Radial Electric Field Structure Research Articles

The structure, evolution and role of the radial edge electric field in H-mode and L-mode on MAST

The first measurements of the structure of the edge radial electric field, Er, in a spherical tokamak (MAST) are presented. Using active Doppler spectroscopy on He+ with 120 lines of sight Er profiles are calculated from the leading terms of the radial momentum balance. A spatial resolution up to Δr ≈ 1.5 mm with a typical time resolution of Δt = 5 ms can be achieved. In L-mode the field is largely determined by the diamagnetic term of the force balance, and fields of only a few kV/m are observed. The measured impurity flow is mostly parallel to B, and is greatly affected by MHD, such as sawteeth or mode locking of tearing modes, or error fields. In H-mode a strong perpendicular flow evolves with poloidal and toroidal velocities up to vHe+ϕθ ≈ -20 km/s, and a deep negative electric field well Ermin ≳ -15 kV/m develops. The profile form is dominated by the diamagnetic term.

Modelling of the Edge Plasma with Account of Self-Consistent Electric Fields

Recent results on the simulation of the edge plasma parameters are discussed with emphasis on the role of self-consistent electric fields. It is demonstrated that simulation with B2SOLPS5.0 code as well as with the other codes is consistent with the neoclassical nature of the radial electric field in the core region 1-2 cm inside the separatrix. Near the separatrix the viscous layer exists, where the impact of the parallel fluxes of the SOL is important and electric field deviates from the neoclassical value. It is shown that the experimental dependence of the L-H transition threshold on the local and global plasma parameters might be explained on the basis of the simulations. The impact of the geometrical factors on the radial electric field structure is discussed. It is shown that the change of the parallel fluxes in the scrape-off layer, which are transported through the separatrix due to turbulent viscosity and diffusivity, should result in variation of the radial electric field. The parallel (toroidal) fluxes in the SOL and mechanisms of their formation are analyzed for different geometry and directions of the magnetic field. Currents in the SOL and in the core region are analyzed and compared with the analytical predictions. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Reduced divertor heat loads, plasma shape effects, and radial electric field structures in JFT-2M HRS H-mode plasmas

A key feature of the ‘high recycling steady’ (HRS) H-mode regimes is the reduction of the transient heat load to the divertor target due to large ELMs. The heat flux to the divertor targets measured by means of a Langmuir probe in the scrape-off layer on the target plate is about ∼0.3 MW m−2 in the typical HRS plasmas, which is very small in comparison with the transient heat load in the ELMy H-mode discharge. In addition, the effects of the plasma shaping on the access to and improved plasma performance of HRS H-mode regimes have been investigated. Furthermore, simultaneous measurements of both density fluctuations and potential profiles at the plasma edge region using a heavy ion beam probe are also presented.

Effect of electrode biasing on the radial electric field structure bifurcation in tokamak plasmas

The mechanism for formation of a steep structure in the radial electric field is a key issue in plasma confinement. Properties of the radial electric field bifurcation are studied taking into account the effect of electrode biasing. The radial electric field structure is determined by the charge conservation equation. From the nonlinear mechanism associated with local current due to ion bulk viscosity, a transition can take place. Various types of radial electric field structures with multiple peaks are allowed for the same boundary condition. The ion orbit loss term breaks the symmetry of the radial current similarly to the ambipolar radial electric field. A radial current driven by the electrode plays the role of a control parameter in a transition similarly to the pressure gradient. A phase diagram is given in the spontaneous drive vs external drive space. Differences in the radial shape of solitary electric field structures are demonstrated in the presence of spatial varying components. This study clarifies the mechanisms of nonlinear structure formation in transport barriers.

Asymmetrical bifurcation of the radial electric field structure induced by electrode biasing in tokamaks

Radial electric field bifurcation induced by electrode biasing near the tokamak edge is considered in this paper. The radial electric field structure is analysed including two non-linear terms in the radial current, namely the neoclassical term and the orbit loss term. Their competition gives various radial electric field structures, and biasing transition is compared with spontaneous transition. Bifurcation has different features for positive and negative biasing, with a larger number of structures available for the negative biasing case. Effectiveness of the solitary radial electric field structures induced by biasing for confinement improvement is discussed.

