ECH Plasmas Research Articles

High-density experiments with hydrogen ice pellet injection and analysis of pellet penetration depth in Heliotron J

A hydrogen cryogenic pellet injection system was fabricated and applied to high-density experiments in a Heliotron J device. The fueling through pellet injection was conducted in Heliotron J by using an injection barrel with a tapered structure for a small pellet with low speed. A single pellet with a speed of 260 ± 30 m s−1 and a typical size of 1.1–1.2 mm was injected into NBI + ECH plasmas and NBI-only plasmas. In both cases, a significant increase in electron density was observed upon pellet injection. The increase in line-averaged electron density due to pellet injection was 4.0 ± 0.7 × 1019 m−3 in both cases. Pellet fueling leads to high stored energy in the high-electron-density regime. A peaked electron density profile with high core density is obtained in NBI-only plasmas after pellet injection, while the edge density is kept at a lower level. An innovative Hα array measurement shows that the pellet has penetrated into the magnetic axis in NBI + ECH plasmas, while the penetration depth in NBI-only plasmas is beyond the magnetic axis, which is deeper than that in NBI + ECH plasmas. The difference in penetration depth cannot be explained by the simulation results of the ablation code, in which the effect of fast electrons/ions on pellet ablation is not considered. These results suggest that the effect of fast electrons/ions should be taken into account in the ablation code in order to explain the present experimental results.

On the correlation between ‘non-local’ effects and intrinsic rotation reversals in Alcator C-Mod

Contemporary predictive models for heat and particle transport in tokamak plasmas are based on the assumption that local fluxes can be described in terms of local plasma parameters, where electromagnetic drift-wave-type turbulence is driven by local gradients and results in cross-field transport. The question of whether or not transport could be dominated by non-local terms in certain circumstances is essential for our understanding of transport in magnetically confined plasmas, and critical for developing predictive models for future tokamaks, such as ITER. Perturbative transport experiments using cold-pulse injections at low density seem to challenge the local closure of anomalous transport: a rapid temperature increase in the core of the plasma following a sharp edge cooling is widely observed in tokamaks and helical devices. Past work in Ohmic plasmas in Alcator C-Mod and in ECH plasmas in KSTAR found that the temperature inversions disappear at higher densities, above the intrinsic toroidal rotation reversal density. These observations suggested that the so-called ‘non-local’ heat transport effects were related to the intrinsic rotation reversal, and therefore to changes in momentum transport. In this work, new experiments and analysis at Alcator C-Mod show that intrinsic rotation reversals and disappearance of temperature inversions are not concomitant in Ohmic plasmas at high plasma current and in ICRH L-modes. This new data set shows that the correlation between transient temperature inversions and intrinsic rotation reversals is not universal, suggesting that ‘non-local’ heat transport and momentum transport effects may be affected by different physical mechanisms.

Gas puff modulation experiment measured by interferometers in Heliotron J

An HCN laser (λ = 337 μm) interferometer with a high time resolution has been developed in a helical device, Heliotron J, for the study of plasma confinement and transport. Using the new interferometer in combination with a microwave interferometer, a gas puff modulation experiment has been performed to clarify the particle transport in ECH and ECH + NBI heated plasmas. Based on the particle balance equation, the diffusion coefficient D and the convection velocity V are evaluated on the assumption of profile shapes for D, V and particle source. The result indicates that ECH plasma has better particle transport characteristics, smaller value on D and V, than the case of NBI heated plasmas. The influence of the source profile shape on this analysis is considered, because there is ambiguity on the edge plasma parameters around LCFS, which determines the source profile shape. Although evaluated values of D and V can depend on the source profiles, the difference still remains within the error bars at the present accuracy in this experimental condition, suggesting that more careful treatment of the assumption on particle source is required for the particle transport study with higher accuracy.

Higher Harmonics in the Perturbative Transport Study in TJ-II ECH Plasma

We analyzed the higher harmonics of temperature perturbation in the modulated electron cyclotron heating experiment on TJ-II plasma. The higher harmonics (e.g., 5th harmonic) exhibited significantly weaker decay in amplitude as they propagated in radius as compared with the prediction by diffusive model. The change in the time derivative of temperature at the onset (and turning-off) of the heating power propagates in radius with very little temporal smoothening.

