Neoclassical Tearing Mode Stabilization Research Articles

Initial results from neoclassical tearing mode stabilization experiment in KSTAR high normalized beta plasmas

Abstract Active stabilization of neoclassical tearing modes (NTMs) is critical for high beta plasma operation in the KSTAR tokamak. In recent device operation, an experiment was conducted to develop a m/n = 2/1 NTM stabilization in high normalized beta (β N) plasmas having β N > 3 by using electron cyclotron current drive (ECCD). The experiment is designed to first demonstrate the direct mode stabilization effect of the device EC system by varying the ECCD deposition location around the mode rational surface to prepare for future NTM stabilization in KSTAR using feedback schemes. In the experiment, the toroidal magnetic field strength, B T, is reduced to 1.5–1.6 T to create a highly localized ECCD profile on a large island width expected to produce a high stabilization effect. To align the ECCD with the mode, B T is varied between discharges by keeping the EC wave injection angles fixed to move the current deposition location across the q = 2 mode rational surface in ∼1 cm steps along the plasma midplane. The result shows a prompt reduction of the mode amplitude by ∼60% with a partial recovery of the loss of stored energy when the ECCD with 0.7 MW power is applied promptly after the mode onset at B T = 1.54 T. This abrupt reduction of the mode amplitude is significantly weaker or disappears in other discharges having slightly different B T in which the ECCD is deposited a few centimeters away. This result indicates a direct interaction between the driven EC current and the radially localized island structure first observed in the device. The EC ray-tracing analysis using the TORAY code shows that the applied EC-driven current aligns with the mode rational surface when the mode amplitude is observed to decrease. The stability of the observed 2/1 NTM is examined by constructing the modified Rutherford equation (MRE). The calculated EC power requirement for complete mode stabilization by assuming perfect alignment of ECCD on the mode is 0.8–1.7 MW by considering the uncertainties in the equation. This MRE-computed mode stability agrees with the experiment. The result projects that the present KSTAR EC system can stabilize the 2/1 NTM disrupting high β N plasmas, and provides the requirement of the mode-ECCD alignment for complete mode stabilization in future experiments in which an accurate alignment is planned to be made by feedback schemes being developed.

Suppression of the m/n = 2/1 tearing mode by electron cyclotron resonance heating on J-TEXT

Stabilization of tearing modes and neoclassical tearing modes is of great importance for tokamak operation. Electron cyclotron waves (ECWs) have been extensively used to stabilize the tearing modes with the virtue of highly localized power deposition. Complete suppression of the m/n = 2/1 tearing mode (TM) by electron cyclotron resonance heating (ECRH) has been achieved successfully on the J-TEXT tokamak. The effects of ECW deposition location and power amplitude on the 2/1 TM suppression have been investigated. It is found that the suppression is more effective when the ECW power is deposited closer to the rational surface. As the ECW power increases to approximately 230 kW, the 2/1 TM can be completely suppressed. The island rotation frequency is increased when the island width is reduced. The experimental results show that the local heating inside the magnetic island and the resulting temperature perturbation increase at the O-point of the island play dominant roles in TM suppression. As the ECW power increases, the 2/1 island is suppressed to smaller island width, and the flow shear also plays a stabilizing effect on small magnetic islands. With the stabilizing contribution of heating and flow shear, the 2/1 TM can be completely suppressed.

Neoclassical tearing mode stabilization by electron cyclotron current drive in EAST tokamak experiments

The stabilization of the m/n= 2/1 neoclassical tearing mode (NTM) by electron cyclotron current drive (ECCD) has been carried out in EAST H-mode discharges, where m/n is the poloidal/toroidal mode number. The experimental results are reported for the first time in this paper. To facilitate the experimental study, the magnetic island (NTM) is generated by a sufficiently large amplitude of the externally applied resonant magnetic perturbation (RMP). After switching off the RMP, the NTM exists due to the bootstrap current perturbation, with the magnetic island width being about 5 cm for the local equilibrium bootstrap current fraction being larger than 10%. By applying the localized ECCD later, the NTM is fully suppressed if the radial misalignment between the magnetic island and the ECCD location is sufficiently small. The stabilizing effect depends on both the radial misalignment and the applied electron cyclotron wave power. More importantly, it is found that the NTM can be avoided when applying ECCD earlier during the ramp-up phase of the RMP amplitude, if ECCD is localized around the O-point of the magnetic island, indicating an efficient way for avoiding locked modes that can lead to the major disruptions of tokamak plasmas.

