Poloidal Plasma Rotation Research Articles

Response of the poloidal rotation to resonant magnetic perturbations in the EAST Tokamak

Abstract In this paper the response of the plasma poloidal rotation to resonant magnetic perturbations (RMPs) is investigated in EAST Tokamak using the multi-channel Doppler Backscattering (DBS) system. It shows that the poloidal rotation spins up towards the ion-diamagnetic drift direction with increasing external perturbation field, which will reduce the edge shear. In ohmically heated discharges, the n=1 RMP can only affect the edge poloidal rotation when the RMP coil current is small, and the influence will gradually reach the inner regions with increasing RMP coil current. At the moment of the n=1 RMP penetration, all the poloidal rotations measured by the DBS will increase significantly, and then they will keep almost unchanged with the increase of the RMP coil current. In H-mode discharges, the poloidal rotation is significantly influenced by the n=2 RMP, and the edge velocity well even reverses, along with ELMs mitigation. However, in the same shot, the n=4 RMP with the same coil current amplitude can hardly affect the poloidal rotations and the behavior of ELMs.

First results of turbulence investigation on Globus-M2 using radial correlation Doppler reflectometry

The first results of investigation of the turbulence structure using Doppler backscattering (DBS) on the Globus-M2 tokamak are presented. A one-channel DBS system with a variable probing frequency within the 18–26 GHz range was installed to investigate the edge plasma at normalized minor radii 0.9–1.1. Radial correlation Doppler reflectometry was used to study the changes in turbulence eddies after the LH transition. Correlation analysis was applied to the phase derivative of complex in-phase and quadrature (IQ) signals of the DBS diagnostic as it contains information about the poloidal plasma rotation velocity. In L-mode, the radial correlation length L r is estimated to be 3 cm and after transition to H-mode reduces to approximately 2 cm. Gyrokinetic modelling in a linear local approximation using code GENE indicates that the instability with positive growth rate at the normalized minor radius 0.75 in L-mode and H-mode on Globus-M2 was the ion temperature gradient (ITG) mode.

A Poloidal CO2 Laser Collective Scattering System for Measuring Electron-Scale Turbulence in the EAST

Anomalous electron thermal transport is an important issue that restricts the development of magnetic confinement thermonuclear fusion, and it is closely related to electron-scale turbulence. This paper introduces the poloidal CO2 laser collective scattering diagnostic system installed on the Experimental Advanced Superconducting Tokamak (EAST) for electron-scale turbulence measurement. The system can measure density fluctuations with four distinct wavenumbers simultaneously ranging from 10 to 30 cm−1 (correspondingly kθρs∼1to5) in two regions (the core region ρ≈0 to 0.4 and the outer region ρ≈0.4to0.8), which realizes the spatial resolution for turbulence measurement. And, the plasma poloidal rotation velocity in these two regions can be calculated using the measured density fluctuation frequency. In addition, the characteristics of small scattering angle and negligible wave refraction effects reduce the size of the ports required for this diagnostic system. These advantages make the diagnostic system an effective tool for measuring electron-scale turbulence and may play an important role in future burning plasma experiments.

Prevention of electron cyclotron current drive triggering explosive bursts in reversed magnetic shear tokamak plasmas for disruption avoidance

An explosive burst excited by a neoclassical tearing mode (NTM) is one of the possible candidates for disruptive terminations in reversed magnetic shear (RMS) tokamak plasmas. For the purpose of disruption avoidance, numerical investigations have been implemented on the prevention of explosive bursts triggered by the ill-advised application of electron cyclotron current drive (ECCD) in RMS configuration. Under the situation of controlling NTMs by ECCD in RMS tokamak plasmas, a threshold in electron cyclotron driven current has been found. Below the threshold, not only are the NTM islands not effectively suppressed but a deleterious explosive burst could also be triggered, which might contribute to major disruption to tokamak plasmas. In order to prevent this ECCD from triggering explosive bursts, three control strategies have been attempted in this work and two of them have been recognized to be effective. One is to apply differential poloidal plasma rotation in the proximity of outer rational surface during the ECCD control process; the other is to apply two ECCDs to control NTM islands on both rational surfaces at the same time. In the former strategy, the threshold is diminished due to the modification of the classical tearing mode index. In the latter strategy, the prevention is accomplished as a consequence of the reduction of the coupling strength between the two rational surfaces via the stabilization of inner islands. Moreover, the physical mechanism behind the excitation of the explosive burst and the control processes by different control strategies have all been discussed in detail.

