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Yamada H.,Japan National Institute for Fusion Science
Nuclear Fusion | Year: 2011

The physical understanding of net-current-free helical plasmas has progressed in the Large Helical Device (LHD) since the last Fusion Energy Conference in Geneva, 2008. The experimental results from LHD have promoted detailed physical documentation of features specific to net-current-free 3D helical plasmas as well as complementary to the tokamak approach. The primary heating source is neutral beam injection (NBI) with a heating power of 23 MW, and electron cyclotron heating with 3.7 MW plays an important role in local heating and power modulation in transport studies. The maximum central density has reached 1.2 × 10 21 m -3 due to the formation of an internal diffusion barrier (IDB) at a magnetic field of 2.5 T. The IDB is maintained for 3 s by refuelling with repetitive pellet injection. In a different operational regime with moderate density less than 2 × 10 19 m -3, a plasma with a central ion temperature reaching 5.6 keV exhibits the formation of an internal transport barrier (ITB). The ion thermal diffusivity decreases to the level predicted by neoclassical transport. In addition to the rotation driven by the momentum input due to tangential NBI, the existence of intrinsic torque to drive toroidal rotation is identified in the plasma with an ITB. This ITB is accompanied by an impurity hole which generates an impurity-free core. The impurity hole is due to a large outward convection of impurities in spite of the negative radial electric field. The magnitude of the impurity hole is enhanced in the magnetic configuration with a large helical ripple and for heavier atoms. Another mechanism for suppressing impurity contamination is identified at the plasma edge with a stochastic magnetic field. A helical system shares common physics issues with tokamaks such as 3D equilibria, transport in a stochastic magnetic field, plasma response to a resonant magnetic perturbation, divertor physics and the role of radial electric field and meso-scale structure. © 2011 IAEA, Vienna. Source

Ida K.,Japan National Institute for Fusion Science | Rice J.E.,Massachusetts Institute of Technology
Nuclear Fusion | Year: 2014

Poloidal and toroidal rotation has been recognized to play an important role in heat transport and magnetohydrodynamic (MHD) stability in tokamaks and helical systems. It is well known that the E × B shear due to poloidal and toroidal flow suppresses turbulence in the plasma and contributes to the improvement of heat and particle transport, while toroidal rotation helps one to stabilize MHD instabilities such as resistive wall modes and neoclassical tearing mode. Therefore, understanding the role of momentum transport in determining plasma rotation is crucial in toroidal discharges, both in tokamaks and helical systems. In this review paper, the driving and damping mechanisms of poloidal and toroidal rotation are outlined. Driving torque due to neutral beam injection and radio-frequency waves, and damping due to parallel viscosity and neoclassical toroidal viscosity (NTV) are described. Regarding momentum transport, the radial flux of momentum has diffusive and non-diffusive (ND) terms, and experimental investigations of these are discussed. The magnitude of the diffusive term of momentum transport is expressed as a coefficient of viscous diffusivity. The ratio of the viscous diffusivity to the thermal diffusivity (Prandtl number) is one of the interesting parameters in plasma physics. It is typically close to unity, but sometimes can deviate significantly depending on the turbulent state. The ND terms have two categories: one is the so-called momentum pinch, whose magnitude is proportional to (or at least depends on) the velocity itself, and the other is an off-diagonal term in which the magnitude is proportional to (or at least depends on) the temperature or/and pressure gradient, independent of the velocity or its gradient. The former has no sign dependence; rotation due to the momentum pinch does not depend on the sign of the rotation itself, whether it is parallel to the plasma current (co-direction) or anti-parallel to the plasma current (counter-direction). In contrast, the latter has a sign dependence; the rotation due to the off-diagonal residual term is either in the co- or counter-direction depending on the turbulence state, but not on the sign of the rotation itself. This residual term can also act as a momentum source for intrinsic rotation. The experimental results of investigations of these ND terms are described. Finally the current understanding of the mechanisms behind the ND terms in momentum transport, and predictions of intrinsic rotation driven by these terms are reviewed. © 2014 IAEA, Vienna. Source

Gorelenkov N.N.,Princeton Plasma Physics Laboratory | Pinches S.D.,ITER Organization | Toi K.,Japan National Institute for Fusion Science
Nuclear Fusion | Year: 2014

The area of energetic particle (EP) physics in fusion research has been actively and extensively researched in recent decades. The progress achieved in advancing and understanding EP physics has been substantial since the last comprehensive review on this topic by Heidbrink and Sadler (1994 Nucl. Fusion 34 535). That review coincided with the start of deuterium-tritium (DT) experiments on the Tokamak Fusion Test Reactor (TFTR) and full scale fusion alphas physics studies. Fusion research in recent years has been influenced by EP physics in many ways including the limitations imposed by the 'sea' of Alfvén eigenmodes (AEs), in particular by the toroidicity-induced AE (TAE) modes and reversed shear AEs (RSAEs). In the present paper we attempt a broad review of the progress that has been made in EP physics in tokamaks and spherical tori since the first DT experiments on TFTR and JET (Joint European Torus), including stellarator/helical devices. Introductory discussions on the basic ingredients of EP physics, i.e., particle orbits in STs, fundamental diagnostic techniques of EPs and instabilities, wave particle resonances and others, are given to help understanding of the advanced topics of EP physics. At the end we cover important and interesting physics issues related to the burning plasma experiments such as ITER (International Thermonuclear Experimental Reactor). © 2014 IAEA. Source

Okamura S.,Japan National Institute for Fusion Science
Contributions to Plasma Physics | Year: 2010

Magnetic configurations of LHD experiments are analyzed from a viewpoint of boundary shape. Effects of Fourier mode components on confinement properties are examined for three configurations with magnetic axis shift. It was confirmed that the basic confinement properties are determined by a small number of components. Contribution of components producing a non-planar axis structure is necessary to obtain the fundamental confinement properties of inward shifted (favorable drift orbits) and outward shifted (creation of magnetic well) configurations of LHD. Further improvement is possible by controlling appropriate Fourier components. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Source

Yasuhara R.,Japan National Institute for Fusion Science | Furuse H.,Japan Institute for Laser Technology
Optics Letters | Year: 2013

The thermal-birefringence-induced depolarization in terbium gallium garnet (TGG) ceramics has been investigated experimentally. The depolarization ratio of 6.1 × 10-4 has been observed at the maximum input power of 117Wcw, which corresponds to a normalized laser power of p = 0.14. As predicted by the previously proposed theory, the amount of depolarization ratio and its slope with respect to the laser power of the ceramic TGG was approximately the same as that previously reported for high-quality-cut (111) single crystal. © 2013 Optical Society of America. Source

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