The Structure of the Radial Electric Field in the Vicinity of the Separatrix and the L-H Transition

The Structure of the Radial Electric Field in the Vicinity of the Separatrix and the L-H Transition

Investigations of Radial Electric Field and Global Circulation Layer in Limiter Tokamaks

An updated version of the 2D multifluid code TECXY is used to study the radial electric field structure and the appearance of a global circulation layer (GCL) inside the separatrix of the limiter tokamaks TEXTOR-94 and Tore-Supra-CIEL. The dependence of the driving forces on device geometry, limiter position, magnetic field orientation, impurity content and other parameters is investigated. The centrifugal force in the vicinity of the limiter head always determines the direction of the poloidal velocity in the GCL. There is good agreement with experimentally measured profiles of the poloidal velocity at the TEXTOR low field side.

Theoretical Analysis of Structure of Radial Electric Field in Helical Toroidal Plasmas.

The structure of the electric field is studied by use of the theoretical model for the anomalous transport diffusivities. In this analysis, the thermal diffusivity is employed based on the theory of self-sustained turbulence due to the balooning mode and the interchange mode, both driven by the current diffusivity. Multiple solutions of Er for the ambipolar condition are obtained in the structure of the radial electric field. The reduction of the anomalous transport coeffient is examined due to the strong gradient of the radial electric field in the phase diagram of the external sources.

Radial Electric Field Structure between Electrodes Separated by a Finite Distance in Tokamak Plasmas

Various types of peaked structure in the radial electric field are predicted theoretically in edge biasing experiments using electrodes. A nonlinear diffusion equation is employed to describe the radial electric field, and is solved in the region bounded by electrodes. Structures with multiple peaks are allowed when the distance between electrodes is finite. The minimum value for the distance between electrodes is obtained. The distance between electrodes, the externally driven current and the applied voltage determine the number of solutions. Multiple transitions and hysteresis are predicted from the relationship between the voltage and the current.

Bifurcation of the radial electric field in the presence of edge polarization in tokamaks

In guiding-center simulations of a tokamak plasma edge, bifurcations of both the radial electric field and the electrode current are observed for a threshold value of the electrode bias voltage, in qualitative agreement with experiments. The simulations are neoclassical and Monte Carlo based, also containing the perpendicular viscosity effect. Above the voltage threshold, an inward moving, solitary radial electric field structure is generated with a concomitant drop in the electrode current. The position, width, and height of the soliton depend on the magnitude and radial variation of the conductivity (and, thus, on local plasma parameters), finite orbit effects, and also on the perpendicular viscosity and neutral density. The obtained Er structures are narrower and higher than the ones measured in the edge polarization experiments in the Jülich tokamak TEXTOR-94 [S. Jachmich et al., Plasma Phys. Controlled Fusion 40, 1105 (1998)].

Modeling of tokamak edge plasma for discharges with neutral beam injection

Edge plasma of tokamak ASDEX-Upgrade is simulated for discharges with unbalanced neutral beam injection. Toroidal momentum transport through the separatrix and further to the divertor plates is simulated and compared with simple analytical solution. Structure of radial electric field near the separatrix is simulated and compared with the predictions of neoclassical theory.

Simulation of edge-plasma profiles and turbulence related toL-H transitions in tokamaks

Results are presented for fluid-equation simulations of transport and turbulence in the edge region of a tokamak using the transport code UEDGE and the turbulence code BOUT. UEDGE calculates two-dimensional plasma and neutral gas profiles using empirical radial transport coefficients, whileBOUT simulates three-dimensional plasma turbulence and resulting transport using experimental or UEDGE plasma profiles. Using UEDGE, the radial electric field structure near the magnetic separatrix is studied for variations of core power, anomalous perpendicular ion viscosity, νa⊥, and prompt ion loss. A BOUT simulation measures the effective νa⊥ from the calculated turbulence-induced radial ion current, giving a value similar to the ion heat diffusivity.

Toroidal mode structure in weak and reversed magnetic shear plasmas and its role in the internal transport barrier

It is becoming clear that anomalous transport in an L-mode tokamak plasma is dominated by radially extended non-local modes in toroidal geometry. This indicates that various improved modes such as the internal transport barrier (ITG) formed in a recent reversed magnetic shear experiment can be achieved by suppressing such a global structure of the toroidal mode. We investigate the role of plasma shear rotation and negative/weak and also reversed magnetic shear on these non-local modes by employing our toroidal simulation code together with a non-local theory. From simulations, in addition to the overall reduction of the wave excitation in the entire negative magnetic shear region, we find that the weak or zero magnetic shear which appears near the minimum q (safety factor) surface breaks up toroidal coupling. This leads to a discontinuity (or gap) in wave excitation around the -surface, leading to the emergence of the transport barrier. The plasma shear rotation which is observed in the experiment enhances the discontinuity so that the performance of the discontinuity is increased. We also investigate the structure of the self-generated radial electric field induced by such ITG modes and the related plasma shear rotation (referred to as the zonal flow) which influence the fluctuation level in the steady state.

