Ni Team

A study of runaway electron confinement in the ASDEX tokamak

The results of runaway electron confinement experiments from ASDEX are analysed to elucidate the structure of electromagnetic turbulence which may cause anomalous electron heat transport in the L-mode confinement regime. From a simple model, the radial correlation length (W) of the magnetic turbulence is determined to be about 1 mm. Using this value and that of the experimentally deduced electron thermal diffusivity, the authors determine the radial magnetic fluctuation level at the plasma edge in the L-mode to be (r/B0) ~ 2 × 10−4. Scalings of W and r/B0 are deduced from parameter scans. From a comparison of these results with the predictions of various theoretical models, it is concluded that skin depth turbulence, electromagnetic drift wave turbulence, rippling modes, and microtearing modes are inferior candidates and that resistive ballooning modes offer the best possibility for a consistent interpretation of the data.

Studies on Impurity Retainment in the ASDEX Divertor

Abstract The retainment and exhaust capability of the ASDEX divertor for neon and argon is investigated during ohmic heating and neutral injection heating. No pronounced influence of the scrape-off layer on the divertor particle outflux is observed, though ionization of neutrals in the divertor throats is most likely. The divertor outfluxes behave as if they are determined by the molecular flow conductances of the divertor throats. The plasma impurity outfluxes show strong top-bottom asymmetries which reverse with the direction of the toroidal field. The exhaust capability can be demonstrated by comparison with limiter discharges. The exhaust efficiency of the double-null configuration is about twice that of a single-null configuration.

Simulation of transport in ASDEX divertor discharges with neutral-injection heating

Cross-field particle and energy transport in diverted ASDEX discharges with up to 2.2 MW neutral-beam heating is studied by testing transport code predictions against experimental data. With neutral injection, enhanced coefficients of electron heat diffusivity and diffusion are found which are independent of density, temperatures and poloidal beta over a large parameter range (χe = 2.9 × 104 cm2s−1 and D = 0.2 χe). The ion heat diffusivity continues to be one to three times the neoclassical value. The period with degraded confinement can be considerably delayed in relation to the beams and is found to be correlated with a characteristic change in the shape of the electron temperature profile due to neutral injection and with the time behaviour of the sawtooth period. Energy and particle confinement do not deteriorate with increasing beam power. It is found that the sawtooth activity does not significantly impair the confinement outside the q = 1 surface. A possible connection between degradation of confinement and non-thermal electron or ion velocity distributions, plasma rotation and drift-wave turbulence is discussed.

H-Mode operating regimes and confinement in ASDEX-Upgrade

The H-mode operating boundaries and confinement regimes in ASDEX Upgrade are described. The L to H mode threshold power is shown to linearly increase with the product of line-averaged electron density times magnetic field. The analyses of the boundaries between different ELM types show that mainly the steady-state ELMing H-mode discharges with type-I ELMs are achieved above the power threshold. The global confinement time of such discharges is analysed. Discharges of the same type with high edge radiation power (80% of the input power radiated) obtained by neon puffing remain in the H-mode and have a good confinement. The importance of strong deuterium puffing in addition to neon is demonstrated. These promising results for power load reduction of the divertor in ITER are discussed.

Erosion and Retention of the Target Plate Material in the ASDEX Divertor

Abstract In this paper we describe experiments undertaken to evaluate the retention of target produced impurities in the ASDEX divertor. The titanium divertor plates have recently been replaced by water-cooled copper plates. Since copper is a material not previously present inside the ASDEX torus, a unique possibility existed to study the transport of the target plate material from the divertor chamber into the main plasma. By spectroscopic measurement of the Cu flux emerging from the target plate and of the Cu density in the main plasma, the retention is assessed. We find a poor retention ( R = 1) for low n e ( n e 13 cm −3 ) but a substantially improved one ( R > 30) at high densities. During additional heating (N1, ICRH) at the same density as in the OH case, the retention is lower, but approaches similar values at slightly higher densities. The retention seems to be independent of the heating power.