Resonant Magnetic Perturbation Coils Research Articles

Non-linear MHD modelling of edge localized modes suppression by resonant magnetic perturbations in ITER

Edge localized modes (ELMs) suppression by resonant magnetic perturbations (RMPs) was studied with the non-linear magneto-hydro-dynamic (MHD) code JOREK for the ITER H-mode scenarios at 15 MA, 12.5 MA, 10 MA/5.3 T. The main aim of this work was to demonstrate that ELMs can be suppressed by RMPs while the divertor 3D footprints of heat and particle fluxes remain within divertor material limits. The unstable peeling–ballooning modes responsible for ELMs without RMPs were modelled first for each scenario using numerically accessible parameters for ITER. Then the stabilization of ELMs by RMPs was modelled with the same parameters. RMP spectra, optimized by the linear MHD MARS-F code, with main toroidal harmonics N = 2, N = 3, N = 4 have been used as boundary conditions of the computational domain of JOREK, including realistic RMP coils, main plasma, scrape off layer (SOL) divertor and realistic first wall. The model includes all relevant plasma flows: toroidal rotation, two fluid diamagnetic effects and neoclassical poloidal friction. With RMPs, the main toroidal harmonic and the non-linearly coupled harmonics remain dominant at the plasma edge, producing saturated modes and a continuous MHD turbulent transport thereby avoiding ELM crashes in all scenarios considered here. The threshold for ELM suppression was found at a maximum RMP coils current of 45 kAt–60 kAt compared to the coils maximum capability of 90 kAt. In the high beta poloidal steady-state 10 MA/5.3 T scenario, a rotating QH-mode without ELMs was observed even without RMPs. In this scenario with RMPs N = 3, N = 4 at 20 kAt maximum current in RMP coils, similar QH-mode behaviour was observed however with dominant edge harmonic corresponding to the main toroidal number of RMPs. The present MHD modelling was limited in time by few tens of ms after RMPs were switched on until the magnetic energy of the modes saturates. As a consequence the thermal energy was still evolving on this time scale, far from the ITER confinement time scale and hence only the form of 3D footprints on the divertor targets can be indicated within this set-up. Also note, that the divertor physics was missing in this model, so realistic values of fluxes are out of reach in this modelling. However the stationary 3D divertor and particle fluxes could be simply extrapolated from these results to the stationary situation considering that a large power fraction should be radiated in the core and SOL and only about 50 MW power is going to the divertor, which is an arbitrary, but reasonable number used here. The 3D footprints with RMPs show the characteristic splitting with the main RMP toroidal symmetry. The maximum radial extension of the footprints typically was ∼20 cm in inner divertor and ∼40 cm in outer divertor with stationary heat fluxes decreasing further out from the initial strike point from ∼5 MW m−2 to ∼1 MW m−2 assuming a total power in the divertor and walls is 50 MW. The heat fluxes remain within the divertor target and baffle areas, however with rather small margin in the outer divertor which could be an issue for the first wall especially in transient regimes when part of the plasma thermal energy is released due to switching on the RMP coils. This fact should be considered when RMPs are applied with a more favorable application before or soon after the L–H transition, although optimization is required to avoid increasing the L–H power threshold with RMPs.

Experimental investigation of electromagnetic GAMs under the influence of 3D magnetic topological structure in EAST

The relationship between geodesic acoustic modes (GAMs) and tearing modes (TMs), or locked modes (LMs), have been investigated in EAST. Firstly, multi-branch electromagnetic GAMs are observed synchronously after the excitation of TMs. Broadened radial coverage is observed for these branches, and these frequencies can be described by the dispersion relation of GAM. Secondly, multi-branch electromagnetic GAMs are generated after the excitation of LMs. The toroidal asymmetric structure of GAMs is excited due to the existence of 3D intrinsic error fields, and the radial distribution of those branches is altered accordingly. Four branches of electromagnetic GAMs are observed in the region that is closer to the ‘X’-point of the locked magnetic islands, which should be merged into two branches for the region that is closer to the ‘O’-point, as confirmed by XUV array measurement. Thirdly, a similar phenomenon is reproduced by the modulation of resonant magnetic perturbation (RMP). The frequency of GAM increases with the current of the RMP coils, and multi-branches of similar modes are generated when the current achieves kA, where the electrostatic and electromagnetic components are altered accordingly. The features of those branches are modulated by the rotation of the RMP. Therefore, the structures of the pair of GAMs can be different, which should be influenced by the different magnetic topological structure.

