Poloidal Electric Field Research Articles

Formation of spiral structures and radial convection in the edge region of a magnetized rotating plasma

The rotation of a cylindrical plasma column in a magnetic field has been studied in the linear section of the new plasma device Mistral. Under suitable conditions we observe a transition to a turbulent regime characterized by strong, bursty fluctuations at the edge of the column. The detection and the study of the spatio-temporal evolution of structures in the turbulent regime have been performed by means of a new enhanced conditional sampling technique. We have collected evidence of the development of a bent tail emanating from the plasma column. The charged particles inside the structure move along a spiral trajectory resulting in a net radial convection of the plasma to the walls. We show experimentally that a poloidal electric field is present inside the structures leading to the observed outwards radial E × B drift, in agreement with the expectations of recent and past theoretical works.

Convective particle transport arising from poloidal inhomogeneity in tokamak H mode

In tokamak high-confinement modes (H modes), a large poloidal flow exists within an edge transport barrier, and the electrostatic potential and density profiles can be steep both in the radial and poloidal directions. The two-dimensional structures of the electrostatic potential, density, and flow velocity near the edge of a tokamak plasma are investigated. The analysis is carried out with the momentum conservation law using the shock ordering. For the case with a strong radial electric field (H-mode case), a particle flux is induced from asymmetry of the poloidal electric field in the transport barrier. This convective transport is found to depend weakly on collisionality, and changes its direction in accordance with the direction of the radial electric field, the toroidal magnetic field, and the plasma current. The divergence of a particle flux is a source of temporal variation of the density, and there are negative divergence regions both in the inward and outward flux cases. Thus this convective particle flux is a new candidate for the cause of the rapid establishment of the density pedestal after the onset of low to high confinement mode (L∕H) transition.

Comparison of plasma turbulence in the low- and high-field Scrape-Off Layers in the T-10 tokamak

Experimental investigation of the Scrape-Off Layer (SOL) turbulence and the perpendicular anomalous particle transport in the high density regime in the T-10 tokamak is presented. Comparison of the plasma parameters measured at the low-field side (LFS) and the high-field side (HFS) of the tokamak shows that the inboard turbulence essentially differs from the outboard one. Turbulence has an intermittent character but the fluctuation level of the inboard ion saturation current is about two times lower than at the LFS. The radial turbulent particle flux and the effective perpendicular diffusion coefficient measured at the LFS are about 3–5 times higher than those observed at the HFS. Enhanced perpendicular particle flux at the LFS is defined by the high density plasma structures formed in the vicinity of the last closed flux surface. The poloidal electric field inside the high density structures has different directions at the inboard and outboard sides of the plasma column. The radial electric drift induced by the poloidal electric field is directed towards the vacuum vessel at both the LFS and the HFS. These experiments support the hypothesis that SOL parallel flow might be caused by an additional mechanism such as ‘ballooning’ transport, which generates a larger flux of particles from the core into the SOL at the outer mid-plane.

Two-Dimensional Structure and Particle Pinch in Tokamak H Mode

Two-dimensional structures of the electrostatic potential, density, and flow velocity near the edge of a tokamak plasma are investigated. The model includes the nonlinearity in bulk-ion viscosity and turbulence-driven shear viscosity. For the case with the strong radial electric field (H mode), a two-dimensional structure in a transport barrier is obtained, giving a poloidal shock with a solitary radial electric field profile. The inward particle pinch is induced from this poloidal asymmetric electric field, and increases as the radial electric field becomes stronger. The abrupt increase of this inward ion and electron flux at the onset of L- to H-mode transition explains the rapid establishment of the density pedestal, which is responsible for the observed spontaneous self-reorganization into an improved confinement regime.

Formation of convective cells during scrape-off layer biasing in the CASTOR tokamak

We describe experiments with a biased electrode inserted into the scrape-off layer (SOL) of the CASTOR tokamak. The resulting radial and poloidal electric field and plasma density modification are measured by means of Langmuir probe arrays with high temporal and spatial resolutions. Poloidally and radially localized stationary structures of the electric field (convective cells) are identified and a related significant modification of the particle transport in the SOL is observed.

ρ s scaling of characteristic turbulent structures in the torsatron TJ-K

The scalings of correlation lengths and times with the drift-scale parameter ρs and other dimensionless parameters are investigated. Using a novel 8×8 Langmuir probe matrix, the two-dimensional structure of drift-wave turbulence has been measured inside the confinement region of the toroidal low-temperature plasma in the TJ-K torsatron [N. Krause et al., Rev. Sci. Instrum. 73, 3474 (2002)]. Using five different gases from hydrogen to argon, ρs could be varied by a factor of 10. For small ion masses, the scalings of the microscopic parameters turn out to be close to the predictions from drift-wave turbulence, which lead to a gyro-Bohm scaling of the diffusivity. Including heavier ions turns the scaling to more Bohm-like. It is shown that ρs scaling studies carried out on the diffusivity can be misleading if residual dependencies on other parameters are present and the cross phase between poloidal electric field and density fluctuations is not constant as in the present case. The measured turbulent diffusivity is consistent with typical values reported for the edge of fusion plasmas.