Controlling the rotation of drift tearing modes by biased electrode in ADITYA-U tokamak

The influence of background plasma poloidal rotation on the rotation frequency of the m/n = 2/1 drift tearing mode (DTM) has been studied in ADITYA-U tokamak. The poloidal rotation velocity of the background plasma in the ion diamagnetic direction is increased or decreased by inducing an outward or inward radial electric field, respectively, through a biased-electrode placed in the edge region of the plasma. The rotation frequency of the preexisting drift tearing mode, rotating in the electron diamagnetic direction, concomitantly decreased or increased with the application of bias depending on its polarity. The positive-bias increases the background plasma rotation in the ion-diamagnetic direction from its pre-bias value, hence decreasing the DTM rotation frequency, whereas the negative bias reduces the plasma rotation velocity in the ion-diamagnetic direction, hence increasing the mode rotation. In addition to that, a short gas puff introduced during the positive and negative bias pulse further reduces the mode frequency, however, with different amplitudes in different bias-polarities. These observations suggest that the background plasma rotation contributes significantly toward the rotation of DTMs, and the rotation frequency of the magnetohydrodynamic modes can be modified by varying the poloidal rotation of the background plasma and/or the diamagnetic drift frequency.

Overview of plasma rotation studies on the TCABR tokamak

An overview of intrinsic plasma rotation studies in Ohmic L-mode discharges carried out in the Tokamak Chauffage Alfvén Brésilien (TCABR) tokamak is presented. Measurements of plasma poloidal and toroidal rotation, and a comparison against neoclassical theory, are presented. The results show that poloidal rotation is in good agreement with neoclassical theory while toroidal rotation is found to be anomalous. A new technique that allows for high temporal resolution measurements of plasma rotation is presented. This technique is used to test two models of intrinsic toroidal rotation: the so-called Helander model (Helander et al 2003 Physics of Plasmas 10 4396) and Rozhansky model (Rozhansky 2013 Perpendicular currents and electric fields in fully and partially ionized magnetized plasma Physics of Plasmas 24 101614). As TCABR is a relatively small device, the influence of the neutrals that form the basis of this model is expected to be enhanced. The results indicate that the mechanism proposed by Helander does not contribute significantly to the intrinsic toroidal rotation in TCABR plasmas. The measurements, however, indicate that the frictional force proposed by Rozhansky might be responsible for part of the intrinsic toroidal rotation observed in TCABR plasmas.

Nonlinear dynamics of the fishbone-induced alpha transport on ITER

The fishbone-induced transport of alpha particles is computed for the ITER 15 MA baseline scenario (ITER Physics Expert Group on Energy Drive 1999 Nucl. Fusion 39 2471) using the non-linear hybrid Kinetic-MHD code XTOR-K. Two limit cases have been studied, in order to analyse the characteristic regimes of the fishbone instability: the weak kinetic drive limit (Berk et al 1999 Phys. Plasmas 6 3102) and the strong kinetic drive limit (Zonca et al 2015 New J. Phys. 17 013052). In both those regimes, characteristic features of the n = m = 1 fishbone instability are recovered, such as a strong up/down-chirping of the mode frequency, associated with a resonant transport of trapped and passing alpha particles. The effects of the n = m = 0 sheared poloidal and toroidal plasma rotation are taken into account in the simulations. The shear is not negligible, which implies that the fishbone mode frequency has a radial dependency, impacting the wave-particle resonance condition. Phase space hole and clump structures are observed in both non-linear regimes, centered around the precessional and passing resonances. These structures remains attached to the resonances as the different mode frequencies chirp up and down. In the non-linear phase, the transport of individual resonant trapped particles is identified to be linked to mode-particle synchronization. On this basis, a partial mechanism for the non-linear coupling between particle transport and mode dominant down-chirping is proposed. The overall transport of alpha particles through the q = 1 surface is of order 2–5\\% of the initial population between the simulations. The loss of alpha power is found to be directly equal to the loss of alpha particles.

