Project Title: Studies of quench locations in ILC cavities
The proposed International Linear Collider (ILC) will use 9-cell 1.3 GHz superconducting RF cavities, operating at 2 K, as accelerating structures for electron/positron beams. The baseline design of ILC calls for more than 16000 9-cell superconducting cavities operating at 31.5 MV/m accelerating gradient with the unloaded quality factor of 1010. The production of these cavities is a significant capital investment and is a large fraction of the ILC cost. The cost estimate of ILC assumes that the cavity production will result in 90 % yield, i.e., 9 produced cavities out of 10 will reach the goal quality factor at the goal gradient. Although the state-of-art superconducting RF cavities exceed these specifications and reach above 40 MV/m accelerating gradient with unloaded quality factor above 1010 at 2 K, in order to produce a state-of-art cavity, niobium material has to undergo a large number of steps, where each step is crucial for the final RF result. Often, however, a cavity is limited at a lower field due to a local defect. We propose developing a thermometry system that will allow an order of magnitude better resolution that the thermometry system currently in use at Jefferson Lab. With such a thermometry system placed on the quench location we can obtain detailed information on the heating distribution around the weld quench location prior to quench and localize the quench spot with a greater precision.
A sophisticated manufacturing and preparation procedure has to be followed, in order to achieve the state-of-art gradient and quality factors. A cavity has to be produced from a high RRR niobium sheet free of metal inclusions or voids, to improve thermal stability. Rolled niobium sheets are shaped by deep drawing and are welded together in ultra-high vacuum to produce a high-quality cavity. The cavity then is chemically polished to remove the damage layer from rolling and deep drawing. After chemical polishing the cavity has to be annealed in the furnace to remove hydrogen contamination. After the furnace treatment the cavity has to be chemically polished again to remove surface contaminants. Finally, after chemistry, the cavity is high pressure rinsed with ultrapure de-ionized water to remove field emitters and is assembled in the ISO 4 cleanroom to avoid contamination.
All preparation steps are crucial to the cavity performance. Electropolishing produces smooth glossy surface, which is crucial for achieving high gradients. Furnace treatment reduces hydrogen content, preventing low field degradation, so-call hydrogen Q-disease. High pressure rinsing with ultrapure de-ionized water removes potential field emitters from the cavity surface. And, cleanroom assembly and slow pump-down is a critical step that prevents possible contamination of the surface. If the preparation process is less-than-optimal at some point, the RF gradient of a superconducting cavity will be...