Initiation and Growth of the Secondary Electron Avalanche along High-Gradient Insulators in Vacuum

High-gradient insulators (HGIs) are materials created by alternating layers of insulating and conducting materials, which can result in a higher holdoff voltage than a monolithic insulator of the same length. We model the HGIs periodic electric field and use both a particle-in-cell direct simulation Monte Carlo code and a test particle approach to parameterize the growth of the secondary electron emission avalanches (SEEAs). To determine the effect of various HGI design parameters on secondary electron trajectories and SEEA growth, the following factors are investigated: layer width, edge layer materials, secondary electron yield, cell coupling, and surface defects as an electron source. We propose an alternative mechanism for initiating surface flashover, which considers surface defects on the downstream side of insulating layers, which differs from the current theory of cell coupling between adjacent insulating layers. Our results indicate surface conditioning is necessary because surface defects are a significant electron source in many cases. Our results reinforce the theory that thin layer widths are better at minimizing the growth of a surface avalanche, even though they will be more susceptible to cell coupling. If an HGI will not be conditioned prior to use, then thin layers are essential because of the significant avalanche growth capable along thicker layers. Finally, a metal layer adjacent to the cathode and an insulating layer adjacent the anode seems the optimal configuration to minimize the probability of flashover.

1 Introduction

High-Gradient insulators (HGIs), constructed by alternating insulating and conducting layers, have been investigated since the mid-1990s to improve holdoff between high-voltage electrodes. Two example HGIs made of ceramic aluminum nitride (AlN) for the insulating layers are shown in Figure 1. The original impetus for the HGI design was the observation that the electric field at which a monolithic insulator will flashover scales with its length as L^−1/2 [1], as will be discussed in Section 1.1. Therefore, it is reasonable to suspect that if short, non-continuous, insulating segments are used, this structure may interrupt the growth of a secondary electron emission avalanche (SEEA) and thus reduce the probability of vacuum surface flashover. Computational results have provided insight into HGI operation, which will be discussed in Section 1.2. Several research groups have confirmed the benefits of an HGI's holdoff voltage but differ in their designs and conclusions. An overview of these prior efforts will be described in Section 1.3. The goal of this study is to explain the effectiveness of different HGI design features towards preventing surface flashover. Figure 1.

High-gradient insulators constructed by sienna technologies. (Left) Constructed with layer widths of a few millimeters and conductive edge layers. (Right) Constructed with layers of a few hundred micrometers in length and insulating edge layers.