Mark C. Williamson

Prototype IVA module

A prospective module has been designed for a future large inductive voltage adder (IVA) machine that would be suitable for the most demanding radiography carried out at AWE Aldermaston, Berkshire, UK. This machine would operate at 13-15 MV. The prototype IVA module (PIM) machine has been built to test a single such module and may also provide the basis for other, lower voltage machines in the future. It consists of a single inductive cavity pulsed by a 10-/spl Omega/ water dielectric Blumlein pulse forming line which is charged by a Marx generator. The Blumlein switch initially installed was a coaxial two stage Rimfire and Trigatron switch. A laser triggering configuration utilizing two radial switches, designed by Titan PSI, is also to be tested. The Blumlein PFL and its switch were tested using a copper sulphate resistive load and achieved a peak output voltage of 1.7 MV. The inductive cavity has been added and a pulse of 1.5-MV peak voltage has been successfully applied to it.

Experiments on PIM in support of the development of IVA technology for radiography at AWE

The PIM machine has been designed and constructed at AWE as part of a program to investigate IVA technology for radiographic applications. PIM, as originally constructed, was a prospective single module of a 14 MV, 100 kA, ten module machine. The design of such a machine is a primary goal of the program as several are required to provide multi-axis radiography in a new Hydrodynamics Research Facility (HRF). Another goal is to design lower voltage machines (ranging from 1 to 5 MV) utilizing PIM style components. The original PIM machine consisted of a single inductive cavity pulsed by a 10 ohm water dielectric Blumlein pulse forming line (PFL) charged by a Marx generator. These components successfully achieved their design voltages and data on the prepulse was obtained showing it to be worse than expected. This information provided a basis for design work on the 14 MV HRF IVA, carried out by Titan-PSD, resulting in a proposal for a prepulse switch, a prototype of which should be installed on PIM by the end of this year. The original single, coaxial switch used to initiate the Blumlein has been replaced by a prototype laser triggered switching arrangement, also designed by Titan-PSD, which it was desired to test prior to its eventual use in the HRF. Despite problems with the laser, which will necessitate further experiments, it was determined that laser triggering with low jitter was occurring. A split oil co-ax feed has now been used to install a second cavity, in parallel with the first, on the PIM Blumlein. This two cavity configuration provides a prototype for future radiographic machines operating at up to 3 MV and a test facility for diode research.

Results of the Installation of a Prepulse Reduction System on the PIM Blumlein

In the PIM pulsed power machine a 1.5 MV, 100 ns duration pulse is generated by a 10 Ohm water Blumlein A prepulse reduction system based on a gas switch has been designed and built by Titan PSD. This has been fitted and reduced the prepulse to less than 3 kV. Prepulse of this order is necessary to drive the focused e-beam diodes used for flash radiography. Details of the prepulse reduction system, the experimental data obtained and comparison with computer models of the machine will be presented.

PIM-a Blumlein driven IVA machine

The PIM machine has been designed and constructed at AWE to develop IVA technology for flash radiography of hydrodynamic experiments. While it was originally conceived as one module of a ten module, 14 MV, 100 kA machine versions operating at up to 3 MV are of interest to satisfy future radiographic applications at AWE. The IVA architecture will enable these machines to be relatively easily configurable in either negative or positive polarity allowing the diode to be either the self magnetic pinch type already in use at AWE or a rod pinch diode to achieve smaller radiographic spots. A Marx generator drives a 1.7 MV, 10 ohm water Blumlein initiated by twin radial laser triggered switches. The Blumlein has been used to drive either one or two parallel inductive cavities to obtain an output of 1.5 or 3 MV with a current of up to /spl sim/150 kA or >50 kA respectively. Prepulse suppression is provided by a gas prepulse switch in the coaxial oil feed from the Blumlein to the cell or cells. The latest results of the testing of the laser triggering system and the prepulse reduction system will be presented.

