F. Braun

Assessment of compatibility of ICRF antenna operation with full W wall in ASDEX Upgrade

The compatibility of ICRF (ion cyclotron range of frequencies) antenna operation with high-Z plasma facing components is assessed in ASDEX Upgrade (AUG) with its tungsten (W) first wall.The mechanism of ICRF-related W sputtering was studied by various diagnostics including the local spectroscopic measurements of W sputtering yield YW on antenna limiters. Modification of one antenna with triangular shields, which cover the locations where long magnetic field lines pass only one out of two (0π)-phased antenna straps, did not influence the locally measured YW values markedly. In the experiments with antennas powered individually, poloidal profiles of YW on limiters of powered antennas show high YW close to the equatorial plane and at the very edge of the antenna top. The YW-profile on an unpowered antenna limiter peaks at the location projecting to the top of the powered antenna.An interpretation of the YW measurements is presented, assuming a direct link between the W sputtering and the sheath driving RF voltages deduced from parallel electric near-field (E||) calculations and this suggests a strong E|| at the antenna limiters. However, uncertainties are too large to describe the YW poloidal profiles.In order to reduce ICRF-related rise in W concentration CW, an operational approach and an approach based on calculations of parallel electric fields with new antenna designs are considered. In the operation, a noticeable reduction in YW and CW in the plasma during ICRF operation with W wall can be achieved by (a) increasing plasma–antenna clearance; (b) strong gas puffing; (c) decreasing the intrinsic light impurity content (mainly oxygen and carbon in AUG). In calculations, which take into account a realistic antenna geometry, the high E|| fields at the antenna limiters are reduced in several ways: (a) by extending the antenna box and the surrounding structures parallel to the magnetic field; (b) by increasing the average strap–box distance, e.g. by increasing the number of toroidally distributed straps; (c) by a better balance of (0π)-phased contributions to RF image currents.

Fast matching of load changes in the ion cyclotron resonance frequency range

In the ion cyclotron resonance frequency (ICRF) range, it is necessary to match the antenna load to the impedance at which the generator delivers its maximum power. This is typically done by adding impedances (mechanical tuners i.e. pieces of transmission lines) at appropriate locations in parallel to the feeder line of the antenna. Large and fast load changes can result from variations in the plasma such as H-mode transitions and ELMs (edge localised modes). For optimum power delivery, those load changes have to be matched dynamically. The required variation of the impedance is performed by a change of electrical length of the tuners. A new type of tuners, fast ferrite tuners (FFT) developed by AFT, relies on the change of magnetisation of ferrites to achieve this. Since there are no mechanically moving parts, the change can be fast. Acceptance tests of a system, developed for General Atomics, using these tuners were performed successfully and first experience was gained on a proof of principle test under plasma conditions.

Interaction of ICRF Fields with the Plasma Boundary in AUG and JET and Guidelines for Antenna Optimization

W sputtering during ICRF on ASDEX Upgrade (AUG) and temperature rise on JET A2 antenna septa are considered in connection with plasma conditions at the antenna plasma facing components and E‖ near‐fields. Large antenna‐plasma clearance, high gas puff and low light impurity content are favorable to reduce W sputtering in AUG. The spatial distribution of spectroscopically measured effective W sputtering yields clearly points to the existence of strong E‖ fields at the antenna box (“feeder fields”) which dominate over the fields in front of the antenna straps. The picture of E‖ fields, obtained by HFSS code, corroborates the dominant role of E‖ at the antenna box on the formation of sheath‐driving RF voltages for AUG. Large antenna‐plasma clearance and low gas puff are favorable to reduce septum temperature of JET A2 antennas. Assuming a linear relation between the septum temperature and the sheath driving RF voltage calculated by HFSS, the changes of the temperature with dipole phasing (00ππ, 0ππ0 or 0π0π) ...

Minority Heating Experiments on the W7‐AS Stellarator

W7‐AS is a modular stellarator with R=2.0m and at the location of the ICRH antennas, an elliptical plasma with a=10cm and b=31cm. The typical parameters of the plasma, formed by 70GHz ECRH, are: Te(0)=1.5keV, Ti(0)=300eV for B=2.55T. Minority D(H) & He(H) ICRH experiments at 37.4MHz have been performed using 2 low field side current strap antennas, shielded by an optically closed TiC coated Faraday screen and TiC coated local carbon bumper limiters. In these experiments the machine had boronized walls. Up to 260kW have been deposited in the plasma, giving some initial plasma heating, however within 50ms the rise in impurities decreases the plasma energy below its pre‐RF level. A rise in the electron density and the production of fast unconfined minority ions is also observed. This is thought to be a result of the too high minority density of ≊10%. The observation of eigenmodes with Q≊200 in the antenna loading supports this hypothesis, suggesting both poor minority absorption and poor tunneling through th...

ASDEX Upgrade results and future plans

ASDEX Upgrade is an ITER shaped divertor tokamak with versatile heating, fueling, exhaust and control systems. All plasma facing components are coated with tungsten layers. Plasma scenarios have been adopted to avoid central tungsten accumulation, which can lead to an H-L transition due to excessive central radiative losses. Compared to a carbon-PFC tokamak, the AUG operation space is slightly more weighted towards higher densities and collisionalities. Actual and future planned extensions aim towards reducing the core collisionality while maintaining good power and particle exhaust. These extensions include a solid tungsten outer divertor target, improved pumping, a higher ECRH power and modified ICRF antennas for reduced tungsten sources. The newest element for advanced plasma control is the first set of 8 magnetic perturbation coils, which already achieved type-I ELM mitigation in various plasma scenarios. Another 8 coils will be installed in autumn 2011 allowing to produce mode spectra with n > 2. In parallel to the improved actuator set, an increasing number of diagnostics is brought into real-time state, allowing versatile profile and stability control.

Analysis of the loading resistance for ICRF heating experiments in ASDEX

The loading resistance has been analyzed for ICRF heating experiments in ASDEX plasmas, and some interesting features have been observed regarding the loading resistance in three particular cases. (1) In H-minority heating experiments, the loading resistance decreases sharply at the L-H transition. However, during the H-phase, which lasts for a few hundred milliseconds, the increasing density induces a variation in the loading resistance. This is reasonably accounted for in terms of an eigenmode effect, and is in agreement with theoretical predictions. (2) In long-pulse second-harmonic heating experiments a slowly changing isotope concentration (H concentration from 70% to 85%) is accompanied by a gradual decrease in loading resistance. (3) In 2 Omega CH heating experiments with repetitive pellet refueling, in which two confinement phases have been identified, it has been observed that the temporal behaviour of the loading resistance is quite different for the two phases.