Multi-Dimensional Structure of the Electric Field in Tokamak H-Mode

In tokamak H-mode, a large poloidal flow exists in an edge transport barrier, and the electrostatic potential and density profiles can be steep both in the radial and poloidal direction. Two-dimensional structures of the potential, density and flow velocity near the edge of a tokamak plasma are investigated. The model includes the nonlinearity in bulk-ion viscosity and turbulence-driven shear viscosity. For the case with a strong radial electric field (H-mode), a two-dimensional structure in a transport barrier is obtained, giving a poloidal shock with a solitary radial electric field profile. The poloidal electric field induces convective transport in the radial direction, and poloidal asymmetry makes the flux-surface-averaged particle flux direct inward with a pinch velocity on the order of 1 [m/s]. A large poloidal flow with radial shear enhances the inward pinch velocity. The abrupt increase of this inward ion and electron flux at the onset of L-to-H-mode transition explains the rapid establishment of the density pedestal at the transition.

Arrangement of emissive and cold probes for fluctuation and Reynolds stress measurements

An arrangement of three emissive probes and one cold probe was used to simultaneously determine the Reynolds stress and the fluctuation-induced flux in the edge region of the tokamak ISTTOK. The emissive probes are arranged in a 90º triangle and inserted into the tokamak so that two of the probes are separated radially and two are separated in the poloidal direction, with one probe being used for both directions. Since emissive probes deliver a better measure of the plasma potential than cold probes, with this arrangement the radial and the poloidal electric field and their fluctuations can be measured simultaneously, so that the Reynolds stress Re can be derived from the data. This setup is radially movable so that also the radial gradient of Re can be determined. In addition, a cold probe is inserted close to one of the outer emissive probes. The fluctuation-induced flux can be determined from a simultaneous measurement of the poloidal field fluctuations and those of the ion saturation current.

Dynamo-free plasma in the reversed field pinch

Transient application of a poloidal electric field to reversed field pinch (RFP) plasmas has led to a period in which dynamo activity (inherent in standard RFP plasmas) nearly vanishes. Measurements of the plasma resistivity, current density and electric field profiles show the edge-applied electric field accommodates Ohm’s law balance without a dynamo term over the entire cross section. Neoclassical theory accurately predicts the resistivity in the RFP, as the predicted resistivity profile (based on measurements of electron temperature, effective ionic charge, and two-dimensional equilibrium effects) is in agreement with the ratio of the parallel electric field and current density profiles.

Electromagnetic Quantities in Black Hole Magnetosphere

Some electromagnetic quantities in the black hole (BH) magnetosphereare discussed by considering the coexistence of the Blandford–Znajekprocess and the magnetic coupling process. These quantities are (i)flux of electromagnetic energy and angular momentum transferred betweenthe BH and the disc, (ii) poloidal currents flowing on the horizon anddisc, (iii) poloidal electric field on the horizon, (iv) toroidalmagnetic field in the BH magnetosphere, and (v) voltage drop across themagnetic coupling region on the horizon. It turns out that thesequantities are determined mainly by three parameters: (i) the positionsrelative to the corotation magnetic surface, (ii) the BH spin, and(iii) the power-law index for the variation of the magnetic field onthe disc.

Radial convection of plasma structures in a turbulent rotating magnetized-plasma column.

The turbulent regime of a rotating magnetized plasma column has been studied. The detection and the spatiotemporal analysis of structures by means of conditional sampling techniques is performed. Because of the overall rotation and centrifugal effects, the structures inside the turbulence move on average along a spiral trajectory leading to a net radial convection of the charged particles to the walls. The development of a poloidal electric field inside the structures has been measured. It leads to the observed outwards radial E x B drift in agreement with the expectations of recent theoretical works.

