Th. Wrubel

Emission in the deep vacuum ultraviolet from a plasma formed by incandescently heating hydrogen gas with trace amounts of potassium carbonate

A hydrogen plasma with intense extreme ultraviolet and visible emission was generated from low pressure hydrogen gas (0.1–1 mbar) in contact with a hot tungsten filament only when the filament heated a titanium dissociator coated with K2CO3 above 750°C. The electric field strength from the filament was about 1 V cm−1, two orders of magnitude lower than the starting voltages measured for gas glow discharges. The emission of the Hα and Hβ transitions as well as the Lα and Lβ transitions were recorded and analysed. The plasma seemed to be far from thermal equilibrium, and no conventional mechanism was found to explain the formation of a hydrogen plasma by incandescently heating hydrogen gas in the presence of trace amounts of K2CO3. The temporal behaviour of the plasma was recorded via hydrogen Balmer alpha line emission when all power into the cell was terminated and an excessive afterglow duration (2 s) was observed. The plasma was found to be dependent on the chemistry of atomic hydrogen with potassium since no plasma formed with Na2CO3 replacing K2CO3 and the time constant of the emission following the removal of all of the power to the cell matched that of the cooling of the filament and the resulting shift from atomic to molecular hydrogen. Our results indicate that a novel chemical power source is present and that it forms the energetic hydrogen plasma that is a potential new light source.

Investigation of the He II and He II transitions at high densities

Measurements of Stark profiles of the He II line at and of the He II line at from a well diagnosed plasma of a gas-liner pinch discharge are reported. The plasma parameters were determined independently by Thomson scattering, and we found - , - 7.5 eV when measurements of the He II line were carried out and - , - 5.4 eV for the He II measurements. We compare widths and shifts of the spectral lines with theoretical approximations and present new empirical scalings. While the width of the He II line shows significant discrepancies with lineshape theories, in the case of the He II line profile we find good agreement with some calculations. In particular, to investigate ion dynamic effects we present an accurate measurement of the dip in the centre of the spectral line.

Radially and axially resolved Thomson scattering in a gas-liner pinch

We performed spatially and temporally resolved Thomson scattering in a plasma generated in a gas-liner z-pinch, where the amount of axially distributed impurity ions seeded in a hydrogen plasma can be controlled. The parameters of the plasma electron density, temperature, concentration of impurity ions as well as the bulk velocity of the plasma are measured along both the radius and the z-axis. The temporal behaviour is studied from shot to shot. Depending on the impurity concentration on the axis and the kind of impurities, the homogeneity and stability of the plasma column along the z-axis are investigated. For small impurity concentrations the impurity ions are located in the homogeneous central part of the plasma offering excellent conditions to test theories on Stark broadening of spectral lines. Modulations of the density and temperature along the axis start to rise when the concentration of the impurity ions exceed a certain limit depending on their charge number. The modulations are related to Rayleigh-Taylor instabilities growing during the late phase of the compression.

The Stark width and shift of the hydrogen Hα line

Measurements of line profiles of the hydrogen H? transition from a well diagnosed gas-liner pinch are reported. The discharge is diagnosed using Thomson scattering and simultaneous spectroscopic measurements are performed giving the plasma parameters 0.5?1018?cm-3?ne?2.5?1018?cm-3 and 6?eV?kBTe?10.5?eV. The Stark width and shift are determined and compared to a selected set of experimental and theoretical data. The difference between the present results and former data, which were measured with the same device, is critically discussed.

Line profile measurements of (4S) 6s 5S–(4S) 6p 5P transitions of Xe III

Abstract We report on measurements of the line profiles and Stark widths of two (6s 5S– 6 p 5 P ) quintet transitions of Xe III. The measurements are performed in a gas-liner pinch discharge. The spectroscopic measurements are made simultaneously with Thomson scattering that give the plasma conditions of electron density, ne=(0.6– 1.2)×10 18 cm −3 and electron temperature, Te=(3– 8) eV . The line widths of the measured transitions are compared to previous investigations on the same transitions and agreement is found. We give an experimental electron density scaling for the Stark width of these transitions for plasma diagnostic purposes.

Broadening of the 3s–3p transition in NeVIII

Abstract We report new measurements of the line width of the 3s–3p transition in NeVIII at 282.07 nm . These measurements were motivated by the discrepancy between a previous experimental investigation and various theoretical (semi-classical and especially quantum-mechanical) calculations. The experiments were carried out at the gas-liner pinch facility at Bochum. The discharge was operated with helium filling, in lieu of the hydrogen that was previously used, with additional small amounts of neon. To improve the overall accuracy in the present experiment, the electron density and temperature were obtained by collective Thomson scattering that is carried out simultaneously with the spectroscopic observation of the line profile. Line profiles were recorded at temperatures between 20 and 70 eV and the electron densities ranged from 1.8 to 5×10 18 cm −3 . The obtained widths are somewhat lower than the widths in the hydrogen plasmas but fit well with the previously found Z-scaling that is also supported by experiments in a low-pressure pulsed arc. Importantly, the discrepancy with quantum-mechanical calculations is not resolved.

Profiles of transitions in Ar VIII

Abstract The gas-liner pinch has been developed into a well-diagnosed plasma device most suitable for line shape studies of multiply ionized atoms. One special feature is the dual gas inlet system, which allows one to seed the final dense plasma column along its axis with test gas ions of interest at variable concentration. Temperatures and densities of electrons, main and test gas ions are measured by collective Thomson scattering employing a ruby laser. Reliable comparisons with theoretical line profiles are thus possible stimulating refined calculations. Furthermore, macroscopic instabilities can be induced by high test gas concentrations offering the possibility of line shape studies under such conditions. In this report line shape measurements of two Ar VIII lines embedded in dense hydrogen plasmas are presented at stable plasma conditions and compared with theoretical calculations.

Investigation of electron-proton drifts with Thomson scattering

Abstract We present a detailed investigation of asymmetric ion features of Thomson scattering spectra observed in the compression phase of a gas-liner pinch discharge. The asymmetries can be explained by drift velocities between electrons and ions, and an appropriate form factor taking into account drift velocities and different atomic species in the plasma can be fitted to the detected spectra. By varying the scattering volume in the midplane of the discharge the magnitude and direction of the drift are measured. These results and pinhole pictures of the whole plasma column show that the drifts are produced by Rayleigh-Taylor instabilities resulting in high radial current density components of the discharge current.

Profiles of the He i 23P-33D line at high densities

Abstract We report on measurements of Stark profiles of the He i 23P-33D transition at λ = 587.6 nm performed in a plasma of the gas-liner pinch discharge. The line shape and peak shift have been measured at densities in the range of ne = 0.5−2.5 × 1018 cm−3 and temperatures of kBT = 4–5.5 eV and compared with theoretical calculations using several approximations. The plasma parameters were determined independently by Thomson scattering. For higher densities the width of the He i 23P-33D transition grows nonlinearly with increasing density, and the shift of the line center shows a change of the sign from a blue to a red shift. For lower densities the Stark width and shift are excellently predicted by the calculations, but, as expected, for higher densities the agreement becomes worse, especially for the shift, when the validity range of the approximations is exceeded.

Spectral line profiles of n=4 to n=5 transitions in C IV, N V and O VI

We report on measurements of Stark profiles of the n=4 to n=5 transitions in the lithium-like ions C IV, N V and O VI. The measurements were performed in a plasma of the gas-liner pinch discharge where electron densities and temperatures are determined independently by 90 degrees Thomson scattering. The comparison of the experimental data with theoretical calculations shows good agreement for all spectra especially when ion dynamics effects are taken into account.