Direction Of Plasma Current Research Articles

Effects of counter-current driven by electron cyclotron waves on neoclassical tearing mode suppression

Through theoretical analysis, we construct a physical model that includes the influence of counter-external driven current opposite to the plasma current direction in the neoclassical tearing mode (NTM). The equation is used with this model to obtain the modified Rutherford equation with co-current and counter-current contributions. Consistent with the reported experimental results, numerical simulations have shown that the localized counter external current can only partially suppress NTM when it is far from the resonant magnetic surface. Under some circumstances, the Ohkawa mechanism dominated current drive (OKCD) by electron cyclotron waves can concurrently create both co-current and counter-current. In this instance, the minimal electron cyclotron wave power that suppresses a particular NTM was calculated by the Rutherford equation. The result is marginally less than when taking co-current alone into consideration. As a result, to suppress NTM using OKCD, one only needs to align the co-current with a greater OKCD peak well with the resonant magnetic surface. The effect of its lower counter-current does not need to be considered because the location of the counter-current deviates greatly from the resonant magnetic surface.

Modeling of toroidal momentum transport induced by neoclassical toroidal viscosity torque for ITER scenarios

Neoclassical toroidal viscosity (NTV) torque caused by resonant magnetic perturbation (RMP) and the induced toroidal momentum transport are investigated for International Thermonuclear Experimental Reactor (ITER) scenarios through numerical modeling. The NTV torque is calculated using the NTVTOK code including the bounce-drift resonant effect, and the toroidal rotation evolution is modeled by solving a toroidal momentum transport equation that couples momentum source (NTV torque) and the momentum diffusion effect. The variation of RMP coil phasing (defined as toroidal phase difference between different rows of RMP coils) results in different types of plasma response and hence different features of NTV torque and toroidal momentum transport. The bounce-drift resonant effect enhances NTV torque and induces more significant toroidal rotation variation than simulations that adopt the bounce-averaged NTV model. With the initial rotation of the ITER design, plasma rotation is braked by NTV torque, but it may be sustained at moderate amplitude due to electron contributions to NTV torque. It is also found that initially static or slowly rotating plasma can be accelerated by NTV torque either toward co- Ip or counter- Ip ( Ip indicates plasma current) direction, indicating that NTV torque can be regarded as a momentum source for plasma with low torque injection; for instance, radio-frequency heated plasma.

Heavy Ion Beam Probing Diagnostics on the TUMAN-3M Tokamak for Study Plasma Potential and Electric Fields in New Operational Regimes

Heavy Ion Beam Probing (HIBP) diagnostic is a powerful tool for electric field studies in the hot dense plasma of modern-day toroidal magnetic confinement devices. On the TUMAN-3M tokamak, the HIBP have been used in regimes with improved plasma confinement to clear up the role of the radial electric field in the transition to good confinement regimes. Recently, a modernization of the TUMAN-3M HIBP diagnostics was performed, aiming to reconfigure it for a work with a reversed plasma current direction and improvement of the overall stability of the diagnostic. The results of the first measurements of the plasma potential in the co-NBI scenario are reported and discussed.

Energetic particle-induced geodesic acoustic modes on DIII-D

Various properties of the energetic particle-induced geodesic acoustic mode (EGAM) are explored in this large database analysis of DIII-D experimental data. EGAMs are n = 0 modes with m = 0 electrostatic potential fluctuations (where n/m = toroidal/poloidal mode number), m = 1 density fluctuations, and m = 2 magnetic fluctuations. The fundamental frequency (∼20–40 kHz) of the mode is typically below that of the traditional geodesic acoustic mode frequency. EGAMs are most easily destabilized by beams in the counter plasma current (counter-I p) direction as compared to co-I p and off-axis beams. During counter beam injection, the mode frequency is found to have the strongest linear dependence (correlation coefficient r = −0.71) with the safety factor (q). The stability of the mode in the space of q and poloidal beta (β p) shows a clear boundary for the mode stability. The stability of the mode depends more strongly on damping rate than on fast-ion drive for a given injection geometry.

