Application Of Resonant Magnetic Perturbations Research Articles

Effect of resonant magnetic perturbations on ELMs in connected double null plasmas in MAST

The application of resonant magnetic perturbations (RMPs) with a toroidal mode number of n = 3 to connected double null plasmas in the MAST tokamak produces up to a factor of 9 increase in edge-localized mode (ELM) frequency and reduction in plasma energy loss associated with type-I ELMs. A threshold current for ELM mitigation is observed above which the ELM frequency increases approximately linearly with current in the coils. The effect of the RMPs is found to be scenario dependent. In one scenario the mitigation is only due to a large density pump out event and if the density is recovered by gas puffing a return to type-I ELMs is observed. In another scenario sustained ELM mitigation can be achieved irrespective of the amount of fuelling. Despite a large scan of parameters complete ELM suppression has not been achieved. The results are compared with modelling performed using either the vacuum approximation or including the plasma response. The requirement for a resonant condition, that is an optimum alignment of the perturbation with the plasma, is confirmed by performing a scan in the pitch angle of the applied field.

Inter-ELM pedestal evolution on MAST and impact of resonant magnetic perturbations

The peak pressure gradient in the pedestal (dPe/dψ) on MAST varies little between edge localized modes (ELMs), although it varies between discharges due factors such as gas fuelling and plasma current. The pressure pedestal width in flux space on the high-field side (HFS), both during the inter-ELM period and amongst different plasma discharges is consistent with a scaling of . In flux space very similar dPe/dψ and Δpe are observed on the HFS and low-field side (LFS) in single null configuration. This symmetry is broken by the application of resonant magnetic perturbations (RMPs). During ELM mitigation by application of RMPs changes in the edge transport barrier position and width are observed. These changes are dependent on the intensity of the RMP and on the toroidal location with respect to the RMP phase. An outward displacement of up to 30 mm and increase in the edge pedestal width of up to 50% with respect to the coils case off are observed. Increased particle transport causes a decrease in ne,ped, and hence Pe,ped, as is observed on other devices. The combination of an increase in Δpe on the LFS and decrease in Pe,ped results in significantly reduced LFS dPe/dψ when these perturbations are applied to the plasma edge. A decrease in dPe/dψ on the HFS is also observed due to RMP; however, this decrease is caused solely by the Pe,ped decrease whilst no expansion of Δpe on the HFS is observed.

Runaway Electron Suppression by ECRH and RMP in KSTAR

We have observed reduction of the runaway electron synchrotron radiation, hard X-ray (HXR) intensity, and HXR energy after applying 110 GHz 2nd harmonic electron cyclotron resonant heating (ECRH) during runaway electron (RE) discharges at low density with startup runaway electrons. However, we did not see a significant reduction of X-rays from 170 GHz 2nd harmonic ECRH at a higher field. A recently installed IR TV camera was used to observe the forward cone of synchrotron radiation from high energy REs in KSTAR. We have observed changes to the synchrotron images and reduction of the HXR by application of resonant magnetic perturbations (RMP) from in-vessel control coils (IVCC) installed inside KSTAR in the n = 1 configuration.

Divertor heat fluxes and profiles during mitigated and unmitigated edge localised modes (ELMs) on the Mega Amp Spherical Tokamak (MAST)

Edge localised modes (ELMs) are a concern for future devices as they can limit the operational lifetime of the divertor. The mitigation of ELMs can be performed by the application of resonant magnetic perturbations (RMPs) which can act to degrade the pressure gradient in the edge of the plasma. Investigations of the effect of RMPs on MAST have been performed in a range of plasmas using perturbations with toroidal mode numbers of n=3, 4 and 6. It has been seen that the RMPs increase the ELM frequency, which gives rise to a corresponding decrease in the ELM energy. The reduced ELM energy decreases the peak heat flux to the divertor, with a three-fold reduction in the ELM energy, generating a 1.5 fold reduction in the peak heat flux. Measurements of the divertor heat flux profile show evidence of strike point splitting consistent with modelling using the vacuum code ERGOS.

