Spectroscopic measurements of the D α and H α line profiles emitted within the edge region of a tokamak plasma, have revealed the existence of a cold central component, broadened mainly by the Zeeman effect arising from the confining magnetic field. Evaluation of the Doppler broadening suggests that the cold component is probably produced by electron impact-induced molecular dissociation, dissociative excitation being one of the few mechanisms which can explain the formation of atoms of kinetic energy around 0.2 eV against a background of comparatively hot electrons and ions. Further analysis of these line profiles, observed along different directions in the equatorial plane and under various tokamak discharge conditions, reveals, in addition to this effective 'cold temperature', an effective 'lukewarm temperature', which we explain in terms of an appreciable collisional heating mechanism. Estimates of the rates for ion-induced dipole and ioninduced quadrupole collisions with excited atoms, yield values of the correct order of magnitude for this observed 'lukewarm temperature'. In addition, measurements of Balmer-α line profiles, radiated from a gas discharge in a magnetic field of similar magnitude, are analysed and their shapes compared with those from the tokamak plasma.

Spectra of the hydrogen isotopes, the major atomic constituents of magnetically confined fusion plasmas, are of particular importance for understanding the physical processes occurring in the plasma edge. A detailed analysis of the Zeeman-split Balmer lines reveals a number of subtle effects related to the formation of the radiating atoms by molecular dissociation, as well as charge-exchange recombination, and their subsequent heating by atom–ion collisions. We discuss and compare two of the possible physical processes whereby the spectra are broadened, through collisions transferring momentum between fast ions from the interior of the plasma and slow (‘cold’) atoms at the plasma edge. The evaluation of momentum transfer cross-sections for these processes is considered, and rate coefficients are compared with typical plasma conditions of interest. The picture of the heating process, as presented in an earlier paper, is modified in several respects, both as regards the calculation of the rate coefficients and the identification of the major channels (pathways between states) of interest.

Spectroscopic measurements of low‐n Balmer line profiles of atomic hydrogen and deuterium, emitted within the edge region of the TEXTOR‐94 tokamak plasma, have revealed the existence of a class of cold excited atoms, whose probable origin has been ascribed to electron impact‐induced molecular dissociation. Associated with these cold radiators are a second group of ‘lukewarm’ atoms, i.e. atoms heated by elastic collisions with hot protons (deuterons) diffusing outward from the plasma interior, as well as a third group of ‘hot’ atoms, produced in the corresponding excited states directly by charge‐exchange recombination between protons (deuterons) and boundary region atoms. A mechanism recently proposed to explain the heating process quantitatively, in terms of elastic atom‐ion collisions, is applied and discussed in this paper.

Microcavities which contain Zn-Cd-Se quantum wells as the resonant medium have been fabricated and tested at blue-green wavelengths. We see clear evidence of coupled-mode behavior at the n=1 heavy-hole exciton in both angle and temperature tuning experiments, with anticrossing (vacuum-Rabi) splittings approaching 20 rneV. The exciton-cavity interaction is consistent with predictions by theory in the strongcoupling regime, and illustrates the impact of the large oscillator strength available in II-VI compounds.Excitons in semiconductor microcavities is a contemporary subject as illustrated by several recent experimental reports that demonstrate the impact of enhanced coupling between extended electronic states in a crystalline solid and resonant electromagnetic waves.Important related theoretical activity has also emerged, ranging from classical reinterpretation of atomic physics pheonomena to full quantum description of the exciton-polariton states in a microcavity.Both experiment and theory have concentrated on the GaAs-based semiconductor heterostructures, in large part due to ready access by experiment to quality epitaxial material. Driven by the prospects of new compact blue-green diode lasers and other optoelectronic applications at short visible wavelengths, strong recent progress has been witnessed in wide-gap II-VI semiconductors, and a range of heterostructure designs is now available to test ideas associated with microcavity effects. One important and motivating difference, in this connection, between the GaAs-and ZnSebased quantum-well systems is that a considerably larger oscillator strength can be obtained in the latter case, as well as the condition where the exciton binding energy satisfies the inequality E )fttoLp kT (where htoLo is the optical phonon energy and kT is evaluated at room temperature).The robustness of such quasi-two-dimensional (2D) excitons in II-VI heterostructures is of relevance to the new bluegreen light emitters; here we demonstrate their impact in a microcavity environment in initial experiments. We use both angle tuning and temperature tuning to show coupled-mode behavior between the n=1 heavy-hole (HH) exciton in a (Zn, Cd)Se quantum well (QW) and the microcavity electromagnetic modes to obtain a quantitative measure of the normal mode (vacuum-Rabi) splitting. The splittings can reach values approaching 20 meV, and are clearly discernible in spite of a background line broadening on the order of 10 meV. Semiclassical theory provides very good agreement with the experiments in which transmission, reflectance, and photoluminescence have been used as optical probes. The II-VI semiconductor optical structure was based on a pseudomorphic (Zn, Cd)Se/Zn(S, Se)/(Zn, Mg)(S, Se) separate confinement heterostructure design, in which three uniformly strained 75-A-thick Zni, Cd"Se QW's provided the quasi-2D exciton confinement (xcd = 0.24), cladded by SiO /TiO DBR stack

Oxygen ion impurity radiation is a potential source of inaccuracy in ion temperature determination with the aid of the commonly used C VI transition n = 8→n' = 7, produced by charge-exchange recombination (CXR) of C6+ ions, since the corresponding transition in O VI cannot be resolved under typical plasma conditions in the tokamak. In order to demonstrate the possible importance of oxygen ion impurity radiation, we have selected a convenient spectroscopic `window' (about 8 Å wide) containing the major Zeeman components of two prominent lines in the visible (multiplet 1), one emitted by C2+ and one by O+. Observations have been performed in this wavelength range, both tangentially and perpendicularly to the magnetic flux surfaces, in the second case with the aid of a special graphite test limiter. Measurements include the case of special plasma discharges in which oxygen gas was introduced from the test limiter. The temperatures of both species are evaluated from the Doppler broadening of the respective Zeeman components, and compared with the results from a model for collisional heating by impact with hot protons (deuterons) in the plasma edge. The spectra and derived results show that impurity identification in tokamak edge plasmas should not be carried out with the aid of spectral lines from highly excited levels populated by CXR, but using lines corresponding to much more species-specific transitions from lower ionization stages. The identification and quantitative analysis should be performed with the aid of carefully measured and calculated Zeeman-(Paschen-Back-) broadened line profiles, since these have features practically unique to the species under investigation. Some allowance may, however, be required for deviation, from a statistical distribution, of population among fine-structure sublevels.