Design and experimental demonstration of feedback adaptive RMP ELM controller toward complete long pulse ELM suppression on KSTAR

Operation of a fusion power plant requires robust edge localized mode (ELM) suppression simultaneously with high plasma performance. In this paper, we describe a novel feedback adaptive resonant magnetic perturbation (RMP) ELM controller designed to address this problem by achieving optimized ELM suppression through the advanced application of 3D RMPs. From real-time Dα data, the controller is able to achieve robust ELM suppression while simultaneously minimizing the applied RMP in order to enhance plasma performance. In real-time, the instantaneous ELM-frequency is analyzed with an adaptive feedback algorithm to determine amplitudes and phases of RMP coil currents that will maximize plasma performance while maintaining ELM suppression. When applied through the KSTAR plasma control system in several experiments using n = 1 RMPs, robust ELM suppression is achieved and sustained in feedback while reducing the RMP strength to ∼65% of its initial value. Minimization of the RMP strength in this manner not only allows for operation of longer discharges due to a decrease in flux consumption but also allows for a strong recovery of up to ∼60% of βN throughout the ELM-free period.

Physical design of a new set of high poloidal mode number coils in the EAST tokamak

An external resonant magnetic perturbation (RMP) field, which is an effective method to mitigate or suppress the edge localized mode (ELM), has been planned to be applied on the ELM control issue in ITER. A new set of magnetic perturbation coils, named as high m coils, has been developed for the EAST tokamak. The magnetic perturbation field of the high m coils is localized in the midplane of the low field side, with the spectral characteristic of high m and wide n, where m and n are the poloidal and toroidal mode numbers, respectively. The high m coils generate a strong localized perturbation field. Edge magnetic topology under the application of high m coils should have either a small or no stochastic region. With the combination of the high m coils and the current RMP coils in the EAST, flexible working scenarios of the magnetic perturbation field are available, which is beneficial for ELM control exploration on EAST. Numerical simulations have been carried out to characterize the high m coil system, including the magnetic spectrum and magnetic topology, which shows a great flexibility of magnetic perturbation variation as a tool to investigate the interaction between ELM and external magnetic perturbation.

Influence of toroidal rotation on plasma response to external RMP fields in tokamak

The type-I edge localized mode (ELM) is a critical event associated with magneto-hydrodynamic(MHD) instabilities occurring in tokamak high-confinement (H-mode) discharges, that leads to huge heat loads on the plasma phasing components (PFC) and may result in material damages. It is important to effectively control large ELMs, in order to ensure safe operation of the future reactor-scale devices such as ITER and DEMO. Resonant magnetic perturbation (RMP) has been experimentally demonstrated to be a mature and robust technique for controlling ELMs. A set of parameters, such as the edge safety factor, the plasma flow, the RMP coil geometry and the spectrum of the applied external field, have been found to play important roles in controlling ELMs by RMP. Furthermore, the plasma pressure is known to affect the plasma response to the RMP field, in particular near the no-wall beta limit. This is because high plasma pressure drives the resonant field amplification of the external field by the plasma response. The ITER 10 MA steady state scenario will be operated near the no-wall stability limit. The new tokamak device HL-2M will also operate in the relatively high-beta regimes. On the other hand, more investigations are still needed to understand the influence of toroidal flow on the high-beta plasma response. This work employs a single fluid toroidal model to compute the plasma RMP response in HL-2M, emphasizing on the roles of two key physical quantities: the plasma resistivity and the toroidal rotation. The former allows penetration of the external RMP field into the plasma, while the latter mainly provides screening effect on the resonant field component. More specifically, the MARS-F code is utilized to study the plasma response to the externally applied <i>n</i> =1 ( n is the toroidal mode number) RMP field for high-beta HL-2M discharges, while varying the plasma toroidal rotation profile. The plasma response is found to (i) substantially modify the poloidal spectrum of the applied vacuum RMP field, (ii) change the amplitude of the resonant radial field amplitude near the plasma edge, and (iii) affect optimal current phasing between the two rows of RMP coils on HL-2M. A sufficiently slow toroidal flow near the plasma edge amplifies the radial field at rational surfaces associated with the perturbation. Since the latter serves as a reliable indicator for controlling the type-I edge localized mode (Type-I ELM) by RMP, varying rotation profile near the plasma edge offers a promising approach to optimize ELM control.

