Joseph Ozelis

High Thermal Conductivity Cryogenic RF Feedthroughs for Higher Order Mode Couplers

The use of higher-order-mode (HOM) pickup probes in the presence of significant fundamental RF fields can present a thermal challenge for CW or high average power SRF cavity applications. The electric field probes on the HOM-damping couplers on the JLab “High Gradient” (HG) and “Low Loss” (LL) seven-cell cavities for the CEBAF upgrade are exposed to approximately 10% of the peak magnetic field in the cavity. To avoid significant dissipative losses, these probes must remain superconducting during operation. Typical cryogenic rf feedthroughs provide a poor thermal conduction path for the probes and provide inadequate stabilization. We have developed solutions that meet the requirements, providing a direct thermal path from the niobium probe, thorough single-crystal sapphire, to bulk copper which can be thermally anchored. Designs, electromagnetic and thermal analyses, and performance data will be presented.

Fabrication and Testing of the SRF Cavities for the CEBAF 12 GEv Upgrade Prototype Cryomodule Renascence

GeV upgrade prototype cryomodule Renascence have been fabricated at JLab and tested individually. This set includes four of the “Low Loss” (LL) design and eight of the “High Gradient” (HG) design. The fabrication strategy was an efficient mix of batch job-shop component machining and in-house EBW, chemistry, and final-step machining to meet mechanical tolerances. Process highlights will be presented. The cavities have been tested at 2.07 K, the intended CEBAF operating temperature. Performance exceeded the tentative design requirement of 19.2 MV/m CW with less than 29 W dynamic heat dissipation. These results, as well as the HOM damping performance are presented.

Utilization of integrated process control, data capture, and data analysis in construction of accelerator systems

Jefferson Lab has developed a web-based system that integrates commercial database, data analysis, document archiving and retrieval, and user interface software into a coherent knowledge management product called Pansophy. Pansophy provides key tools for the successful pursuit of major projects such as accelerator system development and construction by offering elements of process and procedure control, data capture and review, and data mining and analysis. Today, Pansophy is used in Jefferson Labs SNS superconducting linac construction effort as a means for structuring and implementing the QA program, for process control and tracking, and for cryomodule test data capture and presentation/analysis. Development of Pansophy continues, with improvements to data queries and analysis functions that are the cornerstone of its utility. In this paper the present configuration and operational environment of Pansophy is described, along with future development goals. Additionally, specific examples of its use in an accelerator construction project will be presented.

The Jefferson Lab quality assurance program for the SNS superconducting linac accelerator project

As part of a multi-laboratory collaboration, Jefferson Lab is currently engaged in the fabrication, assembly, and testing of 23 cryomodules for the superconducting linac portion of the Spallation Neutron Source (SNS) being built at Oak Ridge National Laboratory. As with any large accelerator construction project, it is vitally important that these components be built in a cost effective and timely manner, and that they meet the stringent performance requirements dictated by the project specifications. A comprehensive Quality Assurance (QA) program designed to help accomplish these goals has been implemented as an inherent component of Jlabs SNS construction effort. This QA program encompasses the traditional spectrum of component performance, from incoming parts inspection, raw materials testing, through to sub-assembly and finished article performance evaluation. Additionally, process and procedure control, vendor performance and oversight, and design and test program reviews constitute complementary areas where QA involvement contributes to successful production performance.

The Use of Integrated Electronic Data Capture and Analysis for Accelerator Construction and Commissioning: Pansophy from the SNS Towards the ILC

Jefferson Lab has extensively used a proprietary web-based system (Pansophy) that integrates commercial database, data analysis, document archiving and retrieval, and user interface software, as a coherent knowledge management product during the construction of the cryomodules for the SNS Superconducting Linac, providing elements of process and procedure control, data capture and review, and data mining and analysis. With near real-time and potentially global access to production data, process monitoring and performance analyses could be pursued in a timely manner, providing crucial feedback. The extensibility, portability, and accessibility of Pansophy via universally available software components provide the essential features needed in any information and project management system capable of meeting the needs of future accelerator construction efforts requiring an unprecedented level of regional and international coordination and collaboration.

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

Error Analysis on RF Measurement Due to Imperfect RF Components

An accurate cavity test involves the accurate power measurement and decay time measurement. The directional coupler in a typical cavity test llrf system usually has low directivity due to broadband requirement and fabrication errors. The imperfection of the directional coupler brings unexpected systematic errors for cavity power measurement in both forward and reflect power when a cavity is not considered a matched load as assumed in a cable calibration. An error analysis will be given and new specification of directional coupler is proposed. A circulator with a low voltage standing wave ratio (VSWR) creates a standing wave between the circulator and cavity. As long as the cavity phase is maintained, the standing wave of a non-matched cavity load will only change the input coupler coupling factor (Qext1), but not to the calculation of the cavity power loss that is independent of the Qext1.