Ground Vibration and Siting of the Cryogenics Facility for Cornell ERL Prototype

A prototype Energy Recovery Linac (ERL) has been proposed and schematic design of the facility is underway. Such a facility necessarily involves the juxtaposition of extremely sensitive superconducting RF cavities and the noisy and powerful motors and compressors used to make the liquid helium the cavity needs. In this paper we give preliminary consideration to the possible deleterious effects on the performance of the superconducting cavities due to microphonics generated by the helium compressors. The compressors are singled out because by far they are the highest power mechanical system in the facility driven by a total of approximately 1100 hp. The cavities for the ERL are expected be especially sensitive to microphonics due to the relatively high Qext of 2.6 × 10, corresponding to a half width half maximum bandwidth of 25 Hz. At some distance from the cryoplant the local cultural noise sources will dominate over those produced by the cryogenic compressors. Siting further away than this distance yields no realizable benefit if the local sources cannot be removed. If at least 300-400 ft were kept between the compressors and the cavities there would be little concern about their influence via ground vibration as other facilities have operated in this mode and either microphonics have not played a role or microphonics are dominated by sources other than ground vibration. [1] However there are strong economic reasons to site the cryogenics facility close to the cavities: to minimize the capital cost of transfer lines, and more importantly to reduce the cryogenic losses in the transfer lines which in turn reduces the size and operating cost of the cryoplant. For the proposed ERL prototype [2] a detuning tolerance also of ±25 Hz was assumed which then determined the optimal Qext and the required klystron power capability of 13 kW per cavity. If instead, 15 Hz were the detuning tolerance the required power would be reduced by about 3 4 kW per cavity. For reference, measurements at the TTF (Tesla Test Facility) on cavities very similar to those to be used in the ERL prototype showed peak to peak detuning range of ±10 Hz. The TTF cavities have a much larger bandwidth of 260 Hz half width at half maximum and a Qext of 2.5×106[3]. The higher detuning tolerance for the prototype was chosen to be conservative because very little was known about the actual microphonic levels that will be present. One can obtain an estimate of the minimum siting distance of the compressors by breaking the problem into three pieces: 1. Determine the amount of vibration caused by the cryogenics plant at a given distance.