Jeremiah Holzbauer

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.

Testing of SSR1 Production Tuner for PIP-II

The PIP-II project at Fermilab is a proton driver linac calling for the use of five different, novel cavity geometries. Prototyping at Fermilab is in the advanced stages for the low-beta single-spoke resonator (SSR1) and associated technologies. A production tuner design has been fabricated and tested, both warm and cold in the Spoke Test Cryostat (STC). This paper will present the detailed studies on this tuner, including slow motor/piezoelectric tuner range and hysteresis as well as dynamic mechanical system characterization.

Modified Slow Tuner Design for Cavity 1 Inside LCLS II Cryomodules

Initial LCLS-II cryomodule testing at Fermilab showed microphonics on the furthest upstream cavity (number 1) at least factor 2 larger than on the rest of the cavities. Testing indicated that this was a difference in the mechanical support of cavity 1, not a local acoustic source. Further investigation pointed to the upstream beam-pipe of the cavity 1. The upstream cavity flange has a solid spool piece connection to the beamline gate valve unlike the other cavities, which all connect through bellows. The gate valve’s weight was, in the original design, supported by sliding system (free in z-axis) connected to large diameter Helium gas return pipe. The tuner design was modified to transform the cavity 1/gate valve interface. The cavity 1 tuner arms were extended and became the support structure for gate valve, eliminating the connection to the helium return pipe. Modification of the tuner design and resulting microphonics mitigations will be presented. INTRODUCTION Testing of the first LCLS II cryomodules at FNAL revealed microphonics of cavity 1 (the most upstream cavity) was consistently worse than other cavities (2-8) by factor of 2-3. This was independent from the overall CM microphonics level, spanning an overall CM microphonics level of 100’s or 10’s of Hz [1]. For all cavities except 1, the cavity beamline flange on the tuner (upstream) side is connected to the neighboring cavity through a beamline bellows. Cavity 1 and the upstream gate valve are connected rigidly, with no beamline bellows (Figure 1). This gate valve is supported vertically by a bracket and sliding system, attached to the 300mm diameter Helium gas return pipe. This sliding system is required to accommodate thermal contractions of the 300mm pipe and cavities string in the horizontal (z-axis) direction. The gate valve acts as large backing weight on the dressed cavity/tuner system, lowering the resonant frequency of the longitudinal modes. In the machine, a beamline absorber will be connected to gate valve, adding even more mass to the cavity 1 system, and this could worsening the microphonics level on the cavity 1 even more. Figure 1: Picture of the “standard” interface between cavity#1 and gate valve. CAVITY 1 MECHANICAL CONNECTION WARM STUDY A simple setup was assembled (Figure 2) to study vibration levels for different configurations of the interface between cavity 1 and Helium gas return pipe. One LCLS II cavity and tuner was installed, mounted to 300mm Helium return pipe. The tuner’s piezo-actuators were used as sensors to monitor the levels of cavity vibration versus different interface configurations. A calibrated impact hammer, equipped with piezo sensor, was used to excite vibration into the mock-up system. Figure 2: Picture of the mock-up for warm study of the proposed gate valve/cavity#1 modifications. Two mitigation options were tested. The integrated response of the different mock-ups are presented in Figure 3. ___________________________________________ * This manuscript has been authorized by Fermi Research Alliance LLC under Contract N. DE-AC02-07CH11359 with U.S. Department of Energy. †jeremiah@fnal.gov 9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW Publishing ISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-WEPML006 WEPML006 2684 Co nt en tf ro m th is w or k m ay be us ed un de rt he te rm so ft he CC BY 3. 0 lic en ce (© 20 18 ). A ny di str ib ut io n of th is w or k m us tm ai nt ai n at tri bu tio n to th e au th or (s ), tit le of th e w or k, pu bl ish er ,a nd D O I. 07 Accelerator Technology T07 Superconducting RF Figure 3: Summary of the warm study with cavity#1 mockup. Integrated response when 300mm He return pipe strike with hammer. BEAMLINE BELLOW BETWEEN CAVITY 1 AND GATE VALVE To mitigate the propagation of mechanical vibration from HGRP to cavity 1 and minimize contribution from the heavy gate valve, we introduced a short beamline bellows between cavity beamline flange and gate valve. In order to replace the sliding bracket that supported the weight of the gate valve, we modified/extended the tuner arms (Figures 4 and 5) [2]. The extended tuner arms took the weight of the gate valve via the cavity 1 helium vessel (Figure 6). This required an additional new element, a gate valve support cage (Figure 5 and 6). This rigidly connected the gate valve to the extended tuner arms. Figure 4: Drawings of the modification of the gate valve/cavity 1 interface, including short bellows introduced between cavity beamline flange and gate valve. Figure 5: 3-D model of the LCLS II tuner with extended arms (right) and gate valve support bracket (left). Figure 6: Picture of the cavity 1 and gate valve assembled on cryomodule 6 with bellows. Extended tuner arms attached to the He vessel of the cavity 1. The support cage is mounted on the arms and supports the gate valve. SOLID CONNECTION BETWEEN CAVITY 1 AND GATE VALVE WITH FLEXABLE JOINTS INTERFACE. The short bellows in the beamline need to be introduced during assembly of the cavity string into clean-room. This modification could not be employed on already assembled cryomodules. A different modification was designed that preserved the solid short spool piece between cavity 1 and gate valve (Figure 7). The same extended tuner arms hold a cage that will support gate valve. The system that supports the gate valve inside cage needs to accommodate cavity’s slow tuner stroke that could be up to 2mm. Flexible joints were introduced to allow slow tuner cavity tuning to nominal frequency. ANSYS simulations were conducted to select the detailed design of the flexible joints. In the new design, the tuner system will operate against cavity plus flexible joints. To preserve the preload on the piezo-actuators below 4kN, the stiffness of the flexible joints need to be below 3kN/mm [2, 3]. 9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW Publishing ISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-WEPML006 07 Accelerator Technology T07 Superconducting RF WEPML006 2685 Co nt en tf ro m th is w or k m ay be us ed un de rt he te rm so ft he CC BY 3. 0 lic en ce (© 20 18 ). A ny di str ib ut io n of th is w or k m us tm ai nt ai n at tri bu tio n to th e au th or (s ), tit le of th e w or k, pu bl ish er ,a nd D O I.

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