Observation of stable superdense core plasmas in the large helical device

In reduced recycling discharges in the Large Helical Device, a super dense core plasma develops when a series of pellets are injected. A core region with density as high as 4.5 x 10(20) m(-3) and temperature of 0.85 keV is maintained by an internal diffusion barrier with very high-density gradient. These results may extrapolate to a scenario for fusion ignition at very high density and relatively low temperature in helical devices.     

Diagnostics for radial electric field measurements in hot magnetized plasmas

Measurements of the radial electric field profile in magnetically confined plasmas have yielded important new insights in the physics of L-H transitions, edge biasing and/or the active control of Internal and Edge Transport Barriers. The radial electric field is not an easy plasma parameter to diagnose. Techniques to measure the radial electric field in the plasma core are the Heavy Ion Beam Probe and the Motional Stark Effect. An indirect method that is quite often applied is to derive the electric field from measurements of the poloidal and toroidal rotation velocities via the radial ion force balance. This paper will first briefly explain the need for detailed measurements of the radial electric field profile. Subsequently, the various diagnostics to measure this parameter will be reviewed. The emphasis will be especially put on recent trends, rather than on an exhaustive overview. Presented at 5th Workshop “Role of Electric Fields in Plasma Confinement and Exhaust”, Montreux, Switzerland, June 23–24, 2002. Partner in the Trilateral Euregio Cluster     

Neutral transport code for rovibrational population calculation of molecular hydrogen in large helical device plasmas

An rovibrationally resolved collisional-radiative model of molecular hydrogen (K. Sawada and M. Goto, Atoms 4: 29, 2016) is included in the authors' neutral transport code for large helical device (LHD) plasmas. The spatial distributions of the electron temperature and density, including the divertor and divertor leg regions, are given to the code. The molecules released from the graphite divertor target are tracked. The initial rotational and vibrational states of the released molecules are provided by simulation using a molecular dynamics model. The rovibrational population produced in an LHD plasma is evaluated.     

Energy confinement time and heat transport in initial neutral beam heated plasmas on the large helical device

The confinement characteristics of large net-current-free plasmas heated by neutral-beam injection have been investigated in the Large Helical Device (LHD). A systematic enhancement in energy-confinement times from the scaling derived from the medium-sized heliotron/torsatron experiments have been observed, which is attributed to the edge pedestal. The core confinement is scaled with the Bohm term divided by the square root of the gyro radii. The comparative analysis using a dimensionally similar discharge in the Compact Helical System indicates gyro-Bohm dependence in the core and transport improvement in the edge region of LHD plasmas.     

Influence of peripheral radial electric field on L-H transition in T-10 tokamak

In this paper the peripheral plasma behaviour during and after L-H transition on the tokamak T-10 is considered. Abrupt changes of Scrape-Off Layer (SOL) plasma parameters (ion saturation current, floating potential, electron temperature) are observed in regimes with transition to the improved plasma confinement. Such changes begin (5–150) ms prior to the L-H transition. The time delay of a transition depends on auxiliary plasma heating powerP _aux and plasma currentI _p (at constant toroidal magnetic field inductionB _t). Presented at the Workshop on the Role of Electric Fields in Plasma Confinement and Exhaust, Budapest, 18–19 June, 2000. This work is supported by Ministry of Atomic Energy of Russia (contract 69F) and by Ministry of Science and Technology of Russia (Federal Program “Controlled Thermonuclear Fusion and Plasma Processes”).     

Observation of the "self-healing" of an error field island in the large helical device

It was observed that the vacuum magnetic island produced by an external error magnetic field in the large helical device shrank in the presence of plasma. This was evidenced by the disappearance of flat regions in the electron temperature profile obtained by Thomson scattering. This island behavior depended on the magnetic configuration in which the plasmas were produced.     