Assessments of the performance of neutral beam injection on the spherical tokamak EXL-50

The numerical simulation of the neutral beam injection (NBI) heating in the spherical tokamak EXL-50 has been performed in order to offer a guideline for the practicable engineering design. A series of parameter scans on injected beam energy, plasma density, electron temperature, plasma current and injection direction have been performed with the NBI heating analysis code NUBEAM. The simulation results show that the injected beam energy should be matched with the plasma density to avoid the excessive shine-through loss. For the scenario with central plasma density of 3.0 × 1019 m−3, the 20 keV beam injection has a power absorption rate about 77%, while for the 50 keV beam injection, only 57% injected power has been absorbed by plasma. When the central plasma density increases to 5.0 × 1019 m−3, a power absorption rate more than 70% can be obtained even for the 50 keV beam injection. As the increase in the electron temperature, the injected power is more likely to heat ions while the total absorbed power by plasma roughly remains stable. The increased plasma current leads to the stronger poloidal magnetic field. A qualitative elucidation on the fast ion trajectory has been given to reveal that the orbit loss has been remarkably suppressed by the enhanced poloidal magnetic field. In addition, a comparison of power absorption rate in the co-current and counter-current injection cases has been carried out, which suggests that counter-current injection leads to more orbit loss and hence less power absorption rate.

Injection performance prediction of the upgraded neutral beam on EAST

The neutral beams(NB) on EAST are being upgraded according to the scheme that the beamline located on Port F will be moved to Port D and adjusted clockwise injection to counterclockwise injection(top view), while the beamline on Port A is unchanged with counterclockwise injection. The two beamlines are modified to counterclockwise injection consistent with the direction of plasma current (co-IP) instead of the primarily balanced-injection (co-IP and counter-IP). The two coming co-IP beamlines are expected to play important roles in plasma heating, neutron emission and current drive in EAST operating scenarios. The injection performances after the beam upgrade are predicted by the simulation codes ONETWO/NUBEAM and TRANSP/NUBEAM to model the discharges on EAST. After the beamlines are upgraded, the bulk ions are heated to a high temperature above 3.2 keV, which is much higher than that in the previous experiments. The orbit loss drops sharply by 83 % and the external charge exchange loss drops by 50 % after the beamline upgrade. The neutron rate induced by the upgraded beamline is greatly enhanced to the order of 1014/s with co-IP injection as opposed to the balanced-injection, and it is also heightened with the increases of plasma density and beam power. The simulation results show that it leads to a significant improvement of the beam-driven current and provides a practical condition for EAST to carry out long pulse and steady-state experiments. The assessment on the injection capabilities of the upgraded neutral beams is an important guiding reference to the subsequent EAST experiments.

Impact of rotation and ion diamagnetic drift on MHD stability at edge pedestal in quiescent H-mode plasmas

MHD stability at edge pedestal in a QH-mode plasma in DIII-D was analyzed by taking into account plasma rotation and ion diamagnetic drift effects. We have found that the coupled rotation and ion diamagnetic drift effects can stabilize a kink/peeling mode in the QH-mode plasma rotating in the direction counter to the plasma current, although it has been recognized the rotation alone destabilizes the mode regardless of its direction. The physics mechanism responsible for the stabilization was identified as reduction of the destabilizing effect by dynamic pressure through the coupling between the rotation and the ion diamagnetic drift. The coupling effect can be harnessed to both stabilize and destabilize the kink/peeling mode by switching the rotation direction, the trend which could be the reason that the QH-mode plasmas in DIII-D favor toroidal rotation counter to the plasma current direction.

Effects of edge safety factor on the toroidal flow velocities during the resonant magnetic perturbations and compact torus injection experiments on the STOR-M tokamak

ABSTRACTThe effects of the edge safety factor on the toroidal flow velocity modification due to resonant magnetic perturbation (RMP) and tangential compact torus injection (CTI) have been investigated on the Saskatchewan Torus-Modified (STOR-M) tokamak. The (l = 2, n = 1) RMP coils are wound against the outer surface of the vacuum vessel. The safety factor has been varied by changing the plasma current at a fixed toroidal magnetic field. As the edge safety factor is reduced, the magnetic perturbations on the q = 2 surface are expected to shift radially outwards, moving closer towards the RMP coils. The evolution of the plasma flow velocity was measured through ion Doppler spectroscopy (IDS). It has been found that RMP changes the flow velocity towards the direction of the plasma current direction and this effect is enhanced when the edge safety factor is reduced. When CT was injected tangentially along the plasma current direction, similar changes in the flow velocity have been found. However, the change is not sensitive to the edge safety factor in the CT injection case.

