Magnetohydrodynamic Activity Research Articles

Fast high resolution echelle spectroscopy of a laboratory plasma

An echelle diffraction grating and a multianode photomultiplier tube are paired to construct a high resolution (R=λ∕δλ≈2.5×104) spectrograph with fast time response for use from the UV through the visible. This instrument has analyzed the line shape of CIII impurity ion emission at 229.687nm over the lifetime (≈100μs) of the hydrogen plasmas produced at SSX. The ion temperature and line of sight average velocity are inferred from the observed thermal broadening and Doppler shift of the line. The time resolution of these measurements is about 1μs, sufficient to observe the fastest magnetohydrodynamic activity.

Plasma rotation evolution near the peripheral transport barrier in the presence of low-frequency MHD bursts in TUMAN-3M tokamak

The experiments described in the paper are aimed at investigating the possible influence of the low frequency magnetohydrodynamic (MHD) activity burst on the Ohmic H-mode in the TUMAN-3M tokamak. During the MHD burst a transient deterioration of improved confinement was observed. The study has been focused on the measurements of plasma fluctuation poloidal velocity performed by microwave Doppler reflectometry. The plasma fluctuation rotation observed before the MHD burst in the vicinity of the edge transport barrier was in the direction of plasma drift in the negative radial electric field. During the MHD activity the measured poloidal velocity was drastically decreased and even changed its sign. Radial profiles of the poloidal velocity measured in a set of reproducible tokamak shots exhibited the plasma fluctuation rotation in the ion diamagnetic drift direction at the location of the peripheral transport barrier. The possible reasons for this phenomenon are discussed.

Characterization of coherent magnetic fluctuations in JFT-2M high recycling steady high-confinement mode plasmas

The most important feature of the “high recycling steady” (HRS) high-confinement mode (H-mode) regime in the JFT-2M tokamak [H. Ninomiya et al., Fusion Science and Technology (2002), Vol. 42, p. 7] is the stationary pedestal condition in the absence of the large edge localized modes (ELMs). Accompanying the HRS H-mode transition, the coherent magnetic fluctuations in the frequency range of the order of 10–100kHz with significant variation are seen on the magnetic probes at the vessel wall. Above all, two types of edge magnetohydrodynamics (MHD) activities, which have associated toroidal mode number of n=1 and n∼7, respectively, seem to be more important for the HRS H-mode plasmas. To investigate their interaction, bispectral analysis is applied for the magnetic probe data, exhibiting the phase-coupled oscillations between two types of edge MHD activities. Parameter similarities and differences with small ELMs and stationary ELM-free H-mode regimes in other devices are also discussed.

Tomographic Analysis of Central MHD Activities and Radiation Losses on the HL-2A and LHD

Results of the two-dimensionally reconstructed distribution of soft X-ray emission in HL-2A and electromagnetic radiation in large helical device (LHD) are presented. Hardware improvements of in-vessel soft X-ray cameras and the development of tomographic software have made detailed and visual studies of soft X-ray emission possible in HL-2A. Several algorithms are employed in order to get as detail as possible in the images while keeping the guiding assumptions to a minimum. Recently success has been achieved in applying a 2D peeling away algorithm for tomographic reconstruction of LHD bolometric data jointly under the China-Japan collaboration. The data analysed so far have been used to study magnetohydrodynamic (MHD) instabilities on HL-2A tokamak and radiation losses on LHD.

Rational surface localization and MHD activity measurements using fast sweep reflectometry on Tore Supra

Investigations on magnetohydrodynamics (MHD) modes using X-mode frequency modulated continuous wave reflectometry have been realized on Tore Supra. Two heterodyne reflectometers operating between 50–75 GHz and 75–110 GHz probe the low field side of the plasma, and the detection of the magnetic islands is performed through the perturbations recorded onto the phase and amplitude of the reflected signal. Fast sweep technique associated with a high repetition rate provides a high radial resolution with a high temporal resolution (up to 20 kHz). The radial localization of rational surfaces is achieved along with the measured density profile. Detection of the time of flight discontinuities in combination with equilibrium code calculations of the current profile allows for the accurate determination of core plasma MHD modes at the q = 1 and q = 2 rational surfaces. Continuous sweeping gives detailed analysis of the radial and poloidal structure of the magnetic island.

