U. Stroth

Characterization of energy confinement in net-current free plasmas using the extended International Stellarator Database

International collaboration on development of a stellarator confinement database has progressed. More than 3000 data points from nine major stellarator experiments have been compiled. Robust dependences of the energy confinement time on the density and the heating power have been confirmed. Dependences on other operational parameters, i.e. the major and minor radii, magnetic field and the rotational transform , have been evaluated using inter-machine analyses. In order to express the energy confinement in a unified scaling law, systematic differences in each subgroup are quantified. An a posteriori approach using a confinement enhancement factor on ISS95 as a renormalizing configuration-dependent parameter yields a new scaling expression ISS04; . Gyro–Bohm characteristic similar to ISS95 has been confirmed for the extended database with a wider range of plasma parameters and magnetic configurations than in the study of ISS95. It has also been discovered that there is a systematic offset of energy confinement between magnetic configurations, and its measure correlates with the effective helical ripple of the external stellarator field. Full documentation of the International Stellarator Confinement Database is available at http://iscdb.nifs.ac.jp/ and http://www.ipp.mpg.de/ISS.

Major results from the stellarator Wendelstein 7-AS (Review Article)

Wendelstein 7-AS was the first modular stellarator device to test some basic elements of stellarator optimization: a reduced Shafranov shift and improved stability properties resulted in β-values up to 3.4% (at 0.9u2009T). This operational limit was determined by power balance and impurity radiation without noticeable degradation of stability or a violent collapse. The partial reduction of neoclassical transport could be verified in agreement with calculations indicating the feasibility of the concept of drift optimization. A full neoclassical optimization, in particular a minimization of the bootstrap current was beyond the scope of this project. A variety of non-ohmic heating and current drive scenarios by ICRH, NBI and in particular, ECRH were tested and compared successfully with their theoretical predictions. Besides, new heating schemes of overdense plasmas were developed such as RF mode conversion heating—Ordinary mode, Extraordinary mode, Bernstein-wave (OXB) heating—or 2nd harmonic O-mode (O2) heating. The energy confinement was about a factor of 2 above ISS95 without degradation near operational boundaries. A number of improved confinement regimes such as core electron-root confinement with central Te ≤ 7u2009keV and regimes with strongly sheared radial electric field at the plasma edge resulting in Ti ≤ 1.7u2009keV were obtained. As the first non-tokamak device, W7-AS achieved the H-mode and moreover developed a high density H-mode regime (HDH) with strongly reduced impurity confinement that allowed quasi-steady-state operation (τ ≈ 65 · τE) at densities (at 2.5u2009T). The first island divertor was tested successfully and operated with stable partial detachment in agreement with numerical simulations. With these results W7-AS laid the physics background for operation of an optimized low-shear steady-state stellarator.

Generation of blobs and holes in the edge of the ASDEX Upgrade tokamak

The intermittent character of turbulent transport is investigated with Langmuir probes in the scrape-off layer and across the separatrix of ASDEX Upgrade Ohmic discharges. Radial profiles of plasma parameters are in reasonable agreement with results from other diagnostics. The probability density functions of ion-saturation current fluctuations exhibit a parabolic relation between skewness and kurtosis. Intermittent blobs and holes are observed outside and inside the nominal separatrix, respectively. They seem to be born at the edge of the plasma and are not the foothills of avalanches launched in the plasma core. A strong shear flow was observed 1 cm radially outside the location where blobs and holes seem to be generated.

Strong terahertz radiation by beating of spatial-triangular lasers in a plasma

Resonant excitation of terahertz (THz) radiation by beating of two spatial-triangular laser beams having different frequencies and wave numbers but the same electric fields is proposed, where the ponderomotive force in the transverse direction is realized due to the beating and spatial variation of the lasers’ fields. This gives rise to a stronger transient transverse current due to a sharp gradient in the laser field, and subsequently THz radiation is excited resonantly in the presence of a periodic density structure. The present scheme yields the THz field ∼105 kV/cm and the efficiency ∼10−2 for the laser intensity ∼1014 W/cm2.

