[en] The disseration deals with the re-emission of hydrogen from pure or borated graphites bombarded with high-energy hydrogen ions at temperatures above 800 K. The re-emitted particles have been detected by mass spectrometry applying the technique of partial pressure variation with the molecules H₂ and CH₄, or D₂, HD and CD₄, respectively. (orig.)

[en] Radio frequency ion sources used in neutral beam injection systems (NBI) of fusion machines are currently supplied by self-excited RF generators. These generators have both a low power efficiency and a limited frequency stability, therefore transistorized amplifiers are being considered for the power supply of the next generation of RF sources. A 75 kW generator, originally designed for broadcasting, has been tested with a negative ion source. High operational reliability and a very good matching to the plasma load has been demonstrated. These results make this generator type a very promising candidate for future NBI systems

[en] Highlights: • NBI is a candidate for a cw tokamak DEMO due to its high current drive efficiency. • The plug-in efficiency must be improved from the present 20–30% to more than 50%. • A suitable candidate is a photo neutralizer with almost 100% neutralization efficiency; basic feasibility studies are underway. • Cw operation with a large availability puts rather high demands on source operation with some safety margins, especially for the components with high power density loads (source back plate and extraction system). • Alternatives to the present use of cesium are under exploitations. - Abstract: The requirements for the heating and current drive systems of a fusion power plant will strongly depend on the DEMO scenario. The paper discusses the R and D needs for a neutral beam injection system — being a candidate due to the highest current drive efficiency — for the most demanding scenario, a steady state tokamak DEMO. Most important issues are the improvement of the wall-plug efficiency from the present ∼25% to the required 50–60% by improving the neutralization efficiency with a laser neutralizer system and the improvement of the reliability of the ion source operation. The demands on and the potential of decreasing the ion source operation pressure, as well as decreasing the amount of co-extracted electrons and backstreaming ions are discussed using the ITER requirements and solutions as basis. A further concern is the necessity of cesium for which either the cesium management must be improved or alternatives to cesium for the production of negative ions have to be identified

[en] An alternative concept of the residual ion dump for the ITER NBI system is presented. The incentive is to avoid the inherent risk of the in-line electrostatic ion removal concept, which is presently foreseen in the NBI design: beam-blocking by enhanced reionization losses and potential amplification of electron currents by electrostatic acceleration. It is proposed to use instead a conventional magnetic ion removal system with remote ion dumps. This method of removing the residual ions has been proven in almost all injection systems world-wide over the last two decades. First calculations show that the maximum power load can be kept below 15 MW/m², sufficiently low to use conventional dump plate designs

[en] For heating and current drive the ITER neutral beam system requires negative hydrogen ion sources capable of delivering above 40 A of D^- ions from a 1.5 x 0.6 m² source for up to one hour pulses with an accelerated current density of 200 A/m². In order to reduce the losses by electron stripping in the acceleration system and the power loading of the grids, the source pressure is required to be 0.3 Pa at an electron/ion ratio ≤1. The development of the source was initially concentrated on filamented arc sources as described in the ITER reference design. As an interesting alternative, IPP Garching is currently developing a RF driven negative hydrogen ion source. RF sources offer substantial advantages: they are basically maintenance-free in operation being quite beneficial for the remote handling requirements of ITER; in contrast to the arc sources having a limited filament lifetime no regular maintenance periods are necessary. In 2005 the IPP RF source has demonstrated its principal suitability for the ITER NBI system: current densities (330 A/m² H^-/230 A/m² D^-) in excess of the ITER requirements have been already achieved on the small test facility 'BATMAN' (Bavarian Test Machine for Negative Ions) at the required source pressure (0.3 Pa) and electron/ion ratio (<1), but with only small extraction area (0.007 m²) and limited pulse length (<5 s). The development concentrates now on the extension of source size and extraction area as well as on the extension of the pulse length. This is done at two other test facilities in parallel: at the large test facility 'MANITU' (Multi Ampere Negative Ion Test Unit) the extraction area can be extended up to 0.03 m² and the pulse length up to 3600 s. In order to demonstrate the required homogeneity of a large RF plasma source as well as the operation of an ITER relevant RF circuit, a so called 'half-size source' - with roughly the width and half the height of the ITER source - was designed and went into operation on a dedicated plasma source test bed ('RADI'). An extensive diagnostic and modelling programme is accompanying those activities. The paper presents the latest results of the RF source development, with an emphasis on deuterium operation and Cs dynamics, on the first results of the operation of the half size ITER source and on the status of the long pulse operation. (author)

