Edge Localized Mode Control Coils Research Articles

A new type of resonant magnetic perturbation for controlling edge localized modes

A new type of resonant magnetic perturbation (RMP), generated by helical coils, is proposed for controlling the edge localized mode (ELM) in H-mode tokamak plasmas. The helical coil optimization utilizes the MARS-F code (Liu et al 2000 Phys. Plasmas 7 3681) computed linear resistive fluid response of the plasma to the applied RMP field. The optimal helical coils are found to be located near the outboard mid-plane of the torus, with relatively simple shape but tilted towards the equilibrium magnetic field line pitch. Compared to the window-frame ELM control coils, the optimal helical coils require 2–4 times less current, in order to achieve the same ELM control performance specified by various figures of merit adopted in this work. The results from the present study show a promising path forward in achieving ELM control with RMP fields in tokamak plasmas.

LOCUST-GPU predictions of fast-ion transport and power loads due to ELM-control coils in ITER

The graphics processing unit (GPU) version of the Lorentz-orbit code for use in stellarators and tokamaks (LOCUST) has been applied to study the fast-ion transport and loss caused by resonant magnetic perturbations in the high-performance Q = 10 ITER baseline scenario. The unique computational efficiency of the code is exploited to calculate the impact of the application of ITER's edge-localised mode (ELM) control coil system on neutral beam heating efficiency, as well as producing detailed predictions of the resulting plasma-facing component power loads, for a variety of operational parameters—the applied fundamental toroidal mode number n 0, mode spectrum and absolute toroidal phase of the imposed perturbation. The feasibility of continually rotating the perturbations is assessed and shown to be effective at reducing the time-averaged power loads. Through careful adjustment of the relative phase of the applied perturbation in the three rows of coils, peak power loads are found to correlate with reductions in neutral beam injection (NBI) heating efficiency for n 0 = 3 fields. Adjusting the phase this way can increase total NBI system efficiency by approximately 2%–3% and reduce peak power loads by up to 0.43 MW m−2. From the point of view of fast-ion confinement, n 0 = 3 ELM control fields are preferred overall to n 0 = 4 fields. In addition, the implementation of 3D magnetic fields in LOCUST is also verified by comparison with the SPIRAL code for a DIII-D discharge with ITER-similar shaping and n 0 = 3 perturbation.

Resistive wall mode stability and resonant field amplification in MAST high beta plasma

The n = 1 (n is the toroidal mode number) resistive wall mode (RWM) instability and the resonant field amplification (RFA) due to a stable RWM response are numerically investigated for an MAST high-pressure plasma scenario, utilizing the MARS-F Liu et al (2000 Phys. Plasmas 7 3681) and MARS-K Liu et al (2008 Phys. Plasmas 15 112503) codes, for the purpose of understanding the potential RWM behavior in MAST-U plasmas. Ideal kink stability analysis identifies a target plasma, with parameters similar to that of the reference equilibrium reconstructed from an MAST high-pressure discharge, that accesses the RWM regime. The unstable n = 1 RWM for the target equilibrium is subject to strong damping by the plasma toroidal flow and/or the drift kinetic effects from thermal particles. As a result, the mode is found to be stable under the experimental flow conditions, or even without flow stabilization if drift kinetic stabilization is included. The stability prediction is robust against variation of the assumed resistive wall minor radius. Active magneto-hydrodynamic (MHD) spectroscopy modeling, using the magnetic coils designed for controlling the edge localized modes (ELMs) in MAST-U as the antenna, shows strong RFA due to a stable RWM response in the target plasma. Maximal amplification, of 6–7 times larger than the applied vacuum field within the plasma, is obtained assuming a coil phasing of 120–180 degrees between the upper and lower rows of the ELM control coils. The MHD-kinetic hybrid model predicts generally higher RFA than the fluid model, in particular near the low-field and high-field sides of the plasma boundary surface. The MARS-F/K modeling thus shows that the RWM in MAST-U is subject to strong flow and kinetic stabilization, but can nevertheless be detected by active MHD spectroscopy.

