Investigation of thermal acoustic oscillations in a superconducting linac cryogenic system

Abstract

During the commissioning stage of a superconducting linear accelerator (linac) three major cryogenic systems were seen to generate thermal acoustic oscillations (TAO): the cryostat liquid helium level sensor interface, the pre-cooling line and the cryogenic distribution valve box with associated transfer tube. By using Rott’s stability curves, published by other researchers elsewhere, an approximate map of the oscillations in the cryogenic systems under investigation was created for both straight and U-shaped tubes. This investigation confirmed that all systems under consideration fall within the critical range and that damping of TAO would be needed to achieve stable operation conditions. The geometry and hence the temperature distribution within the cryogenic system of the accelerator facility could not be altered arbitrarily. Possible TAO damping techniques were narrowed to two options: (a) adding restrictions at the cold end and (b) control of the helium gas pressure. In all systems, before modifications, pressure oscillations were clearly observed within the frequency range 0.25–47 Hz. In two systems TAO were observed with no liquid helium present at the cold end. The TAO investigations provided solutions that resulted in a substantial improvement in the performance of the linac cryogenic and RF stabilization systems, under test.