Review of DIII-D H-Mode Density Limit Studies

Density limit studies over the past 10 yr on DIII-D have successfully identified several processes that limit plasma density in various operating modes. The recent focus of these studies has been on maintenance of the high-density operational window with good H-mode level energy confinement. We find that detachment and onset of multifaceted axisymmetric radiation from the edge (MARFE), fueling efficiency, particle confinement, and magnetohydrodynamic activity can impose density limits in certain regimes. By studying these processes, we have devised techniques with either pellets or gas fueling and divertor pumping to achieve line average density above Greenwald scaling, relying on increasing the ratio of pedestal to separatrix density, as well as density profile peaking. The scaling of several of these processes to next-step devices (e.g., the International Thermonuclear Experimental Reactor) has indicated that sufficiently high pedestal density can be achieved with conventional fueling techniques while ensuring divertor partial detachment needed for heat flux reduction. One density limit process requiring further study is neoclassical tearing mode (NTM) onset, and techniques for avoidance/mitigation of NTMs need additional development in present-day devices operated at high density.

Electron Cyclotron Heating on DIII-D

Electron cyclotron heating (ECH) has proved to be a very flexible system for heating applications in DIII-D. The outstanding characteristics of ECH—controllable heating location, a high degree of localization of the power, ability to heat without introducing particles, and ability to heat only the electron fluid—have been used in a wide variety of experiments to study wave physics and transport, to control magnetohydrodynamic activity, and to improve discharges. These characteristics along with relatively easy coupling to the plasma make ECH a valuable resource for both heating and instability control in burning plasmas.

Measurements of impurity and heat dynamics during noble gas jet-initiated fast plasma shutdown for disruption mitigation in DIII-D

Impurity deposition and mixing during gas jet-initiated plasma shutdown is studied using a rapid (∼5 ms), massive (∼1022 particles) injection of neon or argon into stationary DIII-D H-mode discharges. Fast-gated camera images indicate that the neutral deposition remains fairly localized near the injection port and does not penetrate far into the plasma pedestal. Nevertheless, fast bolometry indicates that high (50–100%) thermal quench radiated power fractions are achieved; this appears to be facilitated through a combination of fast ion mixing and fast heat transport, both driven by large-scale magnetohydrodynamic activity. These experiments suggest that massive gas injection might be viable for disruption mitigation in future tokamaks even if core penetration of jet neutrals is not achieved.

Reynolds and Maxwell stress measurements in the reversed field pinch experiment Extrap-T2R

The complete Reynolds stress (RS) has been measured in the edge region of the Extrap-T2R reversed field pinch experiment. The RS exhibits a strong gradient in the region where a high E × B shear takes place. Experimental results show this gradient to be almost entirely due to the electrostatic contribution. This has been interpreted as experimental evidence of flow generation via turbulence mechanism. The scales involved in flow generation are deduced from the frequency decomposition of RS tensor. They are found related to magnetohydrodynamic activity but are different with respect to the scales responsible for turbulent transport.

Study of MHD activity in the HT-7 superconducting tokamak

An attempt is made to explain the behavior of the magneto-hydrodynamic (MHD) instabilities observed at values of the edge safety factor 3 < q ( a ) < 4 , on the HT-7 superconducting tokamak. For the poloidal-magnetic-field fluctuations observed at the Mirnov coils, Mirnov-coil analysis shows that the MHD activity consists predominantly of m = 2 mode. Estimates are given for the characteristic growth time of the m = 2 mode, τ g , and the related rate of island growth, d W / d t . The experimental value of τ g is found to be smaller than the linear theoretical one, and thus satisfy the condition τ A < τ g < τ S (where ideal timescale is τ A , and the resistive timescale is τ S ). However, the experimental and theoretical calculation of d W / d t are found to be in reasonable agreement with each other.

Characteristics of confinement and stability in large helical device edge plasmas

Recent progress in the heating capability in the large helical device [O. Motojima et al., Phys. Plasmas 6, 1843 (1999)] has allowed the highest average β value (4.1%) obtained in the helical devices, and enables exploration of magnetohydrodynamics (MHD) stability in this β region. MHD activities in the periphery are found to become stable spontaneously from the inner region to the outer region when the averaged β value exceeds a threshold, and then a flattening of the electron temperature profile is observed around the resonant surface. Such a flattening can be formed externally by producing an m∕n=1∕1 magnetic island, and the complete stabilization of the m∕n=1∕1 mode is demonstrated by the moderate island width. In addition, attempts to control peripheral plasmas are also performed by using a limiter and a local island divertor utilizing the m∕n=1∕1 island, to improve plasma confinement and, especially, to stabilize pressure-driven modes in the present study. The stabilization of peripheral MHD modes is obtained with both approaches, and this indicates that these are available to the production of higher-β plasmas without edge MHD activities.