Synergistic effect of Coriolis and centrifugal forces from poloidal flow on internal kink and fishbone modes in tokamak plasmas

The synergy of Coriolis force and centrifugal force is proposed to study the influence of poloidal plasma rotation on internal kink and fishbone modes. A new dispersion relation is established by making use of energy principle when Coriolis and centrifugal forces are taken into account in the momentum equation. The significant discovery is that the destabilizing (stabilizing) effect of poloidal flow on internal kink (fishbone) mode is greatly increased due to the synergy of Coriolis and centrifugal forces. Poloidal flow can neither effectively destabilize internal kink mode nor stabilize fishbone mode with any one of both centrifugal force and Coriolis force being excluded. It is most interesting that the internal kink mode, being stable with positive δWc (perturbed potential energy of bulk plasma), is unstable when poloidal rotation frequency exceeds a threshold. It is difficult for poloidal flow without shear to destabilize the internal kink mode with δWc > 0. The physical mechanism of poloidal flow destabilization of internal kink mode mainly comes from the modification of plasma inertial due to Coriolis and centrifugal forces.

Применение метода многочастотного допплеровского обратного рассеяния для исследования альфвеновских мод в токамаке

The results of the study of toroidal Alfven modes (TAE) using the multi-frequency Doppler backscattering (DBS) in the Globus-M tokamak are presented. The article is focused on the presentation of the Alfven mode registration method for multichannel probing. The possible causes of the observed oscillations of the poloidal plasma rotation velocity at the Alfvén oscillation frequencies are discussed in detail. The data on the spatial distribution of Alfvén modes revealed by multi-frequency DBS are given. The recommendations for the further development of the DBS with the aim of a more detailed study of TAE in tokamaks were determined.

Simulation of plasma turbulence in the periphery of diverted tokamak by using the GBS code

We report on the implementation of diverted magnetic equilibria in GBS and on first simulations in this geometry. GBS is a simulation code used to evolve plasma turbulence in the tokamak periphery by solving the drift-reduced Braginskii's equations. The model equations are written in toroidal coordinates, abandoning flux coordinate systems that are not defined at the X-point. A fourth order finite difference scheme is used for the implementation of the spatial operators on poloidally and toroidally staggered grids. The GBS numerical implementation is verified through the method of manufactured solutions. The code convergence properties are tested on a relatively simple analytical X-point configuration. Finally, the diverted equilibrium from a TCV tokamak discharge is implemented in the new version of GBS. The analysis of the simulation results is focused on blob formation, radial transport, and plasma poloidal rotation mechanisms.

V-band Doppler backscattering diagnostic in the TCV tokamak.

A variable configuration V-band heterodyne Doppler back-scattering diagnostic has been recently made operational in the tokamak configuration variable. This article describes the hardware setup options, flexible quasi-optical launcher antenna, data-analysis techniques, and first data. The diagnostic uses a fast arbitrary waveform generator as the main oscillator and commercial vector network analyzer extension modules as the main mm-wave hardware. It allows sweepable single or multi-frequency operation. A flexible quasi-optical launcher antenna allows 3D poloidal (10°-58°) and toroidal (-180° to 180°) steering of the beam with 0.2° accuracy. A pair of fast HE11 miter-bend polarizers allow flexible coupling to either O or X mode and programmable polarization changes during the shot. These have been used to measure the magnetic-field pitch angle in the edge of the plasma by monitoring the backscattered signal power. Ray-tracing simulations reveal an available k⊥ range between 3 and 16 cm-1 with a resolution of 2-4 cm-1. Perpendicular rotation velocity estimates compare well against ExB plasma poloidal rotation estimates from charge exchange recombination spectroscopy.

Calculation of edge ion temperature and poloidal rotation velocity from carbon III triplet measurements on the COMPASS tokamak

Abstract A high-resolution spectroscopic system for the measurements of the CIII triplet at 465 nm was installed at the COMPASS tokamak. The Doppler broadening and shift of the measured spectral lines are used to calculate the edge ion temperature and poloidal plasma rotation. At first, the spectroscopic system based on two-grating spectrometer and the calibration procedure is described. The signal processing including detection and removal of spiky features in the signal caused by hard X-rays based on the difference in the behaviour of Savitzky-Golay and median filters is explained. The detection and position estimation of individual spectral lines based on the continuous wavelet transform is shown. The method of fitting of Gaussians using the orthogonal distance regression and estimation of the error of estimation of the rotation velocity and ion temperature is described. At the end, conclusions about the performance of the spectroscopic system and its shortcomings based on summary of results calculated from 2033 processed spectral lines measured in 61 shots are drawn and the possible enhancements are suggested.