PIM - A Test-Bed for Determining Accelerator Requirements for Flash Radiography at 1 TO 3 MV

The PIM pulsed power machine, originally a prototype for a 14 MV Inductive Voltage Adder (IVA), is now being used as a test-bed to determine the requirements of future radiographic accelerators in the 1 to 3 MV range. A 1.5 MV, 100 ns duration pulse is generated by a 10 ohm water Blumlein A prepulse reduction system designed and built by Titan PSD has been fitted, reducing the prepulse to less than 3 kV, sufficiently low to successfully drive a number of different types of e-beam diode. In the configuration used for recent experiments the output of the blumlein is split into two in an oil insulated co-axial feed sharing the current between two induction cells which add the voltage generated along a magnetically insulated transmission line (MITL) feeding the diode. The machine is therefore potentially capable of driving a 40 Ohm load at up to 3 MV. Three types of diode have been tested on PIM in this configuration. These are the paraxial, a relatively high impedance diode usually used at higher voltages, and the lower impedance self magnetic pinch and rod pinch diodes. The low impedance diodes load down the machine as configured reducing the available voltage. Some possible modifications to the PIM machine will be discussed that may improve its ability to drive them. The machine is now being reconfigured to drive only one induction cell and therefore provide a 10 Ohm 1.5 MV capability. This will be used to drive a plasma filled rod pinch diode with a view to developing higher brightness X-ray sources with the softer spectrum obtained by operating at ~ 1 MV. The data obtained in this work is being used to improve our ability to model such machines and assess concepts for future accelerators operating in the voltage range mentioned.

Triggering of 1.5 MV SF6 Filled Blumlein Switches

PIM is a water filled Blumlein connected, in parallel, to one or two 1.5 MV, cavities which inductively add voltage on a high impedance vacuum transmission line to a load diode operating at 1-3 MV. The radiographic performance of this IVA pulsed power machine depends on the inductive rise-time of the Blumlein switches. Selection of either one or two simultaneously closing switches on PIM was used to investigate the effects of rise time, in addition to voltage on the diode. Two Blumlein switches, designed and built by Titan PSD were initially tested in a self-break mode at two charging rates, from the Marx, the time to peak voltage being 0.7 and 1.5 mus in the two cases. The breakdown curves are matched with the predicted curves from 0.5 to 1.7 MV. Under this self-break mode it was found that there was a tendency for the second switch to close 7 mus later. A model for this is proposed and its effect on the PFL output pulse is illustrated. Two types of lasers have been installed, the results from closing both switches within 2 ns of each other was investigated.

Investigation of the Self Magnetic Pinch Diode on PIM as a Radiographic Source

PIM is a water filled Blumlein connected, in parallel, to one or two 1.5 MV, cavities which inductively add voltage on a high impedance (100 Ohm) vacuum transmission line. A self-magnetic pinch configured diode was tested on this 1-3 MV IVA pulse power machine and an investigation into the performance of this radiographic diode was made using simple hollow cathodes with a plane, grounded anode configuration. Time dependant resolution of the total radiation obtained and a 2.5 mm diameter (AWE defined) source was demonstrated. Closing either one or two of the PFL switches on PIM was used to investigate the effects of rise time and peak voltage on the diode.

Investigation of the Rod Pinch Diode on PIM as a Radiographic Source

PIM is a water filled Blumlein connected in parallel to one or two 1.5 MV cavities, which inductively add voltage on a high impedance (100 Ohm) vacuum transmission line. A negative polarity rod pinch diode was tested on this 1-3 MV IVA pulse power machine and an investigation into the performance of this radiographic diode was made. Time and spatially resolved investigations into the radiating target geometry were performed and a spot size smaller than 2 mm (AWE defined) was demonstrated. Closing either one or two of the PFL switches on PIM was used to investigate the effects of rise time and peak voltage on the diode.

Transmission Line Code Modelling of the Pulsed Power Accelerator PIM, Including Suggestions to Improve the Output Voltage

PIM is a pulsed power accelerator based around the inductive voltage adder technique. Originally designed to drive high impedance (~100 Omega) electron beam diode loads at approximately 3MV it was found that the output voltage into a lower impedance (~40 Omega) load was closer to 1.5MV, the change in load impedance reflects changing requirements for PIM. Using a transmission line code to understand the power flow around the accelerator, the loss mechanisms have been explored and methods to reduce these suggested.

Modelling the pulsed power, inductive voltage adder, machine: PIM, using a transmission line code

Summary form only given. PIM is an X-ray generating pulsed power machine capable of voltages around 3 MV and currents of 40 kA in 80 ns pulses. It comprises a Marx generator feeding a single Blumlein pulse forming line; the pulse then propagates in parallel into two inductive cells, which add the voltage in series onto a magnetically insulated transmission line (MITL). At the end of the MITL an electron beam diode converts the electron beam energy into Bremsstrahlung X-rays. The work described involved creating a circuit model of the machine capable of being easily modified to aid in the design of modifications optimising the voltage pulse for increased electron beam performance. The code has now been used for this purpose and been extended for use on other machines at AWE.