Positive Radial Electric Field Research Articles

Quasi-coherent mode evolution in discharges with positive radial electric field at the T-10 tokamak

The effect of the high-power ECRH on the quasi-coherent (QC) mode in the plasma of the T-10 tokamak with the magnetic field Bt = 2.2 T, plasma current Ipl = 230 kA and chord-averaged electron density <ne> ≈ 1⋅1019 m-3 is discussed. The most pronounced changes appear in the core plasma region with r < 0.5a. For the ECRH power PECRH = 2.2 MW causing the appearance of the positive radial electric field (in the OH stage, the occurring field is ), the mean frequency of the QC-mode gradually decreases from (in the OH stage) to , and its frequency spread Δf QC decreases from to . For the ECRH power PECRH = 1.7 MW and weak negative radial electric field (), the mean frequency decreases to , and the frequency spread becomes . After the ECRH-induced drop, the QC-mode mean frequency increases with increasing line-averaged density.

Simulations of the radial electric field induced by neutral beam injection in a tokamak

A model based on Poisson’s equation has been proposed to study the radial electric field induced by neutral beam injection (NBI). The radial electric field is produced by charge separation of beam induced electrons and beam ions. The charge separation is produced by redistribution and loss of NBI ions, which is due to the magnetic drift and collision effects. The birth and steady-state distributions of NBI fast ions related to charge separation were numerically computed by the orbit code GYCAVA and the NBI code TGCO. According to our model, the neoclassical polarization density of bulk ions has a great shielding effect on the radial electric field due to charge separation. The simulation results show that NBI ions can produce a significant radial electric field (∼10 kV m−1) in the core region. The counter-current NBI (injection anti-parallel to the plasma current) can produce a negative radial electric field, which is due to the magnetic drift and collision effects of NBI ions. While the co-current injection (injection parallel to the plasma current) produces a positive radial electric field in the core region, which is mainly related to the magnetic drift of NBI ions. The dependence of the radial electric field on the injection direction and the beam energy of NBI has been investigated. As the beam energy increases, the magnitudes of the electric field for counter-injections increase and the magnitude of the electric field for co-current perpendicular injection decrease.

Impurity holes in tokamaks with electron cyclotron resonance heating of the helical core

Long-living internal kink mode, which is often observed between the sawtooth crashes in tokamaks, leads to the formation of a new kind of fast particle orbit, namely, the toroidally trapped superbanana orbits with width Δrsb∼(ξ0r)1/2, where ξ0(r) is the amplitude of a kink displacement (flux surface radius). In the velocity space, such orbits are formed around resonant pitch-angles corresponding to zero toroidal precession of the banana guiding center. High-field side (HFS) electron cyclotron resonance heating (ECRH) applied to such helical cores generates a significant population of fast electrons with resonant pitch angles, which results in the strong non-ambipolar electron flux greatly exceeding the flux of non-resonant thermal ions. Ambipolarity is restored by the strong positive radial electric field (Er), which annihilates this flux. The height of this central “hill” with Er&amp;gt;0 can be comparable in magnitude with the depth of the edge Er&amp;lt;0 “well” supporting H-mode pedestal. This effect can be responsible for the tungsten “hole” observed between sawtooth crashes during HFS ECRH in the ASDEX Upgrade tokamak [Sertoli et al. Nucl. Fusion 55, 113029 (2015)].

Effects of radial electric field on confinement of high energy particles in advanced fusion mirror reactor

The radial electric field Er in a magnetic confined machine, such as the compact fusion reactor (CFR), the field-reserved configuration (FRC), and the tokamak, plays an essential role in affecting the confinement properties of the high energy particles. The parallel velocities of the high energy particles will be accelerated or decelerated by applying a radial electric field, which could change the loss rate of the high energy particles in the magnetic confined machines. Unlike the fourth-order Runge-Kutta method RK4, the recently-developed Boris method can strictly preserve energy conservation of the high energy particles in the case without radial electric field. The orbit of high energy α particle in compact fusion reactor (CFR) is simulated by solving the equations of motion numerically with the Boris Algorithm. The effect of radial electric field on the orbit of the high energy α particle is investigated and the confinement of plasma in different radial electric fields in the CFR machine is studied in the present paper. By changing the strength of the radical electric field and the particles' radical locations in the middle plane of the CFR configuration, the confinement property of the high energy α particle is studied. The numerical results indicate that both the positive radial electric field and negative electric field can significantly affect the confinement of the high energy α particle. When the radial electric field is increased to a threshold, the high energy α particle could be confined in the central region of the CFR machine for a long enough time. The threshold of the radial electric field depends on the initial parameters of the confined particle. Systematic investigations of the radical electronic field effect will conduce to greatly improving the performance of the designed CFR machines.

