Improved measurements of ICRF antenna input impedance at ASDEX upgrade during ICRF coupling studies

A new set of diagnostics has been implemented on ASDEX Upgrade to measure the input impedance of the ICRF antennas, in the form of a voltage and current probe pair installed on each feeding line of every antenna. Besides allowing the measurement of the reflection coefficient Gamma of each antenna port, the probes have two advantages: first, they are located close to the antenna ports (similar to 3 m) and thus the measurements are not affected by the uncertainties due to the transmission and matching network; second, they are independent of matching conditions. These diagnostics have been used to study the behavior of the ASDEX Upgrade antennas while changing the plasma shape (low to high triangularity) and applying magnetic perturbations (MPs) via saddle coils. Scans in the separatrix position R-sep were also performed. Upper triangularity delta(o) was increased from 0.1 to 0.3 (with the lower triangularity delta(o) kept roughly constant at 0.45) and significant decreases in vertical bar Gamma vertical bar (up to similar to 30%, markedly improving antenna coupling) and moderate changes in phase (up to similar to 5 degrees) off on each feeding line were observed approximately at delta(o) >= 0.29. During MPs (in similar to 0.5 s pulses with a coil current of 1 kA), a smaller response was observed: 6% - 7% in vertical bar Gamma vertical bar, with changes in phase of 5 apparently due to R p scans only. As 1 is usually in the range 0.8 - 0.9, this still leads to a significant increase in possible coupled power. Numerical simulations of the antenna behavior were carried out using the FELICE code; the simulation results are in qualitative agreement with experimental measurements. The results presented here complement the studies on the influence of gas injection and MPs on the ICRF antenna performance presented in [4]

On the enhancement of nuclear reaction rates in high-temperature plasma

We argue that the Maxwellian approximation can essentially underestimate the rates of some nuclear reactions in hot plasma under conditions very close to thermal equilibrium. This phenomenon is demonstrated explicitly on the example of reactions in self-sustained DT fusion plasma with admixture of light elements X = Li, Be, C. A kinetic analysis shows that the reactivity enhancement results from non-Maxwellian knock-on perturbations of ion distributions caused by close collisions with energetic fusion products. It is found that although the fraction of the knock-on ions is small, these particles appreciably affect the D+X and T+X reaction rates. The phenomenon discussed is likely to have general nature and can play role in other laboratory and probably astrophysical plasma processes.Comment: 12 pages, 4 figures, to be published in Phys. Lett.