E. Wyndham

An Observation of Energetic Electron Beams in Low-Pressure Linear Discharges

Experimental observations of energetic axial electron beams in a linear Z pinch operating in the pseudospark mode are presented. The device is driven from a fast Marx generator and allows reproducible production of electron beams over a wide pressure range. Evidence of the importance of electrons generated in the cathode recess in the formation of the beams is presented. An electron beam of high energy which is not associated with formation of the discharge is identified. A second beam of high current density and lower energy associated with gas breakdown is also observed.

Ion beam emission in a low energy plasma focus device operating with methane

An investigation of ion beam emission from a low energy plasma focus (PF) device operating with methane is reported. Graphite collectors, operating in the bias ion collector mode, are used to estimate the energy spectrum and ion flux along the PF axis, using the time-of-flight technique. The ion beam signals are time correlated with the emission of soft x-ray pulses from the pinched focus plasma. The correlation of ion beam intensity with filling gas pressure indicates that the beam emission is maximized at the optimum pressure for focus formation at peak current. Ion beam energy correlations for operation in methane indicate that the dominant charge states in carbon ions are C+4 and C+5. The estimated maximum ion energy for H+, C+4 and C+5 are in the range of 200?400?keV, 400?600?keV and 900?1100?keV, respectively, whereas their densities are maximum for the energy range 60?100?keV, 150?250?keV and 350?450?keV, respectively. These results suggest that the ion beams are emitted from a high density, high temperature, short lived focus plasma, at a time which appears to precede the emission of soft x-ray pulses. The properties of the carbon ion beams are discussed in the context of potential applications in materials science.

Theoretical approach for plasma series resonance effect in geometrically symmetric dual radio frequency plasma

Plasma series resonance (PSR) effect is well known in geometrically asymmetric capacitively couple radio frequency plasma. However, plasma series resonance effect in geometrically symmetric plasma has not been properly investigated. In this work, a theoretical approach is made to investigate the plasma series resonance effect and its influence on Ohmic and stochastic heating in geometrically symmetric discharge. Electrical asymmetry effect by means of dual frequency voltage waveform is applied to excite the plasma series resonance. The results show considerable variation in heating with phase difference between the voltage waveforms, which may be applicable in controlling the plasma parameters in such plasma.

Experimental investigations of hotspots in a low energy plasma focus operating in hydrogen-argon mixtures

We present experimental results on the investigation of hotspot formation in PFP-I, a small 3.8 kJ plasma focus device operating in hydrogen-argon mixtures, at pressures from below 0.2 torr upward. A combination of multipinhole and slit-wire X-ray photography is used to measure the characteristic size and temperature of the hotspots, over a range of pressure and gas mixing ratios. Filtered p-i-n diodes and a beam-target detector are used to investigate the time evolution of the hotspots. Typical size for the hottest emitting region, at temperatures between 200 and 400 eV, is found to be around 150 /spl mu/m, with a typical duration of the high temperature phase of the order of 10 ns. In general, the temperature in the final phase of the time evolution of the hotspots reaches values which are nearly twice those of the plasma column where they are formed. Characteristic size of the hotspots is about half of that of the initial plasma column.

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.

Measurements of time average series resonance effect in capacitively coupled radio frequency discharge plasma

Self-excited plasma series resonance is observed in low pressure capacitvely coupled radio frequency discharges as high-frequency oscillations superimposed on the normal radio frequency current. This high-frequency contribution to the radio frequency current is generated by a series resonance between the capacitive sheath and the inductive and resistive bulk plasma. In this report, we present an experimental method to measure the plasma series resonance in a capacitively coupled radio frequency argon plasma by modifying the homogeneous discharge model. The homogeneous discharge model is modified by introducing a correction factor to the plasma resistance. Plasma parameters are also calculated by considering the plasma series resonances effect. Experimental measurements show that the self-excitation of the plasma series resonance, which arises in capacitive discharge due to the nonlinear interaction of plasma bulk and sheath, significantly enhances both the Ohmic and stochastic heating. The experimentally ...

