PRE-PULSE MINIMISATION ON BLUMLEIN-BASED X-RAY MACHINES BY OPTIMISATION OF BALANCE CIRCUIT PARAMETERS

摘要

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 alters conditions in the anode-cathode gap preventing 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 V{sub}(in) (denoted by V{sub}p in the circuit diagram and theory to avoid confusion with the raw pre-pulse voltage V{sub}i), is charged from the output of a Marx bank. An output pulse is created on the inner conductor on closure of an oil switch that shorts the inner and intermediate Blumlein conductors. Prior to closure of this switch, the voltage of the inner Blumlein conductor, denoted by V{sub}i, which is actually the raw pre-pulse voltage, must be kept as close to ground as possible. By convention, this is achieved using a balance circuit, comprising two inductors (L{sub}i & L{sub}o, with mutual inductance M) and two damping resistors (R{sub}i & R{sub}o) that connect to the base of the Marx bank. For ideal electrical balance, the ratios (L{sub}i-M):(L{sub}o-M) and R{sub}i:R{sub}o must equal the ratio C{sub}o:C{sub}i, where C{sub}o is the capacitance between the outer and intermediate Blumlein conductors and C{sub}i is that between the inner and intermediate Blumlein conductors. In practice, machines will have some imbalance and non-zero raw pre-pulse voltage (V{sub}i). These could be kept in check if a convenient way of monitoring and adjusting balance-circuit components existed. To this end, a method is proposed which relies on measurement of both the charging voltage (V{sub}(in) ≡ V{sub}p) and the raw pre-pulse voltage (V{sub}i). These signals can be manipulated and combined mathematically to produce a synthesised raw pre-pulse voltage (V{sub}(is)) which depends on assumed values of the balance circuit parameters (L{sub}i, L{sub}o, R{sub}i, R{sub}o & M) and the known inner and outer Blumlein capacitances (C{sub}i & C{sub}o). The aim is to adjust L{sub}i, L{sub}o, R{sub}i, R{sub}o and M to minimise the difference between the measured (V{sub}i) and synthesised (V{sub}(is)) raw pre-pulse waveforms. Since the method has been implemented in an Excel spreadsheet, the process can be automated using Excel's inbuilt "Solver". An advantage of the approach is that it is independent of Marx bank parameters, in particular the Marx bank series resistance which, being dynamic, can cause problems. However, time derivatives of the waveforms are involved so a suitable method of smoothing and differentiating noisy waveforms is required. These difficulties could be alleviated by improving signal monitoring, particularly by incorporating current monitors (e.g. Rogowski coils) in the balance circuit, which can be set to measure either current or its time derivative. So far, the method has been applied successfully to the AWE radiographic machines Mogul-D and Mogul-E. Note that this paper is related to that by the same authors entitled "Dual-Waveform Ringing Gain Analysis and its Application to Pre-pulse Reduction on Blumlein-based X-ray Machines" [1].