Electrostatic modelling of critical electrical components of the Mevex X-ray machine

Summary form only given. The Opera-3D electrostatic modelling software, Tosca, has been used to model Mevex, a type of X-ray machine used at AWE for flash radiography since the early 1980s. The operating principle of the machine is to switch charge, stored in a Mark bank, into a Blumlein pulse-forming line. This causes the inner line of the Blumlein to rise in voltage to about 700 kV, driving a self-magnetic pinch (SMP) diode to produce a 60-nanosecond X-ray pulse. Currently, the machine design is being refreshed to enhance operability. This study is concerned with possible electrostatic breakdown of critical components, in particular the spiral-inductor, part of the balance circuit, and the high-voltage feed-through from the Marx generator to the Blumlein intermediate conductor. The geometry of the spiral-inductor, though difficult to specify, is generated in the Opera-3D modeller by creating a spiral wire-edge and sweeping a disc along it, from one end to the other, maintaining normality between it and the local direction of the spiral wire-edge. A functional voltage boundary condition is applied to the spiral surface in terms of the length around the spiral wire-edge, which can be determined analytically. The process is not straightforward since a general point on the surface of the spiral does not lie on the wire-edge at its core. In order to implement the boundary condition, an expression for the distance from the general surface point to the spiral wire-edge at its core must be established and minimised. This can be achieved in terms of the angular variable used to specify the spiral wire-edge. Since the resulting equation cannot be solved analytically, Newton-Raphson iteration is used to determine the solution numerically. In practice, this process is performed using a specially written command-interface (COMI) file which is executed automatically when the Opera-3D software is run. Electrostatic field strengths have been determined at the surfaces of the con- uctors of interest, as described above, and shown to be less than required to initiate breakdown. The work described in the poster has not been presented elsewhere.