Off-axis Electron Cyclotron Resonance Heating Research Articles

Expulsion of runaway electrons using ECRH in the TCV tokamak

Abstract Runaway electrons (REs) are a concern for tokamak fusion reactors from discharge startup to termination. A sudden localized loss of a multi-megaampere RE beam can inflict severe damage to the first wall. Should a disruption occur, the existence of a RE seed may play a significant role in the formation of a RE beam and the magnitude of its current. The application of central electron cyclotron resonance heating (ECRH) in the Tokamak à Configuration Variable (TCV) reduces an existing RE seed population by up to three orders of magnitude within only a few hundred milliseconds. Applying ECRH before a disruption can also prevent the formation of a post-disruption RE beam in TCV where it would otherwise be expected. The RE expulsion rate and consequent RE current reduction are found to increase with applied ECRH power. Whereas central ECRH is effective in expelling REs, off-axis ECRH has a comparatively limited effect. A simple 0-D model for the evolution of the RE population is presented that explains the effective ECRH-induced RE expulsion results from the combined effects of increased electron temperature and enhanced RE transport.

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.

Effect of deposition location of electron cyclotron resonance heating on active control of fishbone modes in the HL-2A tokamak

Experiment on suppressing fishbone activities is carried out in HL-2A tokamak by electron cyclotron resonance heating (ECRH). To achieve multiple deposition locations of ECRH, the magnetic field is in a range of 1.22–1.4 T from shot to shot. It is found that the fishbone modes exhibit different characteristics at different radial deposition locations. With the same injected power, the effect of off-axis ECRH is much better than that of on-axis heating. The fishbone modes can be completely suppressed when ECRH is deposited nearby the <i>q</i> = 1 rational surface, but would only mitigate in other cases. Further analysis indicate that injection of high power ECRH leads the electron temperature to increase, then the pressure gradient and plasma current density to change, finally safety factor to change and the minimum safety factor to reach a value larger than 1. Meanwhile, M3D-K simulation results show that the growth rate of fishbone mode declines with the increase of <i>q</i><sub>min</sub>. In other words, the growth of safety factor and disappearance of <i>q</i> = 1 rational surface induced by ECRH contribute to the suppression of fishbone activities. The experimental results reported here may not only help to better understand complex effects of ECRH on magnetohydrodynamic instability, but also provide a physics basis for actively controlling the energetic particle driven modes in the future magnetic confined fusion devices.

Investigation of annular/central collapse events triggered by the double tearing modes in EAST

Annular/central collapses of electron temperature have been investigated in the EAST tokamak. The precursor modes are excited before the annular/central collapse events, and the coexistence of multi magnetic island chains can be observed synchronously for the core concentration of high-Z impurities. The primary magnetic islands for the two cases are located in the same region of 0.25 ⩽ ρ(r/a) ⩽ 0.35, while the secondary magnetic islands for central and annular cases are located inside (ρ ≈ 0.1) and outside (ρ ≈ 0.4) respectively. The similar simulation has been performed by the toroidal resistive MHD code CLT (Zhang et al 2020 Phys. Plasmas 27 122509), and the annular/central collapse events can be reproduced by the magnetic reconnection of m/n = 2/1 double tearing modes, m and n are the poloidal and toroidal mode numbers respectively. The combination of lower hybrid current driven and off-axis electron cyclotron resonance heating is an essential condition for the establishment of reversed magnetic shear. The different penetration depth of high-Z impurity density plays an important role for excitation of the annular/central collapse events, e.g. the steeper gradient of T e-profile can be formed accordingly in the annular region of 0.1 ⩽ ρ ⩽ 0.4, together with excitation of the precursor modes.