Effects of resonant magnetic perturbations on neutral beam heating in a tokamak

The effects of resonant magnetic perturbations (RMPs) on tangential neutral beam heating in the EAST tokamak are studied numerically. RMPs with linear resistive magnetohydrodynamics response are used in the modeling. A variety of representing configurations of RMP coil currents are examined, and their effects on the neutral beam injection (NBI) heating efficiency are compared, in order to find a parameter window where deleterious effects of RMPs on NBI heating efficiency are minimized. It is found that the internal redistribution of fast ions by RMPs induces local accumulation of fast ions, resulting in higher local fast ion pressure than the case without RMPs. It is also found that the toroidal phasing of the RMP with respect to the fast ion source has slight effects on the steady-state radial profile of fast ions. The dependence of fast ion loss fraction on the RMP up-down phase difference shows a similar behavior as the dependence of the radial width of chaotic magnetic field on the phase difference. A statistical method of identifying resonances between RMPs and lost fast ions is proposed, and the results indicate that some resonances between RMPs and lost passing particles may be of non-integer fractional order, rather than the usual integer order.

Nonlinear two-fluid modeling of plasma response to RMPs for the ELM control in the ITER baseline

Numerical modeling, combining the toroidal ideal MHD code GPEC and the nonlinear two-fluid MHD code TM1, was used for comprehensive studies of the plasma response to resonant magnetic perturbations (RMPs) with toroidal mode number n = 1–5 for controlling edge-localized modes (ELMs) in ITER for the standard operation scenario (15 MA Q = 10). Several issues related to RMP ELM control are investigated, including the optimization of the RMP coils configuration, the evaluation of the magnitude of density pump-out and the q 95 windows of ELM suppression. GPEC calculates the magnetic response, which consistently includes the very important edge kink/peeling response to static magnetic perturbations. GPEC two-dimensional scans of the relative coil current phasing among the three rows of internal coils, at fixed coil current amplitude, reveal the optimal phasing for the RMP coil configuration with n = 1–5, respectively. The poloidal half wavelength of resonant mode at the edge of plasma calculated by GPEC indicates that the midplane row coils have the best resonant coupling with the plasma for n = 2, while the upper and lower row coils have the best resonant coupling with the plasma for n = 3. Based on the plasma kinetic equilibrium and the GPEC calculations of the magnetic response, TM1 was used to simulate the conditions for RMP field penetration in the ITER pedestal. TM1 shows magnetic island formation at the foot of ITER pedestal with RMP coil current threshold ranging from 4 kAt to 8 kAt with n = 2 to 4. These magnetic islands at the pedestal-foot lead to density pump-out, the magnitude of which scales as and ranges from 5% to 20% at the pedestal-top when scanning the coil current from 4 to 60 kAt. The density pump-out is found to be weaker for higher n RMP. The nonlinear TM1 simulations also show field penetration at the pedestal-top, where the threshold of RMP coil current depends on the q 95. The alignment of the magnetic island and the location of the pedestal-top decreases the height and width of the pedestal to suppress ELMs. Simulations by two-dimensional scans of RMP coil current and q 95 reveal the accessible q 95 windows of ELM suppression for both n = 3 and 4 RMPs. The predicted q 95 windows of ELM suppression are very similar to the ones in currently operating tokamaks and the required RMP coil current for ELM suppression is less than 40–50 kAt, which is well within the designed capability for ITER. In addition, the simulations indicate that wide q 95 windows of ELM suppression may be accessible in ITER by operating with dominant n = 4 (or n = 5) RMPs.

Modeling of q95 windows for the suppression of edge localized modes by resonant magnetic perturbations in the KSTAR tokamak

The EPEC code is employed to model the q95 windows for n = 2 and n = 1 resonant magnetic perturbation (RMP)-induced edge localized mode (ELM) suppression in typical KSTAR H-mode discharges. The plasma equilibria used in the study are derived by rescaling the experimental plasma equilibrium in KSTAR discharge #18594 measured at time t = 6450 ms. The n = 2 and n = 1 ELM suppression windows predicted by EPEC are comparatively narrow in q95 (i.e., Δq95≃0.1) and are such that (assuming a relative pedestal pressure reduction of 15% is required to trigger ELM suppression), even in the middle of the windows, there is a threshold RMP coil current of about 1–3 kA/turn that must be exceeded before ELM suppression occurs. The n = 2 and n = 1 ELM suppression windows calculated by the EPEC code are consistent with the experimental observations and are also broadly similar to those recently calculated by the TM1 code [Hu et al., Phys. Plasmas 28, 052505 (2021)].