Plasma rotation measurement using UV and visible spectroscopy on Aditya-U tokamak.

A high resolution spectroscopic diagnostic for the measurement of the spatial profile of toroidal rotation velocity (vt) and temperature (Ti) of carbon ions has been developed and implemented on the Aditya-U tokamak. The diagnostic consists of a high resolution 1 m (f/8.7) spectrometer having an 1800 grooves/mm grating coupled with a charged couple device. The setup allows measurements from 5 toroidal lines-of-sights passing through different minor radii in the horizontal mid-plane of the Aditya-U tokamak. The carbon line emissions at 529 nm in visible and 229.6 and 227.09 nm in the ultra violet spectral range are recorded using the setup. Initial results show that typical Aditya-U plasmas have a maximum carbon ion temperature (Ti) of ∼120 eV and a rotation velocity (vt) of ∼-14 km/s in the counter plasma-current direction.

Modification of toroidal flow velocity through momentum Injection by compact torus injection into the STOR-M tokamak

In the Saskatchewan torus-modified (STOR-M) tokamak, tangential compact torus injection (CTI) experiments have been performed with normal (counter-clockwise, CCW, top view) and reversed (clockwise, CW, top view) plasma current directions while the compact torus (CT) injection direction remains in the CCW direction. The intrinsic toroidal flow direction reverses when the discharge current is reversed. However, the change in the toroidal flow direction is always toward the CTI direction (CCW). It has been determined that the momentum in high density and high velocity CT is more than ten times larger than the intrinsic toroidal rotation momentum in the typical STOR-M plasma. Therefore, the modification of the plasma toroidal rotation velocity is attributed to momentum transfer from CT to the tokamak discharge.

Dynamics of a reconnection-driven runaway ion tail in a reversed field pinch plasma

While reconnection-driven ion heating is common in laboratory and astrophysical plasmas, the underlying mechanisms for converting magnetic to kinetic energy remain not fully understood. Reversed field pinch discharges are often characterized by rapid ion heating during impulsive reconnection, generating an ion distribution with an enhanced bulk temperature, mainly perpendicular to magnetic field. In the Madison Symmetric Torus, a subset of discharges with the strongest reconnection events develop a very anisotropic, high energy tail parallel to magnetic field in addition to bulk perpendicular heating, which produces a fusion neutron flux orders of magnitude higher than that expected from a Maxwellian distribution. Here, we demonstrate that two factors in addition to a perpendicular bulk heating mechanism must be considered to explain this distribution. First, ion runaway can occur in the strong parallel-to-B electric field induced by a rapid equilibrium change triggered by reconnection-based relaxation; this effect is particularly strong on perpendicularly heated ions which experience a reduced frictional drag relative to bulk ions. Second, the confinement of ions varies dramatically as a function of velocity. Whereas thermal ions are governed by stochastic diffusion along tearing-altered field lines (and radial diffusion increases with parallel speed), sufficiently energetic ions are well confined, only weakly affected by a stochastic magnetic field. High energy ions traveling mainly in the direction of toroidal plasma current are nearly classically confined, while counter-propagating ions experience an intermediate confinement, greater than that of thermal ions but significantly less than classical expectations. The details of ion confinement tend to reinforce the asymmetric drive of the parallel electric field, resulting in a very asymmetric, anisotropic distribution.

