Warren Schappert

Adaptive compensation of Lorentz force detuning in superconducting RF cavities

The Lorentz force can dynamically detune pulsed Superconducting RF cavities. Considerable additional RF power can be required to maintain the accelerating gradient if no effort is made to compensate for this detuning. An adaptive feed-forward Lorentz Force Detuning (LFD) compensation algorithm developed at Fermilab is described. Systems based on this approach have been used to successfully reduce LFD from several hundred Hz to several 10s of Hz or better.

Improved RF measurements of SRF cavity quality factors

Abstract SRF cavity quality factors can be accurately measured using RF-power based techniques only when the cavity is very close to critically coupled. This limitation is from systematic errors driven by non-ideal RF components. When the cavity is not close to critically coupled, these systematic effects limit the accuracy of the measurements. The combination of the complex base-band envelopes of the cavity RF signals in combination with a trombone in the circuit allow the relative calibration of the RF signals to be extracted from the data and systematic effects to be characterized and suppressed. The improved calibration allows accurate measurements to be made over a much wider range of couplings. Demonstration of these techniques during testing of a single-spoke resonator with a coupling factor of near 7 will be presented, along with recommendations for application of these techniques.

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