J. F. Worley

A high impedance mega‐ampere generator for fiber z‐pinch experiments

At Imperial College a mega‐ampere generator for plasma implosion experiments has been designed, built, and commissioned. With a final line impedance of 1.25 Ω this terawatt class generator has been designed primarily to drive a maximum current of 1.8 MA with a rise time of 150 ns into high inductance z‐pinch loads of interest to radiative collapse studies. This article describes the design and tests of the generator which has a novel configuration of lines and a new design of a magnetically insulated transmission line (MITL). In summary, the generator consists of four Marx generators each of the Hermes III type (2.4 MV, 84 kJ), each connected to 5 Ω pulse forming lines and trigatron gas switches. The power is fed into the matched 1.25 Ω vertical transfer line which feeds a diode stack and a short conical MITL in vacuum which concentrates the power into the z‐pinch load. At 80% charge a current rising to 1.4 MA in 150 ns has been measured in a 15 nH inductive short. Similar results are obtained when using ...

Study of x-ray emission from a table top plasma focus and its application as an x-ray backlighter

A study of a 2 kJ, 200 kA, table top plasma focus device as an intense x-ray source is reported. The x-ray yield from a number of gases, (deuterium, nitrogen, neon, argon, and xenon) is measured as a function of filling pressure and in neon as a function of anode length. In gases with Z<18, the plasma implodes to form a uniform cylindrical column, whereas for Z⩾18, the plasma consists of a number of hot spots. A maximum x-ray yield of 16.6 J and pulse length of 10–15 ns was obtained in neon. The x-ray emission was established to be due to H- and He-like line radiation. The temperature estimated from spectroscopic observations was about 300–400 eV at an electron density of (3–5)×1020 cm−3 in neon. At low pressures in neon, hard x-ray radiation, presumably due to electron beams was dominant. Mesh images of different wire materials were recorded at the optimum pressure in neon as a proof of principle for x-ray backlighting.

Coronal plasma behavior of the Z pinch produced from carbon and cryogenic deuterium fibers

A series of fiber pinch experiments has been carried out on the MAGPIE (mega-ampere generator for plasma implosion experiments) generator (1.8 MA, 150 ns) [Mithell et al., Rev. Sci. Instrum. 67, 1533 (1996)] to study the temporal evolution of the coronal plasma. Analysis of schlieren photographs, axial streak images and gated x-ray photographs gives the radial and axial motion of the coronal plasma. The influence of a current pulse (prepulse) of 30 kA applied 200 ns before the main discharge was also studied. Radial expansion velocities of 5.5×106 cm/s for carbon fiber shots without prepulse and 3.6×106 cm/s for carbon fibers with prepulse were measured. Axial wavelengths (λz) of dominant instabilities in the corona were between 0.05 and 0.2 cm corresponding to ka∼10–20. Comparison of the results obtained with carbon fibers with and without current prepulse and cryogenic deuterium fibers are presented.

Optimization of a high‐voltage trigatron switch

Work has been carried out to optimize the operation of a high‐voltage trigatron switch. It is demonstrated here that the trigatron can operate in two different modes depending on the route of the initial breakdown. This initial breakdown can occur either to the adjacent electrode or to the opposite electrode. It is shown here that, for a given switch, the mode of operation depends only on the ratio of trigger voltage to working voltage. The optimization was based upon the suggestion that the optimum operation of a trigatron would occur when the trigger pin breaks down simultaneously to both the adjacent and the opposite electrodes. This occurs for the trigger to working voltage ratio, which results in equal mean electric fields across the main gap and the trigger gap. The experiments were carried out with working voltages, Vg, between −1 and −2 MV and with trigger voltage to working voltage ratios of between −2% and −8%. It is shown that the minimum delay and jitter figures are indeed obtained with the tr...

Carbon fiber Z-pinch with current prepulse

Results of experiments studying the effect of a ringing 10 kA current prepulse on a 160 kA, 7 μm carbon fibre z-pinch are presented. A prepulse duration of 100–300 ns was found to radically alter the pinch behaviour. A delayed onset in x-ray and optical emission with respect to the main current was observed. There was a marked improvement of axial uniformity in the pinch. A large soft x-ray pulse, ten to twenty times larger than in discharges without prepulse was recorded. An electron temperature of 250±50 eV was measured which was considerably higher than in the discharges without prepulse.

Coronal plasma behaviour in C and D2 fibres on the MAGPIE generator

A series of fibre pinch experiments has been carried out on MAGPIE generator (1.8 MA, 150 ns) to study the coronal plasma. The analysis of schlieren photographs, axial streak images and gated x-ray pictures allows the evaluation the radial and axial motion of the corona plasma. Radial expansion velocity of 5.5×106 cm/s for carbon fibre and 3.6×106 cm/s for carbon with current prepulse was measured. Axial wavelengths of dominant instabilities in the corona were λz=0.05−0.2 cm corresponding to ka∼10–20. Comparisons of the results obtained with carbon fibres with and without current prepulse (30 kA, 200 ns) and deuterium fibres are presented.

The MAGPIE Generator

The construction of MAGPIE, a terawatt pulsed power facility, has been completed at Imperial College, London. The generator consists of four 2.4MV, 86kJ Marx banks each feeding a 5Ω coaxial pulse forming line. The pulse forming lines are connected, via four trigatron switches, to a vertical coaxial transfer line and hence to the load. The generator is specifically designed to drive high impedance loads, enabling radiative collapse experiments to be carried out in cryogenic Hydrogen fibres. This requires the capability to deliver 1.5MA into a 100nH load in 150ns. A review of the project to date is given. This includes the design philosophy behind the generator, emphasising its unique aspects, and an outline of the tests which have been carried out to optimise its performance. Results from the first stage in the commissioning of the generator are also presented. This involved the firing of the complete generator, charged to 60% of maximum voltage, into a 150nH load. Currents of approximately 700kA have been...