The impact of ELM mitigation on tungsten source in the EAST divertor

Divertor tungsten (W) erosion source during edge localized mode (ELM) bursts in EAST is investigated based on optical emission spectroscopy on atomic neutral tungsten line emission at 400.9 nm. Both temporal evolution and total source strength are analyzed and compared in three different ELM mitigation schemes including natural ELMs, resonant magnetic perturbations (RMP) and 2.45 GHz lower hybrid wave (LHW). The mitigation of W source caused by single ELM impact at the divertor target is mainly attributed to the reduction of the W source production in the ELM decay phase. However, with the increase of ELM frequency, the time-averaged intra-ELM W source during an ELM cycle rises and dominates the evolution of the total W source with increasing fraction, which exhibits strong correlation with the core W level in the RMP scheme. In the natural ELM scheme, the pedestal electron temperature is found not only to control the effective W sputtering yield at target during ELMs, thus influencing the intra-ELM W source, but also to be related to the delay time between the divertor ELM WI emission and the core extreme ultraviolet bolometer (XUV) signal which can scale with the parallel ion transit time. Furthermore, the delay time is found to have more consistent dependence on the pedestal plasma collisionality. The rise time of intra-ELM WI emission increases when RMP or LHW is applied, revealing a different ELM mitigation mechanism in comparison with the natural ELM scheme. The temporal profiles of the intra-ELM WI emission in different ELM mitigation schemes are compared and the potential mechanism is discussed. Besides, the ELM mitigation effects with RMP are found to be asymmetric at the outer and inner divertor targets, which is characterized by the different variations of the intra-ELM W source strength and the rise time of WI emission with the ramp-up of RMP coil current. Divertor partial detachment is achieved simultaneously with strong ELM mitigation under a suitable RMP phase difference without additional gas puff.

Development of a new dynamic foveated imager on wide-angle infra-red thermography system to improve local spatial resolution in EAST.

A new dynamic foveated imager has been developed and commissioned on the wide-angle infra-red thermography system in the Experimental Advanced Superconducting Tokamak. This technique improves the local spatial resolution by a factor of 2 while maintaining the wide-angle view to ensure safety operation. Taking advantage of the new foveated imager, heat flux splitting under the application of resonant magnetic perturbation (RMP) coils has been clearly observed. The results indicate that the toroidal asymmetric power load pattern is closely related to the perturbation field induced by the RMP coils.

Toroidal field and q95 scalings on error field penetration in EAST

Toroidal field and q95 scalings of error field penetration are investigated with n = 1 resonant magnetic perturbation coil in EAST. The toroidal field scalings of error field penetration thresholds under fixed q95 are about in both ohmically and lower hybrid wave heated plasmas, where br21 is the vacuum error field at the q = 2/1 rational surface and BT is the toroidal field. These scalings indicate a favorable tolerance on error field in ITER. To make clear the underlying physics on toroidal field scaling, the theoretical analysis is given. By subtituting penetration related scaling parameters into the theory, the obtained theoretical scalings are consistent with the experimental observations using the vacuum penetration thresholds. To further investigate penetration threshold in larger operation region, the q95 scaling on penetration threshold with has also been observed. The rational surface radius rs and magnetic shear s, which are crucial to q95 scaling, are included in the theoretical analysis. The theoretical analysis is also consistent with the experimental scalings using the vacuum penetration thresholds. Moreover, the obtained theoretical scalings are easy to compare with experimental scalings. These theoretical analyses will stimulate the extrapolation of error field tolerance towards future reactors.

Electromagnetic stress analysis of RMP coils during VDEs of the HL-2M tokamak

During tokamak vertical displacement events (VDEs), due to the rapid movement of plasma and rapidly decreasing current, induced currents appear in surrounding conductive components, including Resonant Magnetic Perturbation (RMP) coils. The electromagnetic load caused by the VDEs induced currents constitutes a threat for RMP coils as well as related components. Here the stress condition resulting from the aforementioned electromagnetic load in HL-2M, a large tokamak being assembled at Southwestern Institute of Physics of China, is analyzed using ANSYS. Engineered design of RMP coils in HL-2M is verified, and regions of stress concentration are noticed. To reduce analyzing complexity, a specific method aiming at calculating the transient electromagnetic load on local components around the plasma is proposed. Finite element analysis (FEA), application of Visual Basic Script (VBS) language in ANSYS Maxwell software, and data processing in MATLAB have been used in this analysis.

