Members of the high-gradient cavity team at KEK who recorded the 51 MV/m gradient on 6 September.

A popular conjecture among the experts is that a maxium of approximately 51MV/m should be attainable with a suitably shaped cavity, as limited by the maximum surface current that the niobium material can sustain in its superconducting state. In so doing, the use of the electro-polishing technique, pioneered by the superconducting cavity group at KEK, is considered to be a critical surface treatment process to apply in cavity preparation.

Members of the high-gradient cavity team (WG5) at KEK, led by Professor Kenji Saito, successfully recorded the long awaited 51 MV/m gradient at the Q0-value of 5.88e+9 on 6 September. The test was performed on a single-cell "re-entrant"-type cavity which has been designed, fabricated and post-purified by Professor H.Padamsee's group at Cornell University. The cavities were brought to Japan for surface treatments with the electro-polishing technique and for subsequent tests at KEK.

"Now that we could reproduce the high gradient performance for two days in a row, on 6 September and 7 September, we are reasonably confident", Saito said. "There are all sorts of practical details. Establishing the consistency of our cavity preparation and testing procedure are the keys. We will repeat testing the single-cell cavities, then move on to test the 9-cell -- the one we are considering to actually use for the ILC linac."

Saito also emphasized the importance of the international collaboration, "We are exchanging cavities for surface treatments and high-gradient measurements with colleagues among DESY, Cornell University and Jefferson Laboratory. Also the contributions made by Chinese and Korean colleagues who worked with us have been vital. We expect more exchanges of people and cavities around the world."

Saito's group also tested another single-cell cavity of the "Low-Loss" shape designed and produced at KEK, and achieved 46.5MV/m on 9 September.
--Youhei Morita