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| THP047 |
Prototyping of a Single-Cell Half-Reentrant Superconducting Cavity
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685 |
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- M. S. Meidlinger, J. Bierwagen, S. Bricker, C. Compton, T. L. Grimm, W. Hartung, M. J. Johnson, J. Popielarski, L. Saxton, R. C. York
NSCL, East Lansing, Michigan
- E. Zaplatin
FZJ, Jülich
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As superconducting niobium cavities achieve higher gradients, it is anticipated they will reach a performance limit as the peak surface magnetic field approaches the critical magnetic field. "Low loss" and "reentrant" cavity designs are being studied at CEBAF, Cornell, DESY, and KEK, with the goal of reaching higher gradients via lower surface magnetic field, at the expense of higher surface electric field. At present, cavities must undergo chemical etching and high-pressure water rinsing to achieve good performance. It is not clear whether this can be done effectively and reliably for multi-cell low loss or reentrant cavities using traditional techniques. A "half-reentrant" cavity shape has been developed with RF parameters similar to the low loss and reentrant cavities, but with the advantage that the surface preparation can be done easily with existing methods. Two prototype single-cell half-reentrant cavities are being fabricated at 1.3 GHz; the non-reentrant wall angle is 8 degrees, the beam tube radius is 29 mm, and the cell-to-cell coupling is 1.47%. The half-reentrant cavity design and the results and status of the prototyping effort will be presented.
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| THP066 |
Lorentz-Force Detuning Analysis for Low-Loss, Re-entrant and Half-Reentrant Superconducting RF Cavities
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734 |
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- E. Zaplatin
FZJ, Jülich
- T. L. Grimm, W. Hartung, M. J. Johnson, M. S. Meidlinger, J. Popielarski, R. C. York
NSCL, East Lansing, Michigan
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The RF design of a superconducting elliptical cavity requires a trade-off in the optimization of the cell shape between the region of high electric field and the region of high magnetic field. In practice, the cavity performance may be limited not by the RF characteristics, but by detuning due to the Lorentz force, bath pressure fluctuations, or microphonics; Lorentz force detuning is of concern primarily for pulsed accelerators such as the proposed International Linear Collider. Hence the structural properties must also be taken into account in the cavity design. Several new cavity shapes are being developed in which the surface magnetic field is decreased relative to the TeSLA cavity shape, with the goal of reaching a higher accelerating gradient. This study will compare the Lorentz force detuning characteristics of the TeSLA, "low-loss", "reentrant", and "half-reentrant" cavity middle cells, and explore possible methods for stiffening the structures.
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| THP073 |
High-Current Elliptical Cavity Design and Prototyping
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752 |
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- D. Meidlinger, J. Bierwagen, S. Bricker, C. Compton, T. L. Grimm, W. Hartung, M. J. Johnson, J. Popielarski, L. Saxton
NSCL, East Lansing, Michigan
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Beam instabilities due to undamped higher-order modes (HOMs) in the cavities can limit the performance of high-current superconducting accelerators, such as energy recovery linacs. If the accelerator is designed such that the bunch frequency is equal to the accelerating mode frequency and the beam pipe radius is chosen such that the cutoff frequency is less than twice that of the accelerating mode, all of the monopole and dipole HOMs that can be driven by the beam can be well-damped. A 6-cell elliptical cavity for speed-of-light particles and a 2-cell elliptical injection cavity have been designed for high-current accelerator applications. Both cavities have an aperture 29% larger than the TeSLA cavity, at the expense of peak surface fields about 10% higher for the same gradient. The injection cavity has a geometric β of 0.81 and was designed to accelerate electrons from 50 keV to 1 MeV, and the 6-cell cavity has a geometric β of 1 for further acceleration. Both cavities are designed for the purpose of accelerating hundreds of milliamps without HOM-induced beam breakup and to operate at 2.45 GHz. The cavity designs and prototype injection cavity results will be presented.
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| THP075 |
RF Performance of a Superconducting S-Band Cavity Filled with Liquid Helium
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755 |
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- W. Hartung, J. Bierwagen, S. Bricker, C. Compton, T. L. Grimm, M. J. Johnson, D. Meidlinger, D. Pendell, J. Popielarski, L. Saxton, R. C. York
NSCL, East Lansing, Michigan
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Copper RF cavities filled with hydrogen gas at high pressure have been studied recently by Muons, Inc. and IIT for simultaneous acceleration and ionisation cooling of a muon beam. A further step in this direction would be a superconducting RF cavity filled with liquid helium. One might imagine that this would make the cavity less vulnerable to thermal breakdown, field emission, and multipacting. A disadvantage is that magnetostatic focussing of the beam could not be done simultaneously. Preliminary RF testing has been done on a 2.45 GHz single-cell elliptical cavity filled with liquid helium. Low-field results indicate little or no increase in the power dissipation, consistent with predictions and measurements in the literature. The frequency shift with pressure for a cavity filled with saturated liquid is about 100 times greater than for a cavity under vacuum, consistent with published values of liquid helium permittivity as a function of temperature. Investigation of the high-field performance of a liquid-filled cavity is in progress.
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| THP076 |
Prototyping of a Superconducting Elliptical Cavity for a Proton Linac
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758 |
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- W. Hartung, J. Bierwagen, S. Bricker, C. Compton, T. L. Grimm, M. J. Johnson, D. Meidlinger, J. Popielarski, L. Saxton, R. C. York
NSCL, East Lansing, Michigan
- G. W. Foster, I. G. Gonin, T. K. Khabiboulline, N. Solyak, R. Wagner, V. Yarba
Fermilab, Batavia, Illinois
- P. Kneisel
Jefferson Lab, Newport News, Virginia
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A superconducting cavity has been designed for acceleration of particles travelling at 81% the speed of light (β = 0.81). Possible applications include the proposed Fermilab Proton Driver Linac. The cell shape is similar to the β = 0.81 cavity for the Spallation Neutron Source Linac, but the resonant frequency is 1.3 GHz rather than 805 MHz and the beam tube diameter matches that of the 1.3 GHz cavity for the TeSLA Test Facility. Six single-cell prototypes are being fabricated and tested. Three of these cavities are being formed from standard high purity fine grain niobium sheet. The rest are being fabricated from large grain niobium, following up on the work at Jefferson Lab to investigate the potential of large grain material for cost savings and/or improved RF performance. The fabrication of two 7-cell cavity prototypes (one fine grain, one large grain) is planned. A status report on this prototyping effort will be presented.
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