Preliminary Results From Single Crystal and Very Large Crystal Niobium Cavities

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 to fabricate two 2.2 GHz cavities. 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 rolling process. 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 800 ° C for 3 hrs in UHV and about 200 μm total removals from the inner surface by BCP 1: 1: 1, the cavities showed promising results, however, the Q-drop was still present. In the two cavities made from large grain material accelerating gradients of 30 MV/m have been reached. After “in-situ” baking the Q-drop disappeared. The smaller cavities made from single crystal material showed very low residual resistances and accelerating gradients up to Eacc= 45 MV/m were reached (one of the highest ever achieved), corresponding to a peak surface magnetic fields (Bp) of 160 mT.