Effect of resonant magnetic perturbations with toroidal mode numbers of 4 and 6 on edge-localized modes in single null H-mode plasmas in MAST

The application of resonant magnetic perturbations (RMPs) with a toroidal mode number of n = 4 or n = 6 to lower single null plasmas in the MAST tokamak produces up to a factor of 5 increase in edge-localized mode (ELM) frequency and reduction in plasma energy loss associated with type-I ELMs. A threshold current for ELM mitigation is observed above which the ELM frequency increases approximately linearly with current in the coils. Despite a large scan of parameters, complete ELM suppression has not been achieved. The results have been compared with modelling performed using either the vacuum approximation or including the plasma response. During the ELM mitigated stage clear lobe structures are observed in visible-light imaging of the X-point region. The size of these lobes is correlated with the increase in ELM frequency observed. The characteristics of the mitigated ELMs are similar to those of the natural ELMs suggesting that they are type-I ELMs which are triggered at a lower pressure gradient. The application of the RMPs in the n = 4 and n = 6 configurations before the L–H transition has little effect on the power required to achieve H-mode while still allowing the first ELM to be mitigated.

Towards understanding ELM mitigation: the effect of axisymmetric lobe structures near the X-point on ELM stability

The application of resonant magnetic perturbations (RMPs) with toroidal mode number n = 4 or n = 6 is observed to result in a significant increase in ELM frequency, despite a reduction in the edge pressure gradient. A picture for how type-I ELMs are mitigated, or destabilized, when magnetic perturbations are applied is proposed. Despite the magnetic perturbation incurring a drop in the pedestal pressure gradient, the critical pressure for an ELM to be triggered is dropped even further. Various mechanisms which could cause this degradation of the stability boundary are outlined. The stability of the edge of MAST H-mode plasmas has been tested when lobe structures are present in the separatrix after application of RMPs. The axisymmetric stability analysis presented here shows that when the plasma boundary is simulated with a model for the observed lobe structures included, the ballooning stability is degraded. This degradation in ballooning stability originates from the perturbed field lines dwelling in the region of unfavourable curvature due to the presence of lobe structures rather than the change in the plasma boundary shape.

Interaction between a low-frequency electrostatic mode and resonant magnetic perturbations in MAST

A strong ≈10 kHz mode is detected in both potential and density fluctuations of the edge plasma of the MAST tokamak using a reciprocating probe. The mode is radially localized, with outer limit ≈2 cm inside the separatrix, and is affected on application of resonant magnetic perturbations generated by external coils. A shift in frequency with plasma rotation is found, and a rapid suppression of the mode is observed when it can couple to the imposed n = 3 magnetic perturbations in the rotating frame. Non-linear coupling to high wave number turbulence is evident, and an increase in power of turbulence fluctuations is seen after suppression. These observations are then interpreted in the context of known low-frequency plasma modes present in the toroidal configuration. A possibility that the observed mode is a geodesic acoustic mode is considered and motivated by observations.

The Effect of Externally Applied Resonant Magnetic Perturbations on Fusion Product Dynamics in Toroidal Plasmas: Numerical Simulation

The suppression of edge-localized modes by means of externally induced resonant magnetic perturbations (RMPs) has been investigated extensively on present-day tokamaks. In this paper we examine the modification of the loss of fusion born α-particles as effected by the application of RMPs in tokamak plasmas. This study was performed by means of test-particle simulations. To simplify the calculations we use a toroidal magnetic field model with circular magnetic flux surfaces. The transport properties of energetic α-particles are investigated during a 3 s time interval by tracing the test-particle ensemble with each particle trajectory following by integration of the full orbit equation. Three regimes of particle losses are identified during the evolution of the particle ensemble. A natural consequence of RMP excitation is the formation of magnetic islands together with stochastic magnetic layers at the plasma edge. The formation of these resonant magnetic field structures are associated with irregularities of the energetic α-particle orbits, which can substantially increase the loss of slowing down α-particles from the plasma periphery. At the same time the first orbit losses of fusion alphas are practically unaffected by RMPs.

