On-axis Electron Cyclotron Resonance Heating Research Articles

The influence of electron cyclotron resonance heating on ion-driven fishbone instability

The effect of electron cyclotron resonance heating (ECRH) on fishbone instability is studied based on the generalized energy principle. Off-axis ECRH plays a stabilizing role in fishbone instability, while on-axis ECRH does not distinctly change the growth rate. The frequency of fishbone instability increases (decreases) for off-axis (on-axis) ECRH. The effect of ECRH is greatest when power is deposited near the rational surface. More concentrated power deposition has a better stabilizing effect. Furthermore, the non-resonance effect of trapped energetic electrons is the main factor behind the stabilization effect in the off-axis case, while it has a weak effect in the on-axis case. The frequency of fishbone instability is changed mainly by the Shafranov shift effect on trapped energetic ions since it can change the precessional drift frequency. The Shafranov shift effect can also affect the growth rate because the onset threshold of energetic ion beta is related to the frequency. The effects of magnetic Reynolds number and slowing-down critical energy are weak and can be neglected. This provides the possibility of using off-axis ECRH targeted to the rational surface to control fishbone instability.

Preliminary observation of tungsten-impurity suppression using on-axis ECRH by X-ray crystal spectroscopy in EAST

Impurities degrade tokamak plasma confinement by causing energy loss, diluting fuel concentration, and even terminating discharge in some extreme cases. Previously, the suppression effects of impurity accumulation due to on-axis electron cyclotron resonance heating (ECRH) have been studied on Experimental and Advanced Superconducting Tokamak (EAST) using extreme ultraviolet (EUV) spectroscopy. However, it is difficult to quantify changes in the tungsten (W) impurity profile since W-line emission in the EUV range cannot be easily resolved. X-ray crystal spectroscopy (XCS) is widely used to measure the ion temperature and rotation velocity of plasmas by using line emission in the soft X-ray range. In addition, the XCS can also be used to study the behavior of impurities. An in situ absolute intensity calibration of tangential XCS was conducted by analyzing calculations and measurements of bremsstrahlung radiation. After obtaining the calibration coefficient, the W44+-ion-density profiles were evaluated using Abel inversion operations and the spectral line of W XLV (W44+, 3.9095 Å). Thus, a direct observation of the W44+-impurity concentration suppressed by ECRH was accomplished. Such W44+-density profiles can be used in the future to analyze W transport in combination with impurity transport codes.

Characteristics of electron temperature profile stiffness in electron-heated plasmas on EAST

A very high core electron temperature (T e0 ∼ 10 keV) plasma has been established and stably sustained by applying both lower hybrid wave (LHW) and on-axis electron cyclotron resonance heating (ECRH) in the Experimental Advanced Superconducting Tokamak (EAST). In this work, power balance analysis shows that the increase of ECRH power can increase the normalized T e gradient significantly at the plasma core region (ρ < 0.6), but does not change the T e profile stiffness in the low-density L-mode plasmas. This has been considered to be due to a strong synergistic effect between ECRH and LHW. Furthermore, three distinguishable stages characterized by different T e profile stiffnesses can be identified from the density ramp-up in the electron-heated plasma on EAST. A stronger T e profile stiffness at ρ = 0.3 has been observed in the Stage-II, where the LHW power deposition gradually moves away from the plasma core region, following the electron density increases. Furthermore, the formation of an internal plasma density transport barrier inside ρ ∼ 0.6, accompanied by a sudden drop in core T e and a rise in both core plasma density and ion temperature, has been observed for the first time during the transition from the Stage-II to the Stage-III when the central line-averaged plasma density reaches a threshold of 2.2 × 1019 m−3. This finding strongly affects further development of high-performance gas-fueled electron-heated plasma scenarios in EAST and suggests an advanced operational regime with a wide internal plasma density transport barrier.

2D distributions of potential and density mean-values and oscillations in the ECRH and NBI plasmas at the TJ-II stellarator

