An improved phase-control system for superconducting low-velocity accelerating structures

It is noted that microphonic fluctuations in the RF eigenfrequency of superconducting (SC) slow-wave structures must be compensated by a fast-tuning system in order to control the RF phase. The tuning system must handle a reactive power proportional to the product of the tuning range and the RF energy content of the resonant cavity. The accelerating field level of many of the SC cavities forming the ATLAS linac has been limited by the RF power capacity of the presently used p-i-n diode based fast-tuner. A new system has been developed, utilizing p-i-n diodes operating immersed in liquid nitrogen, with the diodes controlled by a high-voltage V-groove metal-oxide semiconductor (VMOS) FET driver. The system has operated at reactive power levels above 20 kVA, a factor of four increase over an earlier design. The increased capacity will permit phase stabilization of superconducting resonators at higher field levels. In particular, it has enabled the operation of very-low-velocity superconducting structures at gradients of more than 4 MV/m in the ATLAS positive-ion injector linac