Predicting operational windows of ELMs suppression by resonant magnetic perturbations in the DIII-D and KSTAR tokamaks

A newly developed plasma response model, combining the nonlinear two-fluid MHD code TM1 and toroidal MHD code GPEC run in ideal mode, quantitatively predicts the narrow isolated q95 windows (Δq95 ∼ 0.1) of edge-localized mode (ELM) suppression by n = 1, 2, and 3 resonant magnetic perturbations (RMPs) in both DIII-D and KSTAR tokamaks across a wide range of plasma parameters. The key physics that unites both experimental observations and our simulations is the close alignment of essential resonant q-surfaces and the location of the top of the pedestal prior to an ELM. This alignment permits an applied RMP to produce field penetration due to the lower E × B rotation at the pedestal top rather than being screened. The model successfully predicts that narrow magnetic islands form when resonant field penetration occurs at the top of pedestal, and these islands are easily screened when q95 moves off resonance, leading to very narrow windows of ELM suppression (typically Δq95 ∼ 0.1). Furthermore, the observed reduction in the pedestal height is also well captured by the calculated classical collisional transport across the island. We recover observed q95, βN and plasma shape dependence of ELM suppression due to the effect of magnetic islands on pedestal transport and peeling-ballooning-mode stability. Importantly, experiments do occasionally observe wide windows of ELM suppression (Δq95 > 0.5). Our model reveals that at low pedestal-top density multiple islands open, leading to wide operational windows of ELM suppression consistent with experiment. The model indicates that wide q95 windows of ELM suppression can be achieved at substantially higher pedestal pressure with less confinement degradation in DIII-D by operating at higher toroidal mode number (n = 4) RMPs. This can have significant implications for the operation of the ITER ELM control coils for maintaining high confinement together with ELM suppression.

ELM control optimization for various ITER scenarios based on linear and quasi-linear figures of merit

For the purpose of better understanding type-I edge localized mode (ELM) control in ITER with resonant magnetic perturbation (RMP) fields, the plasma response to RMP is computed by a resistive full magneto-hydrodynamic model in toroidal geometry. Five scenarios designed for ITER are considered, ranging from the pre-nuclear to nuclear phases. The plasma response to RMP is quantified by the plasma surface displacement near the X-point of the divertor plasma and at the outboard mid-plane. The optimal coil configurations between two high-Q deuterium-tritium (DT) scenarios (at the same plasma current of 15 MA and the same magnetic field of 5.3 T but different fusion gains, Q = 5 and 10) are predicted to be similar. For the other ITER scenarios with similar edge safety factor q95 ∼ 3 to that of the baseline scenario, the optimal coil phasing is also similar. The optimization results are different for a half-current full-field (7.5 MA/5.3 T) scenario, largely due to the difference in q95. The RMP coil currents are also optimized to tailor the core vs edge toroidal torques exerted by the 3D RMP fields on the plasma column. Torque optimization, with various objective functions proposed in the study, is useful for minimizing the side effects of RMP on the plasma core flow in ITER, while still maintaining the ELM control capability. Full utilization of three rows of ELM control coils in ITER is found to be essential to ensure both flexibility and robustness of ELM control, in terms of both linear and quasi-linear plasma responses.

Toroidal modeling of resonant magnetic perturbations in preparation for the initial phase of ITER operation

A systematic and comprehensive numerical investigation is carried out in order to understand linear and quasi-linear effects of the applied resonant magnetic perturbations (RMPs) on the plasma, during the initial phase of ITER operation at 5 MA plasma current and 1.8 T toroidal field, assuming a limited number of available power supplies (PSs) for controlling the type-I edge localized modes (ELMs) and taking into account uncertainties associated with predicting the plasma toroidal rotation in ITER. Utilizing the plasma surface displacement near the X-point as a proxy for the ELM control capability by RMPs, the coil phasing optimization finds optima that are not sensitive to the variation of the assumed plasma toroidal flow, nor to the choice of toroidal waveform of the coil current distribution. Several assumptions regarding the number of PSs available in this initial phase have been explored, comprising 9 and 13 for current waveforms with n = 3 symmetry: using the equatorial coil row alone, upper and lower rows of coils and the three rows of ELM control coils. The resulting effects on ELM control (through the X-point displacement proxy) and core and edge torques have been evaluated. Optimization and comparison for the three ELM control coil configurations—all three rows, upper and lower rows, and equatorial row—reveal a strong coupling between the core and edge plasma response—both linear and quasi-linear—for these ITER plasma conditions. The equatorial ELM control coil row has the strongest effects on core and edge plasma response, but provides no flexibility in its optimization; this can only be achieved by optimizing the current waveform with the upper and lower rows. A quantitative comparison of the achievable optimization of ELM control and core plasma response with the three configurations of coils has been performed, assuming that nine PSs will be available in the initial ITER operation.

