The refrigeration and cryogenic distribution system for the shortpulse x-ray source Page: 2 of 16
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modules that contain the transverse deflecting RF cavities. The injector linac is the third
largest heat load. The seven superconducting undulator magnets in the hard x-ray
production section are the smallest heat loads connected to the light source refrigeration
plant. The linac and deflecting cavity sections require helium cooling at 1.9 K, 5 K and 40
K. The undulator magnets require two-phase helium cooling at 4.3 to 4.5 K
TESLA Cryogenic Module Modifications Needed to Improve Cavity Cooling
The TESLA cavities have been demonstrated to operate routinely at gradient above 25
MV per meter, and tests at the DESY test facility have shown operations above 35 MV per
meter in some cavities . We propose to operate the superconducting cavities in CW
mode to allow 10 kHz bunch repetition rate, as opposed to the 0.7 % duty factor of the
TESLA design, resulting in significantly larger heat loads. A nine-cell cavity operating in
CW mode at 25 MV per meter will have 63.5 W heat generated as a result of RF current
flow on the inner surface of the cavity. Added to this is 8.5 W heat entering the cavity
niobium body from the input RF power coupler. This dynamic heat load is to be
transferred through the niobium to the cavity outer surface in the super-fluid helium liquid
bath, then to the super-fluid helium surface where boiling occurs at 1.8 K, without
quenching the cavity. In a super-fluid helium test bath there is no problem transferring this
heat from the cavity outer surface to the super-fluid helium surface, however, the transport
of about 72 W from the cavity outer surface through the helium tank, the feed-pipe and the
header-pipe configuration proposed for the TESLA cryogenic modules will require some
With modifications to the helium tank, helium feed pipe and the helium header pipe the
transport of heat within the TESLA cavity cryogenic module is not a problem. The heat
transfer from the cavity surface to the liquid helium surface is limited by the heat transfer
area in the tank (the area of between the cavity convolutions and the cavity helium tank
wall), the total area of the helium feed pipe or pipes, and the area of the cross-sectional
area of liquid helium that is in the feed pipe. In general, the heat transfer limit for the
liquid super-fluid helium in the tank, the feed pipe and the header is about 0.6 to 0.7 Wcm-2
. In TESLA, the area of the space between the cavity convolutions and the tank is about
40 cm2. The area of the feed pipe between the header and the tank is 28 cm-2. The cross-
sectional area of the liquid in a half-full header pipe is about 20 cm2.
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Green, Michael A. & Corlett, John N. The refrigeration and cryogenic distribution system for the shortpulse x-ray source, report, October 20, 2002; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc895382/m1/2/: accessed March 23, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.