Analysis of neoclassical tearing mode stabilization experiment by electron cyclotron injection in KSTAR

We report the neoclassical tearing mode (NTM) stabilization experimental results by injection of the electron cyclotron (EC) beam in KSTAR, and its analysis with integrated modelling. In the KSTAR experiment, NTM was intentionally induced and the EC beam was injected by the plasma control system (PCS) to stabilize it. Here, the EC angle was controlled based on the minimum seeking algorithm for the island growth rate. Although the EC angle was not changed significantly, the island width significantly decreased towards the end of the experiment followed by the recovery of plasma performance. To assess the NTM stabilization experiment from a stability perspective, we performed integrated modelling with TRIASSIC utilizing the reconstructed equilibrium with magnetic diagnostics and motional stark effect diagnostics. As a result, we observed that the NTM is stabilized by EC.

High power mm-wave loss measurements of ITER ex-vessel waveguide components at the FALCON test facility at the Swiss Plasma Center

Many future fusion devices will rely heavily, if not solely, on electron cyclotron (EC) heating subsystems to provide bulk heating, instability control (neoclassical tearing mode (NTM) stabilization), and thermal instability control. Efficient use of the installed heating power (gyrotrons) requires low-loss transmission of the power over 100s of meters since the mm-wave sources need to be installed where the stray magnetic field has a small amplitude. Transmission lines are used to propagate the mm-wave power over this long distance. Quasi-optical techniques (mirrors) are used at W7X and are planned for DTT, for example. Guided components are installed at DIII-D, TCV and elsewhere and are planned at JT60SA and ITER. High power test facilities exist to evaluate the power transmission of assemblies of guided components (transmission lines). The European test facility FALCON was setup by Switzerland and Fusion for Energy (F4E) in Lausanne Switzerland at the Swiss Plasma Center (SPC) in the Ecole Polytechnique Fédérale de Lausanne (EPFL). Operations are funded through a framework contract with F4E. SPC operates the facility. Two ITER-class 170GHz gyrotrons are housed within the facility and used to evaluate the thermal behaviour of components provided by various ITER partners. Loss measurements are presented for miter bends and waveguides of several materials at two different diameters. The results are used to model the expected losses in the ITER ex-vessel waveguides (EW) of all five EC launchers.

Investigation of electron cyclotron current drive efficiency on the J-TEXT tokamak

Electron cyclotron current drive (ECCD) efficiency research is of great importance for the neoclassical tearing mode (NTM) stabilization. Improving ECCD efficiency is beneficial for the NTM stabilization and the ECCD power threshold reduction. ECCD efficiency has been investigated on the J-TEXT tokamak. The electron cyclotron wave (ECW) power scan was performed to obtain the current drive efficiency. The current drive efficiency is derived to be approximately η 0 = (0.06–0.16) × 1019 A m−2 W−1 on the J-TEXT tokamak. The effect of the residual toroidal electric field has been included in the determination of the current drive efficiency, which will enhance the ECCD efficiency. At the plasma current of I p = 100 kA and electron density of n e = 1.5 × 1019 m−3, the ratio of Spitzer conductivity between omhic (OH) and ECCD phases is considered and the experimental data have been corrected. The correction results show that the current drive efficiency η 1 caused by the fast electron hot conductivity decreases by approximately 79%. It can be estimated that the driven current is approximately 24 kA at 300 kW ECW power.