Solitary radial electric field structure in tokamak plasmas

The solitary structure solution of the radial electric field Er in the tokamak plasmas is obtained. It is shown to be stable under an external power supply, like a biased electrode at the edge. The radial gradient is governed by the ion viscosity and the nonlinearlity of the perpendicular conductivity. The radial structure of Er and reduction of turbulent transport, which belong to key issues of the high confinement mode (H-Mode) [F. Wagner et al., Phys. Rev. Lett. 49, 1408 (1982)], are self-consistently determined. A bifurcation from a radially-uniform one to a solitary one occurs at a certain applied voltage, and a hysteresis is associated.

Turbulent transport and structural transition in confined plasmas

The theory of far-nonequilibrium transport of plasmas is described. Analytic as well as simulation studies are developed. The subcritical nature of turbulence and the mechanism for self-sustaining are discussed. The transport coefficient is obtained. The pressure gradient is introduced as an order parameter, and the bifurcation from collisional to turbulent transport is shown. The generation of the electric field and its influence on the turbulent transport are analysed. The bifurcation of the radial electric field structure is addressed. Hysteresis appears in the flux-gradient relation. This bifurcation causes the multifold states in the plasma structure, driving the transition in the transport coefficient or the self-generating oscillations in the flux. The structural formation and dynamics of plasma profiles are explained.

Physics of edge plasmas in enhanced confinement modes

In this article, the physics of various enhanced confinement modes are reviewed. After the discovery of the H-mode, many enhanced confinement modes have been observed. Various kinds of enhanced confinement modes are surveyed first. Typical features of enhanced confinement are the edge pedestal, the density peaking in the core, the modification of the current profile, and the internal transport barrier. Physical processes, which allow these improvements, are explained in terms of the radial electric field structure and magnetic field structure. The influence of the edge and SoL plasma on the core improvement is also explained. An emphasis is made on the role of neutrals in the dynamics of enhanced confinement, illuminating the importance of PSI studies in realizing better enhanced confinement. Perspectives for the steady state plasma are also discussed.

Effect of alpha -particles on radial electric field structure in Torsatron/Heliotron reactor

The radial electric field structure in the fusion reactor having the Torsatron/Heliotron configuration is discussed, taking into account the effects of the orbit loss of alpha -particles. The direct loss of alpha -particles has only a small influence on the profile of the radial electric field. The trapping efficiency of the alpha -particle is also evaluated by using the self-consistent radial electric field.

On Radial Electric Field Structurein CHS Torsatron/Heliotron

The radial electric field structure in helical system is discussed taking into account the effects of the orbit loss and charge exchange loss in addition to the neoclassical fluxes. Analysis is made for the CHS torsatron/heliotron, in which the radial electric field E r was estimated experimentally. It was found that the fast ion orbit loss and charge exchange loss have strong influence on the profile of the radial electric field, particularly near the plasma periphery. The absolute value of E r in the experiments is still more negative than the theoretical prediction.

Edge electric-field profiles of H-mode plasmas in the JFT-2M tokamak.

The structure of the edge radial electric field ${\mathit{E}}_{\mathit{r}}$ inferred from the poloidal rotation velocity is compared with that of the particle and thermal transport barrier for H-mode plasmas in JFT-2M. Both ${\mathit{E}}_{\mathit{r}}$ and its gradient \ensuremath{\partial}${\mathit{E}}_{\mathit{r}}$/\ensuremath{\partial}r in the thermal transport barrier are found to become more negative at the L-H transition. On the other hand, \ensuremath{\partial}${\mathit{E}}_{\mathit{r}}$/\ensuremath{\partial}r is more positive outside of the separatrix. The shear of the radial electric field and poloidal rotation velocity in the H mode is localized within the order of an ion poloidal gyroradius near the separatrix, in the region of ion collisionality ${\ensuremath{\nu}}_{\mathrm{*}\mathit{i}}$\ensuremath{\approxeq}20--40.