The flux of impurity atoms into plasmas from limiting surfaces is considered. It is shown how the flux of an impurity released from a surface can be derived from spectroscopic measurements along a line-of-sight directed at the surface. A theoretical atomic level population model is developed to obtain the "ionization per emitted photon" quantities which link the spectroscopic measurement to the flux. Metastable states and finite density plasma effects are taken into account and observations at visible wavelengths are emphasized. Detailed studies and calculations are performed for C+', C+ ', O+', O+', Cr, Cr+ I , Fe, Fe+ I , Ni and Ni+ I . Tabulations and graphs of relevant quantities are provided. The application of the theory to impurity influxes in the JET Tokamak is described.
Electron energy distribution functions in glow discharges (I=5-50 mA, p=200-1000 Pa) were derived from the intensities of Ar and He spectral lines and N2 molecular bands with different excitation thresholds. The relevant rate coefficients represent weighted integrals in the range from 12 to 24 eV. In molecular gases, structures in the low-energy region was inferred from theoretical calculations. An analytical formula with one experimentally adjusted parameter was used to describe the energy distribution. The total electron density was calculated from the electrical conductivity. The mean electron energies from three spectroscopic criteria, as well as those from an ionization-diffusion model of the discharge, are in close agreement for values of the fitting parameter characteristic for the particular plasma conditions. The high-energy part of the distribution functions is not far from a Druyvesteyn shape both in pure nitrogen and in noble gases. The electron densities in nitrogen and mixture plasmas were also measured from N2+ molecular band intensities. Because of the rapid decrease in electron population at higher energies, the ionization of neutral N2 into the upper N2+ level can be ignored and these bands reflect the N2+ ion ground state population. The excitation rate coefficients were calibrated in pure nitrogen and then applied to mixture plasmas, where the N2+ ion fraction was calculated including charge exchange processes. Agreement with ne results from electrical conductivity is very good over the entire parameter range investigated.
Recent experiments at ASDEX Upgrade have achieved advanced scenarios with high β N (>3) and confinement enhancement over ITER98(y, 2) scaling, H H98y2 = 1.1-1.5, in steady state. These discharges have been obtained in a modified divertor configuration for ASDEX Upgrade, allowing operation at higher triangularity, and with a changed neutral beam injection (NBI) system, for a more tangential, off-axis beam deposition. The figure of merit, β N H ITER89-P , reaches up to 7.5 for several seconds in plasmas approaching stationary conditions. These advanced tokamak discharges have low magnetic shear in the centre, with q on-axis near 1, and edge safety factor, q 95 in the range 3.3-4.5. This q-profile is sustained by the bootstrap current, NBI-driven current and fishbone activity in the core. The off-axis heating leads to a strong peaking of the density profile and impurity accumulation in the core. This can be avoided by adding some central heating from ion cyclotron resonance heating or electron cyclotron resonance heating, since the temperature profiles are stiff in this advanced scenario (no internal transport barrier). Using a combination of NBI and gas fuelling line, average densities up to 80-90% of the Greenwald density are achieved, maintaining good confinement. The best integrated results in terms of confinement, stability and ability to operate at high density are obtained in highly shaped configurations, near double null, with δ = 0.43. At the highest densities, a strong reduction of the edge localized mode activity similar to type II activity is observed, providing a steady power load on the divertor, in the range of 6 MW m −2 , despite the high input power used (>10 MW).
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