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Controlling the high voltage holdoff performance of alumina insulators in vacuum

We use a high voltage device to do a selection between different kinds of high performance aluminas, and to get a control quality test of the surface processing which is applied in the manufacturing process. This work shows how it is possible to step over the difficulties of this kind of measurement to get reproducible results. We describe the apparatus, the specific electrodes, the cleaning process which is necessary before testing samples, and the way of determining the holdoff or flashover voltages, or even the reliability level of samples tested with such a procedure. With the same purpose to select the best alumina materials, another new method is in progress in our laboratories to measure the space charge holding capability of polycrystalline alumina insulators. We will show attractive results on manganese surface doping effect, using a SEM equipment to charge the samples and measure the trapping parameters. This method seems to be very sensitive. However, to get significant results, it is necessary to pay very critical attention on cleaning and thermal processes which are used, when preparing the samples. Next step of this work will be to find the relationship between both methods. This result would be very helpful for the understanding of flashover phenomena.