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 20 mA/cm 2 accelerated Dions at a source filling pressure of ≤ 0.3 Pa and an electron to ion ratio of ≤ 1 from a PINI-size extraction area for pulse lengths of up to 1 hour. The work is progressing along three lines in parallel: (i) optimisation of current densities at low pressure and electron/ion ratio, utilising small extraction areas (< 100 cm 2) and short pulses (< 10 s); (ii); investigation of extended extraction areas (< 300 cm 2) and pulse lengths of up to 3600 s; (iii) investigation of a size-scaling on a half-size ITER plasma source. Three different testbeds are being used to carry out those investigations in parallel. An extensive diagnostic and modelling programme accompanies the activities. The paper contains the recent achievements and the status of preparations in those four areas of development

Development of negative hydrogen ion sources for neutral beam systems is closely linked with an optimisation 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 as well as cesium 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.

These lecture notes are intended to give an introductory course on plasma spectroscopy. Focusing on emission spectroscopy, the underlying principles of atomic and molecular spectroscopy in low temperature plasmas are explained. This includes choice of the proper equipment and the calibration procedure. Based on population models, the evaluation of spectra and their information content is described. Several common diagnostic methods are presented, ready for direct application by the reader, to obtain a multitude of plasma parameters by plasma spectroscopy.

Abstract. Powerful negative hydrogen ion sources are required for heating and current drive at ITER. The physics of production and extraction of high negative ion currents is much more complex than for positive ions. One of the most relevant parameters is the shape of the plasma sheath, which determines the velocity of surface produced negative ions and thus the probability of the ions to reach the extraction system. In order to investigate the influence of hydrogen atoms, positive and negative hydrogen ions and positive caesium ions on the plasma sheath, a 1d3v PIC code for the plasma close to the extraction system has been developed. For typical plasma parameters of such ion sources, surface conversion of impinging atoms is the main negative ion production channel, while conversion of positive ions plays a minor role. Due to the formation of a potential minimum close to the surface, the emission of negative ions into the plasma is space charge limited. As a consequence, the flux of negative ions can be increased only by increasing the density of positive hydrogen ions. At identical plasma parameters, an isotope effect is determined by the mass of the particles only, resulting in lower fluxes of negative deuterium ions compared to hydrogen. A small amount of positive Cs does not change the plasma sheath and the H − flux significantly. Plasma sheath in large caesiated negative hydrogen ion sources2

A fully automated Langmuir probe system capable of operating simultaneously with beam extraction has been developed and commissioned for the negative hydrogen ion source testbeds at IPP Garching. It allows the measurement of temporal and spatial distributions of the plasma parameters within a single plasma pulse (<5 s). This system can operate even in the presence of multi-harmonic RF interference due to a novel transformer-based RF compensation system. Analysis methods of the probe data are described in the paper along with a discussion of errors. Measurements of the plasma parameters for RF powers (30-80 kW) and source pressures (0.3-0.8 Pa) both in plasma generation region and near the plasma grid have been carried out. The plasma generation region has both a high density (>10 18 m −3 ) and hot (T e > 10 eV) plasma with bi-Maxwellian electron energy distribution at low pressures. The plasma found near the plasma grid is very different being of low density ( 10 17 m −3 ) and very cold (T e < 2 eV). This plasma is also strongly influenced by the presence of caesium, the potential of the plasma grid, and if an ion beam is extracted from the source. Caesium strongly reduces the plasma potential of the source and enhances the negative ion density near the plasma grid. Extracting an ion beam is observed to reduce the electron density and increase the potential near the plasma grid. Applying a potential greater than the plasma potential to the plasma grid is found to significantly decrease the electron density near the plasma grid.