2D plasma potential ϕ distribution was measured in the electron cyclotron resonance heating (ECRH) and neutral beam injection (NBI) plasmas of the TJ-II stellarator with the heavy ion beam probe for the whole radial range and wide area of the poloidal angle, and supported by Langmuir probe data at the edge. The whole operation domain for the on-axis ECRH was explored ( = 0.45–0.8 ×1019 m−3, P EC = 220–470 kW), in addition, NBI plasmas with = 0.9–1.3 × 1019 m−3 and P NBI = 510 kW were studied. In ECRH plasmas the density ramp-up is accompanied by the evolution of the potential from the bell-like to the Mexican hat profile, while the density profiles were flat or slightly hollow. The potential has the positive peak at the centre, and LFS-HFS (low field—high field sides) and up-down symmetry. Equipotential lines are consistent with vacuum magnetic flux surfaces. In the high-density NBI scenario, the ϕ profile was fully negative with a minimum up to −300 V at the centre, while at low-density ECRH plasma, ϕ has a maximum up to +0.9 kV at the centre. Fluctuations of potential and density are stronger in low-density scenarios and not poloidally symmetric. At the mid-radius (area of the maximum density), root mean square (RMS) of fluctuations were up to ϕ ∼ 15 V at LFS vs ∼20 V at HFS; RMS n e ∼ 2% at LFS vs ∼3% at HFS. In the NBI plasmas with the density rise, the asymmetry decreases and finally vanishing at = 1.2 × 1019 m−3. 2D distribution of the NBI-induced Alfvén eigenmodes (AEs) shows asymmetric ballooning structure: contrary to broadband turbulence, AE-associated potential perturbation dominates in the LFS with a factor up to 1.7 respect to the HFS. The electrostatic mode, excited in ECRH plasmas by suprathermal electrons also shows asymmetric structures: density perturbation dominates in the top-bottom direction compared to LFS-HFS direction.

Influence of toroidal rotation from electron cyclotron resonance heating in KSTAR

Toroidal rotation behaviors from electron cyclotron resonance heating (ECH) injected Ohmic and high confinement mode (H-mode) plasmas are investigated in KSTAR. The on-axis ECH injection induces core toroidal rotation reversal by neoclassical toroidal viscosity (NTV) damping torque coincided with trapped electron mode (TEM)-driven co-current rotation torque in the countercurrent rotating Ohmic plasmas. In the ECH triggered H-mode plasmas with enhanced heating power, the toroidal rotation in the central region turns to the countercurrent direction and co-current direction in the outer region of the plasma.

Suppression of molybdenum impurity accumulation in the core using on-axis electron cyclotron resonance heating in EAST

Core metal impurity transport in an electron cyclotron resonance heating (ECRH) plasma is investigated for the first time in experimental advanced superconducting tokamak, on the basis of profile measurements of impurity emissions using space-resolved extreme ultraviolet spectroscopy. A significant reduction in the core molybdenum emissivity is observed during the on-axis ECRH phase, accompanied by decreased core electron density and increased electron temperature. The impurity transport study before and during ECRH injection is performed using line emissions of Mo XXIX, Mo XXX, Mo XXXI, and Mo XXXII with the core impurity transport code, STRAHL. It is found that the diffusion coefficient increases substantially and that the value of the diffusion coefficient is about one order of magnitude higher than that of neoclassical transport. Also, the inward convective velocity decreases and even changes its direction in the core region (ρ &amp;lt; 0.5) during on-axis ECRH, which is thought to dominantly contribute to the reduction in core impurity ion density.

Modified Canonical Profile Transport Model for Description of On-Axis Electron-Cyclotron Heating of Tokamak Plasma

The specific power deposited in plasma under on-axis electron-cyclotron resonance heating (ECRH) is characterized by strong peaking, resulting in large-amplitude sawtooth oscillations. To analyze the global properties of plasma, instantaneous experimental profiles of the electron temperature should be averaged over time and space. In the present work, a modified canonical profile transport model for predictive calculation of such averaged profiles is proposed. As an example, the model is used to determine the parameters of plasma with ECRH in the T-15MD tokamak currently under construction.

The effects of electron cyclotron heating and current drive on toroidal Alfvén eigenmodes in tokamak plasmas

Dedicated studies performed for toroidal Alfvén eigenmodes (TAEs) in ASDEX-Upgrade (AUG) discharges with monotonic q-profiles have shown that electron cyclotron resonance heating (ECRH) can make TAEs more unstable. In these AUG discharges, energetic ions driving TAEs were obtained by ion cyclotron resonance heating (ICRH). It was found that off-axis ECRH facilitated TAE instability, with TAEs appearing and disappearing on timescales of a few milliseconds when the ECRH power was switched on and off. On-axis ECRH had a much weaker effect on TAEs, and in AUG discharges performed with co- and counter-current electron cyclotron current drive (ECCD), the effects of ECCD were found to be similar to those of ECRH. Fast ion distributions produced by ICRH were computed with the PION and SELFO codes. A significant increase in Te caused by ECRH applied off-axis is found to increase the fast ion slowing-down time and fast ion pressure causing a significant increase in the TAE drive by ICRH-accelerated ions. TAE stability calculations show that the rise in Te causes also an increase in TAE radiative damping and thermal ion Landau damping, but to a lesser extent than the fast ion drive. As a result of the competition between larger drive and damping effects caused by ECRH, TAEs become more unstable. It is concluded, that although ECRH effects on AE stability in present-day experiments may be quite significant, they are determined by the changes in the plasma profiles and are not particularly ECRH specific.