Energy confinement of hydrogen and deuterium electron-root plasmas in the Large Helical Device

The dependence of the energy confinement and energy transport on the isotope mass is a long-standing open question in the stellarator community. With the recent upgrade of the Large Helical Device to allow for deuterium plasma operation, systematic isotope experiments could be carried out for the first time in a major non-axisymmetric device.Within this framework, electron-cyclotron-resonance heated (ECRH) hydrogen and deuterium plasmas were investigated varying both density and heating power to establish a broad data set. Even at low power the central ECRH heating is sufficient to lead to stellarator-specific core-electron-root-confinement which features a peaked electron temperature profile and a positive radial electric field. For this data set, the energy confinement time and energy transport is investigated in detail and compared to the neoclassical theory.Over the whole data set, the energy confinement time of deuterium is statistically 10%–20% larger than in hydrogen indicating that the ‘isotope effect’ also exists in non-axisymmetric devices. Both the electron and ion temperature are elevated in deuterium compared to hydrogen at the same effective absorbed power and density. From a neoclassical point-of-view, the electron-root and the positive electric field extend over nearly the entire plasma radius. Good agreement is found between the measured and theoretical neoclassical ambipolar electric field. The neoclassical energy-flux can account for up to half the experimental flux implying that turbulence is responsible for a significant fraction of the entire energy-flux.

Study of impurity behaviour for first magnetic configuration changes in W7-X plasmas by means of PHA spectra

Transport properties of impurities in stellarators are sensitive to potential variations on flux-surfaces, and thermodynamic forces from the bulk plasma which might be affected by changes in magnetic field properties. In this contribution, the effect of planar coil current changes on the core impurity behavior in the first operating phase of Wendelstein 7-X (W7-X) is described. Hydrogen plasmas, at different heating power levels were investigated showing Core-Electron-Root-Confinement (CERC) conditions, characterized by peaked Te profiles and positive radial electric fields in the plasma core. Impurity spectra in the x-ray emission range between 1 and 10 keV were measured by the Pulse Height Analysis (PHA) diagnostic. The line-integrated measurements of transitions in sulphur, chlorine and argon are in the focus of this analysis. First observations indicate that the scans of the magnetic configuration did not affect strongly the impurity concentration in the core. Clear differences in the impurity content, however, have been observed when the ECRH power was changed, which affects the bulk plasma properties.

Calculation of the radial electric field with RF sheath boundary conditions in divertor geometry

The equilibrium electric field that results from an imposed DC bias potential, such as that driven by a radio frequency (RF) sheath, is calculated using a new minimal two-field model in the BOUT++ framework. Biasing, using an RF-modified sheath boundary condition, is applied to an axisymmetric limiter, and a thermal sheath boundary is applied to the divertor plates. The penetration of the bias potential into the plasma is studied with a minimal self-consistent model that includes the physics of vorticity (charge balance), ion polarization currents, force balance with , ion diamagnetic flow (ion pressure gradient) and parallel electron charge loss to the thermal and biased sheaths. It is found that a positive radial electric field forms in the scrape-off layer and it smoothly connects across the separatrix to the force-balanced radial electric field in the closed flux surface region. The results are in qualitative agreement with the experiments. Plasma convection related to the net flow in front of the limiter is also obtained from the calculation.

