E. Speth

Overview of the RF source development programme at IPP Garching

The development of a large-area RF source for negative hydrogen ions, an official EFDA task agreement, is aiming at demonstrating ITER-relevant ion source parameters. This implies a current density of 200?A?m?2 accelerated D? ions at a source filling pressure of ?0.3?Pa and an electron-to-ion ratio of ?1 from an extraction area similar to the positive-ion based sources at JET and ASDEX Upgrade and for pulse lengths of up to 1?h. The work is progressing along three lines in parallel: (i) optimization of current densities at low pressure and electron/ion ratio, utilizing small extraction areas (<0.01?m2) and short pulses (<6?s), in this parameter range the ITER requirements are met or even exceeded; (ii) investigation on extended extraction areas (<0.03?m2) and pulse lengths of up to 3600?s and (iii) investigation of a size-scaling on a half-size ITER plasma source. Three different test beds are being used to carry out these investigations in parallel. An extensive diagnostic and modelling programme accompanies the activities. The paper discusses the recent achievements and the status in these three areas of development.

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.

Spectroscopy—a powerful diagnostic tool in source development

The development of negative hydrogen ion sources for neutral beam systems is closely linked with an optimization of negative ion formation in hydrogen plasmas, which requires knowledge of the plasma parameters. Emission spectroscopy is introduced as a non-invasive and in situ diagnostic tool for line of sight averaged plasma parameters. Diagnostic lines and simplified analysis methods for a variety of plasma parameters, such as electron density and electron temperature, gas temperature, atomic and molecular hydrogen density, caesium densities (atoms and ions), and negative ion densities are identified and prepared for direct application. Emphasis is laid on results obtained in RF generated negative ion sources. Correlations of plasma parameters with extracted negative ion current densities are discussed. Stripping losses in the extraction system are quantified by using beam emission spectroscopy.

Physical performance analysis and progress of the development of the negative ion RF source for the ITER NBI system

For heating and current drive the neutral beam injection (NBI) system for ITER requires a 1 MeV deuterium beam for up to 1 h pulse length. In order to inject the required 17 MW the large area source (1.9 m × 0.9 m) has to deliver 40 A of negative ion current at the specified source pressure of 0.3 Pa. In 2007, the IPP RF driven negative hydrogen ion source was chosen by the ITER board as the new reference source for the ITER NBI system due to, in principle, its maintenance free operation and the progress in the RF source development. The performance analysis of the IPP RF sources is strongly supported by an extensive diagnostic program and modelling of the source and beam extraction. The control of the plasma chemistry and the processes in the plasma region near the extraction system are the most critical topics for source optimization both for long pulse operation as well as for the source homogeneity. The long pulse stability has been demonstrated at the test facility MANITU which is now operating routinely at stable pulses of up to 10 min with parameters near the ITER requirements. A quite uniform plasma illumination of a large area source (0.8 m × 0.8 m) has been demonstrated at the ion source test facility RADI. The new test facility ELISE presently planned at IPP is being designed for long pulse plasma operation and short pulse, but large-scale extraction from a half-size ITER source which is an important intermediate step towards ITER NBI.

Design of neutral beam system for ITER-FEAT☆

Abstract The neutral beam (NB) system in ITER-FEAT provides heating and current drive (H&CD) by two NB injectors, each delivering 16.7 MW of D 0 beam to the plasma at 1 MeV. The NB system retains the basic concept of the ITER 1998 design, but there are certain modifications that will be described: the beam transmission is improved by a four beam channel design of the neutralizer and the RID. Also the layout of the NB injector integrated in ITER allows both on- and off-axis current drive. The improved performance of the NB system is discussed from the system efficiency and the current drive capability points of view.

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.

Plasma diagnostic tools for optimizing negative hydrogen ion sources

The powerful diagnostic tool of optical emission spectroscopy is used to measure the plasma parameters in negative hydrogen ion sources based on the surface mechanism. Results for electron temperature, electron density, atomic-to-molecular hydrogen density ratio, and gas temperature are presented for two types of sources, a rf source and an arc source, which are currently under development for a neutral beam heating system of ITER. The amount of cesium in the plasma volume is obtained from cesium radiation: the Cs neutral density is five to ten orders of magnitude lower than the hydrogen density and the Cs ion density is two to three orders of magnitude lower than the electron density in front of the grid. It is shown that monitoring of cesium lines is very useful for monitoring the cesium balance in the source. From a line-ratio method negative ion densities are determined. In a well-conditioned source the negative ion density is of the same order of magnitude as the electron density and correlates with ...

Rf ion sources for fusion applications: design, development and performance

Abstract The use of rf ion sources in neutral beam injection (NBI) systems offers reduced maintenance and cost due to the absence of filaments. For the first time high-power rf positive ion sources have been put in operation recently in a real NBI system on a tokamak. The performance of the new system is equivalent to that of a conventional one with arc discharge sources. Due to the relevance for ITER, rf sources are also being investigated in context with negative ions: 8.5 mA/cm2 H− have been achieved so far in a non-optimised version.

Electron Cyclotron Resonance Heating in the Wendelstein VII-A Stellarator

Plasma build-up and heating of net-current-free plasmas in W VII-A was investigated by ECRH. Experiments were performed at two ECR-frequencies (28 and 70 GHz) and different heating scenarios were investigated such as first harmonic ordinary mode heating and second harmonic extraordinary mode heating. The basic effects predicted by theory, i.e. localized wave absorption and optical thickness of the plasma were verified. The electron heat conduction was found to be governed by neoclassical losses in the plasma core for high enough temperatures, whereas enhanced losses have to be assumed in the outer plasma regions. Generation of a target plasma with sufficient parameters to allow further heating by NBI was successfully demonstrated. Configuration studies showed a beneficial influence of small shear on the confinement, where internal currents have to be taken into account.

Low Pressure and High Power RF Sources for Negative Hydrogen Ions for Fusion Applications (ITER neutral beam injection) (invited)

The international fusion experiment ITER requires for the plasma heating and current drive a neutral beam injection system based on negative hydrogen ion sources at 0.3 Pa. The ion source must deliver a current of 40 A D(-) for up to 1 h with an accelerated current density of 200 Am/(2) and a ratio of coextracted electrons to ions below 1. The extraction area is 0.2 m(2) from an aperture array with an envelope of 1.5 x 0.6 m(2). A high power rf-driven negative ion source has been successfully developed at the Max-Planck Institute for Plasma Physics (IPP) at three test facilities in parallel. Current densities of 330 and 230 Am/(2) have been achieved for hydrogen and deuterium, respectively, at a pressure of 0.3 Pa and an electron/ion ratio below 1 for a small extraction area (0.007 m(2)) and short pulses (<4 s). In the long pulse experiment, equipped with an extraction area of 0.02 m(2), the pulse length has been extended to 3600 s. A large rf source, with the width and half the height of the ITER source but without extraction system, is intended to demonstrate the size scaling and plasma homogeneity of rf ion sources. The source operates routinely now. First results on plasma homogeneity obtained from optical emission spectroscopy and Langmuir probes are very promising. Based on the success of the IPP development program, the high power rf-driven negative ion source has been chosen recently for the ITER beam systems in the ITER design review process.