| Paper | Title | Page |
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| TPPT071 | Preliminary Results on the Simultaneous Excitation of the TM010 and TE011 Modes in a Single Cell Niobium Cavity | 3844 |
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Funding: Work supported by the U.S. DOE Contract No DE-AC05-84ER40150. Simultaneous excitation of both TM010 and TE011 modes has been proposed recently for superconducting photoinjector applications to take advantage of the accelerating field of the TM mode, combined with the focusing magnetic field of the TE mode. Simultaneous excitation of both modes has been carried out on a CEBAF single cell cavity. The cavity has two beam pipe side-ports for each mode for input and pick-up couplers. Coupling to the TE011 mode is done by magnetic loop couplers while for the TM010 mode coaxial antennas are used. The TE011 mode has the property of having zero surface electric field, surface magnetic field orthogonal to the one in the TM010 mode and concentrated in the iris/wall regions of the cavity. The presence of both modes in the cavity at the same time can also be used to investigate the so-called high field Q-drop in the TM010 mode. This paper will present some preliminary result on the test of the single cell cavity at 2K. |
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| TPPT072 | Effects of Electric and Magnetic Fields on the Performance of a Superconducting Cavity | 3874 |
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Funding: Work supported by the U.S. DOE Contract No DE-AC05-84ER40150. A special two-cell cavity was designed to obtain surface field distributions suitable for investigation of electric and magnetic field effects on cavity performance. The cavity design and preliminary results were presented in a previous contribution. The bulk niobium cavity was heat-treated in a vacuum furnace at 1250C to improve the thermal conductivity. Three seamless hydroformed NbCu cavities of the same design were fabricated to investigate the role of the electron beam welds located in high field areas. |
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| TPPT075 | Influence of Ta Content in High Purity Niobium on Cavity Performance | 3955 |
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Funding: Work supported by the U.S. DOE Contract No DE-AC05-84ER40150. In a previous paper* we have reported about initial tests of single cell 1500 MHz cavities made from high purity niobium with three different Ta contents of 160 ppm, ~600 ppm and ~1400 ppm. These cavities had been treated by buffered chemical polishing several times and 100 mm, 200 mm and 300 mm of material had been removed from the surfaces. This contribution reports about subsequent tests following post purification heat treatments with Ti and “in situ” baking. As a result, all cavities exhibited increased quench fields due to the improved thermal conductivity after the heat treatment. After the "in situ" baking at 120C for ~40 hrs the always present Q-drop at high fields disappeared and further improvements in accelerating gradient could be realized. Gradients as high as Eacc = 35 MV/m were achieved and there were no clear indications that the cavity performance was influenced by the Ta content in the material. A multi-cell cavity from the high Ta content material is being fabricated and results will be presented at this conference. *P. Kneisel et al., Linac 2004. |
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| TPPT076 | Preliminary Results from Single Crystal and Very Large Crystal Niobium Cavities | 3991 |
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Funding: Work supported by the U.S. DOE Contract No DE-AC05-84ER40150. We have fabricated and tested several single cell cavities using material from very large grain niobium ingots. In one case the central grain exceeded 7" in diameter and this was used for a 2 GHz cavity. This activity had a dual purpose: to investigate the influence of grain boundaries on the often observed Q-drop at gradients Eacc > 20 MV/m in the absence of field emission, and to study the possibility of using ingot material for cavity fabrication without going through the expensive process of sheet fabrication. The sheets for these cavities were cut from the ingot by wire electro-discharge machining (EDM) and subsequently formed into half–cells by deep drawing. The following fabrication steps were standard: machining of weld recesses, electron beam welding of beam pipes onto the half cells and final equator weld to join both half cell/beam pipe subunits.The cavities showed heavy Q–disease caused by the EDM; after hydrogen degassing at 800C for 3 hrs in UHV the cavities showed promising results, however, a Q-drop above Eacc ~ 20 MV/m was still present. Testing of the cavities is still ongoing – so far accelerating gradients of 30 MV/m have been achieved. |
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| TPPT077 | Testing of HOM Coupler Designs on a Single Cell Niobium Cavity | 4012 |
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Funding: Work supported by the U.S. DOE Contract No DE-AC05-84ER40150. Coaxial higher order mode (HOM) couplers were developed initially for PETRA cavitiesand subsequently for TESLA cavities. They were adopted later for SNS and Jlab upgrade cavities. The principle of operation is the rejection of the fundamental mode by the tunable filter configuration of the coupler and the transmission of the HOMs. It has been recognized recently that, in high average power applications, the pick-up probe of the HOM coupler must be superconducting in order to avoid substantial heat dissipation by the fundamental mode fields and deterioration of the cavity Q. In addition, the thermal conduction of existing rf feedthrough designs is only marginally sufficient to keep even the niobium probe tip superconducting in cw operation. We have equipped a single-cell niobium cavity with different HOM coupler configurations and tested the different designs by measuring Q vs Eacc behavior at 2 K for different feedthroughs and probe tipmaterials |