Osamu Yamagishi

First plasmas in Heliotron J

Results obtained in the initial experimental phase of Heliotron J are reported. Electron beam mapping of the magnetic surfaces at a reduced DC magnetic field has revealed that the observed surfaces are in basic agreement with the ones calculated on the basis of the measured ambient field around the device. For 53.2 GHz second harmonic ECH hydrogen plasmas, a fairly wide resonance range for breakdown and heating by the TE02 mode has been observed in Heliotron J as compared with that in Heliotron E. With ECH injection powers up to ≈ 400 kW, diamagnetic stored energies up to ≈ 0.7 kJ were obtained without optimized density control.

On the particle fluxes and density profiles in helical systems

Quasilinear and neoclassical particle fluxes and their relation to the density profiles in helical plasmas are discussed, by taking the Large Helical Device (LHD) [O. Motojima et al., Nucl. Fusion 43, 1674 (2003)] as a representative example. Various magnetic configurations can be realized in the LHD experiments by changing the coil currents. The neoclassical particle flux shows strong configuration dependence through the significant change of effective helical ripple, while it is insensitive to the change of density profiles. On the other hand, the quasilinear flux shows weak dependence on the magnetic configurations while it strongly depends on the density profiles. As a result of the particle balance in the steady state in the no-source region, a qualitative explanation for the hollow density profiles, which are often observed experimentally in LHD, is proposed. In the appendixes, properties of quasilinear particle flux in a simple model tokamak are discussed, and expressions for neoclassical fluxes in...

Ballooning instabilities in a Heliotron J plasma

Ideal magnetohydrodynamic stability analysis of local pressure-driven modes in an L=1 heliotron, Heliotron J [M. Wakatani et al., Nucl. Fusion 40, 569 (2000)], is investigated by means of three-dimensional (3D) ballooning formalism and the Mercier criterion. In 3D systems such as heliotrons, the ballooning modes are separated into two categories: One is tokamak-like ballooning modes which are localized only in the poloidal direction, and the other is modes inherent to 3D systems which are localized on the specific flux tubes. The tokamak-like ballooning modes change to the Mercier modes in the limit that the mode is sufficiently extended along the field line, but the nonaxisymmetric ballooning mode does not so. The L=1 Heliotron J equilibrium investigated here has weak global shear and the dominant Fourier amplitudes of magnetic-field strength is rather different from the conventional helical systems with L=2 helical coils. Since the weak global shear causes the reduction of integrated local shear along t...

MHD equilibrium and pressure driven instability in L = 1 heliotron plasmas

Free boundary MHD equilibrium properties of Heliotron J are investigated by VMEC, HINT and PIES codes, and ideal MHD stability properties are studied by the Mercier criterion, the ballooning mode equation and the CAS3D global stability code. It is shown by the equilibrium calculations that the change of the plasma boundary shape is substantial in a low shear helical system even if β is relatively low. Preliminary comparison between PIES results and HINT results shows that the β value at which the magnetic island begins to be perceptible is almost the same in both codes, but the island width seems to be different. It is considered that the difference comes from a difference of pressure distribution in real space which is determined through each numerical algorithm. In the stability analysis, the effect of the global magnetic shear on the three-dimensional or helical ballooning mode whose mode structure shows strong poloidal and toroidal mode (helical mode) coupling is investigated. It is found that the positive shear of the rotational transform is favourable for the three-dimensional ballooning mode stability in a low shear helical system.

Global ballooning instabilities in a Heliotron J plasma

The global ballooning stability in a Heliotron J plasma [M. Wakatani et al., Nucl. Fusion 40, 569 (2000)] is studied by use of the CAS3D code (code for analysis of stability in three-dimensional systems) [C. Schwab, Phys. Fluids 5, 3195 (1993)]. The global mode has strong toroidal mode coupling so that the mode structure is typically a helical type. The mode structure of the pressure driven modes, including the interchange mode, is discussed. The comparison between the local and global modes is also shown. The eigenvalue as well as eigenfunction shows good agreement.

Shape effect of the outermost flux surface on effective helical ripple and zonal flow response in an L = 2 heliotron

We calculate two indicators for neoclassical and anomalous transport in the low collisional regime, effective helical ripple eeff and zonal flow response , in an L = 2 heliotron with various shapes of the outermost flux surface. The eeff has a minimum as a function of a parameter representing plasma column twisting, δb. The time average of the damped zonal flow, , shows a similar dependence on δb. We can thus find the optimum configuration for both these indicators in an arbitrary L = 2 heliotron, by choosing the optimum value of δb, together with inherent toroidicity and main helicity of the outermost flux surface. The existence of the optimum is due to the most effective cancellation of the radial drifts of the particles trapped in each helical ripple, rather than the magnetic field symmetry in a whole surface.

Collisionless kinetic-fluid simulation of zonal flows in non-circular tokamaks

Fluid simulation of linear zonal flow damping is done with a closure model based on the collisionless gyrokinetics [Sugama et al., Phys. Plasmas 14, 022502 (2007)]. Simulation results of residual zonal flow for low radial wavenumbers are compared with theoretical formulas for circular and non-circular tokamaks. The effects of the elongation and the triangularity are shown to be properly treated in the closure model. Effects of initial parallel flows on zonal flow evolution are also clarified. An appropriate choice of the initial parallel flow gives a much higher residual level than the conventional result with no initial parallel flow. Besides, the zonal flow simulations are done with the E × B nonlinearity as initial sources, which is evaluated from linear gyrokinetic microinstabilities such as ion temperature gradient modes, trapped electron modes, and electron temperature gradient modes, in order to estimate efficiency of zonal flow generation by the source instabilities.

Kinetic effects on the ideal pressure-driven modes in an L=2 heliotron

The kinetic effects on the ideal ballooning and interchange/Mercier modes are studied in model equilibria for an L=2 heliotron, large helical device (LHD) [A. Iiyoshi et al., Nucl. Fusion 39, 1245 (1999)]. It is shown that the ion finite Larmor radius (FLR) effect stabilizes the modes with high toroidal mode number, n. On the other hand, the finite electron compressibility plays a double role, and stabilizes the low-n modes as the ideal magnetohydrodynamic (MHD) modes, while it destabilizes the high-n modes. It is discussed that the inclusion of the compressibility impacts the stability, and this effect is stronger in LHD than in a comparable tokamak, which is due to the larger magnitude of the local curvature. As a result of the competition between the FLR and the compressibility, it is shown in LHD that the low-n instabilities can become much weaker than that expected by the ideal MHD, while the high-n instabilities are prone to remain unstable near the plasma core region.

Fluid simulation of tokamak ion temperature gradient turbulence with zonal flow closure model

Nonlinear fluid simulation of turbulence driven by ion temperature gradient modes in the tokamak fluxtube configuration is performed by combining two different closure models. One model is a gyrofluid model by Beer and Hammett [Phys. Plasmas 3, 4046 (1996)], and the other is a closure model to reproduce the kinetic zonal flow response [Sugama et al., Phys. Plasmas 14, 022502 (2007)]. By including the zonal flow closure, generation of zonal flows, significant reduction in energy transport, reproduction of the gyrokinetic transport level, and nonlinear upshift on the critical value of gradient scale length are observed.

Microinstabilities, Turbulent Transport, and Structure Formation in Helical Plasmas

Abstract In this paper, microturbulence, turbulent transport, and related structure formation mechanisms in helical plasmas are discussed, with special attention to application to Large Helical Device (LHD) plasmas. The research frontier for the physics of anomalous transport and confinement improvement is illuminated, and future direction of the confinement research is illustrated.