Numerical simulations of NBI fast ion loss with RMPs on the EAST tokamak

Fast ion losses in the presence of resonant magnetic perturbations (RMPs) on the EAST tokamak have been simulated by using the Monte Carlo orbit-following code GYCAVA (Xu et al 2019 Comput. Phys. Comm. 244 40). Fast ions produced by the co-current tangential neutral beam injection (NBI) were considered. The initial distribution of NBI fast ions was numerically computed by the TGCO code and the n = 1 vacuum and response RMP fields were numerically computed by the MHD code MARS-F (Liu et al 2000 Phys. Plasmas 7 3681). It is found that fast ion losses increase almost linearly with the RMP coil current. Poincaré plots show that fast ion losses in the presence of RMPs on EAST are mainly due to the orbit stochasticity induced by RMPs. The collision effect, including the slowing down and the pitch angle scattering, on fast ion losses have also been numerically studied. The synergy effect of RMPs and collision-induced pitch angle scattering can enhance the particle loss of fast ions. The collision effect, including the effective ion charge number effect, on the power loss of fast ions is weak. Fast ion losses increase with the effective ion charge number increasing, which is related to the pitch angle scattering effect. The plasma response to RMPs can enhance or reduce fast ion losses. The plasma response effect on fast ion losses depends on the RMP coil phasing, that is, the phase difference between the upper and lower coils. The heat loads on the outer wall clearly follow the n = 1 periodicity of the RMP coil configuration. Positions of maximum heat loads are closely related to the coil phasing of RMPs. These mean that the magnetic topology of RMPs is responsible for the toroidal distribution of heat loads.

ELM control based on modeling of plasma response to n = 2 and n = 3 resonant magnetic perturbation fields in DIII-D

Toroidal modeling of plasma response to the resonant magnetic perturbation (RMP) fields, with the n = 2 and 3 configurations (n is the toroidal mode number), is carried out for DIII-D discharge utilizing the MARS-F code. In particular, the relative toroidal phase (coil phase) between the lower and upper rows of the I-coil currents is scanned from −180° to 180°, while computing the plasma response. Both the resistive rotating and ideal static plasma response models have been employed to predict the optimum coil phase with typical safety factor (q95) values that are within the windows for suppressing the edge localized modes (ELMs). Six certain criteria, which are constructed from the MARS-F modeling, are proposed and tested to determine the optimum coil phase. For both plasma response models, it is found that the three criteria, namely, the resonant harmonic field including plasma response at the last rational surface, the maximum amplitude of the plasma displacement due to the peeling mode response, and the normal plasma surface displacement amplitude near the X point, can serve as appropriate indicators for determining the optimum coil phase. It is demonstrated that the even parity RMP coil configuration can better control the ELMs within the typical ELM suppression window in DIII-D tokamak.

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.

Test of the ITER-like resonant magnetic perturbation configurations for edge-localized mode crash suppression on KSTAR

KSTAR has demonstrated divertor heat flux broadening during edge-localized mode (ELM) crash suppression using ITER-like three-row resonant magnetic perturbation (RMP) for the first time. To address a couple of critical issues in ITER RMP, robust ELM-crash-suppression methodology has been explored at low q95 and established in KSTAR using low-n RMPs. Taking full advantage of the ITER-like three-row in-vessel control coils (IVCC) in KSTAR, a set of intentionally misaligned RMP configurations (IMC) was tested to investigate whether or not IMC could be compatible with ELM crash suppression, while minimizing electromagnetic loads on RMP coils. As a result, the ITER-like three-row IMCs were found not only to have been compatible with the ELM crash suppression, but also to have broadened the divertor heat fluxes in the vicinity of the outer strike point. In comparison, the two-row RMPs have rarely affected the near scrape-off layer (SOL) heat flux despite slightly broadened profile change in the far-SOL. Since the divertor heat flux broadening reflects the dispersal of the peaked near-SOL heat flux, the experimental outcome is quite favorable to the ITER choice of three rows, instead of two rows. Nonetheless, since the IMC-driven broadening observed in the attached plasmas in KSTAR might appear substantially different in the partially detached plasmas in ITER, additional investigation has been conducted to see if RMP-driven, ELM crash suppression could be compatible with detached plasmas. Although no detached plasmas have been identified with ELM crash suppression yet, significantly reduced divertor heat flux was confirmed in high density, ELM-crash-suppressed plasmas at q95 = 3.4 using n = 2 RMPs. These new findings elevate the confidence level about the ITER RMP system, while the remaining uncertainties need to be further clarified using the three-row IVCCs in KSTAR.

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).

