Reversal Of Toroidal Magnetic Field Research Articles

Alfvén eigenmode structure during off-axis neutral beam injection

The spatial structure of Alfvén eigenmodes on the DIII-D tokamak is compared for contrasting fast ion deposition profiles resulting from on- and off-axis neutral beam injection (NBI). In both cases, poloidal mode rotation and eigenmode twist, or radial phase variation, are correlated with the direction of the normal ion diamagnetic flow and readily inverted with a reversal of toroidal magnetic field, BT. While off-axis NBI results in weakly driven reversed shear induced Alfvén eigenmodes due to reduced fast ion pressure gradient, ∇βfast, in the region of the mode, these marginally unstable modes exhibit a 2D phase structure that is indistinguishable from that observed during on-axis injection. This result is consistent with recent explorations using the non-perturbative codes Gyro and TAEFL that show a weak dependence of eigenmode structure on drive when fast ion density is uniformly reduced by a scalar multiplier. These codes also obtain unstable, counter-propagating modes with the inverted 2D phase structure when BT is kept constant and the diamagnetic flow direction is reversed by making ∇βfast sufficiently positive for an isotropic population of fast ions. While measurements of the spatial profile of fast ion D-α light from the recently upgraded charge exchange recombination diagnostic on DIII-D suggest a strong modification of fast ion pressure towards this limit, no counter-propagating modes have yet been observed in experiment.

Simulation of small size divertor tokamak plasma edge under effect of toroidal magnetic field reversal

Asymmetries between the divertor legs of small size divertor (SSD) tokamak plasma edge are noticed to reverse when the direction of toroidal magnetic field is reversed. In the present paper the small size divertor tokamak plasma edge under effect of toroidal magnetic field reversal is simulated by B2SOLPS0.5.2D fluid transport code. The simulation demonstrate the following results: 1) Parallel (toroidal) flow flux and Mach number up to 0.6 at higher plasma density reverse with reverse toroidal magnetic direction in the edge plasma of small size divertor tokamak. 2) The radial electric field is toroidal magnetic direction independence in edge plasma of small size divertor tokamak. 3) For normal and reverse toroidal magnetic field, the strong ITB is located between the positions of the maximum and minimum values of the radial electric field shear. 4) Simulation result shows that, the structure of radial electric field at high field side (HFS) and low field side (LFS) is different. This difference result from the change in the parallel flux flows in the scrape off layer (SOL) to plasma core through separatrix. 5) At a region of strong radial electric field shear, a large reduction of poloidal rotation was observed. 6) The poloidal rotation is toroidal magnetic field direction dependence.

Response to comment on ‘Magnetic topology effects on Alcator C-Mod scrape-off layer flow’

In his comment to our recent work (Simakov et al 2008 Plasma Phys. Control. Fusion 50 105010), Aydemir has asserted that poloidal plasma flow reversal is not a valid response to toroidal magnetic field reversal in an up–down symmetric tokamak, and that the toroidal plasma flow must reverse instead. We show that this assertion is wrong due to his misunderstanding of the corresponding symmetry transformation.

Quasi-single helicity state by a small positive pulse of toroidal magnetic field in TPE-RX reversed field pinch experiment

By applying a small positive pulse (ΔBta) in toroidal magnetic field, the quasi-single helicity (QSH) state can be obtained with a controllable and reproducible manner in a reversed-field pinch (RFP) experiment on the large RFP machine, TPE-RX [Y. Yagi et al., Fusion Eng. Des. 45, 421 (1999)]. The QSH state in RFP is one of the states where the improved confinement can be observed, and is important for development toward the pure single helicity (SH) state. In the SH state, the dynamo-action for sustaining the RFP configuration will be driven by a single helical mode and its harmonics, and the anomalous plasma loss can be avoided which is caused by the multi-helicity dynamo action in ordinary RFPs. In the operating condition presented here, the reversal of toroidal magnetic field (Bta) is maintained at a shallow value (∼−1mT) for a certain period (∼20ms) after the setting up of the RFP configuration and then the positive ΔBta (≲5mT magnitude and ∼2ms width) is applied to the Bta, which is usually negative during the sustaining phase of RFP. Just after applying the pulse, the m∕n=1∕6 mode (m and n being the poloidal and toroidal Fourier mode numbers, respectively) grows dominantly and the configuration goes into QSH state. This QSH state can be sustained for a long period (up to ∼45ms) almost until the end of discharge by applying a delayed reversal of Bta with appropriate timing and magnitude. The setting up of the QSH states shows a reproducibility of almost 100% with the same timing corresponding to the applied positive pulse. This observation can confirm the interpretation in the former report [Y. Hirano et al., Phys. Plasmas 12, 112501 (2005)], in which it is claimed that the QSH state is obtained when a small positive pulse in toroidal magnetic field spontaneously appears.