Numerical study of m = 2/n = 1 neoclassical tearing mode stabilized by the Ohkawa-mechanism-dominated current drive of electron cyclotron waves

This paper reports a numerical study of the 2/1 neoclassical tearing mode (NTM) stabilized by the Ohkawa-mechanism-dominated current drive (OKCD) of electron cyclotron (EC) waves, and the results are compared with those of the traditional Fisch–Boozer mechanism dominated EC current drive (ECCD). The peak values, radial positions and radial widths of the driven current profiles by EC waves are passed to the modified Rutherford equation to study the effect of OKCD/ECCD on the 2/1 NTM. Well-localized current density profiles and large driven current can be achieved for 2/1 NTM stabilization in a low-aspect-ratio tokamak (R/a ∼ 2.7) by using OKCD. The optimal minimum EC powers are calculated for both OKCD and ECCD to fully stabilize the 2/1 NTM. We compare our results with those of ECCD to stabilize the 2/1 NTM, when the choices of magnetic field strength and gyrotron frequency are such that off-axis deposition on the high-field side is not practical to generate localized current effectively, so that the electron trapping effect is large and important. In this paper, we show that it is better to use lower gyrotron frequencies optimized for the Ohkawa mechanism to obtain a higher current drive efficiency for 2/1 NTM stabilization.

Effect of fast electron transport on neoclassical tearing mode stabilization by electron cyclotron current drive

Recent studies have shown that the stabilization of neoclassical tearing modes (NTMs) by electron cyclotron current drive (ECCD) in ITER may be impeded when the injected electron cyclotron wave beam is broadened by plasma density fluctuations. This paper starts with the analysis of NTM stabilization by ECCD using the modified Rutherford equation (MRE). It is shown that with a wide wave deposition, the most effective approach for mode stabilization is to apply modulated ECCD early during the mode growth when the magnetic island is small, as expected. Numerical simulations based on reduced magnetohydrodynamic equations have been further carried out, and the results show the same trend as those obtained from the MRE. However, the perpendicular transport of fast electrons, taken into account in numerical simulations, is found to significantly degrade the mode stabilization by modulated ECCD. The modulated driven current required for mode stabilization is proportional to the square root of the perpendicular diffusivity of fast electrons and is increased by several times when the diffusivity reaches the anomalous transport level due to plasma turbulence. If the radial misalignment of the driven current from the resonant surface reaches ∼4% of the plasma minor radius, the mode stabilization can be degraded to a great extent, depending on the wave deposition width and the perpendicular diffusivity of fast electrons.

Physics-based control of neoclassical tearing modes on TCV

This paper presents recent progress in studies of neoclassical tearing modes (NTMs) on TCV, concerning the new physics learned and how this physics contributes to a better real-time (RT) control of NTMs. A simple technique that adds a small (sinusoidal) sweeping to the target electron cyclotron (EC) beam deposition location has proven effective both for the stabilization and prevention of NTMs. This relaxes the strict requirement on beam-mode alignment for NTM control, which is difficult to ensure in RT. In terms of the EC power for NTM stabilization, a control scheme making use of RT island width measurements has been tested on TCV. NTM seeding through sawtooth (ST) crashes or unstable current density profiles (triggerless NTMs) has been studied in detail. A new NTM prevention strategy utilizing only transient EC beams near the relevant rational surface has been developed and proven effective for preventing ST-seeded NTMs. With a comprehensive modified Rutherford equation (co-MRE) that considers the classical stability both at zero and finite island width, the prevention of triggerless NTMs with EC beams has been simulated for the first time. The prevention effects are found to result from the local effects of the EC beams (as opposed to global current profile changes), as observed in a group of TCV experiments scanning the deposition location of the preemptive EC beam. The co-MRE has also proven able to reproduce well the island width evolution in distinct plasma scenarios on TCV, ASDEX Upgrade and MAST, with very similar constant coefficients. The co-MRE has the potential to be applied in RT to provide valuable information, such as the EC power required for NTM control with RT-adapted coefficients, contributing to both NTM control and integrated control with a limited set of actuators.

