| Paper | Title | Page |
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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. |