Analysis of stochastic magnetic fields formed by the application of resonant magnetic perturbations on MAST and comparison with experiment

In MAST experiments with applied resonant magnetic perturbations (RMPs), clear reduction in line-averaged density has been observed in a wide range of L-mode plasmas when there is an alignment between the perturbation and the equilibrium magnetic field that maximizes the size of the resonant components of the applied magnetic field, as well as in a few H-mode plasmas but with a much stronger sensitivity to this alignment. This density pump-out is the result of increased particle transport, which is thought to be caused by the formation of a stochastic magnetic field in the plasma edge. This paper presents an analysis of the magnetic field structures formed by the application of n = 3 RMPs on MAST, including various parameters characterizing the degree of stochasticity in the plasma edge. Values for these parameters are calculated and compared with the amount of density pump-out observed in MAST experiments. It is found that density pump-out is fairly well correlated with some of the parameters calculated using vacuum modelling, but none of them provides a single threshold value for pump-out that applies to both L- and H-mode plasmas. Plasma response modelling provides a robust criterion for density pump-out that applies both to L- and H-mode plasmas.

Effect of resonant magnetic perturbations on secondary structures in drift-wave turbulence

Recent experiments showed a decrease of long range correlations during the application of resonant magnetic perturbations (RMPs) [Y. Xu et al., Nucl. Fusion 51, 063020 (2011)]. This finding suggests that RMPs damp zonal flows. To elucidate the effect of the RMPs on zonal structures in drift wave turbulence, we construct a generalized Hasegawa-Wakatani model including RMP fields. The effect of the RMPs is to induce a linear coupling between the zonal electric field and the zonal density gradient, which drives the system to a state of electron radial force balance for large RMP amplitude. A predator-prey model coupling the primary drift wave dynamics to the zonal modes evolution is derived. This model has both turbulence drive and RMP amplitude as control parameters and predicts a novel type of transport bifurcation in the presence of RMPs. The novel regime has a power threshold which increases with RMP amplitude as γc~[δBrB]2.

Runaway electron losses caused by resonant magnetic perturbations in ITER

Disruptions in large tokamaks can lead to the generation of a relativistic runaway electron beam that may cause serious damage to the first wall. To suppress the runaway beam the application of resonant magnetic perturbations (RMPs) has been suggested. In this work we investigate the effect of RMPs on the confinement of runaway electrons by simulating their drift orbits in magnetostatic perturbed fields and calculating the transport and orbit losses for various initial energies and different magnetic perturbation configurations. In the simulations we model the ITER RMP configuration and solve the relativistic, gyro-averaged drift equations for the runaway electrons including a time-dependent electric field, radiation losses and collisions. The results indicate that runaway electrons are rapidly lost from regions where the normalized perturbation amplitude δB/B is larger than 0.1% in a properly chosen perturbation geometry. This applies to the region outside the radius corresponding to the normalized toroidal flux ψ = 0.5.

Rotation and radial electric field in the plasma edge with resonant magnetic perturbation at TEXTOR

In this paper the results of a systematic experimental assessment of the plasma edge rotation and radial electric field with application of resonant magnetic perturbation (RMP) are presented. The results are based on the radially resolved measurement of the poloidal (vpol) and toroidal (vtor) rotation. It is shown that the radial electric field Er can be deduced from the radial force balance when small amplitude resonant magnetic perturbations are applied to the plasma boundary (Br/Btor ∼ 10−4).Both vpol and vtor spin-up in the ion-diamagnetic-drift and co-current direction, respectively, with increasing external perturbation field (Δvpol ∼ 15 km s−1, Δvtor ∼ 2–5 km s−1) yielding an increase in Er by ΔEr,max = 9 kV m−1. The toroidal rotation increases over the whole radius while the poloidal rotation shows distinct local features driving the evolution of the Er-profiles. Depending on the edge safety factor a local (at the q = 5/2 rational surface) increase in the shear rate ΩE×B (ΔΩq=5/2 = 1.4 × 105 s−1) or reduced shearing can occur. Increased shearing is correlated with an improved particle confinement with an increase in the particle confinement time by Δτp = +40%. Increasing the local resonant amplitude by 30% induces a reduced density level, the so-called RMP induced pump-out. At this confinement stage the shear rate decreases by 15% correlated with a significant drop in particle confinement (Δτp = −30%).Field line tracing in the vacuum approximation gives indications towards explaining the threshold behaviour connecting the shearing rate, confinement stages and magnetic topology to the amount of applied RMP. However, this basic approach does not account for plasma response and the results presented are linked in the discussion section to recent results on the link between rotation and plasma response as well as on the transport features of RMP.