Modeling 3D plasma boundary corrugation and tailoring toroidal torque profiles with resonant magnetic perturbation fields in ITER

Plasma response to 3D resonant magnetic perturbations (RMPs), applied for the purpose of controlling type-I edge localized modes (ELMs) in ITER with the baseline ELM control coils, is computed using a toroidal, resistive, full magneto-hydrodynamic model. Considered are five representative ITER plasmas, designed for different phases of the ITER exploration. The plasma response, measured by the plasma boundary corrugation, is found to be similar for the two DT scenarios at full plasma current (15 MA) and full toroidal field (5.3 T) but different fusion gain factors (Q = 5 versus Q = 10), indicating similar ELM control performance with the same RMP coil current configuration. The other plasma scenarios, with proportionally scaled down plasma current and toroidal field, can have different plasma boundary corrugation. The key plasma parameter affecting the response is the plasma toroidal flow near the pedestal region, which significantly varies depending on the transport model assumption for the toroidal momentum. Lower pedestal flow leads to a stronger edge peeling response from the plasma and thus probably a better ELM control. The optimal coil configuration for controlling type-I ELMs is similar for all four ITER plasmas with similar safety factor but different current levels, but is significantly different for the case at half plasma current (7.5 MA) and full field (5.3 T). On the other hand, for the purpose of controlling the radial profile of the plasma toroidal rotation in ITER using 3D fields, the relative amplitude of the toroidal torque density, between the plasma core and edge region, is optimized. Generally, a strong coupling between the core and edge torques is observed, largely due to the middle row ELM control coils. The best decoupling scheme of the core-edge torque distribution thus de-emphasizes the role of the middle row coils. Optimal coil current configurations are found for the ITER 15 MA/5.3 T Q = 10 plasma, that synergistically maximize the plasma edge-peeling response (indication for good ELM control) and the toroidal torque near the plasma edge (good for RMP field penetration through pedestal).

Mechanics of ELM control coil induced fast particle transport in ITER

Using the orbit-following code ASCOT, we have modelled fast ion transport in ITER under the influence of ELM control coils (ECCs), toroidal field ripple, and test blanket modules, with emphasis on how the plasma response (PR) modifies the transport mechanisms and fast ion loads on the divertor. We found that while PR shields the plasma by healing broken flux surfaces at the plasma periphery, it also opens a new loss channel for marginally trapped particles: PR causes strong toroidal variation of the poloidal field near the X-point which leads to de-localisation of banana tips and collisionless transport. The reduction in passing particle losses and the increase in marginally trapped particle losses shift divertor loads from targets to the dome and under-the-dome structures. The plasma response was calculated by both MARS-F and JOREK codes. The new transport mechanism was stronger for PR calculated by JOREK which, unlike MARS-F, explicitly includes the X-point.

Development, optimization and validation of ultrasonic testing for NDE of ELM coils

Institute for Plasma Research (IPR) has been developing technologies appropriate towards realizing fusion relevant Edge Localized Mode (ELM) magnets in Tokamaks such as Joint European Torus (JET) and Steady State Superconducting Tokamak (SST-1). The winding pack of ELM coils have been encased inside the Inconel 625 casing using customized double groove and single bevel weld joints. It is critical to ensure soundness of these weld joints. With this motivation, Non-Destructive Evaluation (NDE) method using ultrasonic technique has been developed and established for inspection of weld joints in ELM control coils. The development of ultrasonic test procedure, test parameters, test validation and optimization using simulation as well as experimental approach has been discussed in this paper. The selection and locations probes along with beams parameters for weld joints have been optimized with commercially available software CIVA- versatile simulation tool developed by CEA (French Atomic Energy Commission), France. Further, the developed ultrasonic test method has been established and validated on mockup box sample of casing for ELM coils. It has been again successfully demonstrated for its performance on 1:1 demo coil equivalent to small ELM coil.