Experimental study of density pump-out effect with on-axis electron cyclotron resonance heating at the T-10 tokamak

The results of T-10 experiments on the ‘density pump-out’ effect research, i.e. the escape of particles from the zone of on-axis electron cyclotron resonance heating (ECRH) to the plasma periphery are presented. The dependences of the pump-out on the central chord line-averaged plasma density , total plasma current and the ECRH power were found. The influence of the gas influx intensity and the conditions of the chamber wall and limiter on the above-mentioned effect is also investigated. It is shown that this effect increases with the growth of the up to a certain critical value , after which the escape of particles from the zone of on-axis ECRH decreases. This critical density increases with the increase of the total plasma current and the ECRH power.

Toroidal rotation profile structure in KSTAR L-mode plasmas with mixed heating by NBI and ECH

The structure of the toroidal rotation profile with mixed heating by neutral beam injection (NBI) and electron cyclotron resonance heating (ECH) has been investigated in KSTAR L-mode plasmas. ECH with varying resonance layer positions was used for heating a mix control. The experimental results show that ECH causes a counter-current rotation increment both for off-axis and on-axis ECH heating. For L-mode plasmas, off-axis ECH produces larger counter-current rotation than on-axis ECH. Analysis of ion heat and momentum transport for the ECH L-mode plasmas shows that the electron temperature gradient is the main reason for the degradation of ion heat confinement and also the main driving force for the non-diffusive momentum flux. As a possible mechanism for the counter-current intrinsic torque with ECH, the transition of the turbulence mode from ion temperature gradient (ITG) to the trapped electron mode (TEM) with the resulting sign change of turbulence driven residual stress is suggested. A linear gyro-kinetic analysis shows the ITG → TEM transition occurs in a localized region during ECH injection, and the trend of TEM excitation is consistent with the observed macroscopic trend of the toroidal rotation.

Control of core argon impurity profile by ECH in KSTAR L-mode plasmas

Experiments on trace argon impurity transport in L-mode discharges were performed on Korea superconducting tokamak advanced research (KSTAR) with electron cyclotron resonance heating (ECH). Ar emission was measured by soft x-ray (SXR) arrays and vacuum UV (VUV) diagnostics. A significant reduction in the core Ar emissivity was observed with core ECH. The reduction was the largest with on-axis heating and became smaller with outward heating positions. The diffusivity and convection velocity of Ar were obtained by analysis of the SXR data with the SANCO impurity transport code for the on-axis ECH and the non-ECH shots. In the on-axis ECH case, both diffusivity and convection velocity increased. Furthermore, the convection changed its direction from inward to outward in the plasma core (r/a < 0.3), resulting in a hollow profile of the total Ar density. Together with the reduction in the SXR signals, the hollow impurity profile in the core and the reversal of the convection velocity consistently confirm that ECH can reduce impurity accumulation in the core region. Neoclassical impurity transport and linear stability of micro-turbulence were calculated and discussed in relation to the possible transport mechanism.

ИССЛЕДОВАНИЕ ВЫНОСА ЧАСТИЦ ИЗ ЦЕНТРА ПЛАЗМЫ ПОСЛЕ ВКЛЮЧЕНИЯ ЦЕНТРАЛЬНОГО ЭЦР-НАГРЕВА В ТОКАМАКЕ Т-10

The study of the particle transport at on-axis electron cyclotron resonance heating (ECRH) is of importance for the future tokamakreactor ITER, since -particles will produce the key contribution into plasma heating in the reactor, the heat being generated in the plasma center. The experiments at tokamaks and stellarators have shown that after on-axis ECRH switching on the additional (both diffusion and convective) carry-over of particles from the heating zone to the plasma periphery appears. It is so the so-called «density pump out» effect. The additional diffusion flux plays the main role in the «density pump out» effect during the first 10—20 ms after the on-axis ECRH switching on, but after this time the convective particle flux becomes dominant. In the paper the experiments exploring the «density pump out» effect in the T-10 tokamak are analyzed. The inverse problem solving was used for reconstruction of the diffusion coefficient and the particle pinching rate (particle flux) as well as for the determination of the particles carry-over from the plasma central area as the function of the ECRH power and the average plasma density.