High-β plasma formation and observation of peaked density profile in RT-1

High-β ECH plasma is generated and stably sustained in a magnetospheric configuration, the Ring Trap 1 (RT-1) device, generated by a levitated dipole field magnet. Geomagnetic-field compensation and optimized operation have realized drastic improvements in plasma properties. The maximum local β value has reached 70% and the pressure profiles have a rather steep gradient near the superconducting magnet. Electrons of the high-β plasma typically consist of 70% hot (∼50 keV) and the rest of cold populations. Confinement time of the hot component plasma is 0.5 s with the optimized neutral gas pressure. By removing the coil support structure, the peaked density profile is observed in the strong field region.

Electron Density Profile Measurement in Heliotron J with a Microwave AM Reflectometer

AbstractA microwave reflectometer is developed for electron density profile measurement in Heliotron J. An amplitude modulation (AM) type system is adopted to reduce density fluctuation effects. The carrier frequency ranges from 33 to 56 GHz, and the modulation frequency is 200 MHz. The X‐mode is selected as the propagation mode in order to measure a hollow density profile which is typically observed in ECH plasmas. A test‐bench examination using an aluminum reflection plate shows that the measured phase shift agrees well with that expected from the change in the plate position. The initial measurement results show that the reconstructed density profile has hollow profile at low density and steep gradient at the edge in ECH plasma. The hollowness is weakened as the averaged density increases (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of Low‐Order Rational Surfaces on the Radial Electric Field of TJ‐II ECH Plasmas

AbstractDynamic magnetic configuration scan experiments have been conducted in order to investigate the influence of the rotational transform on plasma rotation, radial electric field and turbulence. The main magnetic resonances (7/4 and 5/3 in this work) make a positive contribution to the local radial electric field, which depends on the plasma density and plasma radius. A local reduction in the level of density fluctuations due to the influence of low order rational surfaces is also observed. These results could explain local transport changes associated to low order rational surfaces observed in TJ‐II ECH plasmas (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of Supersonic Molecular Beam Injection (SMBI) on Plasma Performance in Heliotron J

AbstractGas fueling by supersonic molecular beam injection (SMBI) is successfully applied to ECH/NBI plasmas in Heliotron J. The stored energy reached ∼4.5 kJ for a heating combination of ECH (∼0.35 MW) and NBI (∼0.6 MW), which is about 50 % higher than the maximum one achieved so far under the conventional gas‐puffing in Heliotron J. After an SMBI pulse for ECH+NBI plasma, two different kinds of perturbation in the radiation profile are caused simultaneously in the center and the peripheral regions. For ECH plasma, an SMBI pulse causes increase or decrease of electron temperature, which seems to depend on the target density (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Construction and Plasma Initiation of the Tokamak-Helical Hybrid Device TOKASTAR-2

TOKASTAR, a hybrid magnetic configuration between tokamak and stellarator, has been proposed for the study of magnetic confinement optimization. The construction of the small device named TOKASTAR-2 was started in April 2008, and completed in March 2009. As a preliminary experiment, spatial distribution of electron temperature (Te) and electron density (ne) are measured in a simple toroidal magnetic field configuration. Initial ECH plasmas with Te ∼ 10 eV, ne ∼ 1016 m−3 are confirmed. In the preliminary experiment of ohmic heating plasma production, plasma current (Ip) of 25 A is observed. The plasma equilibrium is calculated by the TOSCA code, and the magnetic flux surface and optimal vertical field coil currents are clarified for the future experiment.

Sheared flows and transition to improved confinement regime in the TJ-II stellarator

Sheared flows have been experimentally studied in TJ-II plasmas. In low-density ECH plasmas, sheared flows can be easily controlled by changing the plasma density, thereby allowing the radial origin and evolution of the edge velocity shear layer to be studied. In high density NBI heated plasmas a negative radial electric field is observed that is dominated by the diamagnetic component. The shear of the negative radial electric field increases at the L–H transition by an amount that depends on the magnetic configuration and heating power. Magnetic configurations with and without a low order rational surface close to the plasma edge show differences that may be interpreted in terms of local changes in the radial electric field induced by the rational surface that could facilitate the L–H transition. Fluctuation measurements show a reduction in the turbulence level that is strongest at the position of maximum Er shear. High temporal and spatial resolution measurements indicate that turbulence reduction precedes the increase in the mean sheared flow, but is simultaneous with the increase in the low frequency oscillating sheared flow. These observations may be interpreted in terms of turbulence suppression by oscillating flows, the so-called zonal flows.

