Ionization-Induced Self-Channeling of an Ultrahigh-Power Subnanosecond Microwave Beam in a Neutral Gas.

Ionization-induced self-channeling of a ≤500 MW, 9.6 GHz, <1 ns microwave beam injected into air at ∼4.5×10^{3} Pa or He at ∼10^{3} Pa is experimentally demonstrated for the first time. The plasma, generated by the impact ionization of the gas driven by the microwave beam, has a radial density distribution reducing towards the beam axis, where the microwave field is highest, because the ionization rate is a decreasing function of the microwave amplitude. This forms a plasma channel which prevents the divergence of the microwave beam. The experimental data obtained using various diagnostic methods are in good agreement with the results of analytical calculations, as well as particle in cell Monte Carlo collisional modeling.

Stationary striations in plasma, created by a short microwave pulse in a waveguide filled with a neutral gas.

It was observed experimentally that after crossing a waveguide filled with a neutral gas a short powerful microwave pulse leaves a periodic glow of plasma along the waveguide, persisting for several tens of nanoseconds. A theoretical model is presented which in combination with numerical simulations proposes a possible explanation for this phenomenon.

Electric field in a plasma channel in a high-pressure nanosecond discharge in hydrogen: a coherent anti-stokes Raman scattering study.

Experimental results of a study of the electric field in a plasma channel produced during nanosecond discharge at a H2 gas pressure of (2-3)×10(5) Pa by the coherent anti-Stokes scattering method are reported. The discharge was ignited by applying a voltage pulse with an amplitude of ∼100 kV and a duration of ∼5 ns to a blade cathode placed at a distance of 10 and 20 mm from the anode. It was shown that this type of gas discharge is characterized by the presence of an electric field in the plasma channel with root-mean-square intensities of up to 30 kV/cm. Using polarization measurements, it was found that the direction of the electric field is along the cathode-anode axis.

Frequency conversion, "superluminal" propagation, and compression of a powerful microwave pulse in propagating ionization front.

Frequency up-conversion (∼10%) and compression (almost twofold) of a powerful (≤250 MW) microwave pulse in the propagating ionization front produced by the pulse itself in a gas-filled waveguide, is investigated experimentally and analyzed theoretically. Pulse envelope reshaping and group velocity increase manifest themselves in a propagation of the pulse faster than in the empty waveguide. A simple one-dimensional mathematical model allows the adequate interpretation of the experimental results.

Compact high-current pulse generator for laboratory studies of high energy density matter.

We present the design and parameters of a compact and mobile high-current pulse generator, which can be applied in the study of warm dense matter in university laboratories. The generator dimensions are 550 × 570 × 590 mm3, the weight is ∼70 kg, and it consists of four "bricks" connected in parallel. Each brick, made up of 2 × 40 nF, 100 kV low-inductance capacitors connected in parallel, has its own multi-gap and multichannel ball gas spark switch, triggered via a capacitively coupled triggering by a positive polarity pulse of ∼80 kV amplitude and ∼15 ns rise time. At a charging voltage of ∼70 kV, the generator produces a ∼155 kA current pulse with a rise time of ∼220 ns on a ∼15 nH inductive short-circuit load and a ∼90 kA amplitude current pulse in the underwater electrical explosion of a copper wire.

Use of synchrotron-based radiography to diagnose pulsed power driven wire explosion experiments.

We describe the first use of synchrotron radiation to probe pulsed power driven high energy density physics experiments. Multi-frame x-ray radiography with interframe spacing of 704 ns and temporal resolution of <100 ps was used to diagnose the electrical explosion of different wire configurations in water including single copper and tungsten wires, parallel copper wire pairs, and copper x-pinches. Such experiments are of great interest to a variety of areas including equation of state studies and high pressure materials research, but the optical diagnostics that are usually employed in these experiments are unable to probe the areas behind the shock wave generated in the water, as well as the internal structure of the exploding material. The x-ray radiography presented here, performed at beamline ID19 at European Synchrotron Radiation Facility (ESRF), was able to image both sides of the shock to a resolution of up to 8 µm, and phase contrast imaging allowed fine details of the wire structure during the current driven explosion and the shock waves to be clearly observed. These results demonstrate the feasibility of pulsed power operated in conjunction with synchrotron facilities, as well as an effective technique in the study of shock waves and wire explosion dynamics.

Cold Atmospheric Plasma, Created at the Tip of an Elongated Flexible Capillary Using Low Electric Current, Can Slow the Progression of Melanoma.

INTRODUCTION: Cold Atmospheric Plasma Jet (CAPJ), with ion temperature close to room temperature, has tremendous potential in biomedical engineering, and can potentially offer a therapeutic option that allows cancer cell elimination without damaging healthy tissue. We developed a hand-held flexible device for the delivery of CAPJ to the treatment site, with a modified high-frequency pulse generator operating at a RMS voltage of <1.2 kV and gas flow in the range 0.3-3 l/min. The aims of our study were to characterize the CAPJ emitted from the device, and to evaluate its efficacy in elimination of cancer cells in-vitro and in-vivo. METHODS AND RESULTS: The power delivered by CAPJ was measured on a floating or grounded copper target. The power did not drastically change over distances of 0-14 mm, and was not dependent on the targets resistance. Temperature of CAPJ-treated target was 23°-36° C, and was dependent on the voltage applied. Spectroscopy indicated that excited OH- radicals were abundant both on dry and wet targets, placed at different distances from the plasma gun. An in-vitro cell proliferation assay demonstrated that CAPJ treatment of 60 seconds resulted in significant reduction in proliferation of all cancer cell lines tested, and that CAPJ activated medium was toxic to cancer cells. In-vivo, we treated cutaneous melanoma tumors in nude mice. Tumor volume was significantly decreased in CAPJ-treated tumors relatively to controls, and high dose per fraction was more effective than low dose per fraction treatment. Importantly, pathologic examination revealed that normal skin was not harmed by CAPJ treatment. CONCLUSION: This preliminary study demonstrates the efficacy of flexible CAPJ delivery system against melanoma progression both in-vitro and in-vivo. It is envisioned that adaptation of CAPJ technology for different kinds of neoplasms use may provide a new modality for the treatment of solid tumors.