Basic design considerations for a frequency step-tunable electron cyclotron wave system to suppress NTMs in DEMO

An Electron Cyclotron Wave (ECW) system will be used in the European DEMO for the stabilization of Neoclassical Tearing Modes (NTMs). In order to avoid movable mirrors in the harsh environment close to the plasma and to simplify the NTM launcher integration, the tuning of the ECW deposition location can be achieved by launching frequency-tunable ECWs from fixed mirrors while the frequency is tuned in discrete steps of 2–3GHz. An overview of the frequency step-tunable ECW system for NTM stabilization is presented. The design considerations are discussed based on the current DEMO baseline parameters and the status of technologies. A simulation of NTM stabilization with an idealized frequency tunable ECW system on an analytical NTM model is shown. The simulation takes into account a realistic tuning speed based on the present technology and considers the current NTM launcher configurations in DEMO. A simple sweeping strategy is adapted for the control of frequency. Various uncertainties, which will affect the feasibility, need to be further investigated.

Two-fluid model of rf current condensation in magnetic islands

The stabilization of tearing modes with rf waves is subject to a nonlinear effect, termed rf current condensation, that has the potential to greatly enhance and localize current driven within magnetic islands. Here we extend previous investigations of this effect with a two fluid model that captures the balance of diffusive and thermal equilibration processes within the island. We show that the effective power and resulting strength of the condensation effect can be greatly enhanced by avoiding collisional heat loss to the ions. The relative impact of collisions on the overall power balance within the island depends on the ratio of the characteristic diffusion timescale and the electron–ion equilibration time, rather than the latter alone. Although relative heat loss to ions increases with island size, the heating efficiency does as well. In particular, we show that the latter safely dominates for large deposition profiles, as is typically the case for lower hybrid current drive. This supports the possibility of passive stabilization of neoclassical tearing modes without the precise aiming of the rf waves required for electron cyclotron current drive stabilization.

Neoclassical tearing mode stabilization by electron cyclotron current drive for HL-2M tokamak* *Project supported by the National Key Research and Development Program of China (Grant Nos. 2018YFE0303102, 2018YFE0301100, and 2017YFE0301702), the National Natural Science Foundation of China (Grant Nos. 11905109 and 11947238), U.S. DOE SciDAC ISEP, users with Excellence Program (on EAST tokamak) of Hefei Science Center CAS under (Grant

Investigation of neoclassical tearing mode and its suppression by electron cyclotron current drive (ECCD) has been carried out in HL-2M tokamak. The current driving capability of the electron cyclotron wave is evaluated. It is found that the deposition location can be effectively controlled by changing the poloidal angle. The validation of electron cyclotron wave heating and current driving has been demonstrated for the upper launcher port. We show that 3.0 MW and 2.5 MW modulated ECCD can completely stabilize (2,1) and (3,2) NTMs, respectively. The non-modulated ECCD, radial misalignment as well as current profile broadening have deleterious effect on the NTM stabilization. The time required for suppression of (3,2) mode is shorter than that required for the suppression of (2,1) mode. Moreover, the time needed for complete stabilization at different initial island width has been quantitatively presented and analyzed.

Suppression of neoclassical tearing mode instability at the initial stage by electron cyclotron current drive

The suppression of the neoclassical tearing mode (NTM) by the electron cyclotron wave current drive (ECCD) is studied numerically based on the reduced MHD equations. The required current drive for the NTM stabilization is shown to be significantly reduced if ECCD is applied at the initial stage when the magnetic island is not too large. The modulated ECCD is more effective than a continued one to suppress the NTM, as expected. The value of the duty circle of modulated ECCD is also found to affect the result.

Modelling effects of edge density fluctuations on electron-cyclotron current drive used for neoclassical tearing mode stabilization

In this work we study the undesired effects of electron density fluctuations (in the form of blob structures which may exist in the edge region of tokamak plasmas) to the electron-cyclotron wave propagation and current drive in connection to the efficiency of neoclassical tearing mode stabilization. Our model involves the evaluation of the driven current in the presence of density perturbations, by using a combination of a wave solver based on the transfer matrix and electromagnetic homogenization methods for the propagation part prior to and inside the region of these structures (where standard asymptotic propagation methods may not be valid due to the short-wavelength limit breakdown), with a ray tracing code including island geometry effects and current drive computation for the propagation past the perturbed region. The computed driven current is input into the modified Rutherford equation in order to estimate the consequences of the wave deformation (driven by the density fluctuations) to the mode stabilization.

