Toroidal Helical Plasma Research Articles

On violation of local closure of transport relation in high-temperature magnetized plasmas

Rapid propagation of heat modulation and a hysteresis in the gradient-flux relation are found in a global nonlinear simulation of drift-interchange mode turbulence in toroidal helical plasmas. A global mode is excited nonlinearly and induces the turbulence flux in a limited radial region. The nonlinear couplings take a finite temporal duration for redistributing the energy. The mode also has a seesaw effect: increase of the amplitude of the global mode, at the other radii, works to absorb the energy form microscopic modes to suppress the turbulence. Successive excitations of microscopic modes cause the accelerated propagation of change of the heat flux like turbulence spreading after the onset of modulation. Owing to these processes, the hysteresis appears in the gradient-flux relation, which is compared with experiments.

Dual Maxwell Construction and Transport Barrier of Helical Plasmas

The interface of transport in toroidal plasmas is studied. The case, in which the gradient-flux relation is a multiple-valued function, is analyzed by use of the statistical theory of confined plasmas. An analytic formula for the determination of the interface of transport is given, taking an example of the transport in toroidal helical plasmas. The formula is given in a form of a dual Maxwell construction (rule of equal area). The location of the interface in stationary state and the access to the stationary state are discussed.

Activities on Integrated Simulations in LHD

One of the purposes of fusion simulations is to develop a code that could predict the entire temporal behavior of experimentally observed macroscopic physics quantities under continuous external control, which will be used to create the path to helical-type reactor by combining knowledge of reactor design. In this paper an integrated simulation code system for three-dimensional toroidal helical plasmas in the Large Helical Device (LHD) is reported. This code has been developed under the domestic and international research collaborations among universities and institutes. After explaining the structure of the code system, including the transport simulation code TASK3D and the magnetohydrodynamic (MHD) equilibrium and stability code MHD3D, we present typical simulation results: evolution of the rotational transform, MHD stability beta limit, and recent progress in the TASK3D code.

Development of a Hierarchy-Integrated Simulation Code for Toroidal Helical Plasmas, TASK3D

The present status of the development of a hierarchy-integrated simulation code for toroidal helical plasmas, TASK3D, is reported. TASK3D is developed by extending the integrated modeling code for tokamak plasmas, Transport Analyzing System for tokamaK (TASK) [A. Fukuyama et al., Proc. of 20th IAEA Fusion Energy Conf. (Villamoura, Portugal, 2004) IAEA-CSP-25/CD/TH/P2-3]. In order to extend TASK to be applicable for threedimensional configurations, a new module for the radial electric field in general toroidal configurations has been developed and implemented. As a first test for this implementation, numerical simulations for the time evolution of temperature and electric field are conducted on the basis of an LHD experimental result, by a successful combination of a diffusive transport module and the implemented electric field module.

Transport analysis of the effect of zonal flows on electron internal transport barriers in toroidal helical plasmas

The role of zonal flows in the formation of transport barrier in helical plasmas is analysed using the transport code. A set of one-dimensional transport equations is analysed, including the effect of zonal flows. The turbulent transport coefficient is shown to be suppressed when the plasma state takes the strong positive radial electric field. This bifurcation of the turbulent transport is newly caused by the change of the damping rate of zonal flows. It is theoretically demonstrated that the damping rate of zonal flows plays an important role in global confinement in toroidal plasmas.

Reconstruction method of local density fluctuation for heavy ion beam probe measurements

Heavy ion beam probe (HIBP) is an excellent diagnostic to measure the density and potential fluctuations simultaneously in magnetically confined plasmas. However, it has been well known that the density fluctuation measured with the HIBP is not local but contains the fluctuations along the beam orbits. In this article, a method is proposed to evaluate local density fluctuation in the HIBP measurements by removing the well-known path integral effects. The reconstructed density fluctuation amplitude and power spectrum are shown, for example, by applying the proposed method on the density fluctuation measurement data obtained in a toroidal helical plasma, Compact Helical System.