Generation of sub-Mbar pressure by converging shock waves produced by the underwater electrical explosion of a wire array.

We report a demonstration of a generation of sub-Mbar pressure on the axis of the implosion wave produced by an underwater electrical explosion of a cylindrical wire array. The array was exploded by microsecond time scale discharge of a capacitor bank having a stored energy of 4.5 kJ and discharge current amplitude of up to 90 kA. Optical diagnostics were used to determine the time of flight and the trajectory of the converging shock wave. This data were applied for a calculation of the water flow parameters using one-dimensional (1D) and 2D hydrodynamic calculations and the Whitham method. All three methods have shown that the shock wave pressure at 0.1 mm from the axis reaches .

Commissioning of the PRIOR proton microscope.

Recently, a new high energy proton microscopy facility PRIOR (Proton Microscope for FAIR Facility for Anti-proton and Ion Research) has been designed, constructed, and successfully commissioned at GSI Helmholtzzentrum für Schwerionenforschung (Darmstadt, Germany). As a result of the experiments with 3.5-4.5 GeV proton beams delivered by the heavy ion synchrotron SIS-18 of GSI, 30 µm spatial and 10 ns temporal resolutions of the proton microscope have been demonstrated. A new pulsed power setup for studying properties of matter under extremes has been developed for the dynamic commissioning of the PRIOR facility. This paper describes the PRIOR setup as well as the results of the first static and dynamic proton radiography experiments performed at GSI.

Strongly coupled copper plasma generated by underwater electrical wire explosion.

A number of theoretical approaches to the analysis of the parameters of a discharge channel consisting of strongly coupled plasma generated in the process of underwater electrical wire explosion are presented. The analysis is based on experimental results obtained from discharges employing Cu wire. The obtained experimental data included electrical measurements and optical observations from which information about the dynamics of the water flow was extrapolated. Numerical calculation based on a 1D magnetohydrodynamic model was used to simulate the process of underwater wire explosion. A wide range conductivity model was applied in this calculation and good agreement with a set of experimental data was obtained. A method of determining the average temperature of the discharge channel based on this model and experimental results is proposed, and the limits of this method's applicability are discussed.

Simplified model of underwater electrical discharge.

A model of the underwater discharge with initiating wire is presented. The model reveals the nature of similarity parameters which have been phenomenologically introduced in earlier experimental research in order to predict behavior of different discharges. It is shown that these parameters naturally appear as a result of the normalization of differential equations, which determines the process of underwater wire initiated discharge. In these equations the energy conservation law for wire material evaporation and the dependence of plasma conductivity on the energy dissipated in the discharge are implied to calculate the time varying resistance of the discharge gap. The comparison of calculations with the experimental results shows that good agreement is achieved when modification of these parameters is introduced. These new similarity parameters are functions of the original similarity parameters, hence the law of the similarity of underwater electrical discharge is preserved.

Spectroscopic study of plasma evolution in runaway nanosecond atmospheric-pressure He discharges.

Time- and space-resolved visible-emission spectroscopy measurements are applied to study plasma parameters in nanosecond electrical discharges in He gas at pressure of 10(5) Pa, using a 150 kV, 5 ns duration high-voltage pulse. The plasma evolution during the discharge is investigated by applying line-shape analysis of several He I spectral transitions, with the Stark and opacity effects accounted for. The analysis shows that the discharge plasma is not in equilibrium and that significant electric fields of several kV/cm are present in the plasma during the discharge. Regions of plasma with significantly different electron densities are identified and a qualitative model of the plasma formation and evolution is proposed.

Diagnostics of underwater electrical wire explosion through a time- and space-resolved hard x-ray source.

A time- and space-resolved hard x-ray source was developed as a diagnostic tool for imaging underwater exploding wires. A ~4 ns width pulse of hard x-rays with energies of up to 100 keV was obtained from the discharge in a vacuum diode consisting of point-shaped tungsten electrodes. To improve contrast and image quality, an external pulsed magnetic field produced by Helmholtz coils was used. High resolution x-ray images of an underwater exploding wire were obtained using a sensitive x-ray CCD detector, and were compared to optical fast framing images. Future developments and application of this diagnostic technique are discussed.

Coulomb microexplosions of ferroelectric ceramics.

Energetic neutral and extreme ultraviolet emission initiated by the dense plasma propagation along a ferroelectric surface has been found. It was shown that the emission of neutrals is characterized by a large divergence and velocities up to 7 x 10(7) cm/s. This phenomenon is explained by an extremely large electric field with amplitude > or =10(6) V/cm and rise time approximately 10(-10) s which appears at the plasma front due to the fast fall in the driving pulse. This electric field causes microexplosions of the ferroelectric surface due to inertia in the ion polarization response.

Semianalytical solution of the problem of converging shock waves.

An approach to the determination of the self-similarity parameter in the problem of converging strong shock waves is suggested. This approach allows one to obtain analytical expressions that approximate the numerical solution. For adiabatic constants gamma = 6/5-7, the values of the obtained self-similarity parameter differ by <1% from the values determined by the numerical procedure. In addition, accurate analytical characteristics of the reflected shock wave are obtained.