Transient thermo-structural and static magnetic characteristics of 1:1 prototype JET ELM control coils

3D transient thermo-structural analyses and steady state magnetic field analyses of 1:1 prototyped JET Edge Localized Mode (ELM) coils have been carried out. Temperature distribution within the magnet winding as well as the temperature evolution have also been simulated as a function of pulsed transport currents in both large and small ELM coils as per the operational scenarios. The induced thermal stresses along with the shear stress components acting on the winding elements have also been analyzed. The deformations caused by thermal stresses have been calculated for the case, the conductor bundle and the insulation layers within the coils. In addition to thermo-structural analyses, steady state magnetic field analyses have also been carried out in the current carrying ELM coils. These values have been compared with the experimental values. The experimentally obtained values matches well with those obtained in simulations indicating that the prototyped ELM coils can operate successfully in JET operational scenarios. Additionally, the R & D and technologies developed in the context of JET ELM coils have also been validated with the magnet performances experimentally.

Electrical testing and performance evaluation of 1:1 prototype JET ELM control coils

Magnet Technology Development Division at Institute for Plasma research is engaged in extensive R & D for appropriate technologies towards manufacturing of Edge Localized Mode (ELM) magnets for Large Tokamak such as Joint European Torus (JET) as well as for Steady State Superconducting Tokamak (SST-1). Under this project, manufacturing of 1:1 prototype of Large and Small JET ELM control coils (CC) is completed incorporating indigenously developed manufacturing and insulation technologies. Performance evaluation of both the types of coil has been completed for its current carrying capability and insulation resistance as required by various operational scenarios of JET. Experimental setups, test procedures and measurements for electrical characterization of both type of ELM control coil has been discussed in this paper.

Protecting ITER walls: fast ion power loads in 3D magnetic field

The fusion alpha and beam ion with steady-state power loads in all four main operating scenarios of ITER have been evaluated by the ASCOT code. For this purpose, high-fidelity magnetic backgrounds were reconstructed, taking into account even the internal structure of the ferritic inserts and tritium breeding modules (TBM). The beam ions were found to be almost perfectly confined in all scenarios, and only the so-called hybrid scenario featured alpha loads reaching 0.5 MW due to its more triangular plasma. The TBMs were not found to jeopardize the alpha confinement, nor cause any hot spots. Including plasma response did not bring dramatic changes to the load. The ELM control coils (ECC) were simulated in the baseline scenario and found to seriously deteriorate even the beam confinement. However, the edge perturbation in this case is so large that the sources have to be re-evaluated with plasma profiles that take into account the ECC perturbation.

Technology Developments Toward ELM Coil Manufacturing Appropriate for Tokamak

The Institute for Plasma Research (IPR) has been developing technologies appropriate toward realizing fusion-relevant edge-localized mode (ELM) magnets in Tokamaks, such as suitable for Joint European Torus (JET) and Steady State Superconducting Tokamak (SST-1). The development of characteristic winding topologies, developing an appropriate “special-purpose winding facility,” developing a high-temperature resin insulation system, developing vacuum pressure impregnation of high-temperature resin systems in a manufactured winding pack, developing profiled-case manufacturing technologies, and developing technologies at consolidating and encasing the impregnated winding packs are some of the essential aspects of realizing an ELM control coil. In these contexts, under an IN-EU joint initiative, the Magnet Technology Development Division at IPR has developed 1:1 prototypes of JET ELM control coils. These technologies would also be extended to the SST-1 ELM coils. This paper would elaborate on some of the salient technologies developed in support of JET ELM prototype coil realization.