2-D imaging of electron temperature in Tokamak plasmas

By taking advantage of recent developments in millimeter wave imaging technology, an electron cyclotron emission imaging (ECEI) instrument, capable of simultaneously measuring 128 channels of localized electron temperature over a two-dimensional (2-D) map in the poloidal plane, has been developed for the TEXTOR tokamak. Data from the new instrument, detailing the magnetohydrodynamic (MHD) activity associated with a sawtooth crash, is presented.

MHD activity triggered by monster sawtooth crashes on Tore Supra

The crash of monster sawteeth in Tore Supra ion cyclotron resonance heated plasmas is observed to trigger long-lived magneto hydrodynamic (MHD) activity, dominated by a (m = 3, n = 2) magnetic perturbation at the edge. This phenomenon is reminiscent of the triggering of neoclassical tearing modes, although in Tore Supra the MHD activity decays and eventually vanishes. It can be explained by the linear destabilization of the (3, 2) mode as the current sheet developed in the non-linear stage of the internal kink relaxation gets closer to q = 3/2. However, the lifetime of the (3, 2) island is longer than the period of linear instability. We find that the neoclassical drive is essential for explaining the observed lifetime and width of the island, although the overall dynamics is controlled by the relaxation of the current profile on a resistive time scale.

Observation of the low to high confinement transition in the large helical device

The low to high confinement transition has been observed on the large helical device [A. Iiyoshi, A. Komori, A. Ejiri et al., Nucl. Fusion 39, 1245 (1999)], exhibiting rapid increase in edge electron density with sharp depression of Hα emission. The transition occurs in low toroidal field (Bt=0.5–0.75T) discharges and are heated by high power neutral beam injection. The plasma thus has a relatively high value (∼1.5%) of the volume averaged β value. The electron temperature and density profiles have steep gradients at the edge region which has high magnetic shear but is at a magnetic hill. Formation of the edge transport barrier leads to enhanced activities of the interchange type of modes with m=2∕n=3 (m,n are the poloidal and toroidal mode numbers) in the edge region. At present, these magnetohydrodynamic activities limit the rise of the stored energy; the resultant increment of the stored energy remains modest.

MHD-induced energetic ion loss during H-mode discharges in the National Spherical Torus Experiment

Magnetohydrodynamic (MHD) induced energetic ion loss in neutral beam heated H-mode discharges in the National Spherical Torus Experiment (NSTX) is discussed. After H-mode onset, the neutral particle analyser (NPA) spectrum usually exhibits a significant loss of energetic ions mainly for E > Eb/2 where Eb is the beam injection energy, although this loss occasionally extends to lower energy. The magnitude of the energetic ion loss diminishes with increasing tangency radius of the NPA sightline, increasing the toroidal field and the neutral beam injection energy. Modelling suggests that MHD-induced ion loss is enhanced during H-mode operation due to an evolution of the q and beam deposition profiles that feeds both passing and trapped ions into the region of the plasma affected by the low-n MHD activity. It must be emphasized that this loss mechanism is a pressure profile effect that can sometimes occur in unusually high density L-mode discharges, but almost always is observed in H-mode discharges because of the intrinsic broad electron density profile. Analysis of the particle interaction with a model magnetic perturbation supports the energy selectivity of the observed MHD-induced loss. Transport analysis using a fast-ion diffusion model to emulate MHD-induced energetic ion loss shows significant modifications of the heating produced by the neutral beam which changes the inferred power balance, thermal diffusivities and the thermal energy confinement time. In the case cited herein, MHD-induced energetic ion loss increased the thermal energy confinement, τE, by ∼15% and reduced the toroidal beta, βT, by ∼7%. An accounting of energetic ion loss is therefore important for proper analysis of power balance and transport in plasmas exhibiting MHD-induced energetic ion loss.

Preliminary design of the soft x-ray array tomographic diagnostic system for Korea Superconducting Tokamak Advanced Research (KSTAR) plasmas

Analyses of x-ray (SXR) emission from tokamak plasmas provide valuable information about magnetohydrodynamics activities and transport phenomena. In this study, design and optimization of the SXR detector arrays were performed for the KSTAR (Korea superconducting tokamak advanced research) device. Geometrical arrangement of the array system was made by coverage mapping and tomographic reconstruction tests. The space-time tomography algorithm was developed for KSTAR-like plasmas using the fast maximum entropy method combined with singular value decomposition. Spatial setup of the components in each detector array was determined and optimized by geometrical calculation. A curved beryllium window and a developed cooling system will be mounted on the detector arrays. Due to the existence of the complex structures between outer vessel and passive stabilizers, each array is designed to be miniaturized. Hardware-optimization of the array system was cross-checked with the tomographic test results.

