CLASSE, Ithaca, New York
Cornell University has developed and fabricated a novel SRF injector cryomodule for the acceleration of the high current (100 mA), low emittance beam in the Cornell ERL injector prototype. The injector cryomodule is based on superconducting rf technology with five 2-cell rf cavities operated in the cw mode. To support the acceleration of a low energy, ultra low emittance, high current beam, the beam tubes on one side of the cavities have been enlarged to propagate Higher-Order-Mode power from the cavities to broadband RF absorbers located at 80 K between the cavities. Each cavity is surrounded by a LHe vessel and equipped with a frequency tuner including fast piezo-driven fine tuners for fast frequency control. The cryomodule provides the support and precise alignment for the cavity string, the 80 K cooling of the HOM loads, and the 2 K LHe cryogenic system for the high cw heat load of the cavities. In this presentation results of the commissioning phase of this cryomodule will be reported.
CLASSE, Ithaca, New York
Cornell University’s superconducting cavity development program is one contributor to the global collaborative effort on critical SRF R&D for the ILC. We conduct R&D in support of the baseline cavity development as well as several alternate cavity development paths. For the baseline program we are preparing and testing ILC cavities. We have developed a new quench detection system and successfully applied it to ILC 9-cell and 1-cell cavities to find quench producing defects, which were characterized with subsequent optical examination. We have successfully repaired a 9-cell cavity using tumbling to raise the accelerating gradient from 15 to above 30 MV/m. We have identified quench producing defects in single-cell cavities using our large-scale thermometry system and subsequently extracted and inspected the defect region with an SEM. For the alternate R&D, we are developing reentrant cavity shapes with 70 mm and 60 mm apertures, and a simpler, potentially faster and less expensive electropolishing method called vertical electropolishing. We are also assisting in developing new cavity vendors by rapidly testing single-cell cavities they produced to qualify their fabrication methods.
CLASSE, Ithaca, New York
To further the understanding of the r.f. performance of niobium and alternative superconducting materials such as MgB2, high field tests of material samples inserted into host microwave cavities are potentially highly beneficial. In this paper we present results from a detailed design study of such superconducting sample host cavities. The focus of the design work has been on maximizing the ratio of sample surface magnetic field to host cavity maximum surface field, on multiple mode operation to study frequency dependent effects, on mechanical stability of the host cavity under atmospheric pressure, and on choke joints between the sample plates and the host cavity.
CLASSE, Ithaca, New York
A ring-type defect could be a better model for quench caused by the sharp boundary segment of a pure niobium pit. The relationship between quench field and inner radius of a ring-type defect is presented based on calculations of an improved ring-type defect model. Comparison between ring-type defects and disk-type defects model is also presented.
CLASSE, Ithaca, New York
The RF superheating magnetic field of superconducting niobium was measured with a 1.3 GHz re-entrant cavity at several points in the temperature range from 1.9K to 4.2K. This experimental data is used to discriminate between two competing theories for the temperature dependent behavior of the RF superheating field. Measurements were made with <250 μs high power pulses (HPP, ∼1MW). Our test incorporated oscillating superleak transducers to determine the cavity quench locations and characterize changes and the migrations of the quench locations during processing. Using a vertically electropolished cavity, the temperature dependence of the superheating field was found to agree with Ginzburg-Landau predictions to within 10% down to a temperature of 4.2K; whereas prior to this experiment, theory and experiment only agreed at temperatures greater than 6.2K. We also used finite element methods to simulate the internal heating of the cavity, allowing for a more accurate measurement of superheating field as a function of temperature.
CLASSE, Ithaca, New York
Cornell University is currently commissioning a novel, 100 mA electron injector for an energy-recovery linac based X-ray light source. Substantial wakefields will be generated by the short bunch, high current beam when it passes through the injector cryomodule, hosting five superconducting microwave cavities together with six broadband beam pipe microwave absorbers. In this paper we present wakefield and loss factor calculations for all components inside this cryomodule, including RF cavities, microwave absorbers, flanges, gate valves, and beam pipe radius transitions. The dependence on bunch length is discussed as well as a comparison of results from different numerical wakefield codes.
CLASSE, Ithaca, New York
This paper discusses the optimization methods used to design the seven-cell cavities in Cornell's high beam current (100 mA) Energy Recovery Linac. The center cells of the cavity were optimized for high R/Q*G of the fundamental mode to minimize the dynamic cryogenic load, for low electric peak surface fields to minimize the risk of electron field emission, and for increased "cell-to-cell coupling" of the higher order modes to reduce sensitivity to small cell shape errors. The end cells and beam tube sections of the cavity were subsequently optimized to maximize higher order mode damping and thereby increase the beam break up current above the envisioned operating current of 100 mA. The design was then subjected to cell shape perturbations simulating manufacturing variation, and the higher order mode parameter of these imperfect cavities were finally used in beam tracking simulations to determine a realistic estimate for the beam break up current of the ERL main linac.
CLASSE, Ithaca, New York
Broadband RF absorbing materials are frequently utilized in particle accelerator environments. There are stringent requirements placed on some of these absorbers in regard to vacuum compatibility, radiation compatibility, and particulate generation, especially for absorbers in close proximity to the beamline such as in some higher order mode (HOM) load designs. For RF absorbers located directly on the beamline, their DC conductivity must also be large enough to drain away static charge that may be deposited onto them, since such static charge could deflect the particle beam. Ceramics and ferrite materials have often been used for such RF absorbers, and SRF applications tend to extended their use to cryogenic temperatures. Unfortunately, the DC conductivity of these materials often drops precipitously with temperature and they become excellent insulators at cryogenic temperatures. The results of recent DC conductivity tests of several of these RF absorbing materials will be presented.
UCD, Davis, California
CLASSE, Ithaca, New York
There are limited choices available for broadband RF absorbing materials compatible with particle accelerator beamline environments, such as for use in higher order mode (HOM) loads. Ceramics and ferrites that have the vacuum, radiation, and particulate compatibility tend to have strong RF absorption over only a decade of frequency once above 1 GHz. Further, their properties often change considerably at cryogenic temperatures for SRF applications. Mixing carbon nanotubes (CNT’s) into a resin matrix has recently shown promise for providing a broader band of RF absorption. Development work has begun for mixing CNT’s into a ceramic matrix for compatibility with cryogenic accelerator beamlines. The status of the material development and tests will be presented.
STFC/DL/ASTeC, Daresbury, Warrington, Cheshire
CLASSE, Ithaca, New York
FZD, Dresden
STFC/DL, Daresbury, Warrington, Cheshire
LBNL, Berkeley, California
Stanford University, Stanford, Califormia
TRIUMF, Vancouver
Cornell University, Ithaca, New York
DESY, Hamburg
The collaborative development of an optimised cavity/cryomodule solution for application on ERL facilities has now progressed to final assembly and testing of the cavity string components and their subsequent cryomodule integration. This paper outlines the testing and verification processes for the various cryomodule sub-components and details the methodology utilised for final cavity string integration. The paper also highlights the modifications required to integrate this new cryomodule into the existing ALICE cryo-plant facility at Daresbury Laboratory.