Design and Fabrication of a Special Purpose Winding Machine for ELM Control Coils of JET

Under the India-European Union collaboration, the Institute for Plasma Research had successfully completed an engineering initiative aimed at developing a “special purpose winding machine” (SPWM) appropriate for winding 1:1 prototype edge-localized mode (ELM) coils of the Joint European Torus (JET). The typical 1:1 prototype ELM coil winding pack is composed of eight layers of 6.35-mm square cross-section CuCrZr conductors being wound in a saddle configuration having eight turns per layer with strict tolerance requirements. Such an SPWM has been designed and developed with Indian industry complying with the complex shape and tolerance requirements of the ELM coils. The SPWM developed is primarily composed of a feeding conductor spool with appropriate tensioning mechanisms, straightening and shaping units, cleaning unit, insulation unit, press forming units and rotating turn table mandrel, and rotation and synchronization arrangements with electrical control and feedback elements. The SWPM has been successfully demonstrated with winding for both types of 1:1 prototype ELM control coils of JET within a tolerance of ±3 mm. The design consideration for SPWM, critical subsystems of SPWM and their operation principle, winding methodology, and application articulated arm coordinate measuring machine for inspection of winding pack have been discussed in this paper.

Toroidal modelling of RMP response in ASDEX Upgrade: coil phase scan, q95 dependence, and toroidal torques

The plasma response to the vacuum resonant magnetic perturbation (RMP) fields, produced by the ELM control coils in ASDEX Upgrade experiments, is computationally modelled using the MARS-F/K codes (Liu et al 2000 Phys. Plasmas 7 3681, Liu et al 2008 Phys. Plasmas 15 112503). A systematic investigation is carried out, considering various plasma and coil configurations as in the ELM control experiments. The low q plasmas, with (q95 is the safety factor q value at 95% of the equilibrium poloidal flux), responding to low n (n is the toroidal mode number) field perturbations from each single row of the ELM coils, generates a core kink amplification effect. Combining two rows, with different toroidal phasing, thus leads to either cancellation or reinforcement of the core kink response, which in turn determines the poloidal location of the peak plasma surface displacement. The core kink response is typically weak for the n = 4 coil configuration at low q, and for the n = 2 configuration but only at high q (). A phase shift of around 60 degrees for low q plasmas, and around 90 degrees for high q plasmas, is found in the coil phasing, between the plasma response field and the vacuum RMP field, that maximizes the edge resonant field component. This leads to an optimal coil phasing of about 100 (−100) degrees for low (high) q plasmas, that maximizes both the edge resonant field component and the plasma surface displacement near the X-point of the separatrix. This optimal phasing closely corresponds to the best ELM mitigation observed in experiments. A strong parallel sound wave damping moderately reduces the core kink response but has minor effect on the edge peeling response. For low q plasmas, modelling shows that both the resonant electromagnetic torque and the neoclassical toroidal viscous (NTV) torque (due to the presence of 3D magnetic field perturbations) contribute to the toroidal flow damping, in particular near the plasma edge region. For high q plasmas, however, significant amount of torque is also produced in the bulk plasma region, and the contributions from the electromagnetic, the NTV, and the torque associated with the Reynolds stress, all play significant roles.

Effect of plasma response on the fast ion losses due to ELM control coils in ITER

Mitigating edge localized modes (ELMs) with resonant magnetic perturbations (RMPs) can increase energetic particle losses and resulting wall loads, which have previously been studied in the vacuum approximation. This paper presents recent results of fusion alpha and NBI ion losses in the ITER baseline scenario modelled with the Monte Carlo orbit following code ASCOT in a realistic magnetic field including the effect of the plasma response. The response was found to reduce alpha particle losses but increase NBI losses, with up to 4.2% of the injected power being lost. Additionally, some of the load in the divertor was found to be shifted away from the target plates toward the divertor dome.