Edge turbulent transport with lower hybrid current drive in the Hefei Tokamak-7

The edge plasma transport around the last closed flux surface was investigated using Langmuir probes in a lower hybrid current drive (LHCD) experiment in the Hefei Tokamak-7 [J. K. Xie et al., in Proceedings of the 16th International Conference on Fusion Energy, Montreal, 1996 (International Atomic Energy Agency, Vienna, 1997), Vol. 1, p. 685]. Fluctuations of plasma density, plasma temperature, and poloidal electric field in the boundary plasma are reduced, hence the electrostastic turbulence-induced particle flux and heat flux both significantly decrease after the application of LHCD. The suppression of density fluctuation is more sensitive to the lower hybrid wave (LHW) than that of temperature fluctuation, which is in agreement with the model proposed by Ware in the presence of flow shear [A. S. Ware et al., Phys. Plasmas 5, 173 (1998)]. The difference between turbulent convective and conductive fluxes depends on such a discrepancy between normalized density fluctuation and temperature fluctuation. It is speculated that the reduced turbulence-induced transport is possibly attributable to a sheared flow resulting from the varying radial electric field, which is formed possibly due to the loss of the energetic electron generated by LHW. In addition, studies show that the effect of magnetohydrodynamic activity, possibly modified by LHCD, on plasma transport could be another possible candidate for the reduced electrostatic turbulent flux.

Self-Organization Process with Dynamo Activity in Ultra-Low q and Reversed-Field-Pinch Plasmas

Dynamo activity is highly important for formation and confinement of ultra-low q (ULQ) and reversed-field-pinch (RFP) plasmas. These configurations are self-organized by a spontaneous dynamo electric field. In order to investigate the properties of this self-organization process, an inductive poloidal electric field is applied to standard ULQ and RFP plasmas. Synchronized to the application of poloidal electric fields, remarkable responses of plasma parameters are observed. In the ULQ experiment, toroidal plasma current increases with decrease in loop voltage. In this experiment, fluctuation levels are almost constant. On the other hand, dIp/dt decreases with the reduction of plasma fluctuations in the RFP experiment. In these processes, plasmas will self-organize to maintain a state of nearly minimum energy. In general, the self-organization process changes the total dissipation in the system through the enhancement of the plasma fluctuation. To improve the plasma confinement for ULQ and RFP plasmas, it is important to understand the properties of this self-organization process in detail.

On axisymmetric resistive magnetohydrodynamic equilibria with flow free of Pfirsch–Schlüter diffusion

The equilibrium of an axisymmetric magnetically confined plasma with anisotropic resistivity and incompressible flows parallel to the magnetic field is investigated within the framework of the magnetohydrodynamic (MHD) theory by keeping the convective flow term in the momentum equation. It turns out that the stationary states are determined by a second-order elliptic partial differential equation for the poloidal magnetic flux function ψ along with a decoupled Bernoulli equation for the pressure identical in form with the respective ideal MHD equations; equilibrium consistent expressions for the resistivities η∥ and η⊥ parallel and perpendicular to the magnetic field are also derived from Ohm’s and Faraday’s laws. Unlike in the case of stationary states with isotropic resistivity and parallel flows [G. N. Throumoulopoulos and H. Tasso, J. Plasma Phys. 64, 601 (2000)] the equilibrium is compatible with nonvanishing poloidal current densities. Also, although exactly Spitzer resistivities either η∥(ψ) or η⊥(ψ) are not allowed, exact solutions with vanishing poloidal electric fields can be constructed with η∥ and η⊥ profiles compatible with roughly collisional resistivity profiles, i.e., profiles having a minimum close to the magnetic axis, taking very large values on the boundary and such that η⊥>η∥. For equilibria with vanishing flows satisfying the relation (dP/dψ)(dI2/dψ)>0, where P and I are the pressure and the poloidal current functions, the difference η⊥−η∥ for the reversed-field pinch scaling, Bp≈Bt, is nearly two times larger than that for the tokamak scaling, Bp≈0.1Bt (Bp and Bt are the poloidal and toroidal magnetic-field components). The particular resistive equilibrium solutions obtained in the present work, inherently free of—but not inconsistent with—Pfirsch–Schlüter diffusion, indicate that parallel flows might result in a reduction of the diffusion observed in magnetically confined plasmas.

Improved Edge Confinement due to Ion Cyclotron Radio Frequencyboronization in the HT-7 Tokamak

The phenomena of improved edge confinement due to ion cyclotron radiofrequency boronization were observed with a fast reciprocatingLangmuir probe in the HT-7 tokamak. A strong shear layer of radialelectric field was produced in the plasma edge region, which resultedin the formation of an edge transport barrier. Consequently, theedge profiles of electron density were steepened, and both the absoluteand relative fluctuation levels were suppressed by the shearingE×B flows. Concomitant reduction of the coherencebetween electron density and poloidal electric field fluctuations andthe change of their cross-phase resulted in turbulent particle fluxdropping by more than a half at the plasma edge. This demonstrates thede-correlation effect of turbulence and its contribution to edgetransport. The results presented here suggest a link between wallconditions and boundary plasma physics, especially an interactionbetween atomic processes and turbulence through the formation of radialelectric field shear at the plasma edge.

