  The Spallation Neutron Source (SNS) Front-End Systems Group at Lawrence
Berkeley National Lab (LBNL) is developing a Radio Frequency Quadrupole (RFQ)
to accelerate an H- beam from 65 keV to 2.5 MeV at the operating frequency of
402.5 MHz. The 4 section, 3.7 meter long RFQ is a 4 vane structure operating at
6% duty factor. The cavity walls are made from OFE Copper with a GlidCop outer
layer to add mechanical strength. A set of 12 cooling channels in the RFQ
cavity walls are fed and controlled separately from 4 channels embedded in the
vanes. An ANSYS finite-element model has been developed to calculate the
deformed shape of the cavity for given RF heat loads and cooling water
temperatures. By combining the FEA results with a SUPERFISH RF cavity
simulation, the relative shift in frequency for a given change in coolant
temperature or heat load can be predicted. The calculated cavity frequency
sensitivity is -33 kHz per 1 degC change in vane water temperature with
constant-temperature wall water. The system start-up transient was also studied
using the previously mentioned FEA model. By controlling the RF power ramp rate
and the independent wall and vane cooling circuit temperatures, the system
turn-on time can be minimized while limiting the frequency shift.