[en] Erosion of graphite by radiation enhanced sublimation due to neutron bombardment with an energy spectrum such as expected in a fusion reactor is estimated theoretically. Carbon atoms released from their lattice sites in collision cascades initiated by the neutron bombardment are able to diffuse to the graphite surfaces and sublime thermally. The production rate of interstitial carbon atoms is calculated from neutron irradiation damage theory. The calculations result in radiation enhanced sublimation yields of several 10^-4 C/n in the 400 to 1800 K temperature range. The yields are about one or two orders of magnitude larger than the forward physical sputtering yield of neutrons due to collision cascades in the near surface layer. Concerning the erosion of graphite at the vessel walls and the impurity introduction into the plasma in fusion research, both physical sputtering and radiation enhanced sublimation due to fast neutron irradiation are nearly three orders of magnitude smaller than ion and charge exchange neutral atom sputtering. (orig.)

[en] Measurements of the reemitted flux of deuterium implanted with 8 keV into graphite at temperatures between 1100 and 1500 K have been performed. In this temperature range only D₂ is released, no hydrocarbon molecules are found. The flux of deuterium reemitted at fluences where the implanted region is saturated, decreases with increasing temperature. This behaviour can be explained by diffusion of deuterium into inner regions of the graphite target. (orig.)

[en] For the new radial injector of the Wendelstein 7-AS stellerator, two medium size 40 kW RF sources ('W7-AS RF source') have been built and tested. A positive hydrogen ion beam of 28 A at 50 kV with a proton fraction of 60% has been extracted. For future injection systems, in particular those based on negative ions, large area beam extraction and long pulse capability up to cw operation are envisaged. At IPP two different design concepts of RF sources are being investigated in order to meet these requirements: in the first one, the plasma is generated in one or two small circular RF sources ('drivers'), which are mounted onto the back plate of a 30 l expansion volume ('bucket') with magnetic multi-cusp confinement of the plasma ('Type 6-1' and 'Type 6-2' respectively). In the second concept the plasma is generated inside the bucket by an internal RF coil, which is insulated by a quartz tube ('Type 5'). In both cases the applicability for negative as well as for positive ion production is being tested. The next application will be in the positive ion based injectors in the third injector of JET and in the W7X stellerator

[en] This paper reports on the technical implementation and the successful commissioning of the beam current feedback regulation system of the RF neutral beam sources for ASDEX Upgrade. The beam current is regulated by adjusting the RF input power. This is possible due to the fast response of the beam current to changes of the RF input power within a few milliseconds. With beam current feedback regulation, perveanced-matched operation of the sources can be maintained throughout the pulse, thereby increasing the beam quality and the source reliability

[en] Measurements of the re-emitted flux of deuterium implanted into pure and boronized graphite at temperatures between 300 K and 1500 K have been performed. The flux of deuterium, re-emitted after reaching saturation, decreases with increasing temperatures above 800-1000 K. At 1400 K, the re-emission coefficient amounts to about 60% in USB15 boronized graphite compared to about 35% in EK98 graphite. Diffusion of deuterium into deep regions of the target with different diffusion constants of the different target materials is one possible explanation of this behaviour. The CD₄ production is similar in both cases; at 800 K, 20-25% of the deuterium is re-emitted as methane. (orig.)