Nonlinear solitary waves (solitons) in inhomogeneous magnetized warm plasma with negative ions and nonisothermal electrons

The effect of ion temperature together with the concentration of low temperature nonisothermal electrons and their temperature contributions to the excitation of nonlinear solitary waves is studied in an inhomogeneous plasma having negative ions under the influence of a magnetic field. Unlike usual negative-ion-containing plasmas, the present plasma with nonisothermal electrons does not support the rarefactive solitons and only the compressive solitons evolve for two types of modes called the fast mode and slow mode. The fast compressive solitons corresponding to the fast mode propagate with higher amplitude in comparison with the slow compressive solitons, which occur only under a limiting condition on the obliqueness of the wave propagation with the magnetic field. The effect of ion temperature is found to be more significant when fewer negative ions are present in the plasma. Both solitons are found to be sensitive to the concentration and thermal motions of the low temperature electron species. With regard to the dependence of soliton characteristics on the magnetic field, it is observed that both solitons evolve with larger size when the wave propagates at a larger angle to the magnetic field and the magnetic field effect is found to narrow the solitons.

ρs scaling of characteristic turbulent structures in the torsatron TJ-K

The scalings of correlation lengths and times with the drift-scale parameter ρs and other dimensionless parameters are investigated. Using a novel 8×8 Langmuir probe matrix, the two-dimensional structure of drift-wave turbulence has been measured inside the confinement region of the toroidal low-temperature plasma in the TJ-K torsatron [N. Krause et al., Rev. Sci. Instrum. 73, 3474 (2002)]. Using five different gases from hydrogen to argon, ρs could be varied by a factor of 10. For small ion masses, the scalings of the microscopic parameters turn out to be close to the predictions from drift-wave turbulence, which lead to a gyro-Bohm scaling of the diffusivity. Including heavier ions turns the scaling to more Bohm-like. It is shown that ρs scaling studies carried out on the diffusivity can be misleading if residual dependencies on other parameters are present and the cross phase between poloidal electric field and density fluctuations is not constant as in the present case. The measured turbulent diffusiv...

Full-wave calculation of the O–X–B mode conversion of Gaussian beams in a cylindrical plasma

A finite-difference time-domain code is used to obtain the full-wave solution of the O–X mode conversion process for typical parameters of the TJ-II stellarator in a cylindrical geometry. This reduction of the complicated stellarator geometry to a cylindrical geometry is chosen since the conversion process occurs only over a limited radial plasma volume. In the calculations, Gaussian antenna beams are studied with the option of different beam waists in the poloidal and toroidal direction. Optimum conversion efficiency is found if the wavefront of the incident antenna beam is matched to the local curvature of the O–X conversion layer. Finally, the code is used to calculate the complete O–X–B conversion process into a Bernstein wave.

Bispectral experimental estimation of the nonlinear energy transfer in two-dimensional plasma turbulence

Experimental density and potential fluctuation data from a 2D probe array have been analysed to study the turbulent cascade in a toroidally confined magnetized plasma. The bispectral analysis technique used is from Ritz et al (1989 Phys. Fluids B 1 153) and Kim et al (1996 Phys. Plasmas 3 3998). It has been tested on Hasegawa–Wakatani turbulence simulation data and then applied to experimental data from the TJ-K torsatron. The analysis has been done in 2D wavenumber space. Analysis of simulated and experimental data recover the analytically known growth rate and dispersion relation for drift-wave turbulence. The density fluctuations, which at the realistic collisionality are advected as a passive scalar with the vorticity, show power transfer from large to small scales, while the spectral power in potential fluctuations, which represents the energy, is transferred as an inverse cascade to larger scales. This is the first experimental evidence for the dual turbulent cascade in a magnetized plasma.

Electron heat transport studies

Electron transport in fusion plasmas is intensively studied in coordinated experiments and great progress in physics understanding has been achieved during the past years. A threshold in normalized gradient explains most of the observations, both in steady-state and transient conditions. The results convincingly suggest that trapped electron modes (TEM) dominate electron transport at low and moderate collisionality, with electron heating. The stabilization of these modes at high collisionality predicted by theory is found in the experiments. Electron transport is then driven by the ion temperature gradient modes. At low collisionality, if TEM are stabilized by negative shear and Shafranov shift effects, electron internal transport barriers may develop.

Investigation of the parallel dynamics of drift-wave turbulence in toroidal plasmas

The three-dimensional structure of drift-wave turbulence is studied in the core of the toroidal low-temperature plasma in the torsatron TJ-K. The results are compared with simulations from the GEM3 turbulence code. In experiment and simulation, the dimensionless parameters are similar to those of fusion edge plasmas. Arrays with 64 probes are used to measure parallel wavenumbers, propagation velocities and the tilt of the turbulent structures with respect to the field line. A parallel wavelength of 15 m and a parallel velocity in between the ion-sound and the Alfven velocity confirm the three-dimensional nature of drift-wave turbulence. Quantitative agreement between experiment and simulation is found. The comparison with the drift-wave dispersion relation gives evidence for the coupling of the density perturbation to the shear-Alfven wave.