Observation of TAE mode driven by off-axis ECRH induced barely trapped energetic electrons in EAST tokamak

A toroidal Alfvén eigenmode (TAE) is excited by electron cyclotron resonance heating (ECRH) induced barely trapped energetic electrons in experimental advanced superconducting tokamak . This TAE appears in the low density EAST discharges under pure off-axis ECRH heating. After analysing the ECRH power modulation induced local density and temperature oscillations, the location of this TAE mode and the power deposition of ECRH are determined. This edge localized TAE mode drifts in ion-diamagnetic direction may be driven by barely trapped energetic electrons considering the contribution of poloidal bounce effect in the general wave-particle resonance condition. The experimental observations also demonstrate that ECRH power modulation with fixed frequency could be used as an effective diagnostic tool to study the internal properties of MHD modes as well as particle and heat transports.

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.

Parametric dependencies of the experimental tungsten transport coefficients in ICRH and ECRH assisted ASDEX Upgrade H-modes

The profiles of the W transport coefficients have been experimentally calculated for a large database of identical ASDEX Upgrade H-mode discharges where only the radio-frequency (RF) power characteristics have been varied [Angioni et al., Nucl. Fusion 57, 056015 (2017)]. Central ion cyclotron resonance heating (ICRH) in the minority heating scheme has been compared with central and off-axis electron cyclotron resonance heating (ECRH), using both localized and broad heat deposition profiles. The transport coefficients have been calculated applying the gradient-flux relation to the evolution of the intrinsic W density in-between sawtooth cycles as measured using the soft X-ray diagnostic. For both ICRH and ECRH, the major player in reducing the central W density peaking is found to be the reduction of inward pinch and, in the case of ECRH, the rise of an outward convection. The impurity convection increases, from negative to positive, almost linearly with RF-power, while no appreciable changes are observed in the diffusion coefficient, which remains roughly at neoclassical levels independent of RF power or background plasma conditions. The ratio vW/DW is consistent with the equilibrium ∇nW/nW prior to the sawtooth crash, corroborating the separate estimates of diffusion and convection. These experimental findings are slightly different from previous results obtained analysing the evolution of impurity injections over many sawtooth cycles. Modelling performed using the drift-kinetic code NEO and the gyro-kinetic code GKW (assuming axisymmetry) overestimates the diffusion coefficient and underestimates the experimental positive convection. This is a further indication that magneto-hydrodynamic/neoclassical models accounting for 3D effects may be needed to characterize impurity transport in sawtoothing tokamak plasmas.

Peculiar properties of internal transport barrier formation near the q = 1 and q = 2 surfaces in tokamaks

A new type of internal transport barrier (ITB) was found in the T-10 tokamak plasma for the first time. It is created by sawtooth oscillations which are almost damped by off-axis electron cyclotron resonance heating and electron cyclotron current drive (ECRH and ECCD). During the ITB formation, the electron temperature, Te, at a relative radius r/a ≤ 0.45 rapidly rises to a new quasi-steady state, while the turbulence level, which is measured at the ITB, falls below its pre-crash value, and the turbulence spectrum shrinks in a wide range of frequencies. The ITB appears near the q = 1 magnetic surface and exists for approximately 50–70% of the plasma energy confinement time. During this process, accumulation of impurities was not observed. An abrupt and non-local reduction of the electron heat flux (ITB-event) around the q = 1 surface (within 1 ms in the spatial zone with a relative radius 0.2 < r/a < 0.5) was observed for the first time for the case with a gas puffing cut-off in OH T-10 discharges with an increasing high density. In contrast to the ITB that forms during the ECR heating, in this case, impurities begin to accumulate simultaneously with the ITB event. Analysis of the D-III-D experiments with a weak reverse magnetic shear shows that the appearance of a qmin = 2 surface leads to this ITB event in a very wide spatial region (the relative radius of the plasma column is 0 < r/a < 0.7).

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.