Runaway electron drift orbits in magnetostatic perturbed fields

Disruptions in large tokamaks can lead to the generation of a relativistic runaway electron beam that may cause serious damage to the first wall. To mitigate the disruption and suppress the runaway beam the application of resonant magnetic perturbations has been suggested. In this work we investigate the effect of resonant magnetic perturbations on the confinement of runaway electrons by simulating their drift orbits in magnetostatic perturbed fields and calculating the orbit losses for various initial energies and magnetic perturbation magnitudes. In the simulations we use a TEXTOR-like configuration and solve the relativistic, gyro-averaged drift equations for the runaway electrons including synchrotron radiation and collisions. The results indicate that runaway electrons are well confined in the core of the device, but the onset time of runaway losses closer to the edge is dependent on the magnetic perturbation level and thereby can affect the maximum runaway current. However, the runaway current damping rate is not sensitive to the magnetic perturbation level, in agreement with experimental observations.

Edge turbulence and flows in the presence of resonant magnetic perturbations on MAST

The impact of resonant magnetic perturbations (RMPs) on the edge of MAST L-mode plasmas is studied using a mid-plane reciprocating probe equipped with a Gundestrup probe head. A strong impact on the characteristics of the ion saturation current fluctuations is observed just inside the separatrix, with a broadening of the power spectrum and an asymmetrization of the probability distribution functions towards non-Gaussian shapes. No major effect is found in the scrape-off layer (SOL). Floating potential measurements are used to evaluate the modification of the plasma potential profiles. The radial electric field profile flattens when RMPs are applied, leading to a theoretically expected increase in Er inside the separatrix and a decrease in the SOL. A consistent change is observed on the perpendicular and parallel flows which tend to increase inside the separatrix. In contrast, in the SOL, a braking of the rotation is observed with the application of RMPs. All these effects occur only above a threshold in the amplitude of the current applied in the RMP coils and are not toroidally localized.

Resonant magnetic perturbation experiments on MAST using external and internal coils for ELM control

Experiments have been performed on MAST using both external (n = 1, 2) and internal (n = 3) resonant magnetic perturbation (RMP) coils. ELM suppression has not been achieved even though vacuum modelling shows that either set of coils can produce a region (ΔΨpol > 0.17), for which the Chirikov parameter is greater than 1, wider than that correlated with ELM suppression in DIII-D. Although complete ELM suppression has not been achieved, application of RMPs has triggered ELMs in ELM free H-mode periods (n = 3) and increased the ELM frequency in regularly ELM-ing discharges (n = 2, 3). In addition, the application of RMPs in an n = 3 configuration has produced large changes to the edge turbulence in L-mode discharges.

Overview of the results on divertor heat loads in RMP controlled H-mode plasmas on DIII-D

In this paper the manipulation of power deposition on divertor targets at DIII-D by the application of resonant magnetic perturbations (RMPs) for suppression of large type-I edge localized modes (ELMs) is analysed. We discuss the modification of the ELM characteristics by the RMP applied. It is shown that the width of the deposition pattern in ELMy H-mode depends linearly on the ELM deposited energy, whereas in the RMP phase of the discharge those patterns are controlled by the externally induced magnetic perturbation. It was also found that the manipulation of heat transport due to the application of small, edge RMP depends on the plasma pedestal electron collisionality . We compare in this analysis RMP and no RMP phases with and without complete ELM suppression. At high , the heat flux during the ELM suppressed phase is of the same order as the inter-ELM and the no-RMP phase. However, below this collisionality value, a slight increase in the total power flux to the divertor is observed during the RMP phase. This is most likely caused by a more negative potential at the divertor surface due to hot electrons reaching the divertor surface from the pedestal area along perturbed, open field lines.