Hollow cathode effects in charge development processes in transient hollow cathode discharges

A detailed experimental study of space charge formation and ionization growth in transient hollow cathode discharges (THCD) is presented. The experiment was performed with an applied step voltage up to 30 kV, with rise time less than 50 ns. The discharge was operated in different gases, at pressures in the range 50-750 mTorr, with cathode apertures ranging from 1 to 5 mm diameter and 5 to 20 mm long, with 10 cm electrode separation. Spatial charge formation, both in the hollow cathode region (HCR) and inter electrode space, has been studied with a capacitive probe array. Properties of high energy electron beams have been measured with a beam-target scintillator-photomultiplier arrangement. Detailed correlations of the electron beam evolution with the charge probe signals inside and outside the HCR clearly demonstrate the role of the electron beam in the initial formation and late evolution of a virtual anode and, in turn, the field enhanced ionization when the anode potential is brought close to the HCR. These results clearly identify the different regimes in which the Hollow Cathode plays a significant role in ionization growth in the inter electrode space and in the processes which eventually lead to electric breakdown. >

Experimental studies of ion beam anisotropy in a low energy plasma focus operating with methane

We have investigated, with time and space resolution, the ion beam emission from a plasma focus (PF) device, operating in methane, at 20 kV, with 1.8 kJ stored energy. A detector array is used to measure simultaneously the ion beams at five different angular directions with respect to the PF axis (0°, 10°, 15°, 20° and 90°), at a distance of 77 cm from the ion source. Ion beam energy correlations for operation in methane indicate that the dominant charge states on the detector are H+, C+4 and C+5, irrespective of the angular positions. The time integrated ion beam signal and the energy-dispersive x-ray analysis of a carbon films deposited on silicon surface shows the impurity emission from the PF electrode surface. Measured ion fluxes are maximum for the energy range of 15–40 keV, 50–100 keV and 100–300 keV, for H+, C+4, and C+5, respectively. Measurements of the angular distribution of hydrogen and carbon ions reveal a strong angular anisotropy. It is argued that the observed angular anisotropy of the ion beam emission can be explained in terms of ion Larmor radius effects during the z-pinch like plasma formation phase, which is characteristic of PF discharges.

Observations of vacuum spark dynamics from its X-ray emission

The behavior of a medium energy (/spl sim/1 kJ) pulse-power-driven vacuum spark is shown to depend on the electrode material and form of the anode in otherwise similar conditions of operation. The dynamical evolution of the discharge is followed from its soft X-ray emission. The electrode materials compared are titanium and aluminum with a form of anode that is tubular or conical. The use of a tubular anode favors a more uniform sheath and a better formation of a dense Z-pinch and the ensuing hot spots are found to be at least twice as bright. The position of the brightest spots differs according to the material and the electrode shape, and is found to coincide with the shape of the sheath. The energy output is measured and the density of the plasma in the hot spots may be calculated.

Pulsed laser deposition of thin carbon films in a neutral gas background

We studied carbon film deposition using a laser-produced plasma, in argon and helium background gas, at pressures between 0.5 and 700 mTorr. A Nd : YAG, 370 mJ, 3.5 ns, at 1.06 µm, operating at 10 Hz, with a fluence of 6.7 J cm−2 was used. The laser plasma was characterized using space resolved OES and a fast response Faraday cup. The resulting carbon films were analysed using AFM, Raman spectroscopy, XPS and SIMS. The structural properties of the carbon films were found to be strongly correlated with the laser carbon plasma composition. Films with a relatively high content of sp3, characteristic of DLC, were obtained at pressures below 200 mTorr. For these conditions the characteristic carbon ion energies in the expanding laser plasma were of the order of 100 eV. At higher pressures sp2 bonds, associated with amorphous carbon, were dominant, which coincides with a high content of C2 molecules in the laser plasma, and a characteristic carbon ion energy around 20 eV.