Recent ASDEX Upgrade research in support of ITER and DEMO

Recent experiments on the ASDEX Upgrade tokamak aim at improving the physics base for ITER and DEMO to aid the machine design and prepare efficient operation. Type I edge localized mode (ELM) mitigation using resonant magnetic perturbations (RMPs) has been shown at low pedestal collisionality . In contrast to the previous high ν* regime, suppression only occurs in a narrow RMP spectral window, indicating a resonant process, and a concomitant confinement drop is observed due to a reduction of pedestal top density and electron temperature. Strong evidence is found for the ion heat flux to be the decisive element for the L–H power threshold. A physics based scaling of the density at which the minimum PLH occurs indicates that ITER could take advantage of it to initiate H-mode at lower density than that of the final Q = 10 operational point. Core density fluctuation measurements resolved in radius and wave number show that an increase of R/LTe introduced by off-axis electron cyclotron resonance heating (ECRH) mainly increases the large scale fluctuations. The radial variation of the fluctuation level is in agreement with simulations using the GENE code. Fast particles are shown to undergo classical slowing down in the absence of large scale magnetohydrodynamic (MHD) events and for low heating power, but show signs of anomalous radial redistribution at large heating power, consistent with a broadened off-axis neutral beam current drive current profile under these conditions. Neoclassical tearing mode (NTM) suppression experiments using electron cyclotron current drive (ECCD) with feedback controlled deposition have allowed to test several control strategies for ITER, including automated control of (3,2) and (2,1) NTMs during a single discharge. Disruption mitigation studies using massive gas injection (MGI) can show an increased fuelling efficiency with high field side injection, but a saturation of the fuelling efficiency is observed at high injected mass as needed for runaway electron suppression. Large locked modes can significantly decrease the fuelling efficiency and increase the asymmetry of radiated power during MGI mitigation. Concerning power exhaust, the partially detached ITER divertor scenario has been demonstrated at Psep/R = 10 MW m−1 in ASDEX Upgrade, with a peak time averaged target load around 5 MW m−2, well consistent with the component limits for ITER. Developing this towards DEMO, full detachment was achieved at Psep/R = 7 MW m−1 and stationary discharges with core radiation fraction of the order of DEMO requirements (70% instead of the 30% needed for ITER) were demonstrated. Finally, it remains difficult to establish the standard ITER Q = 10 scenario at low q95 = 3 in the all-tungsten (all-W) ASDEX Upgrade due to the observed poor confinement at low βN. This is mainly due to a degraded pedestal performance and hence investigations at shifting the operational point to higher βN by lowering the current have been started. At higher q95, pedestal performance can be recovered by seeding N2 as well as CD4, which is interpreted as improved pedestal stability due to the decrease of bootstrap current with increasing Zeff. Concerning advanced scenarios, the upgrade of ECRH power has allowed experiments with central ctr-ECCD to modify the q-profile in improved H-mode scenarios, showing an increase in confinement at still good MHD stability with flat elevated q-profiles at values between 1.5 and 2.

The features of the global GAM in OH and ECRH plasmas in the T-10 tokamak

Zonal flows and their high-frequency counterpart, the geodesic acoustic modes (GAMs) are considered as a possible mechanism of the plasma turbulence self-regulation. In the T-10 tokamak GAMs have been studied by the heavy ion beam probing and multipin Langmuir probes. The wide range of the regimes with Ohmic, on-axis and off-axis electron cyclotron resonance heating (ECRH) were studied (Bt = 1.5–2.4 T, Ip = 140–300 kA, , PEC < 1.2 MW). It was shown that GAM has radially homogeneous structure and poloidal m = 0 for potential perturbations. The local theory predicts that , that means the frequency increases with the decrease of the minor radius. In contrast, the radial distribution of experimental frequency of the plasma potential and density oscillations, associated to GAM, is almost uniform over the whole plasma radius, suggesting the features of the nonlocal (global) eigenmodes. The GAM amplitude in the plasma potential also tends to be uniform along the radius. GAMs are more pronounced during ECRH, when the typical frequencies are seen in the narrow band from 22 to 27 kHz for the main peak and 25–30 kHz for the higher frequency satellite. GAM characteristics and the range of GAM existence are presented as functions of Te, density, magnetic field and PEC.