Particle confinement control with resonant magnetic perturbations at TEXTOR

Two very contrary particle confinement stages were obtained at TEXTOR-DED by application of resonant magnetic perturbations. On the one hand a spontaneous build up of the total number of particles N tot with correlated increase in the particle confinement time τ p was observed and on the other hand a controlled decrease of N tot and τ p – the so called stochastic particle pump out is seen. Numerical analysis of the perturbed magnetic field topology shows that both domains can be distinguished by the ratio of short connection length field lines touching a specific resonant flux surface (here the q = 5 / 2 surface) to the complete perturbed layer width. During improved particle confinement, the hyperbolic fixed points (X-points) of the pitch resonant islands are directly connected to the DED target followed by an ≲ 40 % increase in τ p . The subsequent increase in the E × B shear rate Ω E × B at the q = 5 / 2 surface and a steepening of ∇ n e ( r ) suggests a reduction of the radial particle transport. On the opposite, complete stochastisation of this island chain, i.e. a predominant diffusive field line characteristics, causes a ≲ 30 % decrease of τ p with a reduction in Ω E × B at the q = 5 / 2 surface and ∇ n e ( r ) indicating enhanced effective outward particle transport.

Suppression of Runaway Electrons by Resonant Magnetic Perturbations in TEXTOR Disruptions

The generation of runaway electrons in the international fusion experiment ITER disruptions can lead to severe damage at plasma facing components. Massive gas injection might inhibit the generation process, but the amount of gas needed can affect, e.g., vacuum systems. Alternatively, magnetic perturbations can suppress runaway generation by increasing the loss rate. In TEXTOR disruptions runaway losses were enhanced by the application of resonant magnetic perturbations with toroidal mode number n=1 and n=2. The disruptions are initiated by fast injection of about 3x10{21} argon atoms, which leads to a reliable generation of runaway electrons. At sufficiently high perturbation levels a reduction of the runaway current, a shortening of the current plateau, and the suppression of high energetic runaways are observed. These findings indicate the suppression of the runaway avalanche during disruptions.

The physics of edge localized modes (ELMs) and their role in power and particle exhaust

In this paper, various types of so-called edge localized mode (ELM), an MHD instability occurring in the edge of H-mode plasmas, are discussed. A phenomenological classification based on experimental observations from several experiments is given. ELMs affect the confinement of both energy and particles, with a stronger degradation of particle confinement. Thus, ELMs provide particle control in the H-mode, allowing steady-state operation at only modest confinement degradation. However, large ELMs may lead to unacceptable power loads on the divertor plates of future large fusion experiments and, thus, active control of ELM type and characteristics is required. At present, three control schemes have been demonstrated, namely dynamic variation of the plasma shape, application of resonant magnetic perturbations and control of the energy flux through the separatrix by additional radiation.

MHD control and ECCD in Compass-D

Electron cyclotron current drive experiments have been carried out in Compass-D using a power of approximately 500 kW at a frequency of 60 GHz. The fundamental resonance was used and the waves were launched from the high field side of the torus through four mirror-antennae. Significant asymmetry in the loop voltage behaviour is observed when comparing co and counter current drive cases, suggesting a driven current approximately 15 kA (in a plasma current of 130 kA). The BANDIT-3D Fokker Planck code has been used to model these discharges revealing a similar value for the predicted driven current at zero loop voltage but strong synergistic effects with the toroidal electric field at the approximately 0.4 Volts/turn typical of the experiments. This synergy appears to be sufficient to explain the inferred net 'co' current drive in most of the 'counter' current drive shots. In H-mode discharges featuring a strongly asymmetric single null separatrix configuration, discrete ELM's sometimes produce sufficient relaxation of the current profile to cause loss of vertical control, leading to vertical displacement events. Application of resonant magnetic perturbations (RMPs) of predominantly n=1 (with various low m values) is seen reliably to increase the frequency of the ELM's, although the exact mechanism underlying this effect has still to be elucidated. A real-time electronic neural network has been used for the first time for feedback control of the equilibrium in a tokamak. The neural network was used for feedback control of the plasma elongation throughout the discharge, while simultaneously monitoring the plasma vertical and horizontal position.