In the axisymmetrized tandem mirror GAMMA 10, thermal-barrier and plug potentials have been formed in the axisymmetric mirror cells at both ends and directly measured with Au neutralbeam probes and end-loss analyzers. Strong end-loss reduction associated with the potential formation results in enhancement of the axial particle confinement time 100 times over the mirror confinement time without plugging, in reasonable agreement with Pastukhov formula. An empirical scaling on nonambipolar radial ion confinement time in the axisymmetrized field configuration is presented.PACS numbers: 52.55.Jd Current tandem mirror researches focus on improvement of confinement properties over the original configuration.

This paper describes the recent progress in divertor simulation research using the GAMMA 10/PDX tandem mirror towards the development of divertors in fusion reactors. During a plasma flow generation experiment in the end cell of the GAMMA 10/PDX, ICRF heating in the anchor cell successfully extended the particle flux up to 3.3  ×  1023 m2 s−1. Superimposing the short pulse of the ECH also attained a maximum heat flux of ~30 MW m−2. We have succeeded in achieving and characterizing the detachment of the high-temperature plasma, which is equivalent to the SOL plasma of tokamaks, by using the divertor simulation experimental module (D-module) in the GAMMA 10/PDX end cell, in spite of using a linear device with a short magnetic field line connection length. Various gases (Ar, Xe, Ne and N2) are examined to evaluate the effect of radiation cooling against the plasma flow at the MW m−2 level in the divertor simulation region and the following results are obtained: (i) Xe gas was most effective in the reduction of heat and particle fluxes (1%, 3%, respectively) and has a stronger effect on electron cooling (down to ~1.6 eV) in the used gas species. (ii) Ne gas was less effective. On the other hand, (iii) N2 gas showed more favorable effects than Ar in the lower pressure range. These results will contribute to the progress in detached plasma operation and in clarifying the radiation cooling mechanism towards the development of future divertors.

Plasma confinement with a transport barrier such as an H mode or an internal transport barrier mode is examined under the constraint due to the conservation of total angular momentum. The results are tested against actual experimental data and the general characteristics of plasma confinement with a transport barrier are well understood in terms of this constraint. This implies that the confinement of tokamak plasmas can be determined by the decay rate of the total angular momentum. It also suggests that the confinement with a transport barrier is good, since electrostatic fluctuations cannot affect this constraint, but that electromagnetic fluctuations such as ELMs can cause the confinement to deteriorate.

Collisionless tearing instabilities and the consequent enhanced transport are investigated theoretically and also numerically, using a two-and-one-half-dimensional particle simulation code in a slab geometry. The effects of electrostatic fields on the instability are also considered. The initial current is found to diffuse along the perturbed magnetic field lines, the observed diffusion along the perturbed magnetic field lines, the observed diffusion coefficient being in good agreement with the theoretical prediction. The growth of the instability is observed to divide into three phases; a linearly unstable phase, a quasi-stable phase, and a nonlinear phase similar to the Rutherford phase. Electrostatic effects have a tendency to enhance the tearing mode growth rate. In multi-mode tearing, a coalescence of magnetic island is observed.

Vortexlike turbulent structures in hot-ion mode plasmas with several keV are observed in the case with a radially produced weak shear of electric fields E(r). However, a strong E(r) shear formation due to a high ion-confining potential phi(c) production clears up these vortices together with plasma-confinement improvement and disappearance of both drift-wave and turbulencelike Fourier spectral signals. These findings are based on three-time progress in phi(c) in comparison to phi(c) attained 1992-2002. The significant advance of phi(c) is well extended in line with proposed potential-formation physics scalings.

No abstract

Off-axis electron-cyclotron heating in an axisymmetric barrier mirror produces a cylindrical layer with energetic electrons, which flow through the central cell and into the end region. The layer, producing a localized bumped ambipolar potential Phi(C), forms a strong shear of radial electric fields E(r) and peaked vorticity with the direction reversal of E(r)xB sheared flow near the Phi(C) peak. Intermittent vortexlike turbulent structures near the layer are suppressed in the central cell by this actively produced transverse energy-transport barrier; this results in T(e) and T(i) rises surrounded by the layer.

No abstract