Magnetic Island Width Research Articles

Excited ion-scale turbulence by a magnetic island in fusion plasmas

The characteristics of ion-scale turbulence in the presence of a magnetic island are numerically investigated using a gyrokinetic model in fusion plasma. We observe that in the absence of the usual ion temperature gradient (ITG) drive gradient, a magnetic island and its flatten effect could drive ITG instability. The magnetic island (MI) not only drives high-n modes of ITG instability but also induces low-n modes of vortex flow. Moreover, as the magnetic island width increases, the width of the vortex flow also increases. This implies that wider islands may more easily induce vortex flows. The study further indicates that the saturated amplitude and transport level of MI-induced ITG turbulence vary with different magnetic island widths. In general, larger magnetic islands enhance both particle and heat transport. When the magnetic island width reaches to 21ρi, the turbulence-driven transport becomes the same level with the cases that ITG is driven by pressure gradients. Our findings indicate the presence of intricate nonlinear effects in the modulation of plasma turbulence by MIs. These effects are of significant importance for comprehending the phenomenon of nonlinear coupling in forthcoming tokamaks such as ITER.

New type of self-sustained divertor oscillation driven by magnetic island dynamics in Large Helical Device

A new self-sustained divertor oscillation is discovered in magnetic island induced detached plasmas in the Large Helical Device. The divertor oscillation is found to be a self-regulation of the width of an edge magnetic island accompanied by detachment-attachment transitions. The modified Rutherford equation combined with an ad-hoc bootstrap current equation is introduced to describe the divertor oscillation as a predator–prey model between the magnetic island width and a remnant X-point bootstrap current. The model successfully reproduces the experimental observations in terms of the oscillation frequency, the phase relation between variables, and the oscillation amplitude.

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.

Effects of diamagnetic drift on nonlinear interaction between multi-helicity neoclassical tearing modes

A numerical study of the diamagnetic drift effect on the nonlinear interaction between multi-helicity neoclassical tearing modes (NTMs) is carried out using a set of four-field equations including two-fluid effects. The results show that, in contrast to the single-fluid case, 5/3 NTM cannot be completely suppressed by 3/2 NTM with diamagnetic drift flow. Both modes exhibit oscillation and coexist in the saturated phase. To better understand the effect of the diamagnetic drift flow on multiple-helicity NTMs, the influence of typical relevant parameters is investigated. It is found that the average saturated magnetic island width increases with increasing bootstrap current fraction f b but decreases with the ion skin depth δ. In addition, as the ratio of parallel to perpendicular transport coefficients χ ∥/χ ⊥ increases, the average saturated magnetic island widths of the 3/2 and 5/3 NTMs increase. The underlying mechanisms behind these observations are discussed in detail.

Bifurcation of coherent vortex flow in a magnetic island through nonlinear parity instability

The topology of the vortex flow associated with the magnetic island plays a significant role in modulating the turbulent transport near the magnetic island. In this paper, self-consistent nonlinear simulations of multi-scale interactions among large scale tearing mode, vortex flow, and small scale ion temperature-gradient (ITG) mode are numerically investigated based on the five-field Landau-fluid model. We found that the coherent vortex flow in a magnetic island has different parities in the nonlinear quasi-steady state, and this can be described by a theoretical framework—nonlinear parity instability. In the ITG stable case, the structure of the vortex flow bifurcates from tearing parity to twisting parity, which is characterized by modulational parity instability, modeled by a four-wave nonlinear coupling process. In the ITG unstable case, the vortex flow stays in tearing parity without parity bifurcation, and the energy is transferred from the twisting parity modes to the tearing parity modes. The impact of the parity instability on the magnetic island width is discussed as well.

Global gyrokinetic simulations of the impact of magnetic island on ion temperature gradient driven turbulence

The effect of island width on the multi-scale interactions between magnetic island (MI) and ion temperature gradient (ITG) turbulence has been investigated based on the global gyrokinetic approach. It is found that the coupling between the island and turbulence is enhanced when the MI width (w) becomes larger. A vortex flow that is highly sensitive to the width of the MI can be triggered, ultimately resulting in a potent shear flow and a consequent reduction in turbulent transport. The shearing rate induced by the vortex flow is minimum at the O-point while it is maximum at both of the two reconnection points of the island, i.e. the X-points, regardless of the island width. There exists a nonmonotonic relationship between zonal flow (ZF) amplitude and island width, showing that the ZF is partially suppressed by medium-sized MIs whereas enhanced in the case of large island. A larger MI can tremendously damage the ITG mode structure, resulting in higher turbulent transport at the X-point whereas a lower one at the O-point, respectively. Such phenomenon will be less distinct at very small island widths below w/a ∼ 8% ≈ 12 (a is the minor radius and the ion gyroradius), where it shows that turbulence near the X-point is hardly affected although it is still suppressed inside the island. Furthermore, the influence of different island sizes on turbulence transport level is also discussed.

