Experimental test of the radial force balance equation in the compact helical system

Ida, K.; Fujisawa, A.; Iguchi, H.; Yoshimura, Y.; Minami, Takashi; Okamura, S.; Nishimura, Satoshi; Isobe, M.; Kado, S.; Liang, Y.; Nomura, I.; Tanaka, K.; Takahashi, C.; Matsuoka, K.

doi:10.1063/1.1331097

Cited by 23 publications

(6 citation statements)

References 13 publications

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“…(Of course this is not the case when the toroidal symmetry is broken in tokamak experiments.) Since the radial electric field is indispensable in reducing ripple losses in helical plasmas, the detailed structure of the radial electric field and poloidal flow has been intensively studied [26][27][28][29][30][31][32]. The mean poloidal flows are measured with charge exchange spectroscopy, while the fluctuating poloidal flows (zonal flows and GAMs) are measured with heavy ion beam probes in helical systems.…”

Section: Relation Between Perpendicular and Parallel Flow And Radial ...mentioning

confidence: 99%

Rotation and momentum transport in tokamaks and helical systems

Ida

Rice

2014

Nucl. Fusion

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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.

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Section: Relation Between Perpendicular and Parallel Flow And Radial ...mentioning

confidence: 99%

Rotation and momentum transport in tokamaks and helical systems

Ida

Rice

2014

Nucl. Fusion

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“…The lowest order radial force balance equation is typically used to relate the minority ion velocity measurements to the velocity of the bulk ion [33]. This equation is 'radial' in the coordinate normal to the flux surfaces and states E + V i × B − ∇p i /Z i en i • ∇ψ = 0, for the ith ion species, where p i is the ion pressure, n i is the density and E is the electric field.…”

Section: Measurement Techniquesmentioning

confidence: 99%

Tokamak rotation sources, transport and sinks

deGrassie¹

2009

Plasma Phys. Control. Fusion

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Toroidal rotation has become a topic of wide interest experimentally and theoretically because of the recognition that it is important for confinement, stability and even access to the H-mode confinement regime. The inception of ITER has generated a focus upon developing a prediction for rotation in burning plasmas where auxiliary injected torque will be relatively small. We will describe experimental results and cite theory, organized by sources of toroidal momentum, transport of this momentum and sinks for momentum, including the effects of some loss of toroidal axisymmetry (AS). We will also describe the so-called 'intrinsic rotation' where rotation under AS conditions is observed in the absence of any auxiliary momentum source, presumably due to effects such as off-diagonal transport elements or turbulent stresses and others. We describe how rotation is measured, and specific areas where the importance of rotation has been established. The predominant source of rotation generation in present devices is the toroidal torque from neutral beam injection. Experiments verify the accuracy of neoclassical (NC) models to describe the torque deposition process in AS conditions. Applied electromagnetic wave power is also found to generate rotation. The radial transport of momentum is found to be much larger than predicted by standard NC theory, having transport rates similar to that of ion thermal energy. Experiments have also verified the existence of a pinch term in the momentum transport, which could generate the interior rotation gradient often observed with intrinsic rotation. The ambient or purposely imposed non-axisymmetric magnetic fields can provide an interior sink for momentum, and that may also drag the rotation to a nonzero offset value. The rotation itself tends to shield out resonant perturbations. Nonresonant (NR) perturbations from toroidal field ripple have long been considered, and the area of NR perturbations has taken on new import for the fields generated by perturbation coils for mitigation of edge localized modes. We consider some of

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“…The radial electric field itself is expected to affect the particle transport especially impurity transport [17][18][19], which is in contrast to the radial electric field shear effect on the energy transport. In the CHS experiment, the impurities tend to accumulate at the plasma axis and the impurity and electron density profiles tend to peaked in plasmas with negative electric field, while the impurity is exhausted and impurity and density profiles are hollow in plasmas with positive electric field [20]. Therefore it is expected that the positive electric field should contribute to suppress the influx of impurities and prevent the radiation collapse.…”

Section: Role Of Radial Electric Field On Particle Transportmentioning

confidence: 99%

Control of the radial electric field shear by modification of the magnetic field configuration in LHD

et al. 2005

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Control of the radial electric field, Er, is considered to be important in helical plasmas, because the radial electric field and its shear are expected to reduce neoclassical and anomalous transport, respectively. In general, the radial electric field can be controlled by changing the collisionality, and positive or negative electric fields have been obtained by decreasing or increasing the electron density, respectively. Although the sign of the radial electric field can be controlled by changing the collisionality, modification of the magnetic field is required to achieve further control of the radial electric field, especially to produce a strong radial electric field shear. In the Large Helical Device (LHD) the radial electric field profiles are shown to be controlled by the modification of the magnetic field by (1) changing the radial profile of the effective helical ripples, εh, (2) creating a magnetic island with an external perturbation field coil and (3) changing the local island divertor coil current.

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Experimental test of the radial force balance equation in the compact helical system

Cited by 23 publications

References 13 publications

Rotation and momentum transport in tokamaks and helical systems

Rotation and momentum transport in tokamaks and helical systems

Tokamak rotation sources, transport and sinks

Control of the radial electric field shear by modification of the magnetic field configuration in LHD

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