Magnetic topology effects on Alcator C-Mod flows

The effect of magnetic topology on ion and impurity flows in a tokamak is considered by investigating the consequences of (i) the reversal of toroidal and poloidal magnetic fields and currents; (ii) a switch from lower to upper X-point operation; (iii) poloidal magnetic field or plasma current reversal, and (iv) toroidal magnetic field reversal. The general symmetries associated with magnetic topology changes in tokamaks are employed to demonstrate that the flux surface flows inside and outside the separatrix observed in Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] can be used to determine the flow features, including neoclassical and turbulent effects and in the presence of charge exchange.

Quasi-single helicity state at shallow reversal in TPE-RX reversed-field pinch experiment

The operating conditions for obtaining a quasi-single helicity (QSH) state with a good reproducibility are found in a reversed-field pinch (RFP) experiment on the large RFP machine, TPE-RX [Y. Yagi et al., Fusion Eng. Des. 45, 421 (1999)]. In these conditions, the reversal of toroidal magnetic field (Bta) is maintained at a very shallow value (∼−0.2mT) after the setting up phase and the following fast current rising phase. After a certain period at this shallow reversal (∼15–25ms), the m∕n=1∕6 mode (m and n being the poloidal and toroidal Fourier mode numbers, respectively) rapidly grows and saturates before the termination of discharge. The growth of this mode dominates the other modes and the QSH state with m∕n=1∕6 is finally achieved. This QSH state can be sustained for a long period (up to ∼45ms) almost until the end of discharge by applying a delayed reversal of Bta with appropriate trigger timing and magnitude. The initial setup of the QSH states shows a reproducibility of almost 100%, but its sustainment for a long period shows a slightly reduced reproducibility (∼85%). The initial rapid growth of the single dominant mode is compared with the numerical results of linear stability and nonlinear three-dimensional (3D) calculations by assuming the experimental magnetic field profile estimated with a standard model. Linear calculations show that the m∕n=1∕6 mode has the maximum growth rate to the ideal magnetohydrodynamic instability and can explain the dominant growth of this mode. The 3D calculations also show a qualitative agreement with the experiment, where under some conditions the m∕n=1∕6 mode becomes dominant after an initial relaxation and continues to the end of the simulation. These results indicate that the present QSH state is the combined result of the linear growth and nonlinear saturation of a particular mode.

High-Density Current Injection to Magnetically Confined Plasmas with a Miniature Plasma Source

Influences of high-density current injection with a miniature plasma source on a reversed-field pinch (RFP) have been studied in the Separatrix Test Experiment-2 (STE-2) device. The current injector has successfully produced a unidirectional field-aligned electron beam. No deterioration or slight improvement of the RFP discharge was observed with injection of poloidal current of up to 1 kA. The deepening of the local edge toroidal magnetic field reversal has been observed in the vicinity of the injection port. The global change in the current profile and the stabilization of m=1 tearing modes will require current injection at multiple toroidal locations with a higher bias voltage. However, the gas throughput from the injector is too high, hindering the addition of more injectors in the STE-2 device. Global current profile control experiments with multiple miniature plasma sources are expected in larger devices.

Finite element modelling of TdeV edge and divertor with E*B drifts

Simulation results are presented from a finite element calculation of transport in the edge and divertor of TdeV. In addition to the usual transport processes, this model also includes a calculation of the electrostatic potential and the associated E*B drifts. The inclusion of E*B drifts is shown to be essential in the calculation of the parallel and toroidal flow velocities. With the inclusion of drifts, the effect of toroidal magnetic field reversal is studied, particularly with regard to in-out asymmetries in the Halpha radiation and power deposited on the plates. Relatively weak asymmetries are calculated on the basis of these drifts alone, and it is concluded that the much stronger asymmetries observed experimentally must be associated with other physical processes not yet included in the model. The transport equations are based on fluid conservation equations for particles, parallel (to the magnetic field) momentum, electron and ion energy. For simplicity, a single ion species is considered and, when modelling recycling, neutral transport is treated in the diffusion approximation. All transport equations are discretized with finite elements on a triangular unstructured mesh that is locally aligned with the flux surfaces. The use of such a mesh allows an accurate representation of the simulation geometry, with boundaries of arbitrary shapes and complexity. In particular, it is possible to account for divertor plates that are not orthogonal to the flux surfaces without any penalty for the validity of the transport equations. The analysis presented here concerns a low density attached ohmic TdeV discharge in a single null geometry, with the X point above the midplane

Numerical simulation of the reversed field pinch dynamo effect in the quasilinear, mean field equilibrium limit

In this paper numerical simulations are presented of the reversed field pinch (RFP) dynamo effect utilizing the resistive magnetohydrodynamic (MHD) equations in the quasilinear approximation. In addition, a mean field equilibrium magnetic field configuration is constructed in cylindrical geometry by dropping inertia on the symmetric radial component of the velocity field. Results are obtained for a variety of operating conditions: flat-top current, current ramp-up, and ramp-down. It is found that the Alfvén time, although present in the equations describing the perturbed quantities, is not important on the long resistive run time; the result is a three-dimensional Ohmic equilibrium with symmetric mean toroidal magnetic field reversal. Using these results, and in view of recent RFP experimental data, a simple correspondence between global MHD behavior in the RFP and the tokamak is given.