On the Pressure Compensation for the B-cell Cavity in the MARK II Cryostat

I. How Tuner Works The B-cell superconducting cavity is tuned by mechanically stretching the cavity along the beam line relative to the cryostat. The drive train consists of a stepping motor followed by a 4.38:1 dual leverage mechanism terminating in a dual parallel flex hinge arrangement (tuner flexure) with mechanical advantage of 3.9:1. The flexure is patterned after the proposed LANL PILAC cavity tuner [1]. The total mechanical advantage of the tuner mechanism is 17:1. Such a flexible linkage system is advantageous because there is no bearings with the inherent alignment and backlash problems. The prototype of the tuner [2] has been built and tested during the Beam Test in August 1994 [3]. As it was reported in [4], the measured value of the frequency sensitivity to pressure (df/dP) is high. It is equal to 250 kHz/Bar for the cavity in the MARK II cryostat. A possible solution with using internal compensation bellows was proposed in [4]. Later another scheme, with external bellows, was proposed as easier to adjust. The bellows volume is connected to the He vessel and therefore pressure inside the bellows is the same as the pressure acting on the cavity. Thus, the compensation force is proportional to the He bath pressure and to the bellows area. In order to decrease the required compensation force and hence the bellows area, it was proposed to apply the force to the same point as the tuner motor to take benefit of the total tuner mechanical advantage. Originally the motor screw had been rigidly connected to the cryostat wall which would prevent any movement of the tuner except that due to the stepping motor. To allow the compensation force to move the tuner, a coil spring was introduced between the screw and the cryostat. The pictorial of the final version of the tuner is shown in Figure 1.