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CLASSE NEWS | 31 May 2009

9-cell cavity repaired by tumbling

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Cavity before and after tumbling. Click image to enlarge.

A test in May 2009 at Cornell's SRF group demonstrated that pit defects can be fixed with tumbling, a major limit to high-voltage operation (Eacc > 20 MV/m)

The 9-cell cavity fabricated by AES was defect limited to 15 MV/m We located the defect and found that it was produced by incomplete consumption of weld undercut. Its location was found by second sound quench location with Cornell Oscillating Superleak Transducers (OSTs). We repaired the pit-defect with tumbling to remove 80micron material from the cavity interior surface, and the cavity performance improved remarkably!

By locating several OSTs in the helium bath around the cavity, the second sound wave front can be observed. The time of arrival of the second sound wave at a given transducer is determined by the time of flight from the heated region, which is centred on the defect causing quench.

While this cavity originally quenched at Eacc = 15 MV/m in the accelerating mode at a weld pit in the first cell, after tumbling and reprocessing the accelerating-mode had Eacc > 30MV/m. The measurement was limited by the available RF power the and the cavity did not quench. When excited in the 5pi/9-mode peak electric fields of 89 MV/m and 1400 Oe of magnetic field were reached in the center cell. This corresponds to Eacc = 37 MV/m in this cell.

This test demonstrates that tumbling is an effective option to repair weld defects, e.g. pits. Another important result of this test is that individual cells in 9-cell cavities processed with Cornell's vertical EP reach fields exceeding 35 MV/m, fields hitherto only reached with a much more complicated horizontal EP setup.