The role of fast ions in spontaneous neoclassical tearing mode instabilities in NSTX

Spontaneous neoclassical tearing modes (spontaneous NTMs) have been observed in the National Spherical Torus Experiment (NSTX). Recently, a TRANSP based model for the modified Rutherford equation analysis was developed to accurately predict the effect of fast ions on the growth of magnetic islands. The goal of this paper is to utilize the TRANSP NTM model to understand the role of fast ions in NSTX spontaneous NTMs. Our analysis shows that when the passing fast ion driven kinetic neoclassical polarization current term is larger than 1% of the bootstrap current term in the modified Rutherford equation, fast ions play a decisive role in the early growth phase of spontaneous NTM. This result is applicable to any tokamak plasmas with a fast ion drift orbit width, or the fast ion poloidal Larmor radius, comparable to the magnetic island width.

Simulations of nonlinear interaction between beta-induced Alfvén eigenmode and tearing mode

The nonlinear interaction between the beta-induced Alfvén eigenmode (BAE) and the tearing mode (TM) observed in the HL-2A experiment is systematically studied with the hybrid kinetic-magnetohydrodynamic code M3D-K. It is found that the nonlinear growth of TM can lead to a gradient buildup on the magnetic island (MI) edge, and then triggers the destabilization of the linearly-stable BAE. Then, the triggered BAE together with TM can produce a significant redistribution of energetic particles. For BAE linearly-dominant-unstable case, the TM activity results in the inward motion of BAE mode structure, and the BAE can have a delay effect on the MI saturation. Furthermore, a high-frequency axisymmetric ‘breathing’ mode is generated by the mode coupling of BAE and TM, agreeing well with the experimental observation, and causes the synchronized periodic oscillation of MI width.

Suppression mechanism of equilibrium magnetic islands in CFQS low-<inline-formula><tex-math id="Z-20231023161017">\begin{document}$\boldsymbol \beta$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20230546_Z-20231023161017.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20230546_Z-20231023161017.png"/></alternatives></inline-formula> plasma

Magnetic island produced in toroidal magnetic confinement plasma has a three-dimensional helical structure because of the rotational transform, especially the equilibrium magnetic surface of the stellarator is three-dimensional helical structure. Thus, the formation and instability of the magnetic island of the Stellarator is a typical issue of the three-dimensional physics and is also one of the key topics of the physics research of the Stellarator. Magnetic islands and related tearing mode physics are major issues in stellarator. The non-inductively current drive, i.e. electron cyclotron current drive (ECCD) can be used as one of the approaches to adjusting the rotational transform, and hence, affecting the generation of magnetic islands. In this study, we use an additional toroidal magnetic field to generate <i>m</i>/<i>n</i> = 5/2 magnetic islands in the low-<i>β</i> operation on the Chinese First Quasi-axisymmetric Stellarator (CFQS) so that the influence of the bootstrap current is negligible. Then, we investigate the suppression mechanism of magnetic islands in low-<i>β</i> plasma by using the HINT code. It is found that in the case of the constant current, when the current direction is positive, with the increase of current, the width of island increases. When the direction of current is reversed, the island is suppressed when the current is larger than 6 kA. The main reason is that the rotational transform is away from <i>ι</i>/2π = 0.4 rational surface and the <i>m</i>/<i>n</i>=5/2 magnetic island does not meet the resonance conditions. In the case of local current profile, the magnetic island width decreases as a result of the enhanced magnetic shear at <i>ι</i>/2π = 0.4 rational surface. Moreover, effects of the direction and the amplitude of the current on the suppression of magnetic islands are also discussed in more detail.

Influence of toroidal rotation on nonlinear evolution of tearing mode in tokamak plasmas

The nonlinear evolution of the tearing mode instability with toroidal rotation is investigated using a three-dimensional toroidal geometry magnetohydrodynamic code (M3D). It is found that toroidal rotation plays a significantly stable role in reducing the width of magnetic island, which is independent of the direction of toroidal rotation. Compared with the stabilizing effect in the linear phase, the stabilizing effect of toroidal rotation is much stronger in the nonlinear phase. The magnetic island is almost suppressed by large enough rotation, even though the linear growth rate is weakly reduced in the linear phase. It is observed that the flow shear has a stabilizing effect, which is significant when the rotation frequency is large. However, it is shown that the magnitude of rotation plays a dominant role in stabilizing the magnetic island when the rotation frequency is small. The Coriolis force, rather than the centrifugal force, is found to play a dominant role in stabilizing the tearing mode in the nonlinear phase. When and tearing modes exist simultaneously, the sheared toroidal rotation is shown to effectively suppress the overlap of magnetic islands and field stochasticity caused by the coupling of neighboring islands.

