[en] Analytical and numerical studies of the linear and non-linear behaviour of resistive tearing and ballooning modes have been completed and applied to two important tokamak physics issues: major disruptions and transport. (author)

[en] The Phaedrus tandem mirror has been operated with min -|B| quadrupole end cells, with axisymmetric end cells and without end cells. ICRF, ECRF and biased (and heated) end rings have been used to achieve plasma potential control and MHD stability. Axial and radial plasma potential control is achieved by modifications to the electron distribution functions using ICRF and ECRF, or by radial end wall bias. Large ion-confining potentials are located near where ω=0.8ωsub(ci). These are not associated with significant increases in ion density. By varying the bias on sets of concentric end rings it has been possible to adjust the shape of the radial potential profile throughout the device. Radial ponderomotive force from the ICRF with ω>ωsub(ci) provides MHD stability to the central cell plasma and allows operation without end cells or with axisymmetric end cells. A heated end ring is also found to enhance the MHD stability of a marginally stable plasma. ICRF with ω<ωsub(ci) is found to be destabilizing but it is found that ECRF can provide some stability to a plasma heated with such ICRF. ECRF is also found to produce electron tails to energies of several keV. Application of too large a radial electric field using the biased end rings also results in unstable plasmas. Azimuthal ponderomotive force from half-turn antennas results in plasma convection across the magnetic field. (author)

[en] Flat-top current pulses of approximately 2 ms have been obtained in ETA-BETA II. In these conditions, reversed field pinch configurations are maintained at THETA>=2 and βsub(theta) approx.= 0.1, and the heating phase is prolonged as compared to the decaying current operation. These discharges at Isub(phi) approx.= 100 kA are characterized by plasma resistances >or approx.0.6 mΩ, fluctuation amplitudes approx.2% with dominant modes m=0.1; n/m approx.=-10; and Zsub(eff)approx.=2. It is found that increasing the density does not cool the discharge, but the confinement improves and β increases until the high density limit is reached. The steady state discharges are discussed in terms of an empirical model for the field profiles from which the q-limits of the configuration are derived and a comparison with the tokamak is drawn. Discharges at Isub(phi)>or approx. 200 kA show an encouraging scaling corresponding to plasma resistances >or approx. 0.25 mΩ and lower fluctuation amplitudes. (author)

[en] The paper reports experimental results on potential formation and end plugging in the axisymmetrized tandem mirror GAMMA 10. The plugging at both ends has been achieved by a combination of neutral beams and gyrotrons. The presence of a plug potential with a thermal barrier in an axisymmetric mirror has been confirmed by direct measurement of the axial potential profile. Enhancement of axial particle confinement has been observed during the end plugging. Non-ambipolar radial transport has been greatly reduced in the axisymmetrized magnetic configuration. The potentials measured by beam probes and end loss analysers are 0.7, 0.4 and 1.1 kV in the central, barrier and plug regions, respectively. Strong end plugging is observed when the central-cell density is higher than the densities in the plug and the barrier, and the plug density remains higher than the barrier density. The plug electron temperature is higher than the central temperature. Hot electrons forming a football-shaped profile have been stably produced in the axisymmetric mirror. The beta value and the fraction of the hot electrons reach up to 5% and 0.8, respectively. Central-cell ion-cyclotron resonance heating can sustain a stable plasma with higher density and ion temperature when resonance surfaces exist in both the anchor and the central cells. (author)

[en] The numerical program HBT has been employed to study global and local stability of high-beta axisymmetric toroidal plasmas with various cross-sections. The stability results of different classes of equilibria for a circular plasma show that high stable betas (<β>/epsilon=0.082) can be found when negative toroidal current densities are admitted in a small region on the inside of the torus. Shaping the cross-section makes further improvements possible. The optimum betas found are in qualitative agreement with Troyon's scaling law. (author)

[en] An explanation is given for the occurrence of tokamak disruptions. Under normal conditions the tearing instability is self-stabilizing through its reduction of the destabilizing current gradient, and a saturated state can then exist. Under certain conditions, however, further growth of the instability is itself destabilizing, and a disruption results. The transition to disruption has the form of a catastrophe. (author)

[en] Energetic trapped particles are shown to introduce a new unstable solution to the internal kink and ballooning modes in tokamaks. Both the real frequencies and growth rates of the instabilities are comparable to the trapped particle precession frequency. Simulations including the excitation and particle-loss mechanisms of the internal kink mode are found to reproduce essential features of the 'fishbone'. Furthermore, the energetic trapped-particle-induced ballooning modes are shown to be consistent with the associated high-frequency oscillations observed experimentally. Several possible stabilizing schemes are considered. (author)

[en] Experimental and theoretical plasma focus study has resulted in the necessity of creating a non-hydrodynamic plasma focus structure model (MKHD model). This model describes the final stage of plasma focus, which starts immediately after maximum plasma compression. It is related to a very limited space near the neck of the sausage instability. The MKHD model is two-dimensional, axially symmetric and collisionless with respect to the ions and magnetohydrodynamic with respect to the electrons; it accounts for the pinch instability of the sausage type (m=0 mode). The MKHD model, first of all, explains the long time of the plasma focus existence and non-thermonuclear peculiarities in the neutron yield. The initial and boundary conditions are formulated in accordance with the experiments and the results of computations in the 2D MHD model. A non-stationary process of plasma focus dynamics is studied numerically for a relatively long time - about 20 ns; this time is, in principle, not restricted. The computations show that the external edge of the neck expands rather slowly (at a speed that is lower than the thermal ion velocity, by an order of magnitude), and the magnetic field energy is converted to the kinetic energy of the chaotic ion motion (which is doubled for the time of computation). A 'supra-thermal' tail (with the deuterium ion energy higher than 10 keV) forms slowly at the ion distribution function; this tail determines a substantial part of the total neutron yield. The formation of stable vortices, which actually determine the structure of the plasma flow during the developed non-hydrodynamic stage of the plasma focus, is also found in the computations. These properties of the development of the sausage instability, as found in the numerical experiment with the MKHD plasma focus model, are in qualitative agreement with the behaviour of an instability of the same type in the MHD models of the Z-pinch

[en] Part A. A technique for determining the pressure distributions over magnetic surfaces limited by ideal-MHD stability is proposed. The limiting tokamak plasma β-values are calculated for different plasma geometries and safety factors at the boundary. Part B. The results of numerical studies on the stability of ballooning helical and tearing modes in a tokamak plasma with hollow current profile are given. (author)

[en] The results of studies on the interaction between relativistic electron beams (REBs) and plasma performed at the Nuclear Physics Institute, Siberian Branch, USSR Academy of Sciences, are reported. It has been demonstrated on the INAR device that injection into the plasma of a beam with a small angular spread leads to a distribution function of heated electrons which is uneven. In addition to the main group of electrons, which have a temperature of some tens of eV, the plasma contains fast electrons with energies between a few hundred eV and several tens of keV. On the GOL-1 device it has been shown that REBs with a large angular spread transfer their energy to a small group of electrons with energies of a few tens of keV. The density of particles in this group decreases with increasing angular spread of the beam. Theoretical estimates of the density and characteristic energy of the hot electrons have been obtained on the assumption that plasma heating is produced by the excitation of Langmuir turbulence, the level of which is limited as a result of elastic scattering of waves by plasma density perturbations generated by ponderomotive force. A brief description is given of the GOL-3 device, which is being constructed at the Nuclear Physics Institute, and experimental results are provided on the production of powerful microsecond REBs on the U-1 accelerator, which serves as the basic beam source module for this device. (author)