Influence of radial electric field on trajectories of plug potential bounce ion in the tandem mirror

Bouncing ions between the plug potentials play an important role in improvement of the axial confinement in the tandem mirror. We examined the influence of the radial electric field on the trajectories of the ions passed through the anchor cell with nonaxisymmetric magnetic configuration on the assumption that the shape of the magnetic flux tube was shifted from the shape of the equipotential surface of the plasma at the mirror throats of the anchor cells. The discrepancy between the shapes enhanced the radial drift of the bounce ion. Radial potential profile of the core plasma was controlled by adjustment of the radially separated endplate potentials, and it was found that the flattened radial potential profile was effective for the decrease of the radial drift. Presented at 5th Workshop “Role of Electric Fields in Plasma Confinement and Exhaust”, Montreux, Switzerland, June 23–24, 2002.     

Influence of ion implantation on the thermal diffusivity of semiconductors

The influence of ion implantation on the thermal diffusivities of semiconductors are studied using the mirage effect. The dependences of the thermal diffusivities on the implantation doses are obtained. For silicon wafers implanted by boron, phosphorus and arsenic ions, with constant implantation energy, the thermal diffusivities decrease with increasing dose, when the doses are less than some critical values. The theoretical calculation results by using a one-dimensional multilayer model are in good agreement with the experimental ones. On the other hand, for gallium-arsenide wafers implanted with silicon ions, it is found experimentally that the thermal diffusivity increases with the implantation dose.     

Sawtooth oscillation in current-carrying plasma in the large helical device

Sawtooth oscillations have been observed in current-carrying helical plasmas by using electron-cyclotron-emission diagnostics in the Large Helical Device. The plasma current, which is driven by neutral beam injection, reduces the beta threshold of the sawtooth oscillation. When the central q value is increased due to the plasma current, the core region crashes, and, when it is decreased, the edge region crashes annularly. Observed rapid mixture of the plasma in the limited region suggests that these sawtooth crashes are reconnection phenomena. Unlike previous experiments, no precursor oscillation has been observed.     

Transport of the ion bounced by the plug potential caused by radial electric field in the tandem mirror

In the tandem mirror, plug potentials are created in the plug/barrier cells by microwave injection, and also anchor cells with a minimum B magnetic field configuration are installed in order to reduce the MHD instability. Existence of the ion bounced by the plug potential (PP-bounced ion) is essential for the ion confinement in the tandem mirror. We study the transport of the PP-bounced ion on the assumption that the cross-sectional shape of the magnetic flux tube is slightly different from the cross-sectional shape of the equi-potential surface at the mirror throats of the anchor cell as small deviation from the equilibrium state. The radial-potential profile of the core plasma is adjusted by controlling the electrostatic potentials of the coaxially separated end plates. We find that the spread type of radial-potential profile is effective for the confinement of the plug-potential-bounced ion.Presented at the Workshop Electric Fields Structures and Relaxation in Edge Plasmas, Nice, France, October 26–27, 2004.     

Effect of electron–ion coupling on the electric microfield distribution in plasmas

The electronic and ionic effective potential of a fully ionized hydrogen plasma containing an impurity of electric charge (+Z _m e ) are calculated in a two‐component plasma model under semiclassical conditions using classical statistical mechanics with a regularized electron–ion interaction. These effective potentials are coupled in a system of nonlinear integral equations (or coupled differential equations), which is solved numerically with two methods, namely the fixed‐point method and the Runge–Kutta method. The Baranger–Moser electric microfield distributions are calculated and compared with those from molecular dynamics simulation. Agreement between theory and simulation is satisfactory, in general.     

Fully self-consistent ion-drag-force calculations for dust in collisional plasmas with an external electric field

The ion drag force on a spherical dust particle immersed in a flowing plasma with an external electric field is self-consistently calculated using the particle-in-cell code SCEPTIC in the entire range of charge-exchange collisionality. Our results, not based on questionable approximations, extend prior analytic calculations valid only in a few limiting regimes. Particular attention is given to the force direction, shown never to be directed opposite to the flow except in the continuum limit, where other forces are of a much stronger magnitude.     

Effect of electric field on the metal-insulator transition with the formation of superstructure

The mechanism of the effect of an electric field on the metal-insulator transition with the formation of superstructure is considered with allowance for the field modulation of the electronic spectrum. Experimental data on the switching effect in vanadium dioxide are interpreted on the basis of this modified model.     

