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
charge distribution used in these experiments were replaced with one that is uniform, i.e., flattop.
In addition, simulations for more recent photoinjector designs (see for example Section 6.6.
PARMELA Simulations), now predict an emittance significantly less than 1 µm.
εn,x (µm)
30
25
20
15
10
5
0
3-2001
8560A81
SLAC
Thermionic Injectors
with Sub-harmonic Bunchers
BOEING
BNL
LANL-APEX
Photoinjectors
LANL-AFEL
BNL/UCLA/SLAC
1980 1985 1990
Year
1995 2000
Figure 6.6 Normalized rms transverse emittance measured by the leading thermoionic (SLAC,
BOEING) and rf photocathode injectors [16]. All data are for bunched beams with
approximately 1 nC of charge.
In the 1-nC regime, the lowest reported projected, normalized, rms emittance for an S-band
gun is 2.4 µm measured at BNL [3] using a two-screen emittance measurement technique with
0.9 nC of charge. The emittance was measured using an approximately flat top transverse laser
(clipped Gaussian) distribution and a 15-ps FWHM approximately Gaussian temporal
distribution. However, the electron beam bunch length downstream of the booster accelerator was
closer to 4 ps FWHM, which was attributed to rf compression in the gun. The beam was also
measured using a quadrupole scan technique, which resulted in an emittance of 3.2 µm with 0.8
nC of charge. The different results in the two measurement techniques were attributed to
background subtraction errors in the quadrupole scan technique. A constant background obtained
with a closed iris was used for background subtraction, while the real background varied with the
electron beam image size due to image intensity changes during the quad scan. Since the beam
size and, therefore, the intensity did not change during the two-screen measurement, the error is
assumed to be less.
Temporal slice emittance measurements were also achieved on the same machine with
roughly 1-ps resolution. By phasing the last booster accelerator section, a linear energy chirp
(energy vs. time) on the beam was created and then spatially filtered in a dispersive section
allowing only a narrow time slice through the slit. The resolution was limited by the minimum
achievable electron beam size primarily due to the intrinsic uncorrelated energy spread in the
beam and the jitter in the rf system. A two-screen emittance measurement setup was located
6-11 ♦ I NJECTOR