a normalized, rms, thermal emittance of 0.3 µm for a 2-mm diameter cathode.
Charge (nC)
1.2
0.8
0.4
4-2001
8560A97
Charge
Cathode Field
0
0 20 40
QE/QE O = e –∆Φ/kTe
T e = 0.14 eV
εn, rms = 0.26 µm
Laser Injection Phase (degrees)
120
80
40
0
60 80 100
Figure 6.7 The measured charge extracted from a Cu photocathode with ≈190 µJ of laser energy
and the corresponding rf field at the cathode are plotted as a function of injection
phase.
6.2.3 Longitudinal Emittance
A significantly larger amount of work has been spent on measuring the transverse emittance
as opposed to the longitudinal emittance. While both of the low emittance experiments at 1 nC
described above reported the electron beam pulse length, neither reported the energy spread. The
BNL experiment, which utilized an energy chirp by intentionally misphasing a linac section,
necessarily modified the longitudinal emittance of the beam, making simultaneous transverse and
longitudinal emittance measurements under nominal operating parameters impossible. The LANL
experiment utilizes an integrated photoinjector so that one can not individually optimize the phase
of the gun and linac to simultaneously optimize the transverse and longitudinal emittances.
Work has begun on the prototype gun to systematically measure the longitudinal
emittance. Preliminary results for a low charge beam of 0.15 nC give 6.4 keV for the
uncorrelated, rms energy spread out of the gun. A detailed study of the uncorrelated longitudinal
emittance of a 144-MHz rf photoinjector indicates that below the space charge limit, the
uncorrelated longitudinal emittance and energy spread vary linearly with the surface charge
density at the cathode [20]. Using this study’s parameterization, the prototype gun experimental
surface charge density would give 4 keV, 40% lower than the observed 6.4 keV. Extrapolating to
the LCLS charge of 1 nC, the low frequency gun study would predict 22 keV for the uncorrelated
rms energy spread out of the LCLS gun. The PARMELA simulation of the LCLS gun at 140
6-14 ♦ I NJECTOR
Field (MV/m)