Pre-Pulse Minimisation on Blumlein-Based X-ray Machines by Optimisation of Balance Circuit Parameters

Abstract

Summary form only given. At AWE, pulsed-power machines are routinely used to drive electron-beam diodes for flash X-ray radiographic applications in the 1-10 MV range. Some diode types are sensitive to relatively low levels of pre-pulse voltage, during the pulse-forming line (PFL) charging phase, due to small anode-cathode gaps and geometries that enhance electric fields. This results in electron emission prior to the main pulse which can prevent proper diode operation. Limiting the pre-pulse voltage at the diode to below the emission level is therefore crucial. A Blumlein PFL, as used on all AWE machines at present, comprises three co-axial cylindrical conductors, the outer one of which is grounded. The intermediate conductor, of voltage Vin, is charged from the output of a Marx bank. An output pulse is created on the inner conductor on closure of an oil-switch which shorts the inner and intermediate Blumlein conductors. Prior to closure of this switch, the voltage of the inner Blumlein conductor, denoted by Vi, which is actually the pre-pulse voltage, must be kept as close to ground as possible. This can be achieved using a balance circuit, comprising two inductors (L1 & L0) and two damping resistors (R1 & R0) which connect to the base of the Marx bank. For ideal electrical balance, the ratios L1:L0 and R1:R0 must equal the ratio C0:C1 where C0 is the capacitance between the outer and intermediate Blumlein conductors and C0 is the capacitance between the inner and intermediate Blumlein conductors. An advantage of the approach is that it is independent of Marx bank parameters, in particular the Marx bank series resistance which, being dynamic, sometimes causes problems. However, time derivatives of the waveforms are involved so a suitable method of smoothing and differentiating noisy waveforms has had to be developed. The method has been applied with some success to certain radiographic machines at AWE.