Characteristics of electron internal transport barrier in Heliotron J

The formation of an electron internal transport barrier (eITB) has been observed for the first time with centrally focused electron cyclotron heating (ECH) microwaves injected into plasma in Heliotron J. When the heating power per electron density () exceeds a threshold of , transient increases of both the central Te and the core Te gradients are observed. A neoclassical (NC) calculation using the Sugama–Nishimura moment method predicts that the large positive radial electric field (Er) is formed in the core region. Heat transport analysis shows a significant reduction of the effective electron thermal diffusivity in the plasma with the eITB related to that without the eITB. The large gap between the experimentally obtained effective thermal diffusivity and the NC thermal diffusivity suggests that the suppression of anomalous transport contributes to the core improved confinement of the eITB plasma. The electron cyclotron emission measurement shows both the transient increase and the hysteresis phenomena during the eITB formation.

Possible influence of near SOL plasma on the H-mode power threshold

A strong effect of divertor configuration on the threshold power for the L-H transition (PLH) was observed in recent JET experiments in the new ITER-like Wall (ILW) [1–3]. Following a series of EDGE2D-EIRENE code simulations with Be impurity and drifts a possible mechanism for the PLH variation with the divertor geometry is proposed. Both experiment and code simulations show that in the configuration with lower neutral recycling near the outer strike point (OSP), electron temperature (Te) peaks near the OSP prior to the L-H transition, while in the configuration with higher OSP recycling Te peaks further out in the scrape-off layer (SOL) and the plasma stays in the L-mode at the same input power. Code results show large positive radial electric field (Er) in the near SOL under lower recycling conditions leading to a large E×B shear across the separatrix which may trigger earlier (at lower input power) edge turbulence suppression and lower PLH. Suppressed Te‘s at OSP in configurations with strike points on vertical targets (VT) were observed earlier and explained by a geometrical effect of neutral recycling near this particular position, whereas in configurations with strike points on horizontal targets (HT) the OSP appears to be more open for neutrals (see e.g. review paper [4]).

Effects of resonant magnetic perturbations on turbulence and transport in DIII-D L-mode plasmas

In this paper we show that resonant magnetic perturbations (RMPs) affect the L- to H-mode power threshold. We find that during the L-mode phase, RMPs cause the particle pinch to reverse from traditionally inward to outward. As a result, the density at the plasma edge increases, while the density in the plasma core is reduced. Linear stability calculations indicate that the plasma transitions from an ion temperature gradient (ITG) to trapped electron mode (TEM) regime at the plasma edge. If the applied RMP current is below the threshold for penetration and island formation, we find that the changes in the edge radial electric field are minimal, while the carbon toroidal rotation brakes over the whole minor radius. Once the RMP field penetrates and the screening plasma response dissappears, the spin-up of the toroidal rotation at the plasma edge results in a positive radial electric field inside the separatrix.

Integrated discharge scenario for high-temperature helical plasma in LHD

The discharge scenario of high temperature plasma with a helical configuration has significantly progressed. The increase of central ion temperature due to the reduction of wall recycling was clearly observed. The peaking of the ion heating profile and the reduction of charge exchange loss of energetic ions play an important role for further improvement of ion heat transport in the ion internal transport barrier (ITB) core. The ion ITB and electron ITB have been successfully integrated due to the superposition of centrally focused electron cyclotron heating to the ion ITB plasma, and the high temperature regime of the ion temperature comparable to the electron temperature (Ti ~ Te) has been significantly extended. The width of the ion ITB formed with electron ITB is wider than the width of electron ITB. The positive radial electric field was observed in the integrated ITB plasma by a heavy ion beam probe, while the negative radial electric field was observed in ion ITB plasmas. The ion temperature gradient decreases with the increase of the temperature ratio (Te /Ti).

Impurity shielding criteria for steady state hydrogen plasmas in the LHD, a heliotron-type device

Impurity behavior has so far been investigated in steady state hydrogen plasmas in the Large Helical Device, which is a heliotron-type device and excellent for steady state operation. There was always found to be an impurity accumulation window, as observed before (Nakamura et al 2002 Plasma Phys. Control. Fusion 44 2121, Nakamura et al 2003 Nucl. Fusion 43 219). To clarify the boundary conditions, the dependences of impurity transport on edge plasma parameters are investigated with a database of steady state hydrogen discharges, and the boundary conditions for the impurity accumulation window are discussed. It is found that two different types of impurity screening effects are essential for preventing intrinsic impurities from entering the core plasma. One of them is due to positive radial electric field at the plasma edge on the low collisionality side and the other is impurity retention caused by friction force in the ergodic layer on the high collisionality side. The classification of steady state discharges on n–T space shows that the impurity behavior can be predicted by the impurity shielding criteria based on each empirical scaling.

