J. Gafert

Scintillator probe for lost alpha measurements in JET

Good confinement of alpha particles in a large magnetic fusion device is a precondition for building a magnetic fusion reactor. The direct measurement of alpha particle losses is of particular interest. Appropriate diagnostics are now being prepared for the Joint European Torus tokamak: a scintillator probe and a set of Faraday cups. Both systems are capable of measuring charged fusion products and ion cyclotron resonance heating tail ions. The design of the lost alpha particle scintillator probe is in the scope of this article. It will allow the detection of particles with a gyroradius between 20 and 140 mm (15% resolution) and a pitch angle between 30° and 86° (5% resolution). As scintillating material P56 will be used. The light emitted by the scintillator caused by charged particles that pass the collimator and hit the scintillator will be detected via a set of optical lenses and a coherent image fiber bundle with a charge coupled device camera and a photomultiplier array. In the following the present...

Edge transport and its interconnection with main chamber recycling in ASDEX Upgrade

Edge profiles of electron temperature and density are measured in ASDEX Upgrade with a high spatial resolution of 2–3 mm with Thomson scattering. In the region of the edge transport barrier in ELMy H-mode, the gradient lengths of Te and ne are found closely coupled, with the temperature decay length two times shorter than the density decay length corresponding to ηe ≈ 2. The ηe constraint allows us to calculate the electron temperature and density profiles from the pressure profile if the density and temperature values are known at one spatial position. The edge density in the region of the barrier foot is closely coupled to the main chamber recycling, with no strong dependence on other parameters. In contrast, the density rise from the outer barrier foot to the pedestal exhibits pronounced dependence on plasma current and shaping, indicating quite different mechanisms determining the absolute density and its gradient.

Volume recombination in divertor I of ASDEX Upgrade

High density operation in the ASDEX Upgrade divertor I with horizontal target plates is reported. Density rampup experiments were carried out to characterize detached plasma conditions in the divertor. During the detached phases, hydrogen continua and spectral line emission from high-n shells were observed in the divertor due to the volume recombination. The spectroscopic measurements provide a consistent picture of the evolution of the divertor plasma parameters during the density ramp. By means of the ADAS atomic physics program package, the rate of volume recombination was evaluated, including the effect of opacity. The relative importance of volume recombination in comparison with the target plasma sink is discussed. Observations indicating differences in volume recombination between the two divertor legs are presented, and the connection of volume recombination to divertor detachment is addressed.

Role of divertor geometry on detachment in ASDEX Upgrade

Abstract The change of the divertor plasma behaviour from Div-I to Div-II for ASDEX Upgrade as measured by a set of reconstructed or newly designed divertor diagnostics is presented. The Div-II configuration is characterised – due to the highly inclined target plates – by reflection of neutrals towards the separatrix. Therefore, detachment in Div-II starts rather early localised close to the separatrix. In contrast, the complete global divertor detachment is practically unchanged, because the outer parts of the SOL stay attached even for high densities. The power distribution on the divertor in Div-II is much broader than in Div-I, resulting in a large reduction of peak power loads both in L and H-mode (factor of 2–4) due to larger divertor radiation losses. The larger losses are caused by larger hydrogen losses, enhancement of carbon radiation due to radial transport and convective energy transport into the radiation zone and larger radial energy transport in the divertor. The new Div-II geometry shows larger neutral gas densities in the divertor for the same line averaged density and a much faster helium exhaust rate ( τ ∗ He /τ E ≈4 ) in H-mode. The neon compression is worse compared with helium. No strong effect on impurity compression and overall divertor performance was seen by puff and pump experiments, neither in Div-I nor in Div-II.

Properties of the new divertor IIb in ASDEX Upgrade

A new divertor configuration (DIV IIb) has been implemented in ASDEX Upgrade. In order to accommodate a large variety of plasma shapes with bottom triangularities (δ bot ) up to 0.48, the outer strike point region was modified and the roof baffle was lowered and diminished at its outer part in comparison with the previous divertor (DIV II). The inner part of the divertor strike point module remains unchanged, but at the divertor entrance a smooth transition to the central column is provided to minimize local hydrogen recycling. According to experiences with power handling in DIV II, ordinary fine grain graphite has been chosen for the outer strike point and, as before the tiles are slightly tilted in toroidal direction to hide the leading edges. A first characterization of DIV IIb reveals that the beneficial behaviour of DIV II is essentially maintained. There is an increase of the power density due to geometrical reasons at the outer target, whereas the divertor radiation for similar magnetic configurations is unchanged. The pumping characteristics for D and He are almost retained, suggesting a large influence of the inner divertor leg, the configuration of which remains unchanged. A significant reduction (20%) of the L-H threshold is observed consistent with larger temperature gradients inside the separatrix just before the transition.