Generation of localized lower-hybrid current drive by temperature perturbations

Despite high demonstrated efficiency, lower-hybrid current drive (LHCD) has not been considered localized enough for neoclassical tearing mode (NTM) stabilization in tokamaks. This assessment must be reconsidered in view of the radiofrequency current condensation effect. We show that an island with a central hot spot induces significant localization of LHCD. Furthermore, in steady state tokamaks where a significant amount of current is provided by LHCD, passive stabilization of NTMs may occur automatically, particularly as islands become large, without requiring precise aiming of the wave power.

Controlled neoclassical tearing mode (NTM) healing by fueling pellets and its impact on electron cyclotron current drive requirements for complete NTM stabilization

Controlled partial stabilization of core m/n = 2/1 neoclassical tearing modes (NTMs) by fueling deuterium pellets is reported in DIII-D and KSTAR H-mode plasmas (m/n are the poloidal/toroidal mode numbers). Analyses of DIII-D data exploring possible physics origins show that an explanation is offered by NTM-turbulence multi-scale interaction, triggered by a sudden increase of local gradients near q = 2 caused by the pellet. Pellet injection from the high-field side allows deep fueling which reaches the island region. In turn, low-k turbulent density fluctuations () increase by in the island region. This can drive transport across the island separatrices, reducing the pressure flat spot at the O-point and diminishing the NTM drive. The Mirnov probe array detects the reduction of the 2/1 magnetic amplitude by up to . Causality between elevated gradients outside of the island, turbulence spreading into the island and reduced NTM drive is qualitatively supported by non-linear gyrokinetic turbulence simulations. These show increased penetration of ion-scale from the background plasma to the O-point region when the background gradient is increased. This interaction has potentially far reaching consequences as it can lead to a reduction of the required electron cyclotron current density () for NTM suppression by 70%, as predicted by the modified Rutherford equation. This beneficial effect of fueling pellets can be important as is the anticipated active NTM control technique for ITER, but its efficiency will be lowered by third harmonic absorption in Pre-Fusion Power Operation-1 (PFPO-1) at half magnetic field.

Non-linear simulations of neoclassical tearing mode control by externally driven RF current and heating, with application to ITER

Neoclassical tearing modes (NTM) must be controlled or suppressed to prevent a degradation of the energy confinement in tokamak plasmas. This can be done applying RF-current via electron cyclotron current drive and -heating at the rational surface where the instability appears. Both the current and heating generated by the RF waves are known to provide a stabilizing effect on the magnetic island. In the present work, we address the issue of neoclassical tearing mode stabilization by heating and current drive in an ITER-like configuration, using a stiff transport model. From a revised generalized Rutherford equation, we revisit the criterion on the RF current and power required to stabilize an NTM, showing that the level of plasma background heating (residual heat sources) in ITER significantly lowers the benefit of the RF heating contribution. Nonlinear magnetohydrodynamic simulations with the XTOR code, where a stiff transport model as well as RF-power and -current drive are implemented, are performed to compute the NTM stabilization efficiency. The stabilization efficiency due to the RF current contribution is found to be less than theoretically predicted in the case of continuous application, but consistent with theory in the modulated control scheme, suggesting an enhanced destabilization at the X-point. The role of RF heating for continuous application is found to be moderate for the range of power envisaged in ITER, essentially because of the detrimental effect of residual heat sources. This numerical work confirms the capability of the ITER RF system to control the NTM.