Impact of ion diamagnetic drift on MHD stability at edge pedestal in JT-60U rotating plasmas

The effect of ion diamagnetic drift on the stability of peeling–ballooning modes in rotating tokamak plasmas has been analyzed numerically. The results show that plasma toroidal rotation can not only destabilize the peeling–ballooning mode but also can reduce the ion diamagnetic drift effect on the mode stability, even though the ion diamagnetic drift effect stabilizes the mode in a static plasma. Plasma poloidal rotation can also destabilize the mode and cancel the ion diamagnetic drift effect, even when the rotation frequency is much smaller than the toroidal one. These impacts of the rotation on the stability can resolve the discrepancy between the result of the numerical stability analysis and the experimental result in type-I ELMy H-mode plasmas in JT-60U. The reduction of the ion diamagnetic drift effect on ELM stability due to plasma rotation is shown to depend on the direction of the rotation, so that the ion diamagnetic drift effect becomes negligible only when the JT-60U plasma rotates in the direction counter to the plasma current.

Doppler reflectometry studies of plasma gradient instabilities in L-2M stellarator

Paper reports on a broadband spectrum analysis of density fluctuations in the edge plasma of the L-2M stellarator, measured using the Doppler reflectometry diagnostics. The evolution of the electron temperature-gradient (ETG) and ion temperature-gradient (ITG) instabilities in the edge plasma of the stellarator is analyzed. Two additional harmonics in the spectrum are found apart from those determined by the Doppler shift caused by poloidal rotation of plasma. The Doppler shifts of these harmonics are determined from the phase velocities of fluctuations generated by the ETG and ITG instabilities. Thus, local measurements of spectra using the Doppler reflectometry enabled us both to evaluate the poloidal rotational velocity of the plasma and describe the evolution of low-frequency plasma instabilities.

The tri-band high-resolution spectrometer for the ITER CXRS diagnostic system

A tri-band high-resolution spectrometer, which was designed for performing diagnostics on the ITER facility using the charge-exchange recombination spectroscopy (CXRS), is described. The CXRS allows measurements of such plasma parameters as the ion temperature, the speed of the toroidal and poloidal plasma rotation, and the concentration of light impurities. The spectrometer is based on three transparent holographic diffraction gratings and is designed to operate simultaneously in three spectral bands: 468 ± 6 nm, 529 ± 6 nm, and 656 ± 8 nm. The results of measuring the main performance parameters of the transparent diffraction gratings and the spectrometer as a whole are presented. It was established that the characteristics of the developed spectrometer satisfy the requirements for the spectroscopic equipment for the ITER CXRS diagnostic system.

Effect of plasma motion on tearing modes in cylindrical plasmas

The effect of equilibrium plasma motion on the resistive m/n = 2/1 tearing mode (TM) in low β plasmas is investigated in cylindrical geometry (with m and n being poloidal and toroidal mode numbers). Without equilibrium plasma motion but with viscosity, the TM stability is mainly determined by the Reynolds number S and reaches maximum near S = 104, which is consistent with previous findings. The poloidal plasma rotation has stabilizing effect on TM; however, the rotation shear has destabilization effect in the low viscosity regime. The axial plasma motion has strong stabilizing effect on TM in the low viscosity regime for Prandtl number Pr < 1, while its shear has slight stabilizing effect with the decrease of growth rate less than 15%. When the axial velocity becomes large enough, the mode frequency tends to be independent of the Prandtl number. In the presence of parallel plasma motion, the growth rate is determined by the axial component at low parallel velocity, while determined by poloidal component at large parallel velocity. The parallel plasma motion drives the TM rotating in the opposite direction. It is shown that the equilibrium motion reduces the growth rate of TM by changing the phase difference and coupling coefficient between potential perturbation and magnetic flux perturbation (deviating from π/2), which results in a lower mode frequency. Compared to the role of velocity shear, the magnitude of plasma velocity itself at the m/n = 2/1 rational surface is dominant in determining the TM characteristics.

Novel approaches for mitigating runaway electrons and plasma disruptions in ADITYA tokamak

This paper summarizes the results of recent dedicated experiments on disruption control and runaway mitigation carried out in ADITYA, which are of the utmost importance for the successful operation of large size tokamaks, such as ITER. It is quite a well-known fact that disruptions in tokamaks must be avoided. Disruptions, induced by hydrogen gas puffing, are successfully avoided by two innovative techniques in ADITYA using a bias electrode placed inside the last closed flux surface and applying an ion cyclotron resonance pulse with a power of ∼50 to 70 kW. These experiments led to better understanding of the disruption avoidance mechanisms and also can be thought of as one of the options for disruption avoidance in ITER. In both cases, the physical mechanism seems to be the control of magnetohydrodynamic modes due to increased poloidal rotation of edge plasma generated by induced radial electric fields. Real time avoidance of disruption with identifying proper precursors in both the mechanisms is successfully attempted. Further, analysing thoroughly the huge database of different types of spontaneous and deliberately-triggered disruptions from ADITYA, a significant contribution has been made to the international disruption database (ITPA). Furthermore, the mitigation of the runaway electron generated mainly during disruptions remains a challenging topic in present tokamak research as these high-energy electrons can cause severe damage to in-vessel components and the vacuum vessel. A simple technique has been implemented in ADITYA to mitigate the runaway electrons before they can gain high energies using a localized vertical magnetic field perturbation applied at one toroidal location to extract runaway electrons.