Confinement transitions in TJ-II under Li-coated wall conditions

This paper presents the latest results on confinement studies in the TJ-II stellarator. The inherently strong plasma–wall interaction of TJ-II has been successfully reduced after lithium coating by vacuum evaporation. Besides H retention and low Z, Li was chosen because there exists a reactor-oriented interest in this element, thus giving special relevance to the investigation of its properties. The Li-coating has led to important changes in plasma performance. Particularly, the effective density limit in NBI plasmas has been extended reaching central values of 8 × 1019 m−3 and Te ≈ 250–300 eV, with peaked density, rather flat Te profiles and higher ion temperatures. Due to the achieved density control, a second type of transition has been added to the low density ones previously observed in ECRH plasmas: higher density transitions characterized by the fall in Hα emission, the onset of steep density gradient and the reduction in the turbulence; which are characteristics of transition to the H mode. Confinement studies in ECH plasmas indicate that lowest order magnetic resonances, even in a low shear environment, locally reduce the effective electron heat diffusivities, while Alfven eigenmodes destabilized in NBI plasmas can influence fast ion confinement.

Initial Results of X-ray Imaging and Energy Spectrum Measurements of Hot Electron Plasmas in RT-1

To acquire spatial profiles and energy spectra of hot electrons in ECH plasmas, we installed a soft x-ray pinhole camera in RT-1. In this publication, we compare the results of an initial experiment using a mechanically supported dipole field coil with the measurements of plasma pressure for different microwave frequencies. The results indicate that the coil support structure was the major loss channel for the high temperature electrons.

Effect of Ellipticity on Thermal Transport in ECH Plasmas in LHD

Effect of ellipticity on thermal transport was investigated in ECH plasmas in LHD. Ellipticity κ is scanned from 0.8 to 1.4 by controlling the quadrupole magnetic field. Experimental data of energy confinement time align with the scaling for all configurations; however, there exist systematic offsets. Performance τEexp /τEISS04 is summarized as 0.94 ± 0.02 for κ = 0.8, 1.41 ± 0.07 for κ = 1.0, and 0.91± 0.03 for κ = 1.4. Local transport analysis based on power balance indicates that the anomalous transport predominates the plasma transport. However, the observed anomaly shows correlation with the change in the effective helical ripple eeff. Physical background of this correlation and the dependence on the poloidal viscous damping rate Cp is discussed. The present experimental comparison suggests a negative evidence for the relevance of Cp.

ECH and HHFW Start-Up Experiments on the TST-2 Spherical Tokamak

Plasma start-up experiments without using inductive field have been performed on the TST-2 spherical tokamak device. A low power ECH (2.45GHz/4kW) and a medium power High Harmonic Fast Wave (HHFW) (21MHz/up to 50kW) were used. Comparisons of various operational parameters were carried out to find the optimum conditions for the toroidal and the poloidal field strengths, poloidal field configurations, ECH power and HHFW power. Using only ECH, currents up to about 0.4 kA were generated. Current increments by up to 0.4 kA were observed when we apply HHFW to ECH plasmas.

ECH Plasma Experiments on an Internal Coil Device with a High Temperature Superconductor Coil

Self-organization related with relaxation phenomenon is playing an important role in various aspects of magnetic confined plasmas. Recently a relaxation theory including the plasma flow has been developed by Mahajan-Yoshida, and a new relaxation state has been identified. The two-fluid relaxation condition is given by β + (V/VA)2 = const. To study a self-organized structure with strong plasma flow, we have introduced an internal coil device. By inducing a radial electric field with appropriate methods, we could drive a toroidal plasma flow, and confine a high beta plasma in a core region. The internal coil device Mini-RT with a high temperature superconductor(HTS) coil(Rc=0.15m, Ic=50kA) has been constructed. The vacuum chamber is 1 m in diameter and ~0.7 m in height. The magnetic field strength near the internal coil is around 0.1 T, and a radio-frequency wave of 2.45 GHz is applied for the plasma production. We have started ECH plasma experiments with the coil supported mechanically. The electron density, which has a peak near the internal coil, is of order 1016 m-3, reaching the cut-off density of the microwave. While, the electron temperature is of order 10 eV with a broad profile. Estimated energy confinement time is of order 10-(5-6) sec. The levitation experiment of the HTS coil has been carried out. The position of the HTS coil is measured with laser sensors, and is feedback-controlled with the levitation coil current. We have succeeded to levitating the HTS coil during one hour with an accuracy of less than 20 ɷm. A preliminary experiment for the plasma production at the floating condition of the HTS coil has been initiated. It is affirmed that the levitation system works well and plasma with separatrix configuration is produced.