New concepts of transport physics in toroidal plasmas

New concepts of transport physics in toroidal plasmas is reviewed. The transport characteristics of non-diffusion and non-locality of transport has been recognized to be important in determining radial structures of density, rotation, and temperature, as well as non-linearity in the flux-gradient relation. The non-diffusive term of momentum transport appears as a ‘spontaneous rotation and intrinsic torque’, while the non-diffusive term of particle transport appears as a ‘particle pinch and particle exhaust’. The sign and magnitude of these non-diffusive terms have been found to be sensitive to the turbulence state, which causes reversal phenomena. In the momentum transport, the spontaneous flow reversal from the co- (parallel to plasma current) direction to the counter- (anti-parallel to plasma current) direction and vice versa have been commonly observed even when the plasma parameter changes slightly. In the particle transport, especially in the impurity transport, reversals of convective radial flux from inward to outward are also observed in toroidal plasmas as phenomena of density peaking/flattening and impurity accumulation/exhaust. Non-locality of transport has been observed in the response to perturbations, such as a core temperature rise associated with the cooling at edge by pellet injection, strong coupling of transport at different radii as seen in the curvature transition of ITB, and spatial propagation of ITB regions. The turbulence with meso-scale and long correlation, which are strong candidates for causing the non-locality of the transport, have been confirmed experimentally and the coupling between the different turbulence scales has also been identified in various devices.

Numerical study on the stabilization of neoclassical tearing modes by electron cyclotron current drive

It is well known that electron cyclotron current drive (ECCD) around the o-point of magnetic island along the plasma current direction can stabilize neoclassical tearing modes (NTMs) in tokamak devices. The effects of the radial misalignment between the island and the driven current, the phase misalignment, and the on-duty ratio for modulated current drive on NTM stabilization are studied numerically in this paper. A small radial misalignment is found to significantly decrease the stabilizing effect. When a sufficiently large phase misalignment occurs for the modulated ECCD, the stabilization effect is also reduced a lot. The optimal on-duty ratio of modulated ECCD to stabilize NTMs is found to be in the range of 60%–70%. A larger on-duty ratio than 50% could also mitigate the effect of phase misalignment if it is not too large. There is no benefit from modulation if the phase misalignment is larger than a threshold.

The construction of an electrode biasing system for driving plasma rotation in J-TEXT tokamak.

A newly designed electrode biasing system has been constructed for driving plasma rotation in J-TEXT tokamak. To reduce the influence to the plasma, the system contains a pneumatic driving system so that it can reciprocate in a single discharge, with a stroke of about 5 cm in 100 ms. The power supply of the system can provide stable and adjustable dc voltage in the range of 0-700 V, with adjustable duration of 10-200 ms; its instantaneous power output can reach up to more than 200 kW. In addition, the power supply can also provide a multi-cycle voltage waveform, with adjustable pulse width and voltage amplitude. When applying a positive bias to the plasma, both an improvement of plasma confinement and the speed-up of plasma-edge toroidal rotation in the same direction of plasma current are observed in the experiments.

Modification of the helical divertor footprint induced by the net toroidal plasma current in the LHD heliotron

In this study, the modification of the helical divertor footprint induced by the net toroidal plasma current was investigated in the Large Helical Device (LHD). The divertor footprint shifted a few centimetres when the net toroidal plasma current, as measured by Rogowski coils, was induced via the neutral-beam injection. The shift direction was related to the plasma current direction, and the shift width showed almost linear dependence on the plasma current divided by the toroidal field strength. Magnetic field line tracing calculation considering the plasma current within a simple model revealed that the modification of the edge field line structure causes the footprint shift. The predicted and experimental shift widths agreed qualitatively.

Poloidal rotation velocity measurement with an MIR system on KSTAR

A multi-channel microwave imaging reflectometry (MIR) system has beencommissioned in the 2012 and 2013 KSTAR campaigns for the measurement of semi 2D (16poloidal and 2 radial channels) electron density fluctuations for transport study onKSTAR. A time delayed cross correlation analysis among 16 poloidal channels has beenapplied to obtain the poloidal rotation velocities of the measured turbulent densityfluctuations. The measured poloidal rotation directions for an 170 GHz ECH assistedohmic plasma was in opposite direction to that of a neutral beam (NB) heated L-modeplasma. This is due to the fact that the intrinsic toroidal rotation in ohmic plasma(counter-clockwise) is in opposite direction to the NB heated plasma (clockwise)with respect to the plasma current direction.

