Coronal Approximation Research Articles

The role of plasma–wall interactions in thermal instabilities at the tokamak edge

Plasma–wall interaction leads to the release of impurities and neutrals of the working gas, which contribute significantly to the energy losses from the plasma edge, and therefore, crucially affects the development of thermal instabilities in fusion devices. An analytical model for impurity radiation is proposed, which takes into account the erosion mechanisms of wall material and the motion of impurity particles across magnetic surfaces. The temperature dependence of radiation losses is found to be very different from that predicted by the coronal approximation often used in considering thermal instabilities. The consequences for the development of poloidally symmetric detachment and multi-faceted asymmetric radiation from the edge (MARFE) are analyzed. It is demonstrated that the MARFE threshold principally depends on the mechanism by which working gas neutrals are released from the wall and on the neutral’s properties, e.g., their ionization rate. The results of density limit experiments in Tokamak Experiment for Technology Oriented Research [Proceedings of the 16th IEEE Symposium on Fusion Engineering, 1995 (Institute for Electrical and Electronics Engineers, Piscataway, NJ, 1995), p. 470] and Joint European Torus [Rebut et al., Fusion Eng. Des. 22, 7 (1993)] are interpreted.

Electron‐Ion Recombination Rate Coefficients and Photoionization Cross Sections for Astrophysically Abundant Elements. IV. Relativistic Calculations for C iv and C v for Ultraviolet and X‐Ray Modeling

The first complete set of unified cross sections and rate coefficients are calculated for photoionization and recombination of He- and Li-like ions using the relativistic Breit-Pauli R-matrix method. We present total, unified (e + ion) recombination rate coefficients for (e + C VI ---> C V) and (e + C V \longrightarrow C IV) including fine structure. Level-specific recombination rate coefficients up to the n = 10 levels are also obtained for the first time; these differ considerably from the approximate rates currently available. Applications to recombination-cascade coefficients in X-ray spectral models of K-alpha emission from the important He-like ions is pointed out. The overall uncertainty in the total recombination rates should not exceed 10-20%. Ionization fractions for Carbon are recomputed in the coronal approximation using the new rates. The present (e + ion) recombination rate coefficients are compared with several sets of available data, including previous LS coupling results, and `experimentally derived' rate coefficients. The role of relativistic fine structure, resolution of resonances, radiation damping, and interference effects is discussed. Two general features of recombination rates are noted: (i) the non-resonant (radiative recombination) peak as E,T ---> 0, and the (ii) the high-T resonant (di-electronic recombination) peak.

Effect of the Dynamics of the Impurity Distribution over the Ionization States on the Radiative Plasma Instabilities and Shock Wave Structure

AbstractWe show that conventional quasistationary (coronal, or improved coronal) approximation for the energy loss due to the impurity radiation in practice can never be applied to the investigations of the impurity radiation driven instabilities in the edge plasmas due to relatively slow evolution of the impurity population over ionization states. We show that taking into account the effect of the evolution of the impurity population over ionization states results in a very significant change of the growth rates of the radiative driven instabilities and the structure of the shock wave in the radiative plasmas and leads to the strict conditions for the existence of the shock wave.

Radiation loss rates in Lyman-alpha for solar conditions

view Abstract Citations (73) References (16) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Radiation Loss Rates in Lyman Alpha for Solar Conditions Athay, R. G. Abstract Mean radiation loss rates per unit volume in Lyman-alpha are computed for solar conditions as a function of the optical thickness of the radiating region. No significant corrections to the optically thin coronal approximation are found until the Lyman-alpha optical thickness at line center exceeds 100. At larger optical thicknesses the corrections can be large as a result of the increased ionization of hydrogen from absorptions in the Balmer continuum. The correction suggested by McClymont and Canfield (1983) is found to be too large for most solar applications. Publication: The Astrophysical Journal Pub Date: September 1986 DOI: 10.1086/164565 Bibcode: 1986ApJ...308..975A Keywords: Energy Dissipation; Lyman Alpha Radiation; Optical Thickness; Radiative Transfer; Solar Radiation; Absorption Spectra; Balmer Series; Hydrogen Ions; Solar Corona; Solar Spectra; Solar Temperature; Solar Physics; RADIATIVE TRANSFER; SUN: CORONA; SUN: SPECTRA; ULTRAVIOLET: SPECTRA full text sources ADS |

Radiative power and electron cooling rates for oxygen in steady-state and transient plasmas at densities beyond the coronal limit.

We have developed a time-dependent, collisional-radiative model to calculate radiative power and electron cooling rates for oxygen at intermediate densities (${10}^{16}$\ensuremath{\le}${n}_{e}$\ensuremath{\le}${10}^{20}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$) where the usual coronal approximation is not valid. Large differences from coronal values are predicted. The behavior of the steady-state radiative-power-loss coefficient, ${L}_{z}$, is investigated as the electron density is increased. Generalized power-loss coefficients applicable to transient plasmas are derived and applied to ionizing and recombining oxygen plasmas. Time-dependent effects are found to play a large role both in terms of the total radiated power and the net electron-energy-loss rate.