ECH effects on toroidal rotation: KSTAR experiments, intrinsic torque modelling and gyrokinetic stability analyses

Toroidal rotation profiles have been investigated in KSTAR H-mode plasma using combined auxiliary heating by co-neutral beam injection (NBI) and electron cyclotron resonance heating (ECH). The ion temperature and toroidal rotation are measured with x-ray imaging crystal spectroscopy and charge exchange recombination spectroscopy. H-mode plasma is achieved using co-current 1.3 MW NBI, and a 0.35 MW ECH pulse is added to the flat-top of H-mode. The core rotation profiles, which are centrally peaked in the pure NBI heating phase, flatten when ECH is injected, while the edge pedestal is unchanged. Dramatic decreases in the core toroidal rotation values (ΔVtor/Vtor ∼ −30%) are observed when on-axis ECH is added to H-mode. The experimental data show that the decrease of core rotation velocity and its gradient are correlated with the increase of core electron temperature and its gradient, and also with the likely steepening of the density gradient. We thus explore the viability of a hypothesized ITG (ITG ion temperature gradient instability) → TEM (trapped electron mode instability) transition as the explanation of the observed counter-current flow induced by ECH. However, the results of linear microstability analyses using inferred profiles suggest that the TEM is excited only in the deep core, so the viability of the hypothesized explanation is not yet clear.

Recent results of the T-10 tokamak

Poloidal asymmetry and radial correlation lengths of turbulence were investigated in T-10 at low field side and high field side by correlation reflectometry. Correlation of plasma confinement with the turbulence type was observed. Improvements in heavy ion beam probe diagnostic enabled us to measure the plasma potential during electron cyclotron resonance heating (ECRH) in a wide range of radial positions and operational regimes. The turbulence appeared to rotate close to E × B velocity. The concept of electron internal transport barrier (e-ITB) formation at low-order rational surfaces under conditions of low density of the rational surfaces was proved by the observation of e-ITB formation near the q = 1.5 surface in discharges with non-central ECRH and current ramp-up. The kinetic phenomena were investigated by means of electron cyclotron emission (ECE) under the strong on-axis ECRH. Lithium gettering of the limiter and the wall allowed us to significantly reduce the impurity level and obtain a recycling coefficient as low as 0.3. The rates of carbon film deposition were measured in the working and cleaning discharges. Second harmonic EC assisted start-up was investigated. ECRH allowed us to control the generation of runaway electrons and the current decay rate after the energy quench at the density limit disruption.

Coupled rotational dynamics of two MHD modes in the T-10 tokamak

The coupled rotational dynamics of the m/n = 2/1 and 3/2 MHD modes in an ohmic discharge and the m/n = 2/1 and 4/1 modes in a regime with on-axis electron-cyclotron resonance heating were studied experimentally. The specific features of rotation of these modes in the presence of an error magnetic field, in particular the dependence of the instantaneous frequency Ω(t) of the MHD mode on its spatial orientation at the current instant, were revealed. In analyzing the results obtained, MHD modes were identified with tearing modes (magnetic islands) having a corresponding spatial structure. A possible mechanism for the mutual influence of magnetic islands resulting in their consistent rotation is discussed.

Preliminary Results of Sawtooth Behaviour and Electron Heat Transport During ECRH Experiment on HL-2A

An electron cyclotron resonance heating (ECRH) system with a maximumoutput power of 1 MW, a frequency of 68 GHz and a duration of1s was operated on HL-2A tokamak. The temperature profile controland the sawtooth behaviour during ECRH experiments are investigated.`Density pumpout' during on-axis ECRH is analysed. Features ofconfinement and electron heat transport during ECRH are studied.

The ballistic jump of the total heat flux after ECRH switching on in the T-10 tokamak

Analysis of experiments with electron cyclotron resonance heating (ECRH) requires a good knowledge of the ECRH power profile. This profile is reconstructed by analysis of the transient process after on-axis ECRH switching on in special experiments with suppressed sawtooth oscillations in the T-10 tokamak. The calculations show that the absorbed ECRH power, , determined by the change in time derivative of the electron temperature at the region of ECRH power input, and the absorbed ECRH power, , determined by the magnetic measurements, are several times different. Depending on the plasma density and plasma current, their relation, , changes from 0.2 to 0.4. Analysis of different explanations for this effect shows that adequate description of the transient process demands introduction of a ballistic jump in the total heat flux just after on-axis ECRH switching on. The effective heat diffusivity increases up to values of 10–15 m2 s−1 in the first 100–200 µs and decreases down to values of 1.5–2.0 m2 s−1 during the following 1–2 ms. Note that such a non-monotone dependence of the effective heat diffusivity cannot be described by the modern critical gradient models. It seems that plasma reacts directly to the deposited power but not to the corresponding consequences (the increase in temperature or gradients). Different physical mechanisms could be proposed for this process (partial destruction of magnetic surfaces, fast transition of information through the turbulent cell connections), but each of them needs further confirmation.