Characterization of the poloidal asymmetries in the ISTTOK edge plasma

Edge plasma in/out and up/down asymmetries have been directly investigated on ISTTOK with a probe system that simultaneously samples the plasma at four poloidal angles. An experimental investigation of the asymmetries is presented as a function of the direction of plasma current and toroidal magnetic field. The edge plasma parameters show significant in/out asymmetries in both equilibrium and fluctuating parameters. Asymmetries favoring the outboard side are observed in density, turbulent particle flux and diffusion coefficient independently of the toroidal magnetic field and plasma current directions, suggesting a ballooning-like transport. This interpretation is supported by the characteristics of the fluctuations as they are found to be more intermittent on the outboard side. In addition, smaller up/down asymmetries reversing with the reversal of the toroidal magnetic field direction and favoring the ion ∇B drift direction are found. Significant poloidal asymmetries were also measured in the parallel flow that can be attributed mainly to Pfirsch–Schluter flows.

Experimental demonstration of an up-down asymmetry effect on intrinsic rotation in the TCV tokamak

A new mechanism has recently been proposed that generates a radial flux of parallel momentum in toroidal plasmas. Namely, by considering up-down asymmetric flux surfaces, the symmetry following the magnetic field can be broken and an additional contribution to the turbulent momentum flux arises, potentially changing the intrinsic rotation profile. These predictions are tested with specific experiments on TCV. The intrinsic toroidal rotation is observed to change by roughly a factor of two when changing the up-down asymmetry of the plasma. More precisely, the toroidal rotation gradient changes in the outer part of the plasma, where the flux surface asymmetry is the highest. The experiments were performed for all combinations of the toroidal magnetic field and plasma current directions, that affect the sign of the predicted up-down asymmetry flux. In each case the variation of the intrinsic rotation profile with the up-down asymmetry is observed in the direction predicted by the theory.

Plasma flow and transport on the tokamak ISTTOK boundary plasma

The ISTTOK boundary plasma velocity near the outer midplane is measured on the parallel and perpendicular directions in four different configurations by reversing independently the toroidal magnetic field and the plasma current directions. The parallel flow is found to not depend significantly on both the toroidal magnetic field and plasma current directions, being always directed towards the nearest limiter in the scrape-off layer. On the contrary, the perpendicular flow is found to follow the Er×B drift direction. The poloidal velocity has also been derived from the correlation of floating potential signals measured on poloidally separated probes and a good agreement with the value derived with the Gundestrup probe is found. Finally, the dynamical interplay between parallel momentum and turbulent particle flux has been investigated and a clear dynamical coupling between these quantities is found in the region inside the limiter.

Observations and analysis of FTU plasmas by video cameras

The interaction of the FTU plasma with the vessel walls and with the limiters is responsible for the release of hydrogen and impurities through various physical mechanisms (physical and chemical sputtering, desorption, etc.). In the cold plasma periphery, these particles are weakly ionised and emit mainly in the visible spectral range. A good description of plasma periphery can then be obtained by use of video cameras. In FTU small size video cameras, placed close to the plasma edge, give wide-angle images of the plasma at a standard rate of 25 frames/s. Images are stored digitally, allowing their retrieval and analysis. This paper reports some of the most interesting features of the discharges evidenced by the images. As a first example, the accumulation of cold neutral gas in the plasma periphery above a density threshold (a phenomenon known as Marfe) can be seen on the video images as a toroidally symmetric band oscillating poloidally; on the multi-chord spectroscopy or bolometer channels, this appears only as a sudden rise of the signals whose overall behaviour could not be clearly interpreted. A second example is the identification of runaway discharges by the signature of the fast electrons emitting synchrotron radiation in their motion direction; this appears as a bean shaped bright spot on one toroidal side, which reverts according to plasma current direction. A relevant side effect of plasma discharges, as potentially dangerous, is the formation of dust as a consequence of some strong plasma–wall interaction event; video images allow monitoring and possibly estimating numerically the amount of dust, which can be produced in these events. Specialised software can automatically search experimental database identifying relevant events, partly overcoming the difficulties associated with the very large amount of data produced by video techniques.