New Accelerating Record for TESLA

For the first time an accelerating field of 35 megavolt per meter (MV/m) has been reached in a 9-cell TESLA cavity.

So far, cavities with accelerating gradients of 25 MV/m - the gradient needed for the 500 gigaelectronvolt (GeV) TESLA linear collider - have been manufactured in industry on a routine basis.

A gradient of 35 MV/m makes it possible to operate the accelerator up to 800 GeV. The accelerating record was achieved in a niobium cavity which - in the frame of a collaboration between KEK (Japan) and DESY - has been electropolished at the Nomura Plating Company in Japan. In contrast to purely chemical etching of the niobium surface, which has been used up to now, electropolishing results in smoother surfaces, which evidently allow higher field strengths. Furthermore, the superconducting properties of niobium are improved by baking the cavity at approx. 100°C prior to measurement. Although the precise reason for this effect is still under discussion, field strengths of 40 MV/m were repeatedly exceeded in one-cell cavities in the course of experimental collaborations between KEK, CEA Saclay, CERN and DESY. Theoretically a maximal accelerating field strength of about 50 MV/m can be expected for niobium.

An electropolishing facility for TESLA cavities will be commissioned at DESY, probably in June 2002.

Plot for the experts:
Excitation curve of a 9-cell cavity in a vertical test. The cavity was submitted to a special vacuum annealing at 1400°C to increase the thermal conductivity. Afterwards the cavity was electropolished and baked out at 105°C for 34 hours under vacuum prior to measurement.

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