| Paper | Title | Page |
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| WE5PFP054 | HOM Survey of the First CEBAF Upgrade Style Cavity Pair | 2123 |
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Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. For the planned CEBAF upgrade ten new cryomodules are required to increase the beam energy to the envisaged 12 GeV. Extensive cavity and cryomodule R&D has been done previously, including the installation of a new cryomodule dubbed “Renascence” in CEBAFs north linac in 2007. It houses both seven-cell low loss and high gradient type of cavities thereby serving as a testbed to address and cope with crucial technological challenges. Based on this experience a final iteration on the upgrade cavity has been performed to improve various aspects of HOM-damping and thermal stability. Two such cavities have been produced and qualified. A thorough cavity HOM-survey has been performed to verify the integrity of the cavities and to guarantee the impedance requirements of each crucial HOM. This paper details the results of HOM-surveys performed for the first two upgrade style low loss cavities tested both individually in a vertical Dewar and horizontally in a dedicated cavity pair cryomodule. The safety margin to the worst beam break-up scenario at 12 GeV has been concluded. |
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| WE5PFP056 | A Family of L-Band SRF Cavities for High Power Proton Driver Applications | 2129 |
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Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. Recent global interest in high duty factor or CW superconducting linacs with high average beam power highlights the need for robust and reliable SRF structures capable of delivering high average RF power to the beam with moderate HOM damping, low interception of halo and good efficiency. Potential applications include proton or H- drivers for spallation neutron sources, neutrino physics, waste transmutation, subcritical reactors, and high-intensity high-energy physics experiments. We describe a family of SRF cavities with a range of Betas capable of transporting beam currents in excess of 10 mA CW with large irises for minimal interception of halo and HOM and power couplers capable of supporting high average power operation. Goals include an efficient cell shape, high packing factor for efficient real-estate gradient and strong HOM damping to ensure stable beam operation. Designs are being developed for low-frequency (e.g. 650-975 MHz), but can easily be scaled to high-frequency (e.g. 1.3-1.5 GHz), depending on the application. We present the results of conceptual design studies, simulations and prototype measurements. |
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| WE5PFP061 | Commissioning of the SRF Surface Impedance Characterization System at Jefferson Lab | 2144 |
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Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. Much remains to be learned regarding the details of SRF performance effects with material variation, including niobium treated in different ways, and different bulk/thin film materials that are fabricated under different conditions. A facility that can measure small samples’ RF properties in a range of 0~180mT magnetic field and 2~20k temperature is necessary in order to answer this question. The Jefferson Lab surface impedance characterization (SIC) system has been designed to attempt to meet this requirement. The SIC system uses a sapphire-loaded cylindrical Nb cavity at 7.5GHz with 50mm diameter flat sample placed on a non-contacting end plate and a calorimetric technique to directly measure the rf dissipation in the sample in response to known rf fields over ~1 cm2. We report on the commissioning of this system and its first uses for characterizing materials. Preliminary tests with Nb thin film sample sputtered on Cu substrate, and bulk Nb sample have been done at low field. The presently available hardware is expected to enable tests up to 20 mT peak magnetic field on the sample CW. Paths to higher field tests have been identified. |
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| WE6RFP009 | Investigations on Absorber Materials at Cryogenic Temperatures | 2799 |
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Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. In the framework of the ongoing CEBAF 12 GeV upgrade program improvements are being made to refurbish cryomodules housing JLab’s original 5-cell cavities. Recently we have started to look into a possible simplification of the HOM-absorber design combined with the need to find alternative material candidates. The absorbers are implemented in two HOM-waveguides immersed in the helium bath and need to operate at 2K. We therefore have built a cryogenic setup to perform measurements on sample load materials to investigate their lossy characteristics and variations from room temperature down to 2K. Initial results are presented in this paper. |
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| TH5PFP093 | Benchmark of Different Electromagnetic Codes for the High Frequency Calculation | 3426 |
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Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. In this paper, we present benchmarking results for high-class 3D electromagnetic (EM) codes in designing RF cavities today. These codes include Omega3P [1], VORPAL [2], CST Microwave Studio [3], Ansoft HFSS [4], and ANSYS [5]. Two spherical cavities are selected as the benchmark models. We have compared not only the accuracy of resonant frequencies, but also that of surface EM fields, which are critical for superconducting RF cavities. By removing degenerated modes, we calculate all the resonant modes up to 10 GHz with similar mesh densities, so that the geometry approximation and field interpolation error related to the wavelength can be observed. |
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| FR5PFP094 | Enhanced Method for Cavity Impedance Calculations | 4523 |
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Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. With the proposal of medium to high average current accelerator facilities the demand for cavities with extremely low HOM impedances is increasing. Modern numerical tools are still under development to more thoroughly predict impedances that need to take into account complex absorbing boundaries and lossy materials. With the usually large problem size it is preferable to utilize massive parallel computing when applicable and available. Apart from such computational issues, we have developed methods using available computer resources to enhance the information that can be extracted from a cavities’ wake potential computed in time domain. In particular this is helpful for a careful assessment of the extracted RF power and the mitigation of potential beam breakup or emittance diluting effects, a figure of merit for the cavity performance. The methods are described as well as examples of their implementation. |