A. Weller

Physics optimization of stellarators

The theoretical and experimental development of stellarators has removed some of the specific deficiencies of this configuration, viz., the limitations in β, the high neoclassical transport, and the low collisionless confinement of α particles. These optimized stellarators can best be realized with a modular coil system. The W7‐AS experiment [Plasma Phys. Controlled Fusion 31, 1579 (1989)] has successfully demonstrated two aspects of advanced stellarators, the improved equilibrium and the modular coil concept. Stellarator optimization will much more viably be demonstrated by W7‐X [Plasma Physics and Controlled Fusion Research, Proceedings of the 12th International Conference, Nice, 1988 (IAEA, Vienna, 1989), Vol. 2, p. 369], the successor experiment presently under design. Optimized stellarators seem to offer an independent reactor option. In addition, they supplement, in a unique form, the toroidal confinement fusion program, e.g., energy transport is anomalous in stellarators too, but possibly more easily understandable in the frame of existing theoretical concepts than in tokamaks.

Electron Cyclotron Heating for W7-X: Physics and Technology

The Wendelstein 7X (W7-X) stellarator (R = 5.5 m, a = 0.55 m, B < 3.0 T), which at present is being built at Max-Planck-Institut für Plasmaphysik, Greifswald, aims at demonstrating the inherent steady-state capability of stellarators at reactor-relevant plasma parameters. A 10-MW electron cyclotron resonance heating (ECRH) plant with continuous-wave (cw) capability is under construction to meet the scientific objectives. The physics background of the different heating and current drive scenarios is presented. The expected plasma parameters are calculated for different transport assumptions. A newly developed ray-tracing code is used to calculate selected reference scenarios and optimize the electron cyclotron launcher and in-vessel structure. Examples are discussed, and the technological solutions for optimum wave coupling are presented. The ECRH plant consists of ten radio-frequency (rf) modules with 1 MW of power each at 140 GHz. The rf beams are transmitted to the W7-X torus (typically 60 m) via two open multibeam mirror lines with a power-handling capability, which would already satisfy the ITER requirements (24 MW). Integrated full-power, cw tests of two rf modules (gyrotrons and the related transmission line sections) are reported, and the key features of the gyrotron and transmission line technology are presented. As the physics and technology of ECRH for both W7-X and ITER have many similarities, test results from the W7-X ECRH may provide valuable input for the ITER-ECRH plant.

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 0.9 T). 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 ≤ 7 keV and regimes with strongly sheared radial electric field at the plasma edge resulting in Ti ≤ 1.7 keV 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 2.5 T). 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.

First island divertor experiments on the W7-AS stellarator

1. Abstract In the past, under limiter conditions, it has been impossible to produce high-power, highdensity, quasi-stationary neutral beam injection (NBI) discharges in W7-AS. Such discharges tended to evince impurity accumulation, lack of density control and subsequent radiation collapse (Normal Confinement). Presently, W7-AS is operating with a modular, open island divertor similar to that foreseen for W7-X. The divertor enables access to a new NBI heated, high density (ne up to 4·10 20 m -3 ) operating regime (High Density H-mode). It is extant above a threshold density, and is characterized by flat density profiles, high energyand low impurity confinement times and edge-localized radiation. The HDH-mode shows strong similarity to ELM-free H-mode scenarios previously observed in W7-AS, but in contrast to these avoids impurity accumulation. These new features enable full density control and quasi steady-state operation over many confinement times (at present only technically limited by the availability of NBI) also under conditions of partial detachment from the divertor targets. In HDH-mode, even in attached discharges, the divertor target load is considerable reduced. This is mainly due to favourable upstream conditions (higher nes), edge localized radiation and increased power deposition width. The benefits of the HDH-mode do not restrict only to hydrogen plasmas. They also occur ‐ albeit in a modified manner ‐ in deuterium plasmas. Undoubtedly, there are clear isotope effects between hydrogen and deuterium discharges. The results obtained in W7-AS render good prospects for W7-X and support the island divertor concept as a serious candidate for devices with magnetic islands at the edge. 2. Results Fig. 1 summarizes the behaviour of the energy confinement time E =W/Pabs, the normalized radiated power Prad/Pabs, and separatrix density nes obtained from quasi-stationary discharges with Pabs=1.4 MW as a function of the line-averaged density ne. E-values in NC follow the scaling E ISS95 =0.26· a 0.4 ·Bt 0.83 ·a 2.21 ·R 0.65 ·ne 0.51 ·Pabs -0.59 , [2], whereas for the HDH-mode one finds E ~ 2· E ISS95 . P rad /P abs grows smoothly with ne until partial plasma detachment, where a jump in the normalized radiated power occurs. The separatrix density n es increases sharply at the NC HDH-mode transition point, then continues to climb with ne and saturates

