Edge radial electric fields were induced in the edge of the TEXTOR tokamak by means of a polarization electrode in order to study their influence on the plasma edge profiles and its confinement. The studies include the generation of H-mode behaviour with either positive or negative polarity. Particle confinement ( T~) of deuterium and of impurity ions as well as energy confinement (73 are investigated. For positive fields which remain below the threshold for the L-H transition, an interesting regime of reduced particle confinement without noticeable energy confinement loss is found. A strong asymmetry in the edge density profiles with respect to the electric field sign is observed at these low polarization voltages. Above the threshold, H-mode behaviour with increased energy confinement and especially particle confinement can be produced with either polarity of the applied electric field. It is, however, found that, whereas the energy confinement in positive H-modes is at least as good as that in negative ones, the ratio T~/ T~ is about three times lower in the former case.

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.

Profiles of the Balmer lines D,(H,), Db(Hs) and D, (H,) have been measured in the scrape-off layer and within the edge of the TEXTOR (upgrade) plasma, under Ohmic conditions and with neutral-beam injection. Each line profile shows a strong Zeeman effect in the vicinity of line centre, and a marked central dip when mainly the ( I components are observed. The line core evidently originates from cold atoms in the edge plasma, excited in the course of molecular dissociation, while the broad pedestal on which the core rests is radiated by excited atoms produced through chargeexchange recombination of deuterons (protons), transported outwards from the much hotter plasma interior, and by atoms heated directly by collisions with the deuterons (protons). Core temperatures of about 0.5 eV and less are obtained from line profile analysis.

With the aid of the factorization method developed by Schrödinger, and Infeld and Hull, we use ladder operators to derive a set of recurrence relations from which electric multipole transition (radial) matrix elements of the type ⟨n′ℓ′|rk|nℓ⟩ in hydrogenic atoms and ions can be obtained with the knowledge of a few particular integrals. These ‘initial values’ for the recurrence relations are very simply derived by elementary algebra, with the aid of the properties of the associated Laguerre function. As a check of these recurrence relations, special cases of transitions with n = n′ are considered, and thereby some apparently new matrix element relations are found. This method of computation, first proposed by Infeld and Hull, is efficient, highly accurate also for very large values of the principal quantum number, and generates no numerical instability as n and n′ both grow large. As illustrations of the method, electric dipole (k = 1, Δℓ = ±1) oscillator strengths and line strengths can readily be derived for principal quantum numbers of several hundred, and, in addition, electric quadrupole (k = 2, Δℓ = 0, ± 2) and electric octupole (k = 3, Δℓ = ±1) matrix elements are derived and tabulated. While these results are ‘exact’ for non-relativistic, hydrogenic atoms and ions, they clearly provide useful approximate values for Rydberg states of any atomic radiator. As an illustration of the potential usefulness of these results, we refer to Griem's formulation of the Stark broadening for transitions between states of high principal quantum number in hydrogen (Rydberg–Rydberg transitions), as recently generalized by Watson, for application to radio astronomical spectra from the H II regions. The present (recurrence relation) method, which is also compared to some similar methods in the literature, is shown to be very convenient, in that customary simplifications of the atomic matrix elements for large n and n′ can be entirely avoided, with very little price of additional complexity in the treatment. The present results are therefore available to assist in any computations involving Rydberg–Rydberg transitions, e.g. the re-evaluation of Stark broadening theory for such highly excited states, should new astronomical data continue to show the need for such work.

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.

The motivation for this work is the problem of providing accurate values of the atomic transition matrix elements for the Stark components of Rydberg–Rydberg transitions in atomic hydrogen and hydrogenic ions, for use in spectral line broadening calculations applicable to cool, low-density plasmas, such as those found in H II regions. Since conventional methods of calculating these transition matrix elements cannot be used for the high principal quantum numbers now easily attained in radio astronomical spectra, we attempt to show that the recurrence relation (ladder operator) method recently employed by Watson (2006 J. Phys. B: At. Mol. Opt. Phys. 39 1889–97) and Hey (2006 J. Phys. B: At. Mol. Opt. Phys. 39 2641–64) can be taken over into the parabolic coordinate system used to describe the Stark states of the atomic (ionic) radiators. The present method is therefore suggested as potentially useful for extending the work of Griem (1967 Astrophys. J. 148 547–58, 2005 Astrophys. J. 620 L133–4), Watson (2006), Stambulchik et al (2007 Phys. Rev. E 75 016401(9 pp) on Stark broadening in transitions between states of high principal quantum number, to physical conditions where the binary, impact approximation is no longer strictly applicable to both electron and ion perturbers. Another possible field of application is the study of Stark mixing transitions in ‘ultracold’ Rydberg atoms perturbed by long-range interactions with slow atoms and ions. Preparatory to the derivation of recurrence relations for states of different principal quantum number, a number of properties and recurrence relations are also found for states of identical principal quantum number, including the analogue in parabolic coordinates to the relations of Pasternack (1937 Proc. Natl Acad. Sci. USA 23 91–4, 250) in spherical polar coordinates.

Experimental results from TEXTOR are presented to provide strong evidence for the feasibility of the "cold radiative plasma mantle", a concept which might be a possible solution for the energy exhaust problem in a fusion reactor. The concept is compared with the high density divertor. The compatibility to other constraints, limitations and open problems are discussed, in particular the issues of stationarity (feed-back control, thermal instabilities, q=2), energy confinement. Heexhaust and fuel dilution.

Examples are presented of Doppler broadening measurements on singlet, doublet and triplet emission spectra from the following impurity ions found in the boundary layer of the TEXTOR tokamak: CII, CIII, CIV, Sin, and Sim. The shapes of these spectral lines are significantly influenced by the confining magnetic field of some 2T, in some cases exhibiting an appreciable Paschen-Back effect which complicates their appearance. It is shown that reliable values for the particular ion temperature can be obtained from the Doppler widths of the various Zeeman components, when the presence of the magnetic field is properly accounted for. Such temperatures derived from partially ionised impurity species should, however, be cautiously interpreted, as the ions in question probably do not exist for long enough in the particular ionisation stage to achieve thermal equilibrium with the background deuterons and protons. This interpretation of our results is supported by a simple one-dimensional model of ionisation and collisional heating processes in the plasma boundary.