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| MO6PFP062 | RF Integration into Helical Magnet for Muon 6-Dimensional Beam Cooling | 274 |
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Funding: Supported in part by USDOE STTR Grant DE-FG02-07ER84825 and by FRA under DOE Contract DE-AC02-07CH11359 The helical cooling channel is proposed to make a quick muon beam phase space cooling in a short channel length. The challenging part of the helical cooling channel magnet design is how to integrate the RF cavity into the compact helical cooling magnet. This report shows the possibility of the integration of the system. |
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| MO6RFP079 | Improved DC Gun Insulator | 557 |
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Funding: Supported in part by USDOE Contract No. DE-AC05-84-ER-40150. Many user facilities such as synchrotron light sources and free electron lasers require accelerating structures that support electric fields of 10-100 MV/m, especially at the start of the accelerator chain where ceramic insulators are used for very high gradient DC guns. These insulators are difficult to manufacture, require long commissioning times, and have poor reliability, in part because energetic electrons bury themselves in the ceramic, creating a buildup of charge and causing eventual puncture. A novel ceramic manufacturing process is proposed. It will incorporate bulk resistivity in the region where it is needed to bleed off accumulated charge caused by highly energetic electrons. This process will be optimized to provide an appropriate gradient in bulk resistivity from the vacuum side to the air side of the HV standoff ceramic cylinder. A computer model will be used to determine the optimum cylinder dimensions and required resistivity gradient for an example RF gun application. A ceramic material example with resistivity gradient appropriate for use as a DC gun insulator will be fabricated by glazing using doping compounds and tested. |
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| TU5PFP017 | RF Cavities Loaded with Dielectric for Muon Facilities | 846 |
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Funding: Supported in part by FRA DOE contract number DE-AC02-07CH11359 RF cavities below 800 MHz are large, so alternative cavities at low frequencies are needed. Novel dielectric loaded RF cavities will allow smaller diameter cavities to be designed; changing the frequency of a cavity design would be as simple as changing the dielectric cylinder insert material or inner radius of the dielectric in the cavity. This paper discusses RF cavities loaded with dielectric material that could be used in various ways for muon facilities. The examples given are for 400 and 800 MHz cavities. Our initial motivation was to use dielectric to reduce the radial size of gas-filled cavities in helical cooling channels, but dielectric-loading has potential use in vacuum cavities for suppression of dark current emission. We also studied cavities that can be used for the phase rotation channel in the front end of a muon collider or neutrino factory. |
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| TU5PFP018 | Tunable RF Cavities Using Orthogonally Biased Ferrite | 849 |
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Funding: Supported by STTR Grant DE-FG02-07ER86320 and FRA DOE contract number DE-AC02-07CH11359 Originally conceived as a solution for FFAG applications, a new compact RF cavity design that tunes rapidly over various frequency ranges can be used to upgrade existing machines. The design being developed uses orthogonally biased garnet cores for fast frequency tuning and liquid dielectric to adjust the frequency range and to control the core temperature. We describe measurements of candidate ferrite and dielectric materials. The first use of the new cavity concept will be for improvements to the 8 GeV Fermilab Booster synchrotron. |
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| TU5PFP019 | Phase and Frequency Locked Magnetrons for SRF Sources | 852 |
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Funding: Supported in part by USDOE Contract. DE-AC05-84-ER-40150 and by FRA DOE contract number DE-AC02-07CH11359 Magnetrons are low-cost highly-efficient microwave sources, but they have several limitations, primarily centered about the phase and frequency stability of their output. When the stability requirements are low, such as for medical accelerators or kitchen ovens, magnetrons are the very efficient power source of choice. But for high energy accelerators, because of the need for frequency and phase stability–-proton accelerators need 1-2 degrees source phase stability, and electron accelerators need .1-.2 degrees of phase stability–-they have rarely been used. We describe a novel variable frequency cavity technique which will be utilized to phase and frequency lock magnetrons. |
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| TU5PFP020 | Doped H2-Filled RF Cavities for Muon Beam Cooling | 855 |
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Funding: Supported in part by USDOE STTR Grant DE-FG02-08ER86350 and and FRA DOE contract number DE-AC02-07CH11359 RF cavities pressurized with hydrogen gas may provide effective muon beam ionization cooling needed for muon colliders. Recent 805 MHz test cell studies reported below include the first use of SF6 dopant to reduce the effects of the electrons that will be produced by the ionization cooling process in hydrogen or helium. Measurements of maximum gradient in the Paschen region are compared to a simulation model for a 0.01% SF6 doping of hydrogen. The observed good agreement of the model with the measurements is a prerequisite to the investigation of other dopants. |
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| TU5PFP021 | Traveling Wave RF Systems for Helical Cooling Channels | 858 |
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Funding: supported in part by USDOE STTR Grant DE-FG02-08ER86350 The great advantage of the helical ionization cooling channel (HCC) is its compact structure that enables the fast cooling of muon beam 6-dimensional phase space. This compact aspect requires a high average RF gradient, with few places that do not have cavities. Also, the muon beam is diffuse and requires an RF system with large transverse and longitudinal acceptance. A traveling wave system can address these requirements. First, the number of RF power coupling ports can be significantly reduced compared with our previous pillbox concept. Secondly, by adding a nose on the cell iris, the presence of thin metal foils traversed by the muons can possibly be avoided. We show simulations of the cooling performance of a traveling wave RF system in a HCC, including cavity geometries with inter-cell RF power couplers needed for power propagation. |
