A. Lorenz

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.

High β plasmoid formation, drift and striations during pellet ablation in ASDEX Upgrade

The ablated material of a frozen hydrogen isotope pellet which is injected into a hot tokamak plasma forms a high β plasmoid. This diamagnetic plasmoid is accelerated to the magnetic low field side of the torus. The high β plasmoid drift was directly observed by an optical diagnostic with high space and time resolution. Spectroscopic measurements of the emitted light allowed the density and temperature of the ablation cloud, and for the first time also of the drifting plasmoids, to be determined. The experiments give a new insight into the dynamics of the formation of striations during the pellet ablation; these striations cause the separation of the ablated material into a sequence of separated, drifting plasmoids. The influence of the drift on the mass deposition profile for high field side pellet injection is discussed. The plasmoid dynamics even influences the radial pellet motion, most probably owing to a rocket effect. The physical principles of the high β plasmoid drift are discussed and compared with the experimental observations.

A system for cryogenic hydrogen pellet high speed inboard launch into a fusion device via guiding tube transfer

Particle deposition deep inside the hot target plasma column by cryogenic hydrogen pellet injection is required for efficient particle refueling of fusion devices such as tokamaks. As the ablation plasmoid is subject to a strong outward drift in hot plasmas, pellet launch from the tokamak inboard side is more useful than from the outboard. The depth of the pellet particle deposition depends on density and temperature of the target plasma, and on the pellet mass and velocity. Plasma operation determines density and temperature values, the maximum affordable density perturbation limits the pellet mass. Consequently, the pellet speed remains the only technically variable parameter allowing improvement of the refueling performance. To achieve this an inboard high-speed pellet injection system based on looping type geometry was designed and built at the midsize tokamak ASDEX Upgrade, and first fueling studies had validated the potential for the required injection velocity increase. Throughout the last two years experimental efforts focused on careful step-by-step optimization of the different system hardware components and the operational procedures. Introducing amongst other features a well pumped, rectangular guide track section, the feasibility for the inboard launch scheme up to an injection velocity of 1 km/s was successfully demonstrated. Detailed off-line tests have confirmed that pellets can withstand controlled mechanical and thermal impact even at this high speed, albeit for the sacrifice of increasing and significant mass losses. In a first application to plasma refueling deep penetration into hot target plasmas and hence, high fueling performance was achieved by deeper pellet born particle deposition and hence enhanced particle sustainment times.

Optimization of pellet scenarios for long pulse fuelling to high densities at JET

Pellet injection was investigated for its fuelling capability to high densities in ELMy H mode discharges at JET. Applying the high field side launch system, optimized refuelling scenarios were developed on the basis of conventional discharge configurations with Ip = 2.5 MA, Bt = 2.4 T, averaged triangularity � δ �≈ 0.34 and mainly neutral beam heating at a level of approximately 17 MW. The accessible operational range was extended with respect to gas puff refuelling by the use of pellet injection. For example, H mode conditions could be maintained at densities beyond the Greenwald level. Plasma energy confinement was observed to become density independent at high densities. Deep pellet particle deposition made possible the uncoupling of edge and core density, allowing more peaked density profiles. When confinement deterioration due to pellet triggered MHD activity or parasitic pellet borne gas was avoided in appropriate pulse schedules, an enhanced particle inventory was achieved while maintaining the plasma pressure profile. In a technical assessment it was found that there is still room for further enhancement of the injection scenario by improved adaptation to high density plasma operation.

Impact strength of cryogenic deuterium pellets for injection into tokamak plasmas

The technical implementation of the potentially very efficient inboard pellet refueling scheme in tokamaks remains so far restricted to low velocities (v=200–300 m/s) due to the fragile nature of cryogenic D2. One specific problem is practically unavoidable pellet impacts in components of the pellet guiding system: first, in a funneling adaptor installed to cope with the angular scatter of pellets from acceleration devices such as centrifuges and, second, inside variably bent guiding tubes required to access the high field side of the torus. In order to inject pellets at speeds close to those available from the ASDEX Upgrade centrifuge (v⩽1200 m/s), knowledge of critical guiding system parameters such as impact angle and speed is needed. In this study design requirements for an optimized guiding scheme are derived by examining grazing incidence pellet impacts on one single and two subsequent flat, rigid plates. The survival of the pellets was found to be determined by the normal impact velocity component....