Edge fluctuation studies in Heliotron J

Low frequency and small-scale fluctuations of density and potential near the last closed flux surface are investigated by using Langmuir probes for the second harmonic ECH plasmas in a helical-axis heliotron device, Heliotron J. The existence of a plasma layer with a radial electric field shear was indicated near the last closed flux surface. Near this layer, the reversal of phase velocity and de-correlation of the fluctuations were observed. On the other hand, it is suggested that a considerable fraction of the fluctuation induced particle flux is carried off through the intermittent events. Preliminary analyses to classify the PDFs of the ion-saturation current fluctuation as stable Lévy distributions demonstrate that the Lévy index decreases from the inner to the outer region of edge plasma, suggesting that the PDFs near the boundary region of Heliotron J are nearly Gaussian, whereas at the outer regions of plasma they become strongly non-Gaussian.

Confinement characteristics of ECH plasmas in Heliotron J

Studies of global energy confinement and toroidal plasma current behaviour for the second harmonic 70 GHz ECH at B = 1–1.5 T are described with emphasis on the magnetic configuration effects in the helical-axis heliotron ‘Heliotron J’. At low densities of , the electron temperature reached Te ≈ 1 keV in the core region, indicating the production of collision-less plasmas of electron collisionality ν* ≪ 0.1, where ν* = ν/(ve/π R0q). For medium densities of , the preferable energy confinement time, 1.5–2 times larger than that of the ISS95 scaling, was obtained under the condition of localized central heating at B ≈ 1.25 T for the standard configuration of Heliotron J. The measurements of the toroidal current under perpendicular microwave injection revealed the change of the current flow direction as a function of the poloidal magnetic field. The measured current behaviour was found to be qualitatively consistent with that of the bootstrap current predicted from neoclassical theory. The observed flow reversal showed that a proper selection of the field configuration could control the bootstrap current in the helical-axis heliotron. In addition, the current control through the electron cyclotron current drive scenario with oblique injection of microwaves was experimentally examined.

Recent H-mode Results on ECH plasmas in Heliotron J

Recent 70-GHz, 0.4-MW ECH experiments in Heliotron J have revealed the existence of the spontaneous confinement transition, like that of the H-mode, at rather low threshold line-averaged densities of 1.2–1.6 · 10 19 m –3 . The transition was discovered in two edge-iota windows: one is 0.54 < ι (a)/ 2π < 0.56 at separatrix discharge plasmas and the other is 0.62 < ι (a)/2π < 0.63 at partial walllimiter discharge plasmas. The energy confinement time for the separatrix discharge plasmas was found to be enhanced beyond the normal ISS95 scaling in the transient H-mode phase, being 50% longer than that in the “before transition” phase.

Behaviour of ion temperature in electron and ion heating regimes observed with ECH, NBI and ICRF discharges of LHD

Ion temperature at the plasma centre has been measured from Doppler broadening of Ti XXI (2.61 Å) and Ar XVII (3.95 Å) x-ray lines using a newly installed crystal spectrometer with CCD detector in ECH, NBI and ICRF plasmas of Large Helical Device (LHD). The ion temperature obtained in a range of 0.6 and 3.5 keV was analysed with electron density and compared with electron temperature. A new parameter range of Ti>Te was found in low-density (ne<1013 cm−3) H-minority ICRF discharges operated in the ion-heating regime (Pi⩾Pe), whereas the ion temperature was roughly equal to the electron temperature in NBI discharges operated in the electron-heating regime (Pi<Pe). The ion temperature in the ICRF and ICRF + NBI discharges was correlated with the input power to bulk ions over the ion density (Pi/ni). As a result, a good correlation was obtained between them and it also indicated a clear rise of the ion temperature for the increasing Pi in a range of Pi/ni<5 (MW/1013 cm−3).

Observation of ablation and acceleration of impurity pellets in the presence of energetic ions in the CHS heliotron/torsatron

Hydrocarbon pellets were injected into NBI and ECH plasmas of the Compact Helical System (CHS) heliotron/torsatron. The ablation of the pellet was observed from horizontal and vertical directions using two CCD cameras, and the velocities of the pellet during ablation were measured with an 11 channel fan array. It was observed that the pellet trajectory was strongly curved in NBI cases, whereas in ECH cases it was straight. When the toroidal direction of the tangential NBI was changed from clockwise to counter-clockwise, the direction of the curved trajectory completely changed. For the first time, it was also found that the pellet was accelerated during the ablation, for example from 270 to 600 m/s. The ablation of the pellet is simulated for the case with fast ions from NBI (40 keV). The results strongly support the idea that the pellet can be mainly ablated by collisions with fast ions. As a result, it is found that the pellet is accelerated by a jet of the ablation cloud resulting from one-side heating due to the toroidally circulating fast ions.