Operational limits of ASDEX Upgrade H mode discharges in the new closed Divertor II configuration

Systematic investigations of gas fuelled, high density H mode discharges with the new closed Divertor II of ASDEX Upgrade are reported. The focus is on operational limits (H-L transition, density limit) over a wide range of externally controllable plasma parameters. It is found that the H mode threshold power dramatically exceeds the generally accepted prediction Pthreshheat ∝ eBt when high densities are approached. Highest densities (density limit) are achieved close to the H-L mode back-transition and are generally associated with complete divertor detachment. The scaling of the density limit is assessed and discussed with a view to detachment based models and the Greenwald limit.

Spectroscopic investigation of the dynamics of ions and neutrals in the ASDEX Upgrade Divertor II

Abstract Detachment at the separatrix is a promising possibility to reduce the peak power load onto the divertor target plates. We investigated this topic spectroscopically at ASDEX Upgrade by measuring particle velocities using Doppler spectroscopy. The experimental data show a reduction of the chord averaged hydrogen velocity along the separatrix by about a factor of 3. Another important subject is flow reversal: after a brief discussion of 2D-code predictions and the conditions for detection we present spectroscopic evidence for flow reversal of C 2+ -ions in the ASDEX Upgrade divertor II. The comparison of these spectroscopic data with B2-EIRENE modeling shows good qualitative and quantitative agreement.

Carbon influx studies in the main chamber of ASDEX Upgrade

Carbon sources in the main chamber of ASDEX Upgrade, especially the 12 guard limiters at the low field side (LFS), were determined spectroscopically using recently installed lines of sight. Absolute photon fluxes were measured for spectral lines in the visible wavelength range referring to all spin systems of C+1 and C+2.A simple transport model for carbon enabled the simulation of the radial distribution of carbon radiation and the determination of the effective inverse photon efficiency, which was used for the evaluation of ion fluxes. The model also predicts the fraction of eroded particles that are transported out of the plasma before further ionization occurs. Comparison of the calculated losses with measurements showed good agreement in L-mode cases, whereas in H-mode cases the CIII/CII radiation ratio was too high by a factor 1.5. The contribution of each spin system to the ion flux was independently measured. For C+1 and C+2 the spin system distribution was found to be close to equilibrium.The line-of-sight-integrated photon fluxes were spatially separated for many lines of sight by Zeeman-analysis and differential measurements. This allowed us to determine the total influx from the high field side and LFS. Surprisingly, the carbon source at the inner heatshield was larger than the carbon influx from the limiter source at the LFS. This is very pronounced for the H-mode case investigated, where 60–80% of the carbon atoms emerge from the heatshield. This source is due to recycling or re-erosion of carbon, which probably originates from the limiters, because ≈85% of the heatshield area consisted of tungsten coated tiles.

Effect of divertor geometry on boundary and core plasma performance in ASDEX Upgrade and JET

A key element in future toroidal magnetic fusion machines like ITER is the design of a divertor, which allows for safe particle and power exhaust in parallel with high bulk plasma performance. Correspondingly, the definition and design of an optimized divertor is a major task within the ongoing international ITER research and development effort. In order to provide a profound physics basis for such a divertor optimization, different divertor geometries are being tested on major tokamaks. This paper describes the effects of these divertor modifications on plasma performance in ASDEX Upgrade and in JET. In conclusion, increasing closure improves divertor performance without limiting the core plasma performance in ELMy H-modes.

The ASDEX Upgrade divertor IIb—a closed divertor for strongly shaped plasmas

A new divertor configuration (DIV-IIb) has been implemented in ASDEX Upgrade. In order to accommodate a large variety of plasma shapes with bottom triangularities (δbot) up to 0.48, the outer strikepoint region was modified and the roof baffle was lowered and diminished at its outer part in comparison with the previous divertor (DIV-II). The inner part of the divertor strikepoint module remains unchanged, but a smooth transition to the central column is provided at the divertor entrance to minimize local hydrogen recycling. An increase in power density is observed due to geometrical reasons at the outer target, whereas the divertor radiation for similar configurations and discharge conditions is unchanged. The pumping characteristics for D and He are almost retained, suggesting a large influence of the inner divertor leg, the configuration of which remains as before. Detachment in L-mode discharges fits well into a scaling deduced from JET data and earlier ASDEX Upgrade data. A significant reduction (20%) of the L–H threshold is observed compared with DIV-II. Its density dependence is weaker than in the previous DIV-II configuration and there are hints for an influence of triangularity on power threshold. Finally, clear evidence for a parasitic plasma below the divertor roof baffle is found.