| Paper | Title | Page |
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| TUP027 | Tests of Superconducting Materials in a High-Q RF Cavity | 305 |
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Superconducting rf is of increasing importance in particle accelerators. We have developed a resonant copper cavity with high quality factor and an interchangeable wall for testing superconducting materials.* A compact TE 01 mode launcher excites the azimuthally symmetric cavity mode, which allows a gap at the detachable wall and is free of surface electric fields that could cause field emission, multipactor, and rf breakdown. The shape of the cavity is tailored to focus magnetic field on the test wall, formed by a material sample. Working at X-band allows us to test small samples in a small available dewar, as well as taking advantage of available high power. We present results of cryogenic experiments conducted with this cavity. Low power tests allow characterization of the cavity parameters and their variation with temperature; high power tests allow determination of field limits for the superconducting samples. We describe our signal processing and analysis. Our experiments begin with reactor-grade niobium, followed by MgB2.
*C. Nantista et al., “Test Bed for Superconducting Materials,” presented at the 2005 Particle Accelerator Conference, Knoxville, Tennessee, May 16-20, 2005; SLAC-PUB-11246. |
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| TUP033 | Performances of High-Purity Niobium Cavities with Different Grain Sizes | 318 |
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| Grain boundaries have for some time been suspected of influencing the performance of rf cavities made from high purity niobium by limiting the temperature dependent BCS surface resistance to a residual resistance because of impurity segregation and by causing field limitations due to flux penetration. We have carried out a comparative study of the rf behavior of 2.2 GHz TM010 cavities of identical shape, fabricated from single crystal niobium, niobium of grain sizes of the order of several cm2 and standard poly-crystalline material. This contribution reports about the results of the measurements of the temperature dependence of the surface resistance Rs(T) and the Q0 vs Eacc behavior at 2 K. From the analysis of the Rs(T) data at low rf fields material parameters such as gap value, mean free path and residual resistance could be extracted. The dependence of the Q-value on rf field was analyzed with respect to the medium field Q-slope, “Q-drop” at high fields and the “quench” fields. The best performance resulted in a breakdown field of ~ 165 mT, corresponding to an accelerating gradient of Eacc ~ 45 MV/m. | ||
| TUP035 | Investigation of Hot Spots as a Function of Material Removal in a Large-grain Niobium Cavity | 324 |
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| The performance of a single-cell cavity made of RRR > 200 large-grain niobium has been investigated as a function of material removal by buffered chemical polishing (BCP). Temperature maps of the cavity surface at 1.7 and 2 K were taken for each step of chemical etching and revealed several “hot-spots”, which contribute to the degradation of the cavity quality factor as a function of the radio-frequency (RF) surface field. It was found that number of “hot-spots” decreased for larger material removal. Interestingly, the losses of the “hot-spots” at different locations evolved differently for successive material removal. The cavity achieved peak surface magnetic fields of about of 130 mT and was limited mostly by thermal quench. By measuring the temperature dependence of the surface resistance at low field between 4.2 K and 1.7 K, the variation of material parameters such as the ratio between the energy gap and the critical temperature, the residual resistance and the mean free path as a function of material removal could also be investigated. This contribution shows the results of the RF tests along with the temperature maps and the analysis of the losses caused by the “hot-spots”. | ||
| THP059 | Coaxial HOM Coupler Designs Tested on a Single-Cell Niobium Cavity | 716 |
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Coaxial higher order mode (HOM) couplers have been developed for HERA cavities and are used in TESLA, SNS and Jlab upgrade cavities. The principle of operation is the rejection of the fundamental mode by the tunable filter of the coupler and the transmission of the HOMs. It has been recognized recently that inappropriate thermal designs of the feed through for the pick-up probe of the HOM coupler will not sufficiently carry away the heat generated in the probe tip by the fundamental mode fields, causing a built-up of the heating of the niobium probe tip and subsequently, a deterioration of the cavity quality factor has been observed in cw operation. An improvement of the situation has been realized by a better thermal design of the feed through incorporating a sapphire rf window [1]. An alternative is a modification of the coupler loop (“F” – part) with an extension towards the pick-up probe. This design has been tested on a single cell niobium cavity in comparison to a “standard TESLA” configuration. by measuring the Eacc behavior at 2 K. The measurements clearly indicate that the modified version of the coupler loop is thermally much more stable than the standard version.
[1] C. Reece et al; http://accelconf.web.cern.ch/accelconf/, paper TPPT082 |
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| THP076 | Prototyping of a Superconducting Elliptical Cavity for a Proton Linac | 758 |
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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. |