Charles Reece

The Mechanism of Electropolishing of Niobium in Hydrofluoric-Sulfuric Acid Electrolyte

Niobium surfaces are commonly electropolished in an effort to obtain optimal smoothness for high-field superconducting radio-frequency cavity applications. We report the use of controlled electrochemical analysis techniques to characterize electropolishing of Nb in a sulfuric and hydrofluoric acid electrolyte. Through the use of a reference electrode, we are able to clearly distinguish the anode and cathode polarization potentials as well as the electrolyte voltage drop, which together sum to the applied power supply voltage. We then identify the temperature and HF concentration dependence of each potential. We also report the use of electrochemical impedance spectroscopy (EIS) on this system. EIS results are consistent with the compact salt film mechanism for niobium electropolishing (EP) in this electrolyte and are not consistent with either the porous salt film or the absorbate-acceptor mechanism. Microscopic understanding of the basic Nb EP mechanism is expected to provide an appropriate foundation with which to optimize the preparation of high-field niobium cavity surfaces.

Improved prototype cryomodule for the CEBAF 12 GeV upgrade

In order to provide a higher performance building block cryomodule for the CEBAF 12 GeV upgrade, modifications have been made to the design of the Upgrade Cryomodule. The prototype cryomodule will be completed in 2004 and be installed for operation in CEBAF. Design changes enable the use of higher gradient cavities to achieve greater than 100 MV per cryomodule while not exceeding the budgeted cryogenic load of 300 W during steady-state operation. They also include refinements based on experience gained during the construction of the first generation upgraded cryomodules as well as the prototype cryomodule for the Spallation Neutron Source. Two cavity designs will be used in the prototype, one optimized for E/sub peak//E/sub acc/ ratio, and the other optimized for minimum cryogenic load. The input waveguides, thermal shield and piping have been redesigned to accommodate the higher expected heat loads. The vacuum connections consist of niobium-titanium flanges, aluminum-magnesium seals and stainless steel clamps to provide reliable UHV sealing. The cavity tuner features one cold motor and two piezoelectric actuators to provide coarse and fine tuning respectively.

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.

Very high residual resistivity ratios of heteroepitaxial superconducting niobium films on MgO substrates

We report residual resistivity ratio (RRR) values (up to RRR-541) measured in thin film Nb grown on MgO crystal substrates, using a vacuum arc discharge, whose 60?160 eV Nb ions drive heteroepitaxial crystal growth. The RRR depends strongly upon substrate annealing and deposition temperatures. X-ray diffraction spectra and pole figures reveal that, as the crystal structure of the Nb film becomes more ordered, RRR increases, consistent with fewer defects or impurities in the lattice and hence longer electron mean free path. A transition from Nb(110) to purely Nb(100) crystal orientation on the MgO(100) lattice occurs at higher temperature.

A system for managing critical knowledge for accelerator subsystems: Pansophy

Accelerator development and construction projects often intentionally push the envelope of well-established technical performance and manageable complexity. In addition, the desire for efficient retention and exploitation of accumulated experience across the multi-decade life cycles of major installations calls for a robust yet user-friendly knowledge management system. To meet these needs, we are deploying a new web-based system at Jefferson Lab: Pansophy. This system is a custom integration of several commercial software utilities, DocuShare/sup TM/, ColdFusion/sup TM/, Matlab/sup TM/, Ingres/sup Tm/, and common desktop programs. Users of the system range from process managers, shop-floor technicians, and test engineers to after-the-fact data miners and operations staff. The system integrates important quality assurance elements of procedural control, automated data accumulation into a secured central database, prompt and reliable data query and retrieval, and online analysis tools, all accessed by users via their platform-independent web browsers. A system overview, a completed pilot project, and implementation experience to date 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.

Enhanced field emission from chemically etched and electropolished broad-area niobium

Electron field emission from broad-area metal surfaces is known to occur at a much lower electric field than predicted by the Fowler–Nordheim law. This enhanced field emission (EFE) presents a major impediment to high electric field operation in a variety of applications, e.g., in superconducting niobium radio-frequency cavities for particle accelerators, klystrons, and a wide range of high-voltage vacuum devices. Therefore, EFE has widely been the subject of fundamental research for years. Although micron or submicron particles are often observed at such EFE sites, the strength and number of emitting sites and the causes of EFE depend strongly on surface preparation and handling. Furthermore, the physical mechanism of EFE remains unknown. To systematically investigate the sources of this emission and to evaluate the best available surface preparation techniques with respect to the resulting field emission, a dc scanning field emission microscope (SFEM) was built at the Thomas Jefferson National Accelerat...

Direct current scanning field emission microscope integrated with existing scanning electron microscope

Electron field emission (FE) from broad-area metal surfaces is known to occur at much lower electric field than predicted by Fowler–Nordheim law. Although micron or submicron particles are often observed at such enhanced field emission (EFE) sites, the strength and number of emitting sites and the causes of EFE strongly depend on surface preparation and handling, and the physical mechanism of EFE remains unknown. To systematically investigate the sources of this emission, a dc scanning field emission microscope (SFEM) has been built as an extension to an existing commercial scanning electron microscope (SEM) equipped with an energy-dispersive spectrometer for emitter characterization. In the SFEM chamber of ultrahigh vacuum (∼10−9 Torr), a sample is moved laterally in a raster pattern (2.5 μm step resolution) under a high voltage anode microtip for field emission detection and localization. The sample is then transferred under vacuum by a hermetic retractable linear transporter to the SEM chamber for indi...

Operational optimization of large-scale SRF accelerators

Unlike other types of accelerator subsystems, because of the flexibility in setting the gradient in each cavity, an SRF linac has many operational degrees of freedom. The overall linac has an operational envelope (beam voltage and current) that depends on acceptable reliability, cryogenic capacity, and RF power budget. For economic and end-user physics reasons, one typically wants to run as close to the edge of the operational envelope as possible. With about 160 cavities in each of the CEBAF linacs, we have been forced to treat this problem in a very general way, and satisfy other non-fundamental needs as energy lock and rapid recovery from failures. We present a description of the relevant diverse constraints and the solution developed for CEBAF.

Superconducting Radio-Frequency Technology R&D for Future Accelerator Applications

Superconducting rf (SRF) technology is evolving rapidly, as are its applications. While there is active exploitation of what one may call the current state-of-the-practice, there is also rapid progress in expanding in several dimensions the accessible and useful parameter space. While state-of-the-art performance sometimes outpaces thorough understanding, the improving scientific understanding from active SRF research is clarifying routes to obtain optimum performance from present materials and opening avenues beyond the standard bulk niobium. The improving technical basis understanding is enabling process engineering to improve both performance confidence and reliability and also unit implementation costs. Increasing confidence in the technology enables the engineering of new creative application designs. We attempt to survey this landscape to highlight the potential for future accelerator applications.