New insights on boundary plasma turbulence and the quasi-coherent mode in Alcator C-Mod using a Mirror Langmuir Probe

A new “Mirror Langmuir Probe” diagnostic, combined with a double-coil scanning magnetic probe, is used to interrogate Alcator C-Mod's quasi-coherent mode (QCM) with unprecedented detail. In ohmic EDA H-modes, the QCM is found to reside in a region of positive radial electric field, with a radial width (∼3 mm) that spans open and closed field line regions. Large amplitude, in-phase sinusoidal bursts (∼100 kHz) in density, electron temperature, and plasma potential are observed, with potential lagging density by ∼16°, producing an outward radial transport velocity of ∼10 m/s. Mode propagation corresponds to the sum of local E × B and electron diamagnetic drift velocities. Poloidal magnetic field fluctuations project to current filaments carrying peak current densities of ∼25 A/cm2. An evaluation of parallel electron force balance (Ohm's law) over a fluctuation cycle indicates a significant electromotive component. Interchange drive is also a contributor in the current continuity (vorticity) equation. Thus, the QCM is primarily a separatrix-spanning electron drift-wave with interchange and electromagnetic contributions.

3-D effects on viscosity and generation of toroidal and poloidal flows in LHD

Three-dimensional effects on plasma flows have been experimentally studied in the large helical device with 3D configurations. Spontaneous toroidal flow without net driving force using the combination of perpendicular neutral beam injection (NBI) heating and balanced tangential NBI heating has been investigated with two magnetic configurations. Co- and counter-directed spontaneous flows have been observed depending on the collisionality. Toroidal flow shear changes the sign at 0.4&amp;lt;reff&amp;lt;0.6 between co- and counter-flowing plasmas, where reff is a averaged minor radius. The detailed flow structures have been also examined at the edge region with stochastic magnetic field. A poloidal flow driven by the positive radial electric field has been observed at the open field region just outside of the plasma boundary. At the stochastic region, a shear of poloidal flow has been observed, while no shear of toroidal flow has been observed. The toroidal flow shear changes only in the nested magnetic flux surface region. A difference of density fluctuation property has been observed between co- and counter-flowing plasmas, where toroidal flow shears are formed.

Inward pinch of high-Z impurity in a rotating tokamak plasma: effects of atomic processes, radial electric field and Coulomb collisions

The transport of high-Z impurity in a toroidally rotating tokamak plasma is investigated analytically and numerically. It is shown that the inward pinch is driven by the atomic processes of ionization/recombination along the particle orbit both in co- and ctr-rotating plasmas. This inward pinch is enhanced by the radial electric field. It is derived that the negative and positive radial electric fields cause the inward pinch and the outward movement (unpinch) of the high-Z impurity, respectively, under the influence of Coulomb collisions with the rotating background plasma. In the ctr-rotation case, the inward pinch becomes significant with increasing toroidal rotation velocity, because the directions of both pinches are inwards. On the other hand, in the co-rotation case, these pinches have opposite directions. Therefore, the unpinch due to the positive radial electric field is decreased by the inward pinch due to the atomic processes. These tendencies are consistent with the tungsten accumulation observed in the JT-60U rotation scan experiment.

Status and Plan of GAMMA 10 Tandem Mirror Program

Recent progress and near future plan of GAMMA 10 efforts are presented. With high power plug electron cyclotron heating (ECH) up to ~400 kW, the ion confining potential of more than 2 kV was confirmed. The drift type low frequency fluctuations were suppressed by the positive radial electric field produced by plug ECH. It is found that the efficient EC heating on mirror devices from a strong B field side requires the minimization of the stray microwave in addition to the 100% X-mode excitation to avoid the enhancement of the ion loss. The development of a gyrotron, the key tool of these ECH experiments, has been made in collaboration with NIFS (National Institute for Fusion Science), More than 1.5 MW for more than 1s has been demonstrated at 77GHz. The plan of the boundary plasma research program with modification of GAMMA 10 is in progress. The new program includes the physics and technology studies of the divertor and SOL plasmas and PWI relevant to torus plasmas like ITER. The high heat flux experiments using the open end mirror throat has been started and we have obtained successful preliminary data, which include the heat flux of 8 MW/m2.