Transport by intermittency in the boundary of the DIII-D tokamak

Intermittent plasma objects (IPOs), featuring higher pressure than the surrounding plasma, are responsible for ∼50% of the E×BT radial transport in the scrape off layer (SOL) of the Doublet III D (DIII-D) tokamak [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] in L- and H-mode discharges. Conditional averaging reveals that the IPOs are positively charged and feature internal poloidal electric fields of up to 4000 V/m. The IPOs move radially with E×BT/B2 velocities of ∼2600 m/s near the last closed flux surface (LCFS), and ∼330 m/s near the wall. The IPOs slow down as they shrink in size from 2 cm at the LCFS to 0.5 cm near the wall. The skewness (i.e., asymmetry of fluctuations from the average) of probe and beam emission spectroscopy data indicate IPO formation at or near the LCFS and the existence of positive and negative IPOs which move in opposite directions. The particle content of the IPOs at the LCFS is linearly dependent on the local density and decays over ∼3 cm into the SOL while their temperature decays much faster (∼1 cm).

Turbulent transport reduction by E×B velocity shear during edge plasma biasing: recent experimental results

Experiments in the tokamaks TEXTOR, CASTOR, T-10 and ISTTOK, as well as in the reversed field pinch RFX have provided new and complementary evidence on the physics of the universal mechanism of E×B velocity shear stabilization of turbulence, concomitant transport barrier formation and radial conductivity by using various edge biasing techniques.In TEXTOR the causality between transport reduction and induced electric fields in the edge has been for the first time clearly demonstrated. The high electric field gradients have been identified as the cause for the quenching of turbulent cells. A quantitative analysis of the measured transport reduction is in good agreement with theoretical predictions. The scaling of plasma turbulence suppression with velocity shear has been established, revealing the density-potential cross-phase as a key element. Reduction in poloidal electric field, temperature, and density fluctuations across the shear layer lead to a reduction of the anomalous conducted and convected heat fluxes resulting in an energy transport barrier that is measured directly.In CASTOR the biasing electrode is placed at the separatrix in a non-intrusive configuration which has demonstrated strongly sheared electric fields and consequent improvement of the global particle confinement, as predicted by theory. The impact of sheared E×B flow on edge turbulent structures has been measured directly using a comprehensive set of electrostatic probe arrays as well as emissive probes. Measurements with a full poloidal Langmuir probe array have revealed quasi-coherent electrostatic waves in the SOL with a dominant mode number equal to the edge safety factor.In T-10 edge biasing is clearly improving the global performance of ECR heated discharges. Reflectometry and heavy ion beam probe measurements show the existence of a narrow plasma layer with strong suppression of turbulence.On ISTTOK, the influence of alternating positive and negative electrode and (non-intrusive) limiter biasing has been compared. Electrode biasing is found to be more efficient in modifying the radial electric field Er and confinement, limiter biasing acting mainly on the SOL.In the RFX reversed field pinch it has been demonstrated that also in RFPs biasing can increase the local E×B velocity shear in the edge region, and hence substantially reduce the local turbulence driven particle flux mainly due to a change in the relative phase between potential and density fluctuations.

Velocity fluctuations and time dependent E×B flows in the JET boundary region

Fluctuations in the phase velocity of fluctuations have been computed from floating potential fluctuations in the JET plasma boundary region. Experimental results reinforce the idea that, in some conditions, fluctuations in the phase velocity are linked with fluctuations in radial–poloidal electric fields opening the possibility to investigate turbulent transport in the plasma core region from measurements of density fluctuations. These results indicate that time dependent radial electric fields and E×B shearing rates might be computed in the plasma core from measurement of density fluctuations.

Evidence of Lévy stable process in tokamak edge turbulence

The time series of floating potential and poloidal electric field fluctuations in the edge plasma of ohmically heated ADITYA tokamak [Phys. Plasmas 4, 4292 (1997)] are analyzed for self-similarity. It is observed that the distribution function of a sum of n data points converges to a self-similar distribution of Lévy scale index, α=1.1–1.3 for n⩽40 and α=1.8–2.0 for larger n. This shows that the scaling properties of small scale fluctuations are non-Gaussian and those of large scale fluctuations are Gaussian. Implication of this observation to our understanding of plasma transport is discussed.

Three-dimensional relativistic electromagnetic subcycle solitons.

Three-dimensional (3D) relativistic electromagnetic subcycle solitons were observed in 3D particle-in-cell simulations of an intense short-laser-pulse propagation in an underdense plasma. Their structure resembles that of an oscillating electric dipole with a poloidal electric field and a toroidal magnetic field that oscillate in phase with the electron density with frequency below the Langmuir frequency. On the ion time scale, the soliton undergoes a Coulomb explosion of its core, resulting in ion acceleration, and then evolves into a slowly expanding quasineutral cavity.