Technique for monitoring fast tuner piezoactuator preload forces for superconducting RF cavities

Abstract

The technology for mechanically compensating Lorentz Force detuning in superconducting RF cavities has already been developed at DESY. One technique is based on commercial piezoelectric actuators and was successfully demonstrated on TESLA cavities [1]. Piezo actuators for fast tuners can operate in a frequency range up to several kHz; however, it is very important to maintain a constant static force (preload) on the piezo actuator in the range of 10 to 50% of its specified blocking force. Determining the preload force during cool-down, warm-up, or re-tuning of the cavity is difficult without instrumentation, and exceeding the specified range can permanently damage the piezo stack. A technique based on strain gauge technology for superconducting magnets has been applied to fast tuners for monitoring the preload on the piezoelectric assembly. The design and testing of piezo actuator preload sensor technology is discussed. Results from measurements of preload sensors installed on the tuner of the Capture Cavity II (CCII)[2] tested at FNAL are presented. These results include measurements during cool-down, warmup, and cavity tuning along with dynamic Lorentz force compensation.

Capture cavity II results at FNAL

“…Furthermore, the piezo correction minimizes the required forward power while keeping the cavity gradient constant during the RF pulse. This has a beneficial impact for the feedback control of the LLRF and optimizes the RF power headroom, which can be critical when driving several cavities from a single klystron, as reported in [3]. In the plot shown above, the additional forward power required to maintain a flat top during the RF pulse peaks around 29% of the total forward power when no piezo …”

“…This section provides an overview of some results obtained with this fast tuner at FNAL. A detailed description of the piezo and its performance on CCII is reported in [3]. The static piezo tuning range is around 1.3 kHz, corresponding to approximately 6.5 microns of cavity motion.…”

“…When used as an actuator, it allows for compensation of any detuning due to the presence of Lorentz forces [4]. As shown in [3,5], exciting the cavity with a controlled mechanical stimulus milliseconds before the arrival of the RF pulse can predetune the cavity and effectively compensate any frequency shift induced by the RF pulse.…”

“…As can be seen in Fig.1, without piezo compensation, the cavity experiences a resonance frequency drift in the order of 275 Hz. With the piezo correction this is reduced to approximately 20 Hz [3]. Furthermore, the piezo correction minimizes the required forward power while keeping the cavity gradient constant during the RF pulse.…”

Integration of piezoelectric stacks in components using powder bed fusion

“…Many of the different state-of-the-art methods have severe drawbacks, which make them unsuitable for the integration in additive manufactured lightweight structures. Most designs are bulky and the integration of the piezoelectric stack was completed either with screws [20][21][22] or with solid springs [22,23] consuming space and reducing the compactness. In the past years, no recent progress to apply piezoelectric stack integration to lightweight structures was seen here.…”