Table of contents

An anomalous particle pinch in tokamaks has been known to exist since the T-3 tokamak. However, it is now known to occur in almost all tokamaks. No satisfactory explanation has been reported for it so far. A theory is given here for an understanding of this pinch based on an idea given earlier by the author. The induction electric field polarizes the trapped particles, producing a polarization charge density in the plane. The polarization electric field so produced crossed with the toroidal magnetic field leads to an drift of all the plasma particles, which has been shown to have a -averaged radially inward component , where is the Ware pinch velocity , and is the inverse aspect ratio. For most tokamaks is found to be of the order of the observed inward drift .

In this letter, we demonstrate the existence of unstable ion temperature gradient driven Alfvén eigenmodes in tokamak plasmas, which are ideally stable with respect to magnetohydrodynamics (MHD). Conditions for the destabilization of such modes are quantitatively discussed on the basis of theoretical analyses of the mode dispersion relation, which is given in a compact analytical form. It is emphasized that instability requires both sufficiently strong thermal ion temperature gradients and that the plasma be sufficiently close to ideal MHD marginal stability.

A diagnostic method has been evaluated for measuring the relative vibrational ground-state population of molecular hydrogen and deuterium. It is based on the analysis of the diagonal Fulcher bands and the Franck-Condon principle of excitation. The validity of the underlying assumptions was verified by experiments in microwave discharges and the method is recommended for application in divertor plasmas in controlled fusion experiments. By attributing a vibrational temperature to the ground-state electronic level and assuming population via the Franck-Condon principle, the upper Fulcher state vibrational distribution can be derived theoretically with as parameter. Comparison with experimentally derived upper-state population gives the corresponding of the ground state. The Franck-Condon factors for the and transitions have been calculated for both hydrogen and deuterium from molecular constants using the FCFRKR code. The method has been applied to low pressure /He and /He microwave plasmas, showing good agreement of experimentally and theoretically derived upper Fulcher state vibrational distributions. The vibrational temperatures range from 3200 K to 6800 K for and 2600 K to 4000 K for . depending on molecular density, pressure and electron temperature, but indicating nearly the same vibrational population for and for comparable plasma conditions.

Current drive efficiency generated by asymmetric synchrotron radiation in tokamaks is investigated. It is shown that experimental measurements of the differential first wall reflectivity determining the asymmetric cyclotron radiation and the related current drive can be envisaged in existing tokamaks using conventional measurements of radiation temperature.

It is shown that the toroidal ion temperature gradient (ITG) mode has an upper ηi (higher ηi) stability regime for experimentally relevant parameter values in addition to the lower regime with the stability threshold at ηi around one ( where L_n and L_T1 are the characteristic lengths for the density and ion temperature gradients). The ITG mode is studied with a focus on the upper ηi stability regime and the β dependence (β = plasma pressure/magnetic pressure). The results of a gyrokinetic and a two-fluid model as well as a semilocal approximation are compared. It is shown that the upper stability threshold is very sensitive to and considerably reduced by finite-β effects. It is also sensitive to finite Larmor radius (FLR) effects and to (L_B is the characteristic length for the toroidal magnetic field gradient). Predictions and comparisons are made with data from a joint European torus (JET) optimized shear discharge and a JET hot ion H-mode (high-performance mode) discharge.

Under detached plasma conditions in the Alcator C-Mod tokamak, the measured spectra show pronounced merging of the Balmer series lines and a photo-recombination continuum edge which is not a sharp step. This phenomenon, known as a smooth discrete-to-continuum (D-C) transition, is typical only for high-density , low-temperature (T_e≈1 eV), and recombining plasma. As we will discuss, this type of transition emphasizes the degree of plasma non-ideality.

A theoretical model capable of treating spectra from a detached divertor plasma, and those with a smooth D-C transition in particular, has been developed. It is comprised of three parts: (i) a collisional-radiative model for the population densities of the excited states, (ii) atomic structure and collision rates for an atom affected by statistical plasma microfields, and (iii) a model for calculating the line profiles and the extended photo-recombination continuum.

The effects of statistical plasma microfields on the population densities of excited states, on the profiles of Balmer series lines, and on the photo-recombination continuum edge are discussed. The changes in spectrum characteristics with plasma parameter variation, leading to the smooth D-C transition, are analysed. The relevance of volumetric plasma recombination to the spectra observed from a detached divertor plasma is discussed. A comparison of the calculated and measured spectra is used to determine the plasma parameters in the recombining plasma region. Along with other properties of measured spectrum, the smooth D-C transition provides evidence in support of the recombining state of the plasma attained under detached divertor conditions.

Combined interferometer and polarimeter systems, using a single detecting element per line of sight, are susceptible to perturbation of the interferometric phase when modulation of the polarization vector is applied. This issue has been investigated extensively for the case of a rotating elliptically polarized probing beam, demonstrating that here a perturbation is inevitable. In this article the analogy between this analysis and earlier work is pointed out, and the underlying physics discussed. It will be demonstrated that schemes have been proposed in which the perturbation has been avoided or kept well within acceptable limits.

Plasma production studies excited by the radio frequency (RF) wave were carried out using various antenna configurations. Six types of side antennae, located outside the large cylindrical chamber, and two types of internal loop antennae were tested. The ion saturation current of a probe and the plasma emission were measured as a function of the filling pressure, the RF power and the magnetic field. With the increase in the power and the magnetic field, increased in most cases, while some of the antennae showed the maximum values under low magnetic field. Among the side antennae, a spiral antenna produced the highest value of in a wide operational window. For the case of the internal loop antennae, the electric field parallel to the magnetic field did not show a significant contribution to plasma production. These results suggest the importance of the loop-like RF-induced electric field. When an external magnetic field was applied, the uniformity of the plasma produced by the internal loop antennae in both the radial and axial directions was improved.

Beam stopping cross section and shine-through for neutral hydrogen beam injection into fusion plasmas have been calculated by using recommended cross sections presently available for atomic processes including multistep collision processes involving excited states. The shine-through thus obtained agrees well with recent experiments of JT-60U. The present calculations show that the multistep processes play a crucial role in the stopping of high-energy neutral hydrogen beams in high-density plasmas. Analytical fits to the stopping cross sections and fitting parameters are also presented for plasma impurities with nuclear charge and Z = 26.

Electron cyclotron resonance heating (ECRH) has become an attractive technique in today's fusion experiments not only as a heating mechanism but also as a diagnostic tool and a way to induce controllable currents through electron cyclotron current drive (ECCD). In this paper we show, using ray tracing simulations, how one can address all these issues with the ECRH system designed for the TJ-II stellarator. This ECRH set up consists of two 500 kW gyrotrons at f = 53.2 Ghz coupled to the plasma through two quasi-optical transmission lines, placed at two stellarator symmetric positions, and equipped with an internal steerable mirror. Using the three-dimensional (3D) Hamiltonian ray tracing code, TRECE, almost full single-pass absorption with localized power deposition is found for both on- and off-axis heating for any magnetic configuration. The simulations also show that it is possible to induce an appreciable ECCD despite the small oblique incidence angles attainable with the movable mirrors because of the rapid variation of the magnetic field direction and strength with the toroidal angle in TJ-II.