New method to improve the accuracy of quench position measurement on a superconducting cavity by a second sound method

Abstract

Quench is a common phenomenon in a superconducting cavity and often limits the accelerating gradient of the cavity. Accurate location of the quench site, typically located at a material or geometrical defect, is the key to improve the cavity accelerating gradient. Here, the second sound propagation in liquid helium II is used to detect the quench location on the cavity. The technique is relatively convenient and complements the traditional temperature mapping which measures the ''prequench'' temperature rise on the cavity using an array of sensors. The speed of the second sound in liquid helium II is roughly 1:7 cm=ms at 2 K which is sufficiently fast to provide a millimeter-size position resolution. However, the dynamics of the quench at the cavity surface are also found to significantly affect the achievable resolution with real cavities. Here we use a dynamic quench model, based on ANSYS, to calculate the quench area and the temperature distribution on the cavity. The detection error caused by the thermal conduction in the niobium was calculated.

Quench Detection on a Superconducting Radio-Frequency Cavity

“…For our further studies on quench detection, it is important and challenging to understand the effect of thermodynamics in cavity surface made of niobium and the transition of heat propagation from the niobium to the superfluid helium. In [19], it was observed that heat could first propagate along the cavity surface and then later excite an entropy wave in the superfluid helium. In particular, in experiments since the speed of propagation is faster in the niobium than in the superfluid helium, the measured travel time at an OST is smaller.…”

“…However, in practice, they have experienced difficulties in exactly locating a quench. For instance, when the real wave velocity is higher than the theoretical one, the radii are too small to have an intersection point on the cavity surface, see [7,19,20,21] and the references therein. The size of the quench is another potential source of inaccuracy.…”

Experimental study of second sound quench detection for superconducting cavities

“…Such premature signals have been experimentally observed by nearly all the laboratories which have used second sound sensors to localize a quench in a superconducting cavity [8,10,11,14,15]. Different hypothesis have been envisaged to explain this difference, and will be discussed later in this paper.…”

“…Using a velocity to describe the processes in niobium was originally meant as an example calculation [35]. It is based on simulated results in [15]. This approach was shown to match real systems surprisingly well as shown in [16].…”

“…Eichhorn et al [14] have supposed that the second sound wave could be generated before the rf system can detect the quench; they have developed a corresponding model, which could explain some tens of μs error, much lower than experimentally observed. Liu [15] and Eichhorn [16] have considered the heat diffusion in niobium, in addition to the second sound propagation, which could cause premature signals, but their models have not been systematically compared to experimental results. The possibility of other effects beside second sound propagation in He II has also been considered, and experiments have been carried out with local heaters as a source for the heat propagation.…”