Density Fluctuation Measurements Using a Frequency Hopping Reflectometer in JT-60U

In JT-60U, a frequency hopping reflectometer has been developed to evaluate the poloidal rotation velocity as a Doppler reflectometer and the radial correlation of density fluctuations in plasmas. The system can measure the radial profile of density fluctuations or the radial correlation profile at two spatial points within 250 ms by combining with the other fixed frequency reflectometer. The radial profiles of the poloidal rotation velocity evaluated from the Doppler-shifted frequency spectrum of density fluctuations show a positive radial electric field in co-rotating plasmas and a negative radial electric field in counter-rotating plasmas. Density fluctuation measurement at the internal transport barrier (ITB) using a correlation reflectometer revealed that long-range correlation increased when ITB was degraded owing to the central heating by electron cyclotron waves.

Ion Heating Experiments and Improvement of Ion Heat Transport in LHD

Ion heating experiments have been intensively carried out in high- and low-Zeff conditions of Large Helical Device plasmas. In high-Zeff plasmas utilizing neon or argon gus puffing, the ion heating power normalized by ion density (Pi/ni) increases with ZeffL and the central ion temperature increases with Pi/ni without saturation. The central ion temperature of 13.5 kV was achieved in an argon-seeded plasma, strongly suggesting the capability of the helical configuration to confine high-performance plasmas. In low-Zeff experiments, improvement of ion heat transport was realized in the core plasmas heated by high-power neutral beam injections. The ion temperature has a peaked profile with steep gradient in the core region (ion internal transport barrier). The transport analysis indicates that the anomalous transport is reduced in the core region, where the negative radial electric field is predicted by the neoclassical ambipolarity. Improvement of ion heat transport with positive radial electric field was also successfully demonstrated utilizing strongly focused electron cyclotron resonant heating, suggesting further improvement of ion heat transport in reactor-relevant plasmas.

Spontaneous Toroidal Flow and Impurity Hole in the High Ion Temperature Plasma on LHD

Spontaneous toroidal flow driven by ion temperature gradient and extreme hollow profile of carbon impurity (denoted as an “impurity hole”) is observed associated with the increase of ion temperature gradient in the large helical device (LHD). Spontaneous toroidal flows driven by radial electric field and ion temperature gradient are studied. The positive radial electric field drives spontaneous flow in the counterdirection at the plasma edge and in the codirection near the magnetic axis. The component of the spontaneous toroidal flow driven by ion temperature gradient is clearly observed and expected to be one of the dominant components of toroidal flows in the high-ion temperature discharges in LHD. The transport analysis of the carbon impurity in the discharge with impurity hole reveals a low diffusion coefficient and the outward convection velocity, whereas the inward convection is predicted by the neoclassical theory at half the minor radius.

Transport Simulation of Helical Plasmas Using the TASK/TX Code

It is widely accepted that the radial electric field strongly affects plasma confinement through the transport process. We have analysed the time evolution of the radial electric field and the radial transport in plasmas in a helical magnetic configuration by extending the TASK/TX code [M. Honda and A. Fukuyama, J. Comput. Phys. 227, 2808 (2008)]. TASK/TX is a one-dimensional dynamic transport code originally developed for axisymmetric plasmas. A set of flux-surface averaged fluid equations is solved simultaneously, rather than a set of diffusion equations based on the flux-gradient relations. TASK/TX consists of one-dimensional two fluids (electron and ion) equations, Maxwell's equations and diffusion equations of neutrals. To apply TASK/TX to helical plasmas, we have included two additional effects; helical neoclassical viscosity force and diffusion due to magnetic braiding. Plasma transport simulation is carried out mainly using the LHD parameters. We obtained negative radial electric field (ion root) with ion heating and positive radial electric field (electron root) with electron heating. Effects of magnetic braiding are also studied.