Formation of an internal transport barrier and magnetohydrodynamic activity in experiments with the controlled density of rational magnetic surfaces in the T-10 Tokamak

Results are presented from experiments on the formation of an internal electron transport barrier near the q = 1.5 rational surface in the T-10 tokamak. The experiments were carried out in the regime with off-axis electron cyclotron resonance (ECR) heating followed by a fast plasma current ramp-up. After suppressing sawtooth oscillations by off-axis ECR heating, an internal transport barrier began to form near the q = 1.5 rational surface. In the phase of the current ramp-up, the quality of the transport barrier improved; as a result, the plasma energy confinement time increased 2–2.5 times. The intentionally produced flattening of the profile of the safety factor q(r) insignificantly affected magnetohydrodynamic activity in the plasma column in spite of the theoretical possibility of formation of substantial m/n = 3/2 and 2/1 magnetic islands. Conditions are discussed under which the flattening of the profile of the safety factor q near low-order rational surfaces leads to the formation of either an internal transport barrier or the development of an island magnetic structure induced by tearing modes.

Analysis of electron heat transport with off-axis modulated ECRH in Tore Supra

Experiments to study inward heat transport phenomena have been performed in the Tore Supra tokamak with off-axis electron cyclotron resonance heating (ECRH). Both power balance and perturbation transport analysis have been done for low-frequency (1 Hz) ECRH modulation experiments. Heat diffusivity and heat pinch have been separately determined by fitting the experimental data of the amplitude and phase of the Fourier transform of the modulated temperature with a linear transport model including convection term. Comparison with the critical gradient model has shown that the heat pinch previously obtained could include a pseudo-pinch due to the non-linearity of the diffusivity and an additional non-diffusive heat pinch. The pinch effect is reduced for higher densities.

Observation of turbulence suppression after electron-cyclotron-resonance-heating switch-off on the HL-2A tokamak

The formation of a transient internal transport barrier (ITB) is observed after the electron-cyclotron-resonance-heating (ECRH) switch-off in the HL-2A plasmas, characterized by transient increase of central electron temperature. The newly developed correlation reflectometer provided direct measurements showing reduction of turbulence in the region of steepened gradients for the period of ITB formation triggered by the ECRH switch-off. Furthermore, the reduction of core turbulence is correlated in time with the appearance of a low-frequency mode with a spectrally broad poloidal structure that peaks near zero frequency in the core region. These structures have low poloidal mode number, high poloidal correlation, and short radial correlation and are strongly coupled with high-frequency ambient turbulence. Observation indicates that these structures play important roles in the reduction of the core turbulence and in improvements of the core transport after the off-axis ECRH is turned off.

Real time control of the plasma current and elongation in tokamaks using ECRH actuators

Real time control of the plasma elongation and plasma current using electron cyclotron resonance heating (ECRH) actuators has been demonstrated on TCV. Tokamak plasmas may be elongated by off-axis ECRH. The associated flattening of the current density profile increases the plasma's vertical stability, enabling higher elongations to be obtained, at lower currents than would be possible in Ohmic conditions. Successful experiments were carried out on TCV to control, in real time, the magnitude of the elongation and the ECRH deposition location using second harmonic ECRH power and ECRH launcher mirror angle actuators. Plasma current control in fully non-inductive plasmas using ECRH actuators to drive ECCD was also demonstrated.

10 kHz repetitive high-resolution TV Thomson scattering on TEXTOR: Design and performance (invited)

In late 2003 a 10kHz multiposition Thomson scattering diagnostic with high spatial resolution became operational on the TEXTOR tokamak. In the initial phase of operation, one burst of 18 pulses of 12J each with a repetition rate of 5kHz could be extracted from the laser system. The installation of a low-dope ruby rod (spring 2005) resulted in a system, which can deliver higher pulse energy and moreover a divergence of better than 0.7mrad, leading to a big improvement in the detection of Thomson scattering photons. Furthermore, the number of laser pulses in one burst could be extended to even more than 30. The achieved laser energy of more than 15J∕pulse makes it possible to measure electron temperature and density profiles with an observational error of 8% on the electron temperature (Te) and 4% on the electron density (ne) at ne=2.5×1019m−3, per spatial element of 7.5mm. The viewing optics enables sampling of either the full plasma diameter of 900mm with 120 spatial channels of 7.5mm each or a 160mm long edge chord with 98 spatial channels of 1.7mm each. The system, which has recently become available for physics exploration, has already been used to study the structure of m=2 magnetic islands and the response of the plasma to off-axis electron cyclotron resonance heating.

