[en] In negative hydrogen ion sources a too high amount or a too rapid increase in co-extracted electrons can prevent achieving the required negative ion current density and restrict the pulse duration. One important measure for reducing and stabilizing the co-extracted electron current is a magnetic filter field. The half-ITER-size NNBI test facility ELISE—in which the filter field is created by a current flowing through the extraction system—is used for performing experiments on the reaction of the source performance on modifying the filter field topology; external magnet bars are attached to the source in different polarities and extensive parameter variations have been done in volume and surface operation. A significant correlation of the extracted ion and electron currents and their temporal stability with the field topology is seen: with the external magnets strengthening the standard filter field a strong reduction of the co-extracted electrons and additionally a reduced increase in these electrons during the pulses is observed. In this configuration it was possible to perform the very first 1 h deuterium pulse in ELISE—an important step toward fulfilling the requirements to the ion source for ITER NBI. (paper)

[en] Highlights: • A test stand for the high vacuum pumping system of the NBI at W7-X is presented. • In W7-X large titanium sublimation pumps (TSPs) will be operated using AC. • AC ohmic heating allows reliable operation of TSPs in the stray B field of W7-X. • The effect of the frequency and waveform of the AC current has been analysed. • AC-operated TSPs in B field perform similarly to DC-operated TSPs at ASDEX-upgrade. - Abstract: A neutral beam injection (NBI) system is being built for the Stellarator experiment Wendelstein 7-X (W7-X) currently under construction at IPP Greifswald. The NBI system consists of two injectors which are essentially a replica of the system present in the Tokamak experiment ASDEX-Upgrade at IPP Garching. A vacuum system with high pumping speed and large capacity is required to ensure proper vacuum conditions in the neutral beam line. For this purpose, large titanium sublimation pumps (TSP) are installed inside the NBI boxes, consisting of 4 m long hanging wires containing Ti and the surrounding condensation walls. The wires are DC ohmically heated up with 142 A to Ti sublimation temperature. A TSP system has been operated since many years in the AUG-NBI system, sublimating Ti in the pauses between the plasma discharges, when no magnetic field is present. However, at W7-X the superconducting coils generate a magnetic field permanently during experimental campaigns, whose stray B field with a maximum of 30 mT, affects the TSPs. Operated with DC, the wires would be deflected against the surrounding panels due to the Lorentz force. A simple possible solution is heating with AC, which reduces the wire deflection amplitude, inducing a risky wire oscillation. The feasibility of the AC operation in an equivalently strong B field such as the stray B field around W7-X has been demonstrated in a test stand for different AC waveforms and frequencies. Several test campaigns have shown no qualitative difference in the pumping properties between AC and DC operation of the TSP and no critical dynamic behaviour of the wires

[en] 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 x 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.

[en] The test facility BATMAN was dedicated since its start in 1996 to the development of radio frequency driven negative hydrogen ion sources for ITER NBI with focus on formation and extraction of negative ions, technological developments and improved concepts. During 2017, the test facility was upgraded in order to replace the former extraction system with a new ITER-like extraction system comparable in size to one ITER beamlet group and having the option to flow a current through the grid creating a magnetic filter field as foreseen for the ITER sources. In addition, the beam diagnostics has been extended: beam emission spectroscopy is located at two positions from the grounded grid (26 cm and 129 cm) with spatial resolution in vertical direction. A newly developed tungsten wire calorimeter is placed just 19 cm downstream the grounded grid to provide quantitative measurements of individual beamlets, whereas the tungsten wire calorimeter at 180 cm distance is still in use for qualitative beam profile diagnostics. Together with a beam dump calorimeter with a crosswise arrangement of thermocouples, beam divergence and uniformity can be studied. This is accompanied by modeling the beamlet transport from the extraction system up to the calorimeter. Results from the first experimental campaign are reported, being very promising for detailed understanding of features measured on large beams.

[en] 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 fuer 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.