X-mode reflectometry for magnetohydrodynamic activity associated with q=1 surface measurements on Tore Supra

Tore Supra is equipped with two 20 μs fast sweep X-mode reflectometers operating between 50–110 GHz dedicated to density profile determination and an X-mode fixed frequency reflectometer operating between 105–155 GHz for density fluctuation measurements. Heterodyne and sine–cosine detection provide measurements of the reflected signal with high sensitivity. Operating profile reflectometer in burst mode (5 μs dead time between two consecutive sweeps) allows quasi-simultaneous measurements at fixed frequency over a broad frequency band. Thus, information on plasma fluctuations, such as magnetohydrodynamic (MHD) activity, up to 20 kHz as well as a radial localization of the modes is accessible. The temporal evolution of the q=1 rational surface during sawtooth crash activity has been recorded in the plasma center with high spatial resolution. In addition, a direct comparison between signals associated with a central MHD mode from both profile and fluctuation reflectometers, positioned at different toroidal angles, allows one to determine the plasma toroidal velocity.

“Optical” soft x-ray arrays for fluctuation diagnostics in magnetic fusion energy experiments

We are developing large pixel count, fast (⩾100 kHz) and continuously sampling soft x-ray (SXR) array for the diagnosis of magnetohydrodynamics (MHD) and turbulent fluctuations in magnetic fusion energy plasmas. The arrays are based on efficient scintillators, high thoughput multiclad fiber optics, and multichannel light amplification and integration. Compared to conventional x-ray diode arrays, such systems can provide vastly increased spatial coverage, and access to difficult locations with small neutron noise and damage. An eight-channel array has been built using columnar CsI:Tl as an SXR converter and a multianode photomultiplier tube as photoamplifier. The overall system efficiency is measured using laboratory SXR sources, while the time response and signal-to-noise performance have been evaluated by recording MHD activity from the spherical tori (ST) Current Drive Experiment-Upgrade and National Spherical Torus Experiment, both at Princeton Plasma Physics Laboratory.

Effect of MHD on edge electrostatic fluctuation with lower hybrid current drive in the HT-7 tokamak

The close correlation between the edge electrostatic fluctuations around the last closed flux surface and the magnetohydrodynamic (MHD) activity in the case of lower hybrid current drive (LHCD) is observed and analysed in the HT-7 tokamak. The turbulent transport is primarily driven by a coherent MHD mode, indicating that the electrostatic fluctuations are strongly correlated with MHD activity. The results supply direct evidence that the edge electrostatic fluctuations in tokamak plasmas can be decreased by suppressing internal tearing mode activity and thus reduce the radial plasma transport. The coherent electrostatic fluctuations may be possibly attributable to the radial plasma transport accompanied by the internal tearing mode activity. Studies show that the smaller electrostatic fluctuations at a frequency of 10 kHz may be mainly ascribed to the suppressed MHD activity, possibly due to the formation of an internal transport barrier and flow shear in the edge region. It is speculated that the change of mode region resulting from the mode coupling/transform, excited by the modified current profile by LHCD, could also provide some contribution to the MHD reduction.

Equilibrium and Stability of High-β Plasmas in Wendelstein 7-AS

One of the major goals for Wendelstein 7-AS (W7-AS) was the testing of the theoretical basis for the optimized configuration of Wendelstein 7-X (W7-X), currently under construction in Greifswald, Germany. In the last experimental campaign of W7-AS, volume-averaged β values >3% have been achieved. The underlying experimental changes leading to these results are briefly reviewed. The equilibrium characteristics expected from magnetohydrodynamic (MHD) theory are modeled in a simplified picture and compared with three-dimensional equilibrium calculations. A wide range of parameters has been covered in the experiments with and without net toroidal currents. Experimental data are compared with free-boundary equilibrium calculations and exhibit good agreement. The high-β equilibria usually showed only small MHD activity. The most prominent activities are low-frequency pressure-driven modes connected with low-order rationals also expected from numerical calculations using the CAS3D code, and Alfvén modes driven by energetic particles from the tangential neutral beam injection. Comparison of experimentally measured frequencies and mode structures from soft-X-ray tomography with theoretical predictions also shows the improving understanding of these modes in stellarators. The agreement of experiment and theory gives confidence in the predictions for W7-X.