Anthony Crawford

Magnetic Flux Expulsion Studies in Niobium SRF Cavities

With the recent discovery of nitrogen doping treatment for SRF cavities, ultra-high quality factors at medium accelerating fields are regularly achieved in vertical RF tests. To preserve these quality factors into the cryomodule, it is important to consider background magnetic fields, which can become trapped in the surface of the cavity during cooldown and cause Q0 degradation. Building on the recent discovery that spatial thermal gradients during cooldown can significantly improve expulsion of magnetic flux, a detailed study was performed of flux expulsion on two cavities with different furnace treatments that are cooled in magnetic fields amplitudes representative of what is expected in a realistic cryomodule. In this contribution, we summarize these cavity results, in order to improve understanding of the impact of flux expulsion on cavity performance. INTRODUCTION How strong is the impact of residual magnetic fields on the Q0 of a superconducting RF cavity? Trapped flux degrades Q0 and necessitates additional cryogenic capacity for cooling at a given accelerating gradient. With magnetic shielding and active compensation to reduce the residual axial field to ∼5 mG, what will the impact on Q0 be? Recent discoveries have shown that: • Spatial thermal gradients during cooldown can significantly improve expulsion of magnetic flux [1] • Flux expulsion behavior can be substantially enhanced through UHV furnace treatment [2] In this contribution, we study two newly fabricated cavities produced using high RRR niobium from the same production group. Only one of these cavities is given high temperature furnace treatment at temperatures higher than 800 C. The impact on flux expulsion behavior is measured, as is the impact on Q0 in a magnetic field that is of similar strength to what would be expected in an accelerator cryomodule. MEASUREMENT TECHNIQUE The setup for measuring flux expulsion, after the method in [3], is shown in Fig. 1. An axial magnetic field is applied to a cavity during cooldown, and fluxgate magnetometers at the middle of the cell measure the magnetic field before BNC and after BSC the superconducting transition. Thermometers measure the temperature at the top, bottom, and middle ∗ This work was supported by the US Department of Energy † sposen@fnal.gov of the cavity cell. The temperature difference between the top and bottom of the cell is used to represent the thermal gradient. If the applied field is fully trapped in the cavity wall when the cavity passes through the superconducting transition temperature, the field should not change (BSC /BNC=1). If the field is fully expelled by the superconductor, simulations show that the field should be enhanced by a factor of approximately 70% (BSC /BNC=1.7). An uncertainty of 0.1 was assumed for BSC /BNC due to the exact distance of the fluxgate probe from the cavity surface, its alignment relative to the applied field and non-uniformities in the field. An uncertainty of 0.2 K was assumed for the temperature measurement in each probe, due to thermal impedance between cavity and thermometer and non-uniformity in temperature around the cavity. Figure 1: Apparatus used to measure flux expulsion (left) and simulation used to determine the magnetic field enhancement factor for full expulsion. Two fine grain 1.3 GHz single cell cavities, AES024 and AES025, were fabricated by the same vendor using high RRR niobium from the same production run. Only AES025 was given 900 C furnace treatment for 3 hours. Then both received bulk EP, 800 C degas, and ‘2/6’ nitrogen doping with 5 micron EP (which is the baseline recipe for the cavities for the LCLS-II project [4]). During cooldown in vertical test, spatial temperature gradient was measured from the bottom to the top iris when the bottom iris reached 9.2 K. For each cavity, many cooldown-warmup cycles were run. Unless RF data was taken, cooldown was stopped at 6 K. Proceedings of IPAC2016, Busan, Korea WEPMR009 07 Accelerator Technology T07 Superconducting RF ISBN 978-3-95450-147-2 2277 C op yr ig ht

Furnace N2 Doping Treatments at Fermilab

The Fermilab SRF group regularly performs Nitrogen (N2) doping heat treatments on superconducting cavities in order to improve their Radio Frequency (RF) performances. This paper describes the set up and operations of the Fermilab vacuum furnaces, with a major focus on the implementation and execution of the N2 doping recipe. The cavity preparation will be presented, N2 doping recipes will be analyzed and heat treatment data will be reported in the form of plot showing temperature, total pressure and partial pressures over time. Finally possible upgrades and improvements of the furnace and the N2 doping process are discussed.

