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
along with two H bends that symmetrically feed rf power to the full cell of the LCLS 1.6-cell rf
gun [23]. A diagnostics port in the magic tee will monitor rf power asymmetries. The coupling
hole size will need to be reduced compared to the prototype gun to maintain the same rf coupling
coefficient. Such a symmetric rf feed is planned for the LCLS gun.
6.3.4 120 Hz Operation
The prototype gun was originally designed for low repetition rates, but it is estimated that it
could be operated up to 40 Hz. The stored energy in the cavity fields is 9.1 J for the LCLS design
of 140 MV/m. The LCLS gun requires 14 MW of power from the klystron and has a filling time
of approximately 670 ns assuming a Q0 of 12000 and a critically coupled cavity resulting in an
average heat load of 3.8 kW at 120 Hz. A slightly modified gun was designed and built by a
BNL/KEK/SHI collaboration for operation up to 100 Hz [24]. Water cooling channels were
added in the vicinity of the irises and satisfactory operation at 100 MV/m with a 4-µs wide rf
pulse at 50 Hz has been demonstrated at the University of Tokyo at Tokai [25]. At these operating
conditions the gun is dissipating roughly 1 kW of power.
At 120 Hz, the prototype gun with unmodified cooling channels would not operate at design
specifications due to thermo-mechanical distortions. There are two possible solutions to this
problem and some combination of the two will be adopted. The first solution, already utilized by
the BNL/KEK/SHI collaboration, is to study the energy deposition in the rf gun and provide
appropriate cooling at the optimal locations to minimize thermal gradients without compromising
structural integrity [24]. One must also carefully consider the thermal distortions in the gun and
not allow significant frequency shifts due to thermal expansion or introduction of higher order
modes due to asymmetric cavity distortions.
Alternatively the thermal load can be reduced by a factor of three or more, i.e., to less than
the 1-kW level already tested at Tokai, by properly shaping the rf pulse [26] and/or overcoupling
the cavity to reduce the filling time. An overcoupled cavity with coupling coefficient of 1.5
instead of unity will dissipate 20% less power while only requiring 4% higher peak rf power to
achieve an identical field in the gun. Using 30 MW of rf power instead of 14 MW to drive the
gun, the rf field will build up to the desired value of 140 MV/m in only 750 ns instead of 2.8 µs.
Once the desired accelerating voltage is reached, a fast (100 ns) rf attenuator on the klystron input
could be used to stabilize the voltage at the desired value during beam extraction. The total rf
pulse duration can thus be limited to about 1 µs with a corresponding reduction in the heat load of
a more than a factor of three.
6.3.5 Photocathode
The choice of cathode material is a function of several restrictions including gun emittance,
laser power at a given wavelength, longevity under rf processing or operation, and gun cavity
construction. The use of a cathode plug or insert in an S-band gun has so far limited the cathode
field to about 110 MV/m [27], whereas simulations indicate the transverse emittance drops with
increasing field up to about 140 MV/m. However, a load-lock coupled gun, which utilizes a back
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