D. Wünderlich

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.

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.

PIC code for the plasma sheath in large caesiated RF sources for negative hydrogen ions

Powerful negative hydrogen ion sources are required for heating and current drive at ITER. The physics of the production and extraction of high negative ion currents is much more complex than that 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 particle in cell code (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 with hydrogen. A small amount of positive Cs does not change the plasma sheath and the H− flux significantly.

The development of the radio frequency driven negative ion source for neutral beam injectors (invited).

Large and powerful negative hydrogen ion sources are required for the neutral beam injection (NBI) systems of future fusion devices. Simplicity and maintenance-free operation favors RF sources, which are developed intensively at the Max-Planck-Institut für Plasmaphysik (IPP) since many years. The negative hydrogen ions are generated by caesium-enhanced surface conversion of atoms and positive ions on the plasma grid surface. With a small scale prototype the required high ion current density and the low fraction of co-extracted electrons at low pressure as well as stable pulses up to 1 h could be demonstrated. The modular design allows extension to large source dimensions. This has led to the decision to choose RF sources for the NBI of the international fusion reactor, ITER. As an intermediate step towards the full size ITER source at IPP, the development will be continued with a half-size source on the new ELISE testbed. This will enable to gain experience for the first time with negative hydrogen ion beams from RF sources of these dimensions.

Magnetic filter field dependence of the performance of the RF driven IPP prototype source for negative hydrogen ions

The ITER neutral beam system requires a negative hydrogen ion beam of 48 A with an energy of 0.87 MeV and a negative deuterium beam of 40 A with an energy of 1 MeV. The beam is extracted from a large RF driven ion source with the dimension of 1.9 × 0.9 m2. An important role for the transport of the negative hydrogen ions to the extractor and the suppression of the co-extracted electrons is the magnetic filter field in front of the extractor. For the large ITER source the filter field will be generated by a current of up to 4 kA flowing through the first grid of the extractor. The extrapolation of the results obtained with the small IPP RF prototype source, where the filter field has a different 3D structure as it is generated by permanent magnets, is not straightforward. Furthermore, the filter field is by far not optimized due to the technical constraints of the RF source. Therefore, a frame that surrounds the ion sources and hosts permanent magnets was constructed for a fast and flexible change of the filter field. First results in hydrogen show that a minimum field of 3 mT in front of the extractor is needed for a sufficiently large number of extracted negative hydrogen ions, whereas sufficient co-extracted electron suppression is achieved by a source integrated magnetic field of more than 1.0 mTm.

Negative hydrogen ion transport in RF-driven ion sources for ITER NBI

The injection of energetic neutral atoms is a major component of plasma heating in fusion experiments. In order to fulfill the requirements of the ITER neutral beam injection (NBI), a RF-driven ion source for negative ions has been developed at the MPI f?r Plasma Physik (IPP Garching). Negative hydrogen ions are generated on a converter surface by impinging neutral particles and positive ions under the influence of magnetic fields and the plasma sheath potential. A 3D negative ion trajectory calculation including a Monte Carlo description of reactions and collisions with plasma particles was used to calculate the total and spatially resolved extraction probabilities for realistic field topologies and geometries of the large scale extraction system LAG. The experimentally observed increase in extracted ion current by the use of chamfered aperture collars agrees with the results of the ion transport simulation. Profiles of the extraction probability on the converter show that most of the extracted negative ions are created in the vicinity of the plasma grid apertures. These areas of intensified extraction probability are influenced by the magnetic field configuration. The ion extraction probability is affected by the long ranging magnetic filter field. The short ranging electron deflection field, however, which is generated by magnets near the converter surface, does not significantly influence the extraction probability.

A novel diagnostic technique for H¯(D¯) densities in negative hdyrogen ion sources

A new diagnostic method for the determination of negative hydrogen ion densities H− (D−), namely the Hα/Hβ line ratio method, is presented and applied to high power (P≈100 kW) negative ion sources operating at low pressure (p < 1 Pa). The basis of the method is the mutual neutralization process which enhances Balmer line radiation selectively. Detailed parameter studies carried out with a collisional radiative model show that the Balmer line ratio Hα/Hβ is very well suited to obtain negative ion densities in low temperature hydrogen plasmas with typical plasma parameters of Te ≈ 1–5 eV and ne ≈ 1016–5 × 1018 m−3. The Hα/Hβ line ratio method has the great advantage that it can be accomplished easily with the non-invasive and in situ plasma diagnostic method of optical emission spectroscopy. The method is applied to the RF ion sources of IPP Garching, currently developed for the neutral beam injection system of ITER. Line-of-sight averaged negative hydrogen densities in the range of 1016–1017 m−3 are measured close to the extraction system in hydrogen and deuterium discharge. Absolute values depend on pressure, power and caesium conditioning of the source. Negative ion densities measured with this novel technique in front of the grid correlate very well with the extracted negative ion current densities.

Neutral depletion in an H- source operated at high RF power and low input gas flow

This paper was published with an incorrect entry in table 5. The full corrected table 5 is shown in the accompanying PDF file

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.