Ion heating and high-energy-particle production by ion-cyclotron heating in the large helical device

Ion-cyclotron heating was applied to the Large Helical Device. When the proton-cyclotron resonance was near the saddle point of the magnetic field-strength plane, strong ion-cyclotron damping occurred. Under these conditions efficient plasma heating was achieved for more than one minute. A high-energy ion tail was observed, and the effective tail temperature was determined by a balance between the wave acceleration and the electron-drag relaxation. There was no apparent sign of particle orbit loss effect in the investigated density range of 0.8-1.3x10(19) m(-3).     

Characteristics of electron heat transport of plasma with an electron internal-transport barrier in the large helical device

Associated with the transition from ion root to electron root, an electron internal transport barrier (ITB) appears in the large helical device, when the heating power of electron cyclotron resonance heating exceeds the threshold power. The incremental thermal diffusivity of electron heat transport chi(inc)(e) in the ITB plasma is much lower than that in the plasma with the heating power below the threshold, and the thermal diffusivity chi(e) decreases with increasing of heating power [dchi(e)/d(P/n(e))<0] in helical ITB plasmas.     

Analysis of bifurcation phenomena in the electron internal transport barrier in the large helical device

The electron internal transport barrier (eITB) formation in the Large Helical Device (LHD) is studied with the transport code TOTAL and a GyroBohm-like model. The reduction of anomalous transport by the E x B shear has been introduced by means of the factor [1 + (tauomega(ExB))(gamma)](-1). Simulation results show a clear critical transition between plasma regimes with rather flat electron temperature profiles (non-) to a steeped one (with eITB) when average density is low enough. With the aim of studying the eITB formation as a phase transition phenomenon, the electron average density is taken as the control parameter and the E x B shearing rate as the order parameter. Results show how the eITB formation in LHD is compatible with a continuum phase transition with critical exponent beta = 0.40.     

Specific heat,thermal diffusivity and thermal conductivity of Ca doped NaNO3 crystals near the transition point

Thermal characteristics of calcium doped NaNO₃ crystals near the transition point were investigated. Their values near the transition point decrease proportionally to the increasing calcium concentration in crystal. In the same way decreases the critical temperature. Thermal conductivity changes can be explained by the presence of an order-disorder scattering mechanism.Dúbravská cesta, Bratislava, Czechoslovakia.     

Experimental measurements and modelling of the structure of the radial electric field in RFX

The experimental determination of the radial electric fieldE _r and the associatedE ×B drift velocity at the edge of RFX is presented and possible mechanisms responsible for its generation are discussed.E _r measurements by means of an array of Langmuir probes and those deduced from Doppler spectroscopy of impurity lines agree fairly well. In particular the rotation velocity of the plasma edge has been determined from the Doppler shift of a C III emission line. The observation of other ions characterised by different radial positions, such as B IV and C V, has allowed an estimate of the velocity shear. Typical values of plasma rotation at the edge are around 10 km/s while the velocity shear is of the order of (10⁵−10⁶)s⁻¹ in the spontaneous layer, a few cm thick, usually observed in standard discharges. Plasma rotation has been artificially modified by both positive and negative edge biasing and the associated increase or decrease of the fluid velocity is well in agreement with the radial electric field change. The modification ofE _r during edge biasing and Pulsed Poloidal Current Drive (PPCD) are also reported and interpreted within a momentum balance model. Analytical and self-consistent Monte Carlo models at the plasma edge suggest that impurities have a relevant role in the generation of the radial electric field, due to their relatively large Larmor radius. Presented at the Workshop on Role of Electric Fields in Plasma Confinement and Exhaust, Budapest, 18–19 June 2000.     

A note on the ion sound theory in an infinite plasma with electric field

Stability of the electroacoustic wave in an infinite one dimensional plasma with a DC electric field is analysed. Residual charged particle-neutral collisions are modelled by a BGK term. With the ion collisions weak, the effect of ion acceleration in the DC electric field is taken into account. Conditions for unstable propagation are discussed.