Electrical breakdown studies with Mycalex insulators

ELECTRICAL BREAKDOWN STUDIES WITH MYCALEX INSULATORS* W. Waldron, W. Greenway, S. Eylon, E. Henestroza, S. Yu Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA Abstract Insulating materials such as alumina and glass- bonded mica (Mycalex) are used in accelerator systems for high voltage feedthroughs, structural supports, and barriers between high voltage insulating oil and the vacuum beam pipe in induction accelerator cells. Electric fields in the triple points should be minimized to prevent voltage breakdown. Mechanical stress can compromise seals and result in oil contamination of the insulator surface. We have tested various insulator cleaning procedures including ultrasonic cleaning with a variety of aqueous-based detergents, and manual scrubbing with various detergents. Water sheeting tests were used to determine the initial results of the cleaning methods. Ultimately, voltage breakdown tests will be used to quantify the benefits of these cleaning procedures. Figure 1: Test #1 insulator and electrode configuration. INTRODUCTION Glued Mycalex insulators have been used in the induction cells and the injector column of the second axis of the Dual Axis Radiographic Hydrodynamic Test (DARHT) facility at Los Alamos National Laboratory [1,2]. Mycalex samples were tested in vacuum to determine the electrode geometry and the acceptable fields at the insulator triple points. The effect of oil contamination on voltage holding, and cleaning methods have also been considered. HOCKEYPUCK TESTS Test #1 The pulser used for these tests was an RC discharge circuit with a time constant of 24 ms. In this configuration, the insulators were glued between flowerpot electrodes to lower the fields at the insulator triple points (Figure 1). For a peak test voltage of 90 kV, the peak field at the electrode is 125 kV/cm (Figure 2). The observed voltage breakdowns around 90 kV were between the two electrodes (0.48 gap) and not along the insulator surface. The electrodes did condition to hold 90 kV. Figure 2: Test #1 electric field distribution with the arrow pointing to the highest field region. Test #2 In this configuration, the insulators were glued between flat electrodes which created a uniform field of 30 kV/cm on the insulator surface for a 100 kV test voltage. The insulator held above 100 kV and there were no breakdowns. Test #3 This work has been performed under the auspices of the US DOE by UC-LBNL under contract DE- AC03-76SF00098. In this configuration, the insulators were glued between rectangular electrodes which hung over the insulator sample to create a triple point with a field of 200 kV/cm for a 110 kV test voltage (Figure 3). This sample was tested to 110 kV without any voltage breakdowns.