Mass transfer in long pellet guiding systems at ASDEX upgrade and JET

Abstract Guiding systems used for transfer of cryogenic pellets to the tokamak inboard currently undergo optimisation to possibly extend the operational range of Inboard pellet injection scenarios. Work reported here has been concentrating on experimental studies and calculations of pellet mass loss during transfer in a guiding tube as a function of pellet parameters such as velocity and ice composition, and guiding track characteristics such as vacuum conductance, geometry, size and shape of cross-section. Results from ASDEX Upgrade and ORNL systems suggest, that centripetal loads on the pellets due to small radii bends in the systems strongly increase the erosion of pellet ice. A deleterious effect of accumulated gas due to sublimated D2-ice blocking the tube and eroding subsequent pellets has been found. However, an efficient vacuum pumping set up for a given pellet repetition rate can significantly improve the mass conservation with respect to closed tube systems.

Refuelling performance improvement by high speed pellet launch from the magnetic high field side

Limited available pellet velocities have so far restricted the refuelling performance of efficient launch schemes from the tokamak magnetic high field side (HFS). Although pellet injection during H mode has resulted in more peaked density profiles and enhanced performance with respect to gas puff refuelling, prompt particle and energy losses caused by pellet induced ELM bursts have still limited the extension of the operational area. Now, the preliminary version of a new optimized pellet injection set-up at ASDEX Upgrade allows for significantly higher injection speeds when launching pellets from the magnetic HFS. Intact pellets with velocities up to vP = 560 m/s were successfully injected instead of the vP = 240 m/s available with the previous set-up. The velocity increase results in a deeper pellet penetration and seems to follow the vP1/3 scaling derived from a multimachine study using conventional pellet launch from the torus outside. The inward shift of the pellet particle deposition profile with respect to the penetration depths turned out to be approximately the same for both launch velocities. The respectively achieved deeper particle deposition inside the plasma column reduced the particle and energy loss rates during the immediate post-pellet phase. Thus, further enhancement of tokamak operation in the high density regime seems feasible by means of high speed pellets launched from the torus inner side.

Status of the diagnostics development for the first operation phase of the stellarator Wendelstein 7-X

An overview of the diagnostics which are essential for the first operational phase of Wendelstein 7-X and the set of diagnostics expected to be ready for operation at this time are presented. The ongoing investigations of how to cope with high levels of stray Electron Cyclotron Resonance Heating (ECRH) radiation in the ultraviolet (UV)/visible/infrared (IR) optical diagnostics are described.

Design and implementation of a high speed guiding system for inboard pellet refuelling

Abstract Cryogenic pellet injection from the magnetic high field side (HFS) of a tokamak is the currently favoured scheme to refuel steady state H-mode discharges. However, in currently operating tokamaks access to the HFS has to be gained via strongly curved guide tubes severely restricting injection speeds and leading to shallow plasma penetration where a large portion of the injected material is lost in the edge gradient layer within a few milliseconds after injection. Therefore, at ASDEX Upgrade an optimized pellet guiding system for speeds close to 1000 m/s has been designed and put into operation. The 17 m long track consists of: (i) a funnel installed to catch pellets scattered by the accelerator; (ii) a preliminary teflon tube section; and (iii) a guiding track transferring the pellets in a diagnostic port into the plasma. Using a looping geometry the system is designed to keep semi-permanent contact between pellet and track by steering the pellets in a controlled trajectory. Emphasis is also put on efficient evacuation of all sections to avoid pressure build-up in the system due to the Leidenfrost effect. In first proof-of-principle plasma discharges a scan of pellet injection at v =240–880 m/s and plasma parameter studies at v =560 m/s were completed.

Implementation of earned value management tools in the wendelstein 7-X project

The stellarator project W7-X, which is currently under construction, has recently implemented several earned value management tools to enable a tighter monitoring of internal processes such as the W7-X assembly process, the diagnostic engineering, and the manufacture of in-vessel components. Specification and implementation of these tools has posed several challenges characteristic of large-scale research projects that are subject to a number of changes during their life cycle. After putting these tools in operation, they have proven as early and transparent performance indicators for project control.