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| WE5PFP008 | RF Breakdown of Metallic Surfaces in Hydrogen | 2000 |
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Funding: Supported in part by USDOE STTR Grant DE-FG02-08ER86350 Supported in part by USDOE STTR Grant DE-FG02-08ER86352 and in part by FRA DOE contract number DE-AC02-07CH11359 In earlier reports, microscopic images of the surfaces of metallic electrodes used in high-pressure gas-filled 805 MHz RF cavity experiments were used to investigate the mechanism of RF breakdown of tungsten, molybdenum, and beryllium electrode surfaces. Plots of remnants were consistent with the breakdown events being due to field emission, due to the quantum mechanical tunnelling of electrons through a barrier as described by Fowler and Nordheim. In the work described here, these studies have been extended to include tin, aluminium, and copper. Contamination of the surfaces, discovered after the experiments concluded, have cast some doubt on the proper qualities to assign to the metallic surfaces. However, two significant results are noted. First, the maximum stable RF gradient of contaminated copper electrodes is higher than for a clean surface. Second, the addition of as little as 0.01% of SF6 to the hydrogen gas increased the maximum stable gradient, which implies that models of RF breakdown in hydrogen gas will be important to the study of metallic breakdown |
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| WE5PFP009 | RF Breakdown Studies Using a 1.3-GHz Test Cell | 2003 |
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Funding: Supported in part by USDOE STTR Grant DE-FG02-08ER86352 and FRA DOE contract number DE-AC02-07CH11359 Many present and future particle accelerators are limited by the maximum electric gradient and peak surface fields that can be realized in RF cavities. Despite considerable effort, a comprehensive theory of RF breakdown has not been achieved and mitigation techniques to improve practical maximum accelerating gradients have had only limited success. Recent studies have shown that high gradients can be achieved quickly in 805 MHz RF cavities pressurized with dense hydrogen gas without the need for long conditioning times, because the dense gas can dramatically reduce dark currents and multipacting. In this project we use this high pressure technique to suppress effects of residual vacuum and geometry found in evacuated cavities to isolate and study the role of the metallic surfaces in RF cavity breakdown as a function of magnetic field, frequency, and surface preparation. A 1.3-GHz RF test cell with replaceable electrodes (e.g. Mo, Cu, Be, W, and Nb) and pressure barrier capable of operating both at high pressure and in vacuum been designed and built, and preliminary testing has been completed. A series of detailed experiments is planned at the Argonne Wakefield Accelerator. |
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| WE5PFP042 | Rugged Ceramic Window for RF Applications | 2089 |
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Funding: Supported in part by USDOE SBIR Grant DE-FG02-08ER85171 High-current RF cavities that are needed for many accelerator applications are often limited by the power transmission capability of the pressure barriers (windows) that separate the cavity from the power source. Most efforts to improve RF window design have focused on alumina ceramic, the most popular historical choice, and have not taken advantage of new materials. Alternative window materials have been investigated using a novel Merit Factor comparison and likely candidates have been tested for the material properties which will enable construction in the self-matched window configuration. Window assemblies have also been modeled and fabricated using compressed window techniques which have proven to increase the power handling capability of waveguide windows. Candidate materials have been chosen to be used in fabricating a window for high power testing at Thomas Jefferson National Accelerator Facility. |
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| WE5PFP043 | Beam Pipe HOM Absorber for 750 MHz RF Cavities | 2092 |
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Funding: Supported in part by USDOE Contract. DE-AC05-84-ER-40150 Superconducting HOM-damped (higher-order-mode-damped) RF systems are needed for present and future storage ring and linac applications. Superconducting RF (SRF) systems typically contain unwanted frequencies or higher order modes (HOM) that must be absorbed by ferrite and other lossy ceramic-like materials that are brazed to substrates mechanically attached to the drift tubes adjacent to the SRF cavity. These HOM loads must be thermally and mechanically robust and must have the required broadband microwave loss characteristics, but the ferrites and their attachments are weak under tensile stresses and thermal stresses and tend to crack. A HOM absorber with improved materials and design will be developed for high-gradient 750 MHz superconducting cavity systems. RF system designs will be numerically modeled to determine the optimum ferrite load required to meet the broadband loss specifications. Several techniques for attaching ferrites to the metal substrates will be studied, including full compression rings and nearly-stress-free ferrite assemblies. Prototype structures will be fabricated and tested for mechanical strength. |
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| WE5PFP044 | High Power Co-Axial SRF Coupler | 2095 |
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Funding: Supported in part by DOE SBIR grant DE-FG02-08ER85171 Supported in part by USDOE Contract DE-AC05-84-ER-40150 There are over 35 coupler designs for SRF cavities ranging in frequency from 325 to 1500 MHz. Two-thirds of these designs are coaxial couplers using disk or cylindrical ceramics in various combinations and configurations. While it is well known that dielectric losses go down by several orders of magnitude at cryogenic temperatures, it not well known that the thermal conductivity also goes down, and it is the ratio of thermal conductivity to loss tangent (SRF ceramic Quality Factor) and ceramic volume which will determine the heat load of any given design. We describe a novel robust co-axial SRF coupler design which uses compressed window technology. This technology will allow the use of highly thermally conductive materials for cryogenic windows. The mechanical designs will fit into standard-sized ConFlat® flanges for ease of assembly. Two windows will be used in a coaxial line. The distance between the windows is adjusted to cancel their reflections so that the same window can be used in many different applications at various frequencies. |