Overview of KSTAR research progress and future plans toward ITER and K-DEMO

A decade-long operation of the Korean Superconducting Tokamak Advanced Research (KSTAR) has contributed significantly to the operation of superconducting tokamak devices and the advancement of tokamak physics which will be beneficial for the ITER and K-DEMO programs. Even with limited heating capability, various conventional as well as new operating regimes have been explored and have achieved improved performance. As examples, a long pulse high-confinement mode operation with and without an edge-localized mode (ELM) crash was well over 70 and 30 s, respectively. The unique capabilities of KSTAR allowed it to improve the capability of controlling harmful instabilities, and they have been instrumental in uncovering much new physics. The highlights are that the L/H transition threshold power is sensitive to the resonant magnetic perturbation (RMP) and insensitive to non-resonant magnetic perturbation. Co-Ip offset rotation dominated by an electron channel predicted by general neoclassical toroidal viscosity theory was confirmed. Improved heat dispersal in a divertor system using three rows of rotating RMP was demonstrated and predictive control of the ELM-crash with a priori modeling was successfully tested. In magnetohydrodynamic physics, validation of the full reconnection model (i.e. q0 > 1 right after the sawtooth crash) and self-consistent validation of the anisotropic distribution of turbulence amplitude and flow in the presence of the 2/1 island with theoretical models were achieved. The turbulence amplitude induced by RMP was linearly increased with the slow RMP coil current ramp-up time (i.e. the magnetic diffusion time scale). The Dα spikes (i.e. ELM-crash amplitude) was linearly decreased with the turbulence amplitude and not correlated with the perpendicular electron flow. In the turbulence area, a non-diffusive ‘avalanche’ transport event and the role of a quiescent coherent mode in confinement were studied. To accommodate the anticipation of a higher performance of the KSTAR plasmas with the increased heating powers, a new divertor/internal interface with a full active cooling system will be implemented after a full test of the new heating (neutral beam injection II and electron cyclotron heating) and current drive (CD) (Helicon and lower hybrid CD) systems. An upgrade plan for the internal hardware, heating systems and efficient CD system may allow for a long pulse operation of higher performance plasmas at βN > 3.0 with f bs ~ 0.5 and Ti > 10 keV.

Numerical study of tearing mode seeding in tokamak X-point plasma

A detailed understanding of island seeding is crucial to avoid neoclassical tearing modes and their negative consequences like confinement degradation and disruptions. In the present work, we investigate the growth of 2/1 islands in response to magnetic perturbations. Although we use externally applied perturbations produced by resonant magnetic perturbation (RMP) coils for this study, the results are directly transferable to island seeding by other MHD instabilities creating a resonant magnetic field component at the rational surface. Experimental results for 2/1 island penetration from ASDEX Upgrade are presented extending previous studies. Simulations are based on an ASDEX Upgrade L-mode discharge with low collisionality and active RMP coils. Our numerical studies are performed with the 3D, two-fluid, nonlinear MHD code JOREK. All three phases of mode seeding observed in the experiment are also seen in the simulations: first, a weak response phase characterized by large perpendicular electron flow velocities followed by a fast growth of the magnetic island size accompanied by a reduction of the perpendicular electron velocity and finally the saturation to a fully formed island state with perpendicular electron velocity close to zero. Thresholds for mode penetration are observed in the plasma rotation as well as in the RMP coil current. A hysteresis of the island size and electron perpendicular velocity is observed between the ramping up and down of the RMP amplitude consistent with an analytically predicted bifurcation. The transition from dominant kink/bending to tearing parity during the penetration is investigated.

Effects of edge safety factor on the toroidal flow velocities during the resonant magnetic perturbations and compact torus injection experiments on the STOR-M tokamak

ABSTRACTThe effects of the edge safety factor on the toroidal flow velocity modification due to resonant magnetic perturbation (RMP) and tangential compact torus injection (CTI) have been investigated on the Saskatchewan Torus-Modified (STOR-M) tokamak. The (l = 2, n = 1) RMP coils are wound against the outer surface of the vacuum vessel. The safety factor has been varied by changing the plasma current at a fixed toroidal magnetic field. As the edge safety factor is reduced, the magnetic perturbations on the q = 2 surface are expected to shift radially outwards, moving closer towards the RMP coils. The evolution of the plasma flow velocity was measured through ion Doppler spectroscopy (IDS). It has been found that RMP changes the flow velocity towards the direction of the plasma current direction and this effect is enhanced when the edge safety factor is reduced. When CT was injected tangentially along the plasma current direction, similar changes in the flow velocity have been found. However, the change is not sensitive to the edge safety factor in the CT injection case.