Survey of magnetohydrodynamic instabilities in the advanced stellarator Wendelstein 7-AS

Magnetohydrodynamic (MHD) instabilities in the Wendelstein 7-AS stellarator (W7-AS) [G. Grieger et al., Phys. Fluids B 4, 2081 (1992)] are characterized experimentally in various plasma parameter regimes and heating scenarios. The observations are compared with theoretical predictions for particular cases. In the high-β range (〈β〉⩽2%) no clear evidence of a stability β-limit could be found yet. In the lower β regime fast particle driven global Alfven modes are the most important instabilities during neutral beam injection (NBI). Besides of coherent modes with almost no effect on the plasma performance additional Alfven modes appear at higher frequencies up to 400 kHz, which show nonlinear phenomena-like bursting, frequency chirping, and MHD induced energy and fast particle losses. The activity of edge localized modes (ELMs) is investigated in NBI heated discharges. The issue of current driven instabilities and their potential stabilization by a stellarator field has been investigated with regard to the de...

Detailed investigation of the vacuum magnetic surfaces on the W7-AS stellarator

The fluorescent rod technique is a rather new and sensitive method of experimentally mapping vacuum magnetic surfaces in toroidal confinement devices. It was applied to the modular low shear W7-AS stellarator before starting plasma experiments. Magnetic island widths down to less than 5% of the plasma radius could be recorded, these being caused by relative perturbation field amplitudes of down to 10-5. The almost complete Fourier spectrum of the resonant perturbation field components is derived for values of the rotational transform iota between 1/4 and 2/3. The results obtained on W7-AS demonstrate once more that the existence of magnetic islands is a common phenomenon in toroidal confinement devices, and that the assumption of closed, nested flux surfaces in transport theories may not always be justified. On the basis of the measured perturbation field spectrum, the enhancement of the radial transport caused by the vacuum field islands is estimated with a simple transport model. The predictions of the model are compared with the experimental dependence of the diamagnetic energy content on iota in W7-AS

The source of polarized heavy ions (PSI) at the Heidelberg MP tandem

Abstract The new polarized source at the Heidelberg MP tandem is described. The source produces beams of 6 Li , 7 Li and 23 Na ions, which are preaccelerated to 150–200 keV and injected into the MP tandem. The results of the first test runs are presented. A 44 MeV 7 Li beam has been produced and the polarization was detected in a nuclear reaction. A Na beam will be accelerated during one of the next few runs. Improvements of the polarization by laser optical pumping are discussed.

The W7-X project: scientific basis and technical realization

The Wendelstein 7-X Stellarator (W7-X) is the next step device in the stellarator line of IPP Garching. A new branch of IPP is being built at Greifswald, Germany, to house W7-X. The design of W7-X is based on physics principles, which are discussed in the light of experimental results from the W7-AS stellarator. The experiment aims at demonstrating the inherent steady state capability of stellarators at reactor relevant plasma parameters and is therefore equipped with a modular superconducting twisted coil system. The 3D magnetic configuration of W7-X asks for a special divertor solution for steady state heat removal and decoupling of the vessel wall from the plasma. The status of the design and construction of W7-X including heating systems, divertor and diagnostics is presented.

MHD study of the reactor-relevant high-beta regime in the Large Helical Device

In the Large Helical Device, the volume averaged beta value βdia of 5%, which is the highest value in all heliotron/stellarators and relevant to the reactor requirement, was achieved by optimizing the magnetic configuration from the viewpoint of magneto-hydrodynamic (MHD) characteristics, transport and heating efficiency of the neutral beam. This beta value was instantaneously obtained by pellet injection and maintained for more than 10τE, whereas the steady-state plasma with a maximum βdia of 4.8% was sustained for 85τE by gas-puff fueling. While it is theoretically predicted that stochastization of the peripheral magnetic field structure develops with an increment of βdia, no serious degradation of the global confinement has been observed in the present βdia range. The several low-order MHD activities located in the periphery were enhanced with the beta value and sometimes affect the local profiles. The amplitude of the mode in the periphery strongly depends on the magnetic Reynolds number, which is close to that of the growth rate and/or the radial mode width of the resistive interchange instability.

Maximum entropy based reconstruction of soft X-ray emissivity profiles in W7-AS

The reconstruction of 2-D emissivity profiles from soft X-ray tomography measurements constitutes a highly underdetermined and ill-posed inversion problem, because of the restricted viewing access, the number of chords and the increased noise level in most plasma devices. An unbiased and consistent probabilistic approach within the framework of Bayesian inference is provided by the maximum entropy method, which is independent of model assumptions, but allows any prior knowledge available to be incorporated. The formalism is applied to the reconstruction of emissivity profiles in an NBI heated plasma discharge to determine the dependence of the Shafranov shift on beta , the reduction of which was a particular objective in designing the advanced W7-AS stellarator