High-resolution spectroscopy diagnostics for measuring impurity ion temperature and velocity on the COMPASS tokamak

High-resolution spectroscopy is a powerful tool for the measurement of plasma rotation as well as ion temperature using the Doppler shift of the emitted spectral lines and their Doppler broadening, respectively. Both passive and active diagnostic variants for the COMPASS tokamak are introduced. The passive diagnostic focused on the C III lines at about 465nm is utilized for the observation of the poloidal plasma rotation. The current set-up of the measuring system is described, including the intended high-throughput optics upgrade. Different options to increase the fiber collection area are mentioned, including a flower-like fiber bundle, and the use of micro-lenses or tapered fibers. Recent measurements of poloidal plasma rotation of the order of 0–6km/s are shown. The design of the new active diagnostic using a deuterium heating beam and based on charge exchange recombination spectroscopy (C VI line at 529nm) is introduced. The tool will provide both space (0.5–5cm) and time (10ms) resolved toroidal plasma rotation and ion temperature profiles. The results of the Simulation of Spectra code used to examine the feasibility of charge exchange measurements on COMPASS are shown and connected with a selection of the spectrometer coupled with the CCD camera.

Indirect measurement of poloidal rotation using inboard–outboard asymmetry of toroidal rotation and comparison with neoclassical predictions

An alternative experimental spectroscopic measurement of poloidal plasma rotation in toroidally confined plasmas is proven effective in the TCV tokamak. Charge exchange recombination measurements of the toroidal rotation profile over the full mid-plane plasma diameter are used to infer the complete bi-dimensional flow structure of the intrinsic C6+ impurity, which includes its poloidal component. For divergence free flows, the difference between the toroidal rotation frequency ft = ut/R at the inboard and outboard locations on the same flux surface is proportional to the poloidal rotation. This indirect measurement provides increased accuracy as the measured quantity ft,in − ft,out≈4qup/Raxis(q is the local safety factor) is larger than the intrinsic uncertainties of a direct spectroscopic measurement of poloidal velocity. The method is applied in a variety of TCV ohmic and electron cyclotron heated L-mode plasmas in the banana-plateau collisionality regime . In the radial range of normalized poloidal flux ρψ < 0.8, an impurity poloidal velocity of up = 0.5–2.5 km s−1 is observed, always in the electron diamagnetic drift direction. The measurements are compared with neoclassical calculations and they agree in magnitude and sign to within <1 km s−1.

Two Point Correlation Technique for the Measurements of Poloidal Plasma Rotation by Heavy Ion Beam Probe

The paper proposes a method to measure poloidal rotation velocity of toroidal plasma vpol using Heavy Ion Beam Probe (HIBP) with a multi-slit energy analyzer. The method is based on calculation of phase shift between broadband density turbulence measured simultaneously in two sample volumes, located at the same magnetic surface but separated poloidally. Oscillatory component of HIBP beam current is used as a density turbulence characteristic. HIBP is capable to provide the temporal evolution of the vpol in a fixed radial position and also the vpol profile by periodic radial scan. Method was verified in real plasma experiment in ECRH and NBI discharges on the TJ-II stellarator. Result shows that in low density discharges (ne ≈ 0.3-0.5 × 10 19 m −3 ) absolute values of local vpol is about 4-6 km/s, oriented in the ion diamagnetic drift direction. When HIBP operates for radial scans, it is conventionally measuring the plasma potential profile, and so provides the radial electric field Er and velocity of E × B drift (vE×B) at the same time as plasma rotation. Experimental data shows that in low density ECRH plasma the rotation velocity coincides with E × B velocity within achieved experimental accuracy. When the density is increasing, both vE×B and vpol tends to decrease and then change the sign at threshold plasma densities in the range of ne ≈ 0.7-1 × 10 19 m −3 . With this new proposed technique HIBP becomes the new effective tool to study plasma rotation and turbulence characteristics in toroidal plasmas.