Physics of internal transport barrier of toroidal helical plasmas

The role of zonal flows (ZFs) in the formation of an internal transport barrier in a toroidal helical plasma is analyzed. The turbulent transport coefficient is shown to be suppressed when the plasma state changes from the branch of a weak negative radial electric field to the strong positive one. This new transition of turbulent transport is caused by the change of the damping rate of the ZFs. It is clearly demonstrated, theoretically and experimentally, that the damping rate of the ZFs governs the global confinement of toroidal plasmas.

Turbulence and transport characteristics of a barrier in a toroidal plasma

Turbulence and zonal flow at a transport barrier are studied with twin heavy ion beam probes in a toroidal helical plasma. A wavelet analysis is used to extract turbulence properties, e.g. spectra of both density and potential fluctuations, the coherence and the phase between them and the dispersion relation. Particle transport estimated from the fundamental characteristics is found to clearly rise with their intermittent activities after the barrier is broken down. Time-dependent analysis reveals that the intermittency of turbulence is correlated with the evolution of the stationary zonal flow.

Spectrograph of electric field fluctuation in toroidal helical plasma

This is a report on fluctuation measurements using twin heavy ion beam probes in CHS. The observation shows that dozens of coherent modes with intermittent nature co-exist in an electron cyclotron heated plasma. The modes are found to have a long-distance correlation in the toroidal direction. The radial structure of the modes is evaluated in potential fluctuation. The characteristics are consistent with those of the geodesic acoustic modes.

Observation of core electron temperature rise in response to an edge cooling in toroidal helical plasmas

The first observation of a significant rise of core electron temperature in response to edge cooling in a helical plasma has been made on the Large Helical Device [O. Motojima et al., Phys. Plasmas 6, 1843 (1999)]. When the phenomenon occurs, the electron heat diffusivity in the core region is reduced abruptly without changing local parameters in the region of interest. Therefore the phenomenon can be regarded as a so-called “nonlocal” electron temperature rise observed so far only in many tokamaks.

Modelling of Density Limit Phenomena in Toroidal Helical Plasmas

The physics of density limit phenomena in toroidal helical plasmas based on an analytic point model of toroidal plasmas is discussed. The combined mechanism of the transport and radiation loss of energy is analyzed, and the achievable density is derived. A scaling law of the density limit is discussed. The dependence of the critical density on the heating power, magnetic field, plasma size and safety factor in the case of L-mode energy confinement is explained. The dynamic evolution of the plasma energy and radiation loss is discussed. Assuming a simple model of density evolution, of a sudden loss of density if the temperature becomes lower than critical value, then a limit cycle oscillation is shown to occur. A condition that divides the limit cycle oscillation and the complete radiation collapse is discussed. This model seems to explain the density limit oscillation that has been observed on the Wendelstein 7-AS (W7-AS) stellarator.

Observation of bifurcation properties of radial electric fieldsusing a heavy ion beam probe

The bifurcated nature of the potential profile of a toroidal helical plasma is investigated in CHS using a heavy ion beam probe.The measurements reveal that three main branches of potential profile exist in ECR heated plasmas with a low density of n̄e ≈ 0.5 × 1019m-3.The branches with higher central potential exhibit a rather strong radial electric field shearthat should result in fluctuation reduction and formation of a transport barrier.An analysis of the pulsating behaviour in the potential suggests the existence of a fine structure in the potential profiles between the main branches. Lissajous and histogram expressions are useful in extracting the bifurcation characteristics of the potential structure.

Experimental study of the bifurcation nature of the electrostatic potential of a toroidal helical plasma

The bifurcation nature of the electrostatic structure is studied in the toroidal helical plasma of the Compact Helical System (CHS) [K. Matsuoka et al., Proceedings of the 12th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Nice, 1988 (International Atomic Energy Agency, Vienna, 1989), Vol. 2, p. 411]. Observation of bifurcation-related phenomena is introduced, such as characteristic patterns of discrete potential profiles, and various patterns of self-sustained oscillations termed electric pulsation. Some patterns of the electrostatic structure are found to be quite important for fusion application owing to their association with transport barrier formation. It is confirmed, as is shown in several tokamak experiments, that the thermal transport barrier is linked with electrostatic structure through the radial electric field shear that can reduce the fluctuation resulting in anomalous transport. This article describes in detail spatio-temporal evolution during self-sustained oscillation, together with correlation between the radial electric field and other plasma parameters. An experimental survey to find dependence of the temporal and spatial patterns on plasma parameters is performed in order to understand systematically the bifurcation property of the toroidal helical plasma. The experimental results are compared with the neoclassical bifurcation property that is believed to explain the observed bifurcation property of the CHS plasmas. The present results show that the electrostatic property plays an essential role in the structural formation of toroidal helical plasmas, and demonstrate that toroidal plasma is an open system with a strong nonlinearity to provide a new attractive problem to be studied.