Stability of a weakly collisional plasma with runaway electrons

We investigate the problem of the tearing stability of a post-disruption weakly collisional plasma where the current is completely carried by runaway electrons. We adopt here a two fluid model which takes into account also ion sound Larmor radius and electron inertia effects in the description of the reconnection process. In the past, it has been demonstrated in [Helander et al. Phys. Plasmas 14, 12, (2007)] that in the purely resistive regime the presence of runaway electrons in plasma has a significant effect on the saturated magnetic island width. In particular, runaway electrons generated during disruption can cause an increase of 50% in the saturated magnetic island width with respect to the case with no runaway electrons. These results were obtained adopting a periodic equilibrium magnetic field that limited the analysis to small size saturated magnetic islands. Here we present our results to overcome this limitation adopting a non-periodic Harris’ type equilibrium magnetic field. Preliminary results on the effects of the ion sound Larmor radius effects will also be presented.

Dependence of nonlinear coupling among turbulence, geodesic acoustic modes and tearing modes on magnetic island width in the HL-2A edge plasmas

Statistical spectral features of the dependence of geodesic acoustic modes (GAMs) and their nonlinear couplings with ambient turbulence on the magnetic island (MI) width (W) in the edge region of HL-2A tokamak plasmas are analyzed. Experimental observations have indicated that the modulation influence as well as the strength of nonlinear interactions between GAMs and turbulence generally shows a gradual decay while the couplings between MIs and the latter are increased simultaneously as the MI becomes larger. The MIs mainly reduce the couplings between GAMs and potential fluctuations, whereas the changes in the nonlinear interactions between density fluctuations and MIs are more evident. Moreover, it is found that there exists a nonmonotonic relationship between the turbulence correlation length and island width, in which it exhibits a minimum around W ∼ 3.7 cm, suggesting that the MI around such a scale would have a significant suppression effect on turbulent transport. These findings promote the understanding of the nonlinear interactions between MIs and turbulence in the edge of fusion plasmas.

A dynamo effect of multiple tearing modes on Taylor relaxation

The dynamo effect of multiple tearing modes in a force-free plasma is investigated using resistive magnetohydrodynamics equations. In a steady state, two tearing modes are considered. It is found that the dynamo effect is related to the distance between the two rational surfaces and the magnetic island width. The λ=j·B/|B|2 profile is flatter for closer rational surfaces and wider magnetic islands. The case of an arbitrary number of tearing modes is also considered, and it is found that the λ profile in a finite plasma region can be flattened by the dynamo effect if there are enough tearing modes. This indicates that λ can be flattened in the entire plasma region, which makes it clear that the dynamo effect actually flattens λ rather than the current density. In the growth stage, the case of a growing tearing mode and two saturated modes is considered. The calculation shows that the middle tearing mode makes connections between the two modes on each side, playing the role of a mediator. Our results provide a more clear explanation for the dynamo effect of multiple tearing modes as a possible mechanism behind the Taylor relaxation process.

Beta-induced Alfvén eigenmodes and geodesic acoustic modes in the presence of strong tearing activity during the current ramp-down on JET

The analysis of the current ramp-down phase of JET plasmas has revealed the occurrence of additional magnetic oscillations in pulses characterized by large magnetic islands. The frequencies of these oscillations range from 5 to , being well below the toroidal gap in the Alfvén continuum and of the same order as the low-frequency gap opened by plasma compressibility. The additional oscillations only appear when the magnetic island width exceeds a critical threshold, suggesting that the oscillations could tap their energy from the tearing mode (TM) by a non-linear coupling mechanism. A possible role of fast ions in the excitation process can be excluded, being the pulse phase considered in the observations characterized by very low additional heating. The calculation of the coupled Alfvén–acoustic continuum in toroidal geometry suggests the possibility of beta-induced Alfvén eigenmodes (BAEs) rather than beta-induced Alfvén–acoustic eigenmodes. As a main novelty compared to previous work, the analysis of the electron temperature profiles from electron cyclotron emission has shown the simultaneous presence of magnetic islands on different rational surfaces in pulses with multiple magnetic oscillations in the low-frequency gap of the Alfvén continuum. This observation supports the hypothesis of different BAE with toroidal mode number n = 1 associated with different magnetic islands. As another novelty, the observation of magnetic oscillations with n = 2 in the BAE range is reported for the first time in this work. Some pulses, characterized by slowly rotating magnetic islands, exhibit additional oscillations with n = 0, likely associated with geodesic acoustic modes (GAMs), and a cross-spectral bicoherence analysis has confirmed a non-linear interaction between TM, BAE and GAM, with the novelty of the observation of multiple triplets (twin BAEs plus GAM), due to the simultaneous presence of several magnetic islands in the plasma.

