Vibration stability studies of a superconducting accelerating module quadrupole operating at 4.5 K

Abstract

A new method for low level low frequency vibration measurements in the cryogenic environment is presented. Commercial moving coil seismometers (geophones) have been used to investigate the mechanical stability of the quadrupole of a third generation (so-called Type-III) FLASH cryomodule (named Module 6) in fully operating conditions. Geophones were able to operate at 4.5 K without any loss of performance, providing nanometer level resolution even in the 1-10 Hz frequency band, region not covered by existing data because of the reduced sensitivity of the cooled piezoelectric accelerometers [1]. A preliminary evaluation of the impact of the refrigeration system and of the high power RF on the vibration level of the quadrupole is also presented. The results are of interest for the design of linear accelerator (linac) cryomodules of the International Linear Collider (ILC) [2] and of the European X-ray Free Electron Laser (XFEL) [3], for which the design is a further evolution of the FLASH Type III.

Vibration Property of a Cryogenic Optical Resonator within a Pulse-Tube Cryostat

“…Geophone has small size, low cost, and is very sensitive to low audio frequency vibration, but its specified working temperature is only down to around . A proper calibration can bring the working temperature of the geophones into the cryogenic temperature [ 25 ]. To extend the working temperature of geophones and employ them in vibration measurement at 4 K, we first re-calibrate the geophones, taking into account the induction effect of the vibration sensors at low temperature.…”

“…The working principle of geophone is based on Faraday’s law: a sensing coil is hung in the casing shield through springs while a fixed magnet is mounted to the bottom of the shield; therefore, the vibrational motion can be sensed when the coil moves with respect to the fixed magnet. The output voltages of the geophone, , is related to the vibrational motion through velocity sensitivity [ 25 , 26 ]: …”

“…By analyzing the electric circuit, calibration parameters can be fitted from the equation: where is the source voltage, is the coil resistance, is the shunt resistance, L is the coil inductance, and h is a simplified description which is related to the and , . At room temperature, is rather small in comparison with and, therefore, the coil inductance is normally neglected in the calibration [ 25 , 26 ].…”

Vibration diagnostics instrumentation for ILC

“…The best charge-mode accelerometer available off-the-shelf (model 7703A-1000 from Endevco, Co.) is currently installed in TTF cryomodules, aboard the quadrupole helium vessel. Unfortunately, the loss of a factor of ∼3 in charge sensitivity, when cooled down to 4.5 K, further lowers their relatively poor resolution, finally lying in the µg region between 1 and 10 Hz [9]. For these reasons, these sensors look marginally or not sufficiently sensitive to detect vibration levels of interest for linac diagnostics in that frequency band.…”