Electron Thermal Transport Barrier and Density Fluctuation Reduction in a Toroidal Helical Plasma

A thermal transport barrier resulting in a high central electron temperature of $\ensuremath{\sim}2\mathrm{keV}$ is established in the core of electron-cyclotron-resonance heated plasmas of the Compact Helical System Heliotron/Torsatron. The formation of the barrier is correlated with the reduction in the density fluctuations and with a structural change of the radial electric field profile at the barrier location. The results suggest that the decrease in fluctuation should contribute to a reduction of anomalous transport and a drastic increase in electron temperature at the barrier.

Nonlinearity of Radial Electric Field and Self-Excited Oscillation in a Toroidal Helical Plasma

Nonlinearity of Radial Electric Field and Self-Excited Oscillation in a Toroidal Helical Plasma

Electric pulsation and profile quantization in CHS heliotron/torsatron

Radial electric field is a key physical quantity in determining the structure of toroidal helical plasmas, owing to the nonlinear dependence of transport on the radial electric field. It has been observed that the potential profiles show various interesting patterns in space and time in the compact helical system (CHS) plasmas. In particular, the phenomenon of electric pulsation, clearly demonstrates the nature of bifurcation of radial electric field in toroidal helical plasmas.

Measurement of Space Potential Fluctuations During MHD Activities in a Toroidal Helical Plasma with a Heavy Ion Beam Probe

A heavy ion beam probe (HIBP) is a unique diagnostic instrument which can measure local plasma potential and its fluctuations. However, in the presence of MHD activities, acceleration or deceleration of both primary and secondary beams are caused by the fluctuating vector potential ∂ A /∂ t along the beam trajectories (path integral effect). Such effect possibly induces a contamination to space potential measurement. We have evaluated those effects during a bursting MHD instability in the Compact Helical System (CHS), which is considered to be an m / n =2/1 interchange instability. In this procedure, the vector potential profile is estimated both theoretically and experimentally. Contribution from path effect is found to be less than several percent to the measured space potential fluctuation. Then space potential fluctuation is found to be localized around q =2 magnetic surface, which is consistent with a model for the interchange mode.

Discovery of Electric Pulsation in a Toroidal Helical Plasma

Discovery of Electric Pulsation in a Toroidal Helical Plasma

Dynamic Behavior of Potential in the Plasma Core of the CHS Heliotron/Torsatron

During combined electron cyclotron and neutral beam heating, an abrupt drop and rise in potential by about one half of the central electron temperature ({approximately}400 V) was observed in the core of the compact helical system heliotron/torsatron plasma. Drastic changes of radial electric field between positive and negative states occur over periods as short as 60{mu} s, which is much shorter than the energy confinement time scale of about a few milliseconds. A nonlinear relation between the radial electric field and radial current is obtained. This is the first experimental observation of a microsecond time scale spontaneous transition based on an electric field bifurcation in toroidal helical plasmas. {copyright} {ital 1997} {ital The American Physical Society}

Direct observation of potential profiles with a 200 keV heavy ion beam probe on the Compact Helical System

In this paper we present space potential profiles directly observed in a toroidal helical plasma of the Compact Helical System (CHS) [K. Matsuoka et al., Proceedings, 12th International Conference on Plasma Physics and Controlled Nuclear Fusion, Nice, 1988 (International Atomic Energy Agency, Vienna, 1989), Vol. 2, p. 411], using a 200 keV heavy ion beam probe. The potential profiles exhibit widely varied characteristics, including positive and negative polarities for electron cyclotron and neutral beam-heated plasmas, respectively. The behavior of high-energy particles in the CHS plasmas are deduced from loss cone diagrams evaluated from the observed potential profiles.