The xenon spectrum, excited in ECR- and NBI-heated plasmas with central electron densities of around 10(20) m(-3) and central electron temperatures from 0.7 to 2.5keV, has been studied photoelectrically with a multichannel grazing-incidence spectrometer. Besides numerous well-known lines of Zn- and Cu-like Xenon, more than 50 additional lines which have not yet been published in the literature have been found and partly identified,.

Electron cyclotron resonance heating (ECRH) at the W7-AS stellarator is performed two systems of comparable heating power at 70 GHz (0.8 MW, 3 s) and 140 GHz (0.9 MW, 0.4 s). Experiments with 140 GHz open a new parameter window with high density operation up to 1.1 × 1020 m−3. H mode transitions were observed in high density discharges at 2.5 T and at lower density at 1.25 T. The operational window for the stellarator H mode with emphasis on the density and power threshold and the influence of gas puffing on the H transitions are discussed. Perturbation experiments with modulated ECRH power were performed to determine the heat transport and the power deposition profile. Density control is achieved in combined heating with neutral beam injection (NBI) despite the beam particle fuelling, whereas with NBI alone a steady density rise is observed. The impact of combined heating on the impurity confinement was investigated. Electron cyclotron current drive (ECCD) was studied in different magnetic field configurations and the influence of trapped particles on the ECCD efficiency was examined experimentally and compared with theory.

For a limiter defined SOL, as given by small values of the rotational transform, the edge topology of W7-AS is characterized by large flux bundles distributed regularly over the poloidal angle. These bundles are poloidally decoupled to a good approximation, as indicated by 2D resolved Langmuir probe data. Therefore, for iota less-than-or-equal-to 0.4, a 1D radial plasma model is a reasonable first step to provide a qualitative description of the transport in the W7-AS boundary layer. The toroidal variation of the flux bundle geometry is taken into account by averaging the transport equations along the corresponding field lines. The effects of the neutral gas sources and the sensitivity of the diffusion coefficient to the unknown T(i) profile are discussed. A iota scan of the diffusion coefficient shows a minimum at iota congruent-to 0.35, where smooth magnetic surfaces exist throughout the SOL. This suggests that topological effects related to perturbations at the ''natural'' 5/m resonances may be responsible for the observed iota dependence of the diffusion coefficient.

Optimum confinement is realized in WENDELSTEIN 7-AS (low shear modular stellarator, R = 2 m, a congruent-to 0.18 m) by wall conditioning and by properly adjusting the parameters determining the magnetic field configuration. In particular low order rational values of the rotational transform have to be excluded from the confinement region or sufficient shear must be established by internal currents. The effective heating of net current free plasmas by ECRF (P less-than-or-similar-to 0.8 MW, 70 GHz) and neutral beam injection (NBI, P less-than-or-similar-to 1.5 MW, 45 kV) involves different plasma parameters and transport regimes. Stationary plasmas are generally produced by ECRF, whereas density and impurity control is a severe problem during NBI. This has initiated different kinds of impurity and particle control scenarios (carbonization, boronization and edge cooling). Thus, < beta > less-than-or-similar-to 1.1% (1.25 T) could be achieved. An extended parameter range with electron temperatures of 200 eV less-than-or-equal-to T(e) less-than-or-equal-to 3 keV, ion temperatures of 100 eV less-than-or-equal-to T(i) less-than-or-equal-to 0.7 keV and electron densities of 10(19) less-than-or-equal-to n(e) less-than-or-equal-to 3 . 10(20) m-3 was accessible. The characteristics of the energy confinement (confinement times up to 35 ms are observed in low power/low density ECRF heated and up to 25 ms in high power/high density NBI heated plasmas) and the particle and impurity transport are described and related to the specific heat and particle sources. The investigations comprise the analysis of electron and ion heat conductivity, particle transport modelling based on H-alpha measurements at relevant locations around the torus and impurity transport studies by laser blow-off experiments. The influence of the ambipolar electric field is discussed.

The plasma edge region of the W 7-AS stellarator has a complex structure due to "natural" islands. Guiding centre orbit calculations by a Monte Carlo code indicate that there are locally fixed patterns of enhanced particle flow to the wall. Power loss simulations by the DEGAS code satisfactorily reproduce measured power depositions on the main limiters, but in the total power balance including radiation a significant deficiency is found. The role of limiters with respect to impurity fluxes is discussed on the basis of spectroscopic data. A long-term collector probe analysis shows inhomogeneous poloidal and toroidal distributions of deposited materials which are influenced by plasma shots as well as conditioning discharges.

Spectra of helium hydride were observed after neutralization of a mass-selected HeH+ beam. The molecules were produced in a fast beam, and so a special setup had to be used to avoid Doppler broadening and a careful calibration procedure had to be applied to determine the line positions and linewidths. In an earlier paper, we reported the first observation of a discrete spectrum of helium hydride, which was discovered by means of an emission band near 8000 Å. In this paper, a detailed analysis of this band for all four stable isotopic mixtures is given. For the deuterides several vibrational bands were observed, which allowed equilibrium molecular constants to be determined. These constants agree with the results of recent ab initio calculations. The similarity of these constants to those of HeH+ in the ground state confirms that the observed states are Rydberg states. Comparison of the molecular constants for different isotopic mixtures shows deviations from the Born–Oppenheimer approximation. Born–Oppenheimer breakdown parameters were derived. Both the upper and lower states show predissociation to the repulsive ground state of helium hydride. The line intensities give evidence of a dependence of the electronic transition moment on the interatomic distance.

The complete series of stable Rydberg states (14 ⩽ n ⩽ 65) of H 3 was studied for the first time by using laser excitation of a fast (15 kV) molecular beam and detection by field ionization. The observed spectrum is particularly simple because its rotational and vibrational temperatures are zero kelvin as a result of non-radiative decay processes. Furthermore, it was shown that these high Rydberg states are long-lived and their lifetime depends sensitively on n , l and external electric fields. The ionization potential and quantum defect of this series are discussed taking l -uncoupling into account.