I. H. Mitchell

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 ...

The dynamics of wire array Z-pinch implosions

Wire array Z-pinch dynamics are studied in experiments with 16-mm diameter arrays of between 8 and 64, 15-μm diameter aluminum wires, imploded in 200–260 ns by a 1.4-MA current pulse. Side-on laser probing shows early development of noncorrelated m=0-like instabilities with an axial wavelength ∼0.5 mm in individual wires. End-on interferometry (r-θ plane) shows azimuthal merging of the plasma with a density of 1017 cm−3 in 90–65 ns for 8–64 wires, respectively. At the same time low-density plasma reaches the array axis and forms a precursor pinch by 120–140 ns. At 0.7–0.85 of the implosion time a global m=0 instability with a wavelength of 1.7–2.3 mm was detected in soft x-ray gated images, laser probing, and optical streaks. The time when the instability reaches the observable level corresponds to the number of e-foldings for the growth of the classical Rayleigh–Taylor instability of ∫γ dt∼5.6–7. The scaling of this number with the number of wires is consistent with the instability growth from the seed l...

Investigation of the plasma jet formation in X-pinch plasmas using laser interferometry

A two-frame Mach–Zender interferometer is used to investigate the dynamics of X pinches formed from two 10 μm aluminum wires at current levels of 100 kA. Particularly, the columns of plasma that form on the interelectrode axis of the X pinch are studied quantitatively. It is demonstrated that the plasma which forms these columns does not come solely from expansion of the corona from the limbs of the X pinch but rather predominantly from the crossing point region. The results suggest that the plasma column is indeed a jet which consists of several components.

Two-dimensional magneto-hydrodynamic modeling of carbon fiber Z-pinch experiments

A two-dimensional magneto-hydrodynamic simulation incorporating cold start conditions is used to explain the early phase of carbon fiber Z-pinch experiments. The rapid development of large scale, nonlinear m=0 perturbations in the plasma corona is reproduced. X-ray bright spot formation in the necks of the instability is followed by bright spot bifurcation and fast axial motion. Bright spot bifurcation is found to be due to axial components of the j×B force and occurs off-axis due to the presence of a residual core of unionized carbon. Artificial diagnostic images are generated from the simulations data to allow direct comparison with experimental x-ray imaging and laser probing diagnostics. The accurate reproduction of the experimental images provides confirmation that the experimentally observed features are a repercussion of the non-linear development of the m=0 instability in an ionizing medium.

Time-resolved energy measurement of electron beams in fiber Z-pinch discharges

The emission of hard x rays with energies much greater than the applied anode-cathode voltage is a common feature of Z-pinch plasmas. Here time-resolved measurements of such emission from fiber Z pinches at the mega-ampere current level are reported. The x-ray spectrum measured by an array of detectors is used to calculate the energy of the electron beam producing the emission. Pulses of between 20 and 100 ns duration were observed and electron-beam energy measurements of around 2 MeV obtained. It is thought that these high-energy beams are generated by the plasma becoming resistive at the time of x-ray emission.

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.

The dynamics of bifurcating bright-spots in fiber Z-pinch plasmas

Results are presented from the diagnosis of the optical and x-ray emission from “bright-spots” in carbon fiber Z-pinch experiments using the MAGPIE (Mega-Ampere Generator for Plasma Implosion Experiments) generator [I. H. Mitchell et al., Rev. Sci. Instrum. 67, 1533 (1996)]. Inhomogeneities evolve very rapidly within the plasma with bright-spots becoming detectable after 15–20 ns. After a short (∼4 ns) duration formation phase, these bright-spots exhibit highly dynamic behavior. Bifurcation of the bright-spots is observed giving rise to rapid axial motion at 1–3×105 ms−1. The post-bifurcation bright-spots persist for up to 40 ns. Analysis of cross-filtered, time integrated, x-ray pinhole images yield bright-spot parameters during the formation phase (diameter ∼80 μm, temperature 250–300 eV, ion number densities ∼2×1026 m−3). With a spatial resolution of 175 μm, the strong temperature and density gradients within the post-bifurcation spots can be resolved in gated x-ray images with 2 ns exposure times. Aft...

Observations of the plasma dynamics of a vacuum spark from its soft x-ray emission

Experimental observations of the plasma dynamics in a vacuum spark are presented which permit measurements of the electron density and temperature during a large part of the compression phase of the pinch. The vacuum spark is generated by a low-impedance pulse forming line with a maximum current in excess of 100 kA. The discharge is operated in the hybrid mode in a titanium plasma. A laser focused onto the cathode provides the preionizing source. Soft x-ray emission from the current sheath is observed with an x-ray framing camera well before maximum compression. These observations are compared with holographic interferograms, both showing the formation of an axisymmetric rhombic boundary to the plasma sheath. The temperature and density of both the sheath and the internal plasma are observed until the formation of hot spots at the time of maximum compression. The temperature evolution of the hot spots is presented, showing a repeatable behavior that depends on the axial position.

Observations of plasma dynamics in the vacuum spark

Experimental observations are presented of a vacuum spark driven by a low impedance pulse forming line delivering 100 kA to the load. A pulsed laser is used to form a preionizing plasma on the cathode. The combination of axial and radial optical streak camera observations, together with the time and space resolved soft x‐ray emission, permit the evolution of the plasma dynamics, density and temperature to be measured. Three kinds of behavior are observed according to axial position. A close correlation is found between the x‐ray and the optical emission, with the observation of micropinch formation. A discussion is presented in which the behavior of the vacuum spark under differing operating conditions is compared.

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...