Ari Palczewski

Secondary ion mass spectrometry for superconducting radiofrequency cavity materials

Historically, many advances in superconducting radio frequency (SRF) cavities destined for use in advanced particle accelerators have come empirically, through the iterative procedure of modifying processing and then performance testing. However, material structure is directly responsible for performance. Understanding the link between processing, structure, and performance will streamline and accelerate the research process. In order to connect processing, structure, and performance, accurate and robust materials characterization methods are needed. Here, one such method, secondary ion mass spectrometry (SIMS), is discussed with focus on the analysis of SRF materials. In addition, several examples are presented, showing how SIMS is being used to further understanding of material-based SRF technologies.Historically, many advances in superconducting radio frequency (SRF) cavities destined for use in advanced particle accelerators have come empirically, through the iterative procedure of modifying processing and then performance testing. However, material structure is directly responsible for performance. Understanding the link between processing, structure, and performance will streamline and accelerate the research process. In order to connect processing, structure, and performance, accurate and robust materials characterization methods are needed. Here, one such method, secondary ion mass spectrometry (SIMS), is discussed with focus on the analysis of SRF materials. In addition, several examples are presented, showing how SIMS is being used to further understanding of material-based SRF technologies.

First Results of the SRF Wafer Test Cavity for the Characterization of Superconductors

The wafer test cavity was designed as a short sample test system that could create a reproducible environment for the testing of superconducting materials above the Bardeen-Cooper- Schrieffer limit of niobium. The results of the sapphire test cavity showed that the dielectric was too lossy, and thus, the original design had to be altered to make operation feasible with current hardware and achieve ~200 mT. The new design was fabricated at Thomas Jefferson National Accelerator Facility and was cryogenically tested. After four tests, the cavity was able to produce a 6.6-mT field with a Q of 3.96 * 108. Although lower than anticipated, in comparison to other TE01 cavities, this result is quite encouraging. Multipacting and coupling were limitations, but current work is pursuing the elimination of these complications. This document will expound upon the new design, mathematical simulations, testing of the cavity, complications, results, and future work.

Analysis of BCS RF Loss Dependence on N-Doping Protocols

We present a study on two parallel-path SRF cavities (one large grain and one fine grain, 1.3 GHz) which seeks to explain the correlation between the amount of nitrogen on the inner surface of a “nitrogen doped” SRF cavity and the change in the temperature dependant (packaged into term BCS) RF losses. For each doping/EP, the cavities were tested at multiple temperatures (2.0 K to 1.5 K in 0.1 K steps) to create a Q0 vs. Eacc vs. T matrix which then could be used to extract temperature dependant and independent components. After each test, the cavities were thermally cycled to 120 K and then re-cooled and retested to assess if evidence of hydrogen migration might appear even at a small level. In addition, TD-5 was also tested at fixed low field (Q0 vs. T) to fit standard BCS theory. In parallel, SIMS data was taken on like-treated samples to correlate the amount of nitrogen within the RF surface to the change in the temperature dependant fitting parameter “A”. INTRODUCTION During the last one and a half years, while developing guidance for the nitrogen doping protocols for the LCLSII cryomodules, JLab has systematically doped over 20 single and multi-cell cavities, with most cavities being doped more than once. The wide range of cavity dopings was done to better understand the feasibility of nitrogen doping for project. For all test RF measurements each cavity was tested at multiple temperatures (Q0 vs. Eacc vs. T) in order to enable decomposition of the RF losses into temperature dependant and temperature independent portions. Initial analysis of RF losses on multiple cavities was presented at IPAC 2015 [1]. These results suggest that there is a correlation between the doping/electropolish parameters and the temperature dependent RF losses, but it is unclear what mechanism would explain this correlation. Another mystery that arose during the initial phase of development was the occurrence of lower than expected temperature independent losses after a surface reset of 35 to 50 μm. These so-called “re-baselined” cavities had performance similar to standard preparation EP cavities but with the Q0 vs. Eacc @ 2.0 K curve shifted up. This suggested that there was still a substantial amount of nitrogen left in the niobium and that the nitrogen may play more than one role in the niobium. In this paper we present a new study on two cavities which seeks to explain the correlation between the temperature dependant portion of the surface resistance with doping/EP as well as the higher than expected Q0 at 2.0 K after surface reset. CAVITY HISTORY The two cavities chosen for this study were RDT-13 and TD-5; both cavities are 1.3 GHz TESLA shaped single cell cavities. RDT-13 uses the symmetric long end cell design (geometry factor of 279) made out of fine grain niobium RRR>250 from Tokyo-Denki. The cavity had been doped multiple times before this study, with a 40-45 μm chemistry reset between doping and an 80 μm reset before this study. After its last doping (800°C 3hrs N1A10 EP5), the cavity had a rather strange temperature independent component to its Q0 vs. Eacc performance, similar to a medium field Q0 slope. At the time this was presumed to be caused by a “bad” EP, and therefore 80 μm was taken off the inner surface to ensure what was thought to be a full surface reset, i.e. no doping left. TD-5 is symmetric center cell design (geometry factor of 270) cavity made out of large grain niobium RRR>300 from Tokyo-Denki. After manufacturing and before the baseline test, the cavity was post purified at 1250°C with titanium. The full cavity histories after half-cell machining are presented in Table 1. TEST PLAN This study was designed to follow two different cavities through a single nitrogen doping followed by multiple EP removals with RF tests. After the first RF test for each EP, each cavity was warmed up to 120 K for a minimum of 5 hours and then re-cooled and tested. Such incremental steps with removal by EP continued until there was a positive slope in the temperature dependant portion of the surface resistance. In addition, after the first EP removal of 5 μm EP, the outsides of the cavities were BCP’ed removing 10 μm and retested. The full test outline is shown in Table 2. RESULTS This study was designed to follow two different cavities through a single nitrogen doping followed by multiple EP removals with an RF test after each removal. The RF data is presented in multiple ways; Q0 vs. Eacc at 2.0 K, temperature independent and dependant surface resistance vs field, as well as low field Q0 vs. T on TD-5. ___________________________________________ * Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177 with supplemental funding from the LCLS-II Project U.S. DOE Contract No. DE-AC02-76SF00515 #ari@jlab.org MOPB039 Proceedings of SRF2015, Whistler, BC, Canada ISBN 978-3-95450-178-6 174 C op yr ig ht

Characterization of superconducting radiofrequency breakdown by two-mode excitation

We show that thermal and magnetic contributions to the breakdown of superconductivity in radiofrequency (RF) fields can be separated by applying two RF modes simultaneously to a superconducting surface. We develop a simple model that illustrates how mode-mixing RF data can be related to properties of the superconductor. Within our model the data can be described by a single parameter, which can be derived either from RF or thermometry data. Our RF and thermometry data are in good agreement with the model. We propose to use mode-mixing technique to decouple thermal and magnetic effects on RF breakdown of superconductors.