Integrated High-Power Tests of Dressed N-doped 1.3 GHz SRF Cavities for LCLS-II

New auxiliary components have been designed and fabricated for the 1.3 GHz SRF cavities comprising the LCLS-II linac. In particular, the LCLS-II cavity’s helium vessel, high-power input coupler, higher-order mode (HOM) feedthroughs, magnetic shielding, and cavity tuning system were all designed to meet LCLS-II specifications. Integrated tests of the cavity and these components were done at Fermilab’s Horizontal Test Stand (HTS) using several kilowatts of continuous-wave (CW) RF power. The results of the tests are summarized here. INTRODUCTION The LCLS-II 4 GeV superconducting linac [1] is based on XFEL/ILC technology intensively developed over the last couple of decades. A major difference however is that LCLS-II operates in the CW regime, whereas the XFEL/ILC will operate in pulsed mode. This required modifications to or complete re-design of some of the basic components: cavity Helium vessel, tuner, power coupler, and other cryomodule parts in order to accommodate the much higher cryogenic loads expected in the CW regime. To accelerate the production of two pre-production cryomodules, it was decided to use existing ILC bare cavities and fundamental power couplers, which led to some constraints. The major LCLS-II modifications of the dressed cavity and auxiliaries are as follows:  Nitrogen doped cavity to reduce losses in CW regime. LCLS-II requirements: Q0 > 2.7 x 10 at the nominal gradient of 16 MV/m.  Helium vessel with a larger diameter two-phase connection to accommodate higher heat flux, and two helium supply inlets to provide more uniform thermal gradients during cooldown, which are crucial to effective magnetic flux expulsion, and hence low surface resistance.  Two layers of magnetic shielding to reduce residual magnetic field at the cavity below 5mG.  New end-lever tuner design which had to remain compatible with the “short-short” version of the ILC cavity adopted for the pre-production cryomodule. This design must also fit the “short-long” XFEL version of the cavity, which was adopted for production cryomodules.  Design of the fundamental power coupler (FPC) was modified to fulfil LCLS-II requirements: loaded Q=4 x 10 and average power up to 6.4kW (includes 1.6kW of reflected power). Major modifications include reduction of the antenna length by 8.5mm and increase in the thickness of copper plating on the inner conductor of warm section to reduce coupler temperature. To minimize the risks to the project all technical solutions and new designs have to be prototyped and tested in a cryomodule. Testing was focused on the most critical components and technical solutions, and performed in the Horizontal Test Stand cryostat (HTS) under conditions approximating the final cryomodule configuration. An integrated cavity test was the last stage of the design verification program. In this test a nitrogen doped cavity (AES021), previously qualified in a vertical cryostat, was dressed and fully assembled with all components (fundamental power coupler, two-layer magnetic shielding, XFEL-type feedthroughs, end-lever tuner). All components were previously individually tested in the HTS with cavities, but not as a complete integrated system. One major goal of this integrated test was to demonstrate that high Q0 values demonstrated in vertical test can be preserved even when additional sources of heating from the power coupler and tuner and potential additional external magnetic fields from auxiliary components are present. Other important studies related to design verification included thermal performance and power handling of the power coupler, heating of HOM couplers and tuner components, tuner performance, sensitivity to microphonics, and frequency control. Data from this test program allows component design to be verified and certain other aspects of cryomodule design (e.g., component thermal anchoring) to be finalized. TEST PREPARATION AND CAVITY CONFIGURATION Dressed cavity AES021 was tested previously in a vertical test stand (VTS) without HOM feedthroughs. HOM feedthroughs were later installed in a clean room and after a brief high pressure water rinse, a pumping manifold was installed, the cavity evacuated, and successfully leak checked. The cavity field probe was not removed or replaced. The cavity was transported to a different clean room for installation of the coupler cold section. No additional cleaning of the cavity surfaces took place either as part of or subsequent to coupler installation. HOM feedthroughs were later installed in a clean room and after brief high pressure water rinsing, a pumping manifold was installed and cavity was leak tight. Cavity was transported to assembly clean room for ___________________________________________ # solyak@fnal.gov N. Solyak , T. Arkan, B. Chase, A. Crawford, E. Cullerton, I. Gonin, A. Grassellino, C. Grimm, A. Hocker, J. Holzbauer, T. Khabiboulline, O. Melnychuk, J. Ozelis, T. Peterson, Y. Pischalnikov, K. Premo, A. Romanenko, A. Rowe, W. Schappert, D. Sergatskov, R. Stanek, G. Wu, FNAL, Batavia, IL 60510, USA MOPB087 Proceedings of SRF2015, Whistler, BC, Canada ISBN 978-3-95450-178-6 342 C op yr ig ht