S. Büscher

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.

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.

Line shape measurements employing independently diagnosed pinch plasmas

Line shape studies for comparison with theoretical calculations rely on well-diagnosed plasmas. At Bochum we have utilized for some time the gas-liner pinch facility, where the plasma parameters are obtained independently from spectroscopic measurements by collective Thomson scattering. Both spectroscopy and laser scattering allow spatially resolved investigations. Characteristics and potentials, but also deficiencies, of the device are discussed for conditions where the plasma is stable or unstable. Specific attention is paid to recent investigations. The profile of the HeII Balmer-alpha line was studied at densities from 0.2×1018 to 3×1018 cm−3 and electron temperatures from 5 to 35 eV, where two different plasma formation regimes could be created: a high density and a high temperature mode. The Stark width shows a ne2/3 dependence, and it is compared with different theoretical approximations. Attempts are reported to study the broadening of the 2s-2p doublet of CIV, a possible benchmark experiment, whe...

Balmer-α transition of He II: measurements and calculations

We analyse measurements of the Stark-broadened profile of the He II Balmer-α transition at densities and temperatures up to ne = 3×1018 cm-3 and kBTe = 30 eV, respectively, and compare the results with calculations and existing experimental data. Theoretical spectra calculated according to the frequency fluctuation model (FFM) agree within the measured spectral range with the experimental ones when ion dynamics is taken into account, whereas calculations according to the model microfield method (MMM) underestimate the width. Concerning the Stark shift, the present experimental values are consistent with existing shifts measured at lower temperatures but due to the experimental error bars it is not possible to distinguish between predictions based on the MMM or the standard theory, even though these predictions differ by a factor of two.

Line profile measurements in a gas-liner pinch discharge

We report on recent line shape measurements performed in a gas-liner pinch discharge. The 2s3s−2s3p singlet and triplet transitions in Ne VII were measured which are of interest for astrophysical line formation calculations. The measured widths of the Lorentzian line shapes are in good agreement with semi-classical calculations which include electron quadrupole broadening. The Stark widths of the 3s 2S−3p2 P0 transitions in lithiumlike N V and F VII were measured optically thin. At high plasma densities (⩽×1019 cm−3) the widths which are dominated by electron collisional broadening show deviations from linear scaling. Furthermore, we investigated spectral line shapes of transitions between levels of high principal quantum numbers of lithiumlike ions. The measurement of the n=4 to n=5 transition in F VII, where forbidden transitions and various (n−1,l)-(n,l′) fine structure components overlap, show excellent agreement with a suitable line shape code. Finally, we present measurements of the line shapes of P...