Control of neoclassical tearing modes and integrated multi-actuator plasma control on TCV

The control of 2/1 neoclassical tearing modes (NTMs) with electron cyclotron (EC) waves has been studied both experimentally and numerically on TCV. Dynamic evolutions of NTMs along with time-varying deposition locations of the control beam have been studied in detail. The prevention of NTMs by means of preemptive EC (i.e. the control beam is switched on before the mode onset) has also been explored. A small sinusoidal sweeping with full amplitude of 0.07 (normalized to the minor radius) has been added to the control beam in two of the experiments to facilitate the comparison between NTM stabilization and prevention. It is shown that the prevention of NTMs is more efficient than NTM stabilization in terms of the minimum EC power required. Interpretative simulations with the modified Rutherford equation (MRE) have been performed to better quantify various effects, with coefficients well defined by dedicated experiments. Specifically, in order to obtain more insight on the dominant dependencies, a simple ad-hoc analytical model has been proposed to evaluate the time-varying classical stability index in the test discharges, based on the -triggered nature of these 2/1 NTMs. This allows simulating well the entire island width evolution with the MRE, starting from zero width and including both NTM stabilization and prevention cases for the first time. The exploration of NTM physics and control has facilitated the development of an NTM controller that is independent of the particular features of TCV and has been included in a generic plasma control system (PCS) framework. Integrated control of 2/1 NTMs, plasma (the ratio of plasma pressure to magnetic pressure) and model-estimated safety factor q profiles has been demonstrated on TCV.

Integrated current profile, normalized beta and NTM control in DIII-D

There is an increasing need for integrating individual plasma-control algorithms with the ultimate goal of simultaneously regulating more than one plasma property. Some of these integrated-control solutions should have the capability of arbitrating the authority of the individual plasma-control algorithms over the available actuators within the tokamak. Such decision-making process must run in real time since its outcome depends on the plasma state. Therefore, control architectures including supervisory and/or exception-handling algorithms will play an essential role in future fusion reactors like ITER. However, most plasma-control experiments in present devices have focused so far on demonstrating control solutions for isolated objectives. In this work, initial experimental results are reported for simultaneous current-profile control, normalized-beta control, and Neoclassical Tearing Mode (NTM) suppression in DIII-D. Neutral beam injection (NBI), electron-cyclotron (EC) heating & current drive (H&CD), and plasma current modulation are the actuation methods. The NBI power and plasma current are always modulated by the Profile Control category within the DIII-D Plasma Control System (PCS) in order to control both the current profile and the normalized beta. EC H&CD is utilized by either the Profile Control or the Gyrotron categories within the DIII-D PCS as dictated by the Off-Normal and Fault Response (ONFR) system, which monitors the occurrence of an NTM and regulates the authority over the gyrotrons. The total EC power and poloidal mirror angles are the gyrotron-related actuation variables. When no NTM suppression is required, the gyrotrons are used by the Profile Control category, but when NTM suppression is required, the ONFR transfers the authority over the gyrotrons to the NTM stabilization algorithm located in the Gyrotron category. Initial experimental results show that simultaneous control of different aspects of the plasma dynamics may improve the overall control and plasma performances. Also, the potential of the ONFR system to successfully integrate competing control algorithms is demonstrated.

Induction and stabilization of neoclassical tearing modes on HL-2A tokamak

The induction and stabilization characteristics of neoclassical tearing modes (NTMs) on Hl-2A tokamak are presented. NTMs are induced by sawtooth oscillations of the m/n = 1/1 mode via toroidal coupling and by gas puffing fuelling in the experiments of HL-2A tokamak. The evolutions of NTMs are modeled with a two dimensional non-linear cylindrical tearing mode code and compared with the experimental observations. The stabilization of NTMs with electron cyclotron current drive (ECCD) is numerically studied. It is found that a driven current as low as 0.015 of total plasma current can stabilize or even fully suppress NTMs. On the other hand, a small radial misalignment decreases the stabilizing effect considerably. In addition, a phase misalignment blocks the NTM stabilization and results in island width saturation at a moderate value. The effects of island rotation frequency on NTM stabilization are explored. Furthermore, it is found that the stabilizing effect can be improved efficiently by applying ECCD when the island width is less than a threshold value.