Prefire probability of the switch type Fast LTD

In this paper, we present the most recent test of the multigap gas switches which are one of the key elements of the fast LTD stages with oil insulation. Inside these switches, the charge voltage is distributed between the multiple gaps with the use of a corona discharge. The evident advantages of this technique are low cost and extreme compactness of the voltage divider because it just consists of few needles that are soldered to the switch electrodes. At the same time, for proper operation of the switch, special features of the corona discharge have to be taken into account. These features include the nonlinear volt-ampere characteristics of the corona discharge that must be the same in each serial gap; the influence of UV radiation and free charged particles, that appear during the corona discharge, on the breakdown voltage of the gaps; and the variation of the shape of the needles that may burn during the switch operation thus limiting its life time. Each of these features (and probably some others that might be not as evident at this moment) may influence the voltage distribution between the gaps and therefore be a reason for the switch prefire. Below we present the design of the switch type Fast LTD, the breakdown voltage of the switch gaps, the volt-ampere characteristics of the corona discharge, and the statistics on the switch jitter and prefire probability.