Performance analysis of superconducting rf cavities for the CERN rare isotope accelerator

The first cryomodule of the new HIE-ISOLDE rare isotope accelerator has recently been commissioned with beam at CERN, with the second cryomodule ready for installation. Each cryomodule contains five superconducting low-beta quarter wave cavities, produced with the technology of sputtering a thin niobium film onto the copper substrate ($\mathrm{Nb}/\mathrm{Cu}$). This technology has several benefits compared to the bulk niobium solution, but also drawbacks among which the most relevant is the increase of surface resistance with accelerating field. Recent work has established the possible connection of this phenomenon to local defects in the $\mathrm{Nb}/\mathrm{Cu}$ interface, which may lead to increased thermal impedance and thus local thermal runaway. We have analyzed the performance of the HIE-ISOLDE cavities series production, as well as of a few prototypes’, in terms of this model, and found a strong correlation between the rf properties and one of the model characteristic quantities, namely the total surface having increased interface thermal impedance.

Figure 1

The HIE-ISOLDE QWR cavities, before (top left) and after (bottom left) coating, and 3D illustration of the cavity geometry (right).

Figure 2

$Q(E)$ data of all cavities of interest for this study, namely the 10 cavities installed in the first two cryomodules, the reference cavity QP1.4 and the test cavity QS7.1. Details of the nomenclature are explained in the text.

Figure 3

FIB-SEM cross section images of HIE-ISOLDE coating at the top of the cavity with $-80\mathrm{V}$ bias (left) and $-120\mathrm{V}$ $\text{bias}+\text{wide}$ cathode (right) showing the densification of the layer.

Figure 4

FIB-SEM cross section images of HIE-ISOLDE coating at the middle of the antenna with $-80\mathrm{V}$ bias (left) and $-120\mathrm{V}$ bias (right) showing the delamination of the layer.

Figure 5

Assumed step-like dependence for $R_s$ vs accelerating field $E_{\text{acc}}$.

Figure 6

Typical result of the fitting procedure of the thermal model (cavity QP1.4 in this example).

Figure 7

Plot of $R_s^1$ as a function of $A$. The insert allows an expanded view of all cavities except QS7.1.

Figure 8

$R_s(E_{\text{acc}})$ curves for QS7.1 and QS7.2 (left) and their distribution function $f(R_{\mathrm{Nb}/\mathrm{Cu}})$ as a function of the interface thermal impedance $R_{\mathrm{Nb}/\mathrm{Cu}}$ (right). The lines are only intended as a guide for the eye.