[en] The development at IPP of a large-area rf source for negative hydrogen ions, an official EFDA task agreement, aims to demonstrate ITER-relevant source parameters. This implies a current density of 20 mA cm^-2 accelerated D^- at a source-filling pressure of <0.3 Pa, an electron to ion ratio of <1, and for pulse lengths of up to 1 h. The principle suitability concerning current density, pressure, and electron content has been demonstrated with the test facility Bavarian Test Machine for Negative Ions but with only small extraction area (70 cm²) and for pulse length of <6 s. The further development concentrates now on long pulse operation at the test stand Multi-Ampere Negative Ion Test Unit (MANITU), which became operational this spring. For source size extension from 70 to 1000 cm² MANITU and a third test facility, currently under development, called RADI will be used. This article will report on the latest results of the work in progress. A critical issue for ITER is reliable source operation at high current densities. Therefore the procedure used to obtain in a reproducible manner source operation at the ITER target values will be detailed and discussed

[en] Each of the neutral beam injectors in the experimental devices ASDEX Upgrade and Wendelstein 7-X can be equipped with up to four positive ion sources with an injected neutral beam power of 2.5 MW per source. For the conditioning of the system, a movable calorimeter can be placed in the path of the neutral beam to dump the heat load. The main heat load on the calorimeter is absorbed by a set of calorimeter panels (CP). Its design, originally from 1988, specified the lifetime of the CPs to 25,000 heating cycles and ruled out any plastic deformation due to thermal cyclic loading. This was predicted with the tools available at the time but years of operation show that plastic deformation of the CPs already occurs at the very beginning of the calorimeter full power operation. After some years mechanical fatigue has led a few times to water leaks and consequently to NBI system shutdowns for repair. For this reason, careful inspection of the CPs is performed frequently and deformed CPs are exchanged for new ones. This paper presents the analysis of the causes that lead to the failure of the old CPs and proposes an optimized design for new CPs to be manufactured in the near future to substitute the old ones. The new design aims at minimizing mechanical fatigue from thermal cycling by reducing maximum surface temperature and thermo-mechanical stresses and improving material properties.

[en] IPP Garching is developing H^-/D^- RF ion sources for the ITER neutral beam system. On the MANITU testbed the experiments are focussed on long pulse H^-/D^- beam extraction with a 100 kW prototype source. The negative ion production is based on surface conversion of atoms and positive ions on Caesium layers. In long pulses with H^- beam extraction the ion currents were stable but with too high fraction of co-extracted electrons. The electron current could be lowered considerably by avoiding copper impurities from the Faraday screen in the plasma which was achieved by coating of the inner surfaces of the source with Molybdenum. A positive bias potential with respect to the source applied to the plasma grid, the bias plate or to a metal rod installed near the plasma grid enables regulation of the electron current during long pulses. In this way low values consistent with the ITER requirements can be achieved without significant loss of ion current.

[en] IPP Garching is currently developing a RF driven negative ion source for the ITER neutral beam injection system as an alternative to the presently foreseen filamented source. The RF source has demonstrated the ITER requirements concerning negative ion current and electron/ion ratio at the required source pressure for small pulse length and small extraction area. The next goals are the extension to long pulses and large area sources at two further test facilities. The Half-Size source at the test bed RADI, recently commissioned at the IPP Garching, has roughly half the size of the ITER source. It is devoted to demonstrate the required plasma homogeneity of a large RF driven source, to test an ITER relevant RF circuit and to show the scalability of the IPP RF source. Having no large area extraction the source performance will be demonstrated with an extensive diagnostic and modeling program. This paper will present the results of the first plasma discharges and describe the main technical features of RADI. (author)

[en] The ITER neutral beam system will be equipped with radio-frequency (RF) negative ion sources, based on the IPP Garching prototype source design. Up to 100 kW at 1 MHz is coupled to the RF driver, out of which the plasma expands into the main source chamber. Compared to arc driven sources, RF sources are maintenance free and without evaporation of tungsten. The modularity of the driver concept permits to supply large source volumes. The prototype source (one driver) demonstrated operation in hydrogen and deuterium up to one hour with ITER relevant parameters. The ELISE test facility is operating with a source of half the ITER size (four drivers) in order to validate the modular source concept and to gain early operational experience at ITER relevant dimensions. A large variety of diagnostics allows improving the understanding of the relevant physics and its link to the source performance. Most of the negative ions are produced on a caesiated surface by conversion of hydrogen atoms. Cs conditioning and distribution have been optimized in order to achieve high ion currents which are stable in time. A magnetic filter field is needed to reduce the electron temperature and co-extracted electron current. The influence of different field topologies and strengths on the source performance, plasma and beam properties is being investigated. The results achieved in short pulse operation are close to or even exceed the ITER requirements with respect to the extracted ion currents. However, the extracted negative ion current for long pulse operation (up to 1 h) is limited by the increase of the co-extracted electron current, especially in deuterium operation. (paper)