Charge Exchange Processes between Excited Helium and Fully Stripped Ions

We made a classical trajectory Monte Carlo (CTMC) calculation of state selective cross sections for processes between some light ions and excited helium. The results, useful for analysis of spectroscopic data of fusion devices, are in good agreement with theoretical predictions of scaling laws.Comment: LaTex, 8 pages, 4 figures (available on request to the authors), DFPD/94/TH/57, to be published in Phys. Rev.

Charge exchange and ionisation in N7+^{7+}, N6+^{6+}, C6+^{6+} - H(n=1,2n=1, 2) collisions studied systematically by theoretical approaches

The introduction of gases like nitrogen or neon for cooling the edge region of magnetically confined fusion plasmas has triggered a renewed interest in state selective cross sections necessary for plasma diagnostics by means of charge exchange recombination spectroscopy. To improve the quality of spectroscopic data analysis, charge exchange and ionisation cross sections for N$^{7+}$ + H($n=1,2$) have been calculated using two different theoretical approaches, namely the atomic-orbital close-coupling method and the classical trajectory Monte Carlo method. Total and state resolved charge exchange cross sections are analysed in detail. In the second part, we compare two collision systems involving equally charged ions, C$^{6+}$ and N$^{6+}$ on atomic hydrogen. The analysis of the data lead to the conclusion that deviations between these two impurity ions are practically negligible. This finding is very helpful when calculating cross sections for collision systems with heavier not completely stripped impurity ions.Comment: 21 pages, 10 figures, 6 data table

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 Γ of each antenna port, the probes have two advantages: first, they are located close to the antenna ports (∼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 Rsep were also performed. Upper triangularity δo was increased from 0.1 to 0.3 (with the lower triangularity δu kept roughly constant at 0.45) and significant decreases in |Γ| (up to ∼30%, markedly improving antenna coupling) and moderate changes in phase (up to ∼5°) of Γ on each feeding line were observed approximately at δo ≥ 0.29. During MPs (in ∼0.5 s pulses with a coil current of 1 kA), a smaller response was observed: 6% - 7% in |Γ|, with changes in phase of ∼5° apparently due to Rsep scans only. As |Γ| 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]