Self-Sustained Divertor Oscillation Driven by Magnetic Island Dynamics in Torus Plasma

A new type of self-sustained divertor oscillation is discovered in the Large Helical Device stellarator, where the peripheral plasma is detached from material diverters by means of externally applied perturbation fields. The divertor oscillation is found to be a self-regulation of an isolated magnetic field structure (the magnetic island) width induced by a drastic change in a poloidal inhomogeneity of the plasma radiation across the detachment-attachment transitions. A predator-prey model between the magnetic island width and a self-generated local plasma current (the bootstrap current) is introduced to describe the divertor oscillation, which successfully reproduces the experimental observations.

Design and installation of divertor biased target system on the HL-2A TOKAMAK

Resonant magnetic perturbations (RMPs) have been established as an efficient way of stabilizing edge localized modes (ELMs). Driving the scrape-off layer (SOL) currents can also generate RMP for ELMs control. A divertor biased target system (2 × 2 array) has been recently designed and installed on the HL-2A tokamak to drive the helical currents. There are two groups of biased targets (the high field side and the low field side, respectively) with each consisting of 2 targets separated from each other by 180° along the toroidal direction. Computed perturbation of magnetic field is found the resonant radial vacuum field component for n = 1 generated by the helical current in the q95 surface can reach 12.5 Gs/kA. Computation of the width of vacuum magnetic islands and the magnetic field topology for n = 1,2 RMP evidently indicates utilizing the divertor biased targets system can mitigate or suppress the ELMs.

Influence of resonance magnetic perturbation on the asymmetric magnetic perturbation induced double tearing modes

In this paper, we have studied the influence of resonance magnetic perturbation (RMP) on the double tearing modes induced by asymmetric magnetic perturbation in a 2D geometry based on the MHD model. After studying the four cases, we have found that with RMP, no matter which rational surface the magnetic perturbation is in: (1) the width of the outer magnetic island will be greater than that of the inner magnetic island at the end of the mode evolution; (2) The influence of the RMP on the outer magnetic island is far greater than the inner magnetic island; (3) RMP will shorten the time required for the magnetic island to reach saturation.

Experimental observation of the localized coupling between geodesic acoustic mode and magnetic islands in tokamak plasmas

The localized coupling among geodesic acoustic mode (GAM), tearing modes (TMs) and twin counter-propagating beta-induced Alfvén eigenmodes (BAEs) waves has been investigated in the experimental advanced superconducting tokamak. Before the appearance of TMs, typical continuous GAM is observed through the multi-channel Doppler backscattering (DBS) diagnostic. The twin BAEs can be excited after the burst of magnetic islands, which are localized to the q = 4 rational surface that is confirmed by the measurement of DBS array, where the GAM and twin BAEs are observed synchronically at R ≈ 2.23 m (normalized radius ρ ≈ 0.8). One reasonable excitation mechanism is proposed that the twin BAEs can be excited by the nonlinear interaction between GAM and magnetic islands. As the width of magnetic islands increases, the electromagnetic twin BAEs increase synchronically with the decreasing of electrostatic GAM, strongly suggesting that the electromagnetic components are pumped from three-wave interaction between electrostatic GAM and magnetic islands.

Simulation studies of the radiation-driven tearing mode in tokamaks

The influence of bremsstrahlung and cyclotron radiation on the evolution and saturation of the resistive tearing mode is investigated by using a three-dimensional toroidal magnetohydrodynamic code. It is found that accumulation of impurities (such as tungsten) in magnetic islands has a destabilizing effect on the tearing mode due to an imbalance of local radiative cooling and Ohmic heating inside the island. The asymmetry current effect and the nonlinear mode coupling play an important role on evolution of the radiation-driven tearing mode instability. As a result, the magnetic island width could increase over 20% of the minor radius (disruption scale). Along with higher mode burst, multi-mode interaction makes magnetic field lines become stochastic. Moreover, the core pressure is reduced due to the radiation cooling inside the magnetic island. Consequently, the whole discharge area is shifted towards the high-field side, which leads to a strong intensification of the mode.

Structure bifurcation induced by wide magnetic islands

Magnetic islands resulting from magnetic reconnection play significantly important roles in critical issues of magnetic fusion plasmas, because they essentially change magnetic topology and significantly degrade plasma confinement. In this paper, we report that the wide islands can lead to a structure bifurcation of flow parity inside islands, then induce a state with a large vortex flow. The mechanism is identified as an instability in the configuration with magnetic islands, in which the eigenmode parity is contrary to the conventional tearing mode and the radial profile of zonal flow also changes. The critical values of the width of magnetic island for the bifurcation are obtained and discussed. The dependence of bifurcation on various physical parameters is studied in detail. It is found that tearing instability parameter and diamagnetic drift flow have significant impacts on the bifurcation. Moreover, the characteristics of the bifurcation show the qualitative agreement with the experimental observation on the Large Helical Device.