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[en] The CEBAF cryogenic system consists of 3 refrigeration systems: Cryogenic Test Facility (CTF), Central Helium Liquefier (CHL), and End Station Refrigerator (ESR). CHL is the main cryogenic system for CEBAF, consisting of a 4.8 kW, 2.0 K refrigerator and transfer line system to supply 2.0 K and 12 kW of 50 K shield refrigeration for the Linac cavity cryostats and 10 g/s of liquid for the end stations. This paper describes the 9-year effort to commission these systems, concentrating on CHL with the cold compressors. The cold compressors are a cold vacuum pump with an inlet temperature of 3 K which use magnetic bearings, thereby eliminating the possibility of air leaks into the subatmospheric He
[en] A new type of rf structure for the deflection and crabbing of particle beams is presented. The structure is comprised of a number of parallel TEM-resonant lines operating in opposing phase from each other. One of its advantages is its compactness compared to conventional crabbing cavities operating in the TM110 mode, thus allowing low frequency designs. The properties and characteristics of this type of structure are presented.
[en] Hall A at Jefferson Lab has recently completed three experiments done using the technique of parity-violating electron scattering. Taken together these experiments are a good demonstration of the versatility of this approach. Looking forward, there are two very large scale parity-violation experiments approved to run in Hall A in the 12 GeV era. These experiments represent a significant increase in precision and technical requirements.
[en] Operating on a frequency band occupying several nonoverlapping channels, IEEE 802.11 is now widely used in wireless mesh networks (WMNs). Many multichannel MAC protocols are proposed to improve the spatial reuse in the network under the assumption that the transmissions on nonoverlapping channels do not interfere with each other. Some joint routing and channel assignment algorithms are also designed to increase the network throughput based on the premise that we can switch between different channels freely. Although simulations show that great improvements on network throughput can be observed in both cases, two fundamental questions remain: (1) Can we really use multiple nonoverlapping channels freely in WMNs? (2) If we can, what will be the cost when we switch channels dynamically and frequently? In this paper, by conducting extensive experiments on our testbed, we attempt to answer these questions. We find that in spite of interference between both overlapping and nonoverlapping channels, we can still use multiple channels in mesh networks under certain conditions but with care. We also show that the channel switching cost is actually very significant in WMNs. We recommend not to switch the channels too frequently when designing the channel assignment algorithms, and those channel assignment algorithms selecting one channel for each packet are not really beneficial.
[en] The CEBAF superconducting linear accelerator incorporates cryogenic refrigeration equipment at three locations within the site: the Central Helium Liquefier, located in the center of the accelerator; the experimental end station refrigerator; and the test laboratory refrigerator located in the Cryogenic Test Facility (CTF) adjacent to the test laboratory. The CEBAF cryogenic system will provide 2K refrigeration to the linacs of the accelerator and test laboratory and 4.5K refrigeration for the end station experimental halls. The Central Helium Liquefier and the test laboratory systems will produce 45K supercritical gaseous helium for shield refrigeration. Liquid nitrogen shields will also be incorporated in the test laboratory and end stations. 6 refs., 5 figs
[en] The Thomas Jefferson National Accelerator Facility (Jefferson Lab) is producing ten 100+MV SRF cryomodules (C100) as part of the CEBAF 12 GeV Upgrade Project. Once installed, these cryomodules will become part of an integrated accelerator system upgrade that will result in doubling the energy of the CEBAF machine from 6 to 12 GeV. This paper will present a complete overview of the C100 cryomodule production process. The C100 cryomodule was designed to have the major components procured from private industry and assembled together at Jefferson Lab. In addition to measuring the integrated component performance, the performance of the individual components is verified prior to being released for production and assembly into a cryomodule. Following a comprehensive cold acceptance test of all subsystems, the completed C100 cryomodules are installed and commissioned in the CEBAF machine in preparation of accelerator operations. This overview of the cryomodule production process will include all principal performance measurements, acceptance criterion and up to date status of current activities.
[en] Jefferson Labzs CEBAF Accelerator Facility uses most of the currently available vacuum pumping systems for its successful operations.These vacuum systems include turbo, ion, NEG and cryo pumps.The theoretical lowest pressure obtainable with these pumps is different and also the effective pumping speeds of all these pumps at lowest pressures are likely to vary depending on the prevailing conditions. Consequently, the ultimate pressure of a given vacuum system pumped by each of the above mentioned pumps could be expected to vary under the same operating conditions. In this paper the results of the analysis of above pumps with respect to their lowest achievable ultimate pressures are presented.z This work was supported by U.S. Department of Energy Contract No. DE-AC05-84ER40150