LCLS Conceptual Design Report - Stanford Synchrotron Radiation ...

ε n,rms (µm)
3
2
1
4-2001
8560A1
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
98 MV/m 123 MV/m 147 MV/m 172 MV/m
E o = 143 MV/m
ε n,rms = 1.06 µm
B z = 3.193 kG
0
2 3 4
B z (kG)
Figure 6.10 Emittance versus cathode field strength and peak solenoidal magnetic field.
Prototype guns have been operated for limited periods with fields up to 140 MV/m. However,
as the field is increased above 100 MV/m, not only does the dark current typically increase rather
dramatically (quickly exceeding the photocurrent), but the frequency and intensity of rf
breakdowns also increases. RF breakdowns tend to leave pits in the cathode surface that lead to
nonuniform emission. At the GTF, using a Cu cathode, 120 MV/m has been the typical operating
field.
PARMELA simulations (See Section 6.6.4, Sensitivity Study) indicate the ratio of the field in
the full cell to half cell should be close to unity. Unbalanced fields can lead to large correlated
energy spreads exiting the gun (as designed into thermionic rf guns) and subsequent emittance
growth. The LCLS gun will incorporate calibrated field probes in both cells to set and monitor
the field ratio.
6.3.3 Symmetrization
The emittance growth due to multipole modes of Ez in a gun cavity with a conventional
asymmetric rf coupler (as for the earlier BNL 1.5-cell gun) is estimated to contribute ~1 µm to the
transverse emittance [22]. This growth is dominated by the dipole mode and the contribution
from the higher order modes is less than 0.1 µm. Therefore the higher order modes can be
neglected. Symmetrization in the prototype gun is achieved by including a second identical
“coupling” hole (which is also used for vacuum pumping) directly across from the rf waveguide
coupling hole. This reduces the field amplitude dipole term by an order of magnitude over the
unsymmetrized case. Further symmetrization of the dipole field phase can be achieved by
utilizing a symmetric power feed instead of a single-sided feed. This is done with a magic tee
6-18 ♦ I NJECTOR