Behavior of small-scale density fluctuations in discharges with off-axis electron-cyclotron resonance heating in the T-10 tokamak

In experiments on off-axis electron-cyclotron resonance heating in the T-10 tokamak, a steep gradient of the electron temperature was observed to form for a short time at a relative radius of ρ ≈ 0.25 after the heating power was switched off. Small-scale fluctuations of the electron density were studied with the help of correlation reflectometry. It was found that, in a narrow region near ρ ≈ 0.25, the amplitude of the density fluctuations was two times lower than that in the ohmic heating phase. Quasi-coherent fluctuations were suppressed over a period of time during which the steep temperature gradient existed. Measurements of the poloidal rotation velocity of turbulent fluctuations show that there is no velocity shear after the heating is switched off. An analysis of the linear growth rates of instabilities shows that the ion-temperature-gradient mode is unstable at ρ ≈ 0.25 throughout the entire discharge phase. The effect observed can be explained by an increase in the distance between the rational surfaces near the radius at which the safety factor is q = 1 due to the temporary flattening of the q profile after the off-axis electron-cyclotron resonance heating is switched off.

Reduced core transport in T-10 and TEXTOR discharges at rational surfaces with low magnetic shear

It has been observed in the T-10 tokamak that immediately after off-axis electron cyclotron resonance heating (ECRH) switch-off, the core electron temperature stays constant for some time, which can be as long as several tens of milliseconds, i.e. several energy confinement times (τE), before it starts to decrease. Whether or not the effect is observed depends critically on the local magnetic shear in the vicinity of the q = 1 rational surface, which should be close to zero. It is hypothesized that a small shear can induce the formation of an internal transport barrier. Measurements of density fluctuations in the transport barrier with a correlation reflectometer show immediately after the ECRH switch-off a clear reduction in the fluctuation level, corroborating the above results. The delayed temperature decrease has also been observed in similar discharges in the TEXTOR tokamak; however, the delay is restricted to ∼ 1 × τE.

Recent experimental results from the HL-1M tokamak and progress in the HL-2A project

Recent experimental results from the HL-1M tokamak and progress in the HL-2A project are presented in this paper. In HL-1M, strong fishbone instability was observed during off-axis electron cyclotron resonance heating (ECRH). This is the first observation of fishbone instability purely driven by energetic electrons produced by ECRH. The molecular beam injection (MBI) was first proposed and demonstrated in HL-1M. Recently, new results of the MBI experiment were obtained by increasing the pressure of the gas. A stair-shaped density increment was obtained with high-pressure multi-pulse MBI just like the density evolution behaviour during multi-pellet injection. It is shown that injected particles penetrated into the core region of the plasma. HL-2A is a divertor tokamak constructed at SWIP based on the original ASDEX main components. The mission of the HL-2A project is to explore the physics issues involved in an advanced tokamak. For the first phase, divertor (edge plasma) and confinement research will be emphasized. The major parameters of HL-2A are R = 1.65 m, a = 0.4 m, Bt = 2.8 T, IP = 0.48 MA. The main parameters and characteristics of the subsystems such as power supply, pumping, diagnostics and auxiliary heating are presented in this paper. The first plasma of HL-2A was obtained at the end of 2002.

Electron Heat Transport in an HL-1M Tokamak ECRH Plasma

Electron temperature profiles, the normalized temperature gradientlength and the electron heat diffusivity calculated by the heat pulsetransfer method in a plasma confinement region, are investigatedduring electron cyclotron resonance heating (ECRH) in an HL-1M tokamak.The electron temperature profiles remain almost constant. Thenormalized temperature gradient length is in the range of 7–13.Electron heat diffusivity in the ECRH plasmas is larger than that inthe ohmically heated plasmas. Electron heat diffusivity during off-axis(ρ<0.3) ECRH discharge is similar to that during on-axis ECRHdischarge.