Modelling toroidal rotation damping in ITER due to external 3D fields

The linear and quasi-linear plasma response to the n = 3 and n = 4 (n is the toroidal mode number) resonant magnetic perturbation (RMP) fields, produced by the in-vessel edge localized mode control coils, is numerically studied for an ITER 15 MA H-mode baseline scenario. Both single fluid and fluid-kinetic hybrid models are used. The inclusion of drift kinetic effects does not strongly alter the plasma response compared to the fluid approximation for this ITER plasma. The full toroidal drift kinetic model is also used to compute the neoclassical toroidal viscous (NTV) torque, yielding results close to that of an analytic model based on geometric simplifications. The computed NTV torque from low-n RMP fields is generally smaller than the resonant electromagnetic torque for this ITER plasma. The linear response computations show a weak core kink response, contrary to a strong kink response often computed for plasmas from present day tokamak devices. Initial value quasi-linear simulations, including various torque models, show a localized damping of the plasma toroidal flow near the edge, as a result of the applied RMP fields. This localized rotation damping can be weak or strong depending on whether a weakly unstable edge localized peeling mode is present. No qualitative difference is found between the n = 3 and n = 4 RMP field configurations, in both the linear and non-linear response results.

ITER edge-localized modes control coils: the effect on fast ion losses and edge confinement properties

The magnetic perturbations due to in-vessel coils, foreseen to mitigate edge-localized modes (ELMs) in ITER, could also compromise the confinement of energetic ions. We simulate the losses of fusion alpha particles and neutral beam injection-generated fast ions in ITER under the influence of the 3D perturbations caused by toroidal field coils, ferritic inserts, test blanket modules and ELM control coils (ECCs) with the ASCOT code. The ECCs are found to stochastize the magnetic field deep inside the pedestal in the 15 MA inductive reference operating scenario. Such a field is found insufficient to confine not only the fast but also the thermal ion population, leading to a strongly reduced fast ion source in the edge. Therefore, even with a stochastic edge, no high fast ion power loads are expected. However, the plasma response has not yet been included in the calculation of ITER magnetic background data, and it is probable that the perturbation is currently overestimated.

Analysis of edge magnetic field line structure in ITER due to in-vessel ELM control coils

In this work we evaluated the ITER ELM coils design based on two metrics: the Chirikov vacuum magnetic island overlap parameter, and the vacuum Field Line Loss Fraction. The study was performed for a range of current amplitudes for three different n=4 waveforms: square, cosine and sine. The results indicated that ITER ELM coils are designed with a high level of flexibility to accommodate different operation scenarios (H-mode and Steady State) with different values of q95 and q-profiles. The magnetic island overlap analysis showed that ITER ELM coils are capable of matching the DIII-D I-coil spectrum. The Field Line Loss analysis showed that edge vacuum stochastization might be achieved that is similar or greater than in DIII-D. Fault analysis of the coils indicated that ITER ELM coils are robust and show good characteristics even with 11% of dead coils.

The limits and challenges of error field correction for ITER

Significant progress has been made in interpreting the effects of non-axisymmetric “error” fields on a plasma through ideal MHD stability and a dominant “least stable” ideal mode through which the fields couple to the tearing resonant surface. However, in contrast to expectations from such theories, experiments have found limited success in correcting error fields, with single correction coil arrays giving benefits of between 0% and ∼50% correction (in terms of improvement to a low density locked mode limit), dependent on the structure of the error and correcting fields. With additional coils up to ∼70% is possible. It was unclear whether this represented an intrinsic stability or control limit, or higher order toroidal or poloidal harmonic effects. Thus, studies on the DIII-D tokamak explored correction of a proxy error field, using two differently structured coil arrays. This enabled the principles of error correction to be tested at high amplitudes and operational densities, with known pure n=1 fields. Results showed substantial residual effects from the corrected n=1 field, with improvements of only ∼50% in the low density locked mode limit. This suggests that n=1 error fields must couple to more than one surface in the plasma, and this is conjectured to be through more than one ideal mode, thereby requiring precise correction. For ITER, updated predictions of field error have been obtained and compared with revised scalings for tearing mode thresholds, indicating 50% or better error field correction will be needed. This will likely require more than one well coupled correction coil array and sets a challenge for theory to model the behavior, in order to clarify the plasma response and braking mechanisms, and so the effectiveness of ITER's correction coils and the possible need for support from its edge localized mode control coils.