Search

The impact of the poloidally inhomogeneous particle source on the onset of Stringer spin-up (SSU) and geodesic acoustic mode (GAM) is investigated. Using a gyrofluid model with Hammet–Perkins closure, it was found that Landau damping stabilizes both waves and subsequently makes a threshold. To capture the full effects of Landau damping, a gyrokinetic model is adopted and results are compared with those from the gyrofluid model. Both models predicted the same value of the threshold for SSU, while for the case of GAM, the gyrofluid model overestimates the threshold value. Considering maximal throughput of the ITER pellet fueling system, the source intensity is calculated at a similar or slightly lower level compared to the source threshold for SSU.

We derive expressions for the gyro-averaging operator that is applicable to electrostatic fluctuations in a spatially inhomogeneous magnetic field. Both low and high wavenumber limits are considered. The gyro-averaging operator for the former case is represented by sums of Bessel functions with different orders. A simplified expression is provided as a Padé approximant in the low wavenumber limit. This form could be used in practical computations based on the gyrofluid formulation. In the high wavenumber limit, we find that the operator naturally involves fractional derivatives whose physical interpretations are yet to be explored. Discussions are made of a potential impact of this asymptotic expression in the high wavenumber limit.

From global gyrokinetic simulations of toroidal ion temperature gradient-driven (ITG) turbulence, we identify two distinguished regimes where zonal flow generation and its radial pattern formation is governed by either phases or amplitudes of the turbulence spectrum. When the unstable region is wider than the correlation length of ITG modes, the zonal flow structure changes in a turbulence time scale. We newly uncover that the radial phases of ITG modes determine the evolution of the zonal flow structure. Synchronization of mode phases induced by the global zonal flow drives a fine-scale zonal flow pattern. With a narrow unstable region comparable to the mode correlation length, the phase effect almost vanishes. In this regime, we recover that the modulational instability promptly amplifies a coherent zonal flow, leading to a lower turbulence saturation level. This finding explains the reduction of turbulent transport at a narrow width of the strong gradient region, which has been attributed to the system size effects in ion-scale gyrokinetic turbulence.

Nonlinear effects of light impurities on the zonal flow activities and electrostatic ITG (ion temperature gradient) mode are investigated with gyrokinetic simulations. For the investigation, a new multiple-ion-species gyrokinetic Poisson solver is implemented numerically. Benchmark tests for the new solver show good agreements with theoretical and previous simulation results. Nonlinear ITG simulations with and without light impurities are compared. To isolate nonlinear effects of impurities on ITG, simulation parameters are set to exhibit approximately identical spectra of linear growth rates for the admixed and pure deuterium cases (i.e., the cases with and without the impurities). With an intermediate safety factor (∼1.4), the admixed case shows smaller heat transport and more robust E × B staircase structures than the pure deuterium case. The locations of the transport suppression and staircases are strongly correlated, which indicate that light impurities have stabilizing effects on ITG by enhancing the staircase-like E × B shearing. Especially, the radial correlation length of the fluctuations is significantly reduced for the admixed case. On the other hand, the stabilizing effect of impurities is weakened with a high safety factor (∼5). In those cases, strong geodesic acoustic mode activity is observed, and the electric field is dominated by oscillating components instead of stationary staircases.

We present a possible mechanism for the generation of strong E × B flow shear relevant to internal transport barrier formation in tokamak plasmas. From gyrokinetic calculations, we show that strong E × B flow shear can be generated by finite orbit width (FOW) effects associated with a non-uniform heat source and is sufficient to lead to transport barrier formation in the core region with a moderate power level. Two FOW effects inducing neoclassical polarization are shown to be responsible for this: (1) the radial drift of particle orbit center due to the variation of the heat source within orbit width and (2) the non-uniformly evolved orbit width by the non-uniform heating.

Advanced operation scenarios such as high poloidal beta (β P) or high q min are promising concepts to achieve the steady-state high-performance fusion plasmas. However, those scenarios are prone to substantial Alfvénic activity, causing fast-ion transport and losses. Recent experiments with the advanced operation scenario on KSTAR tokamak have shown that the electron cyclotron current drive (ECCD) is able to mitigate and suppress the beam-ion driven toroidal Alfvén eigenmodes (TAEs) for over several tens of global energy confinement time. Co-current directional intermediate off-axis ECCD lowers the central safety factor slightly and tilts the central q-profile shape so that the continuum damping in the core region increases. Besides, the rise of central plasma pressure and increased thermal-ion Landau damping contribute to TAE stabilization. While the TAEs are suppressed, neutron emission rate and total stored energy increase by approximately 45% and 25%, respectively. Fast-ion transport estimated by TRANSP calculations approaches the classical level during the TAE suppression period. Substantial reduction in fast-ion loss and neutron deficit is also observed. Enhancement of fast-ion confinement by suppressing the TAEs leads to an increase of non-inductive current fraction and will benefit the sustainment of the long-pulse high-performance discharges.

On the basis of a three-field flute-reduced magnetohydrodynamic model, which mainly describes the edge instabilities by shielding a major part of the J × B force in the flute reduction, we study the stability of ballooning modes in the edge pedestal, highlighting the role of an equilibrium parallel current gradient. This effect, which is designated as the current gradient driven (CGD) term in this paper, is shown to have an influence on the stability of finite-n pedestal ballooning modes due to the existence of a highly localized bootstrap current. An analysis in the ideal limit shows that the CGD term destabilizes the ballooning modes regardless of the sign of its gradient, especially near the stability boundaries. An inclusion of the finite Larmor radius (FLR) effect via ion diamagnetic flow and finite resistivity results in a coupling of the FLR effect and the current gradient. In this particular regime where the deviation from the ideal stability is considerable, this coupling effect is shown to dominate stability in intermediate n ( 20

We present an approximate expression for an isotropic alpha particle slowing down distribution function that is applicable down to the lowest velocity limit. The effect of non-uniform electron and ion temperature, i.e., the different rate of energy equilibration, is retained in this expression. Application of the extended slowing down distribution function shows that keeping the non-uniformity effect may have an influence on the alpha particle–background plasma interaction by the modification of the energetic particle contents in comparison with the conventional slowing down distribution function.

We calculate a set of ITER poloidal field (PF) coil currents at the initial magnetization phase using the quadratic programming algorithm. The optimization is performed by exploiting the total magnetic energy of the ITER PF system as a cost function to be minimized. We also develop a simple analytic model which can make a rapid evaluation of temperature margin of superconducting PF coils for a given set of coil currents. This calculation is necessary for a quick examination of the compatibility of the PF scenario with engineering constraints. For an easy adaptation to the future integrated ITER simulator, all modules are developed using the ITER Integrated Modeling Analysis Suite (IMAS) data structure and the Kepler framework.

We investigate the finite Larmor radius (FLR) effects that are associated with an external heat source or a sink in the full-f formulation of plasma dynamics. Specifically, we show that a gyrocenter or an additional vorticity source due to the FLR effect should be replenished in full-f turbulence simulations. A quantitative energetics analysis in particle phase space demonstrates that an additional vorticity source is required to satisfy the total energy conservation. A physics picture is presented to elucidate the mechanism of the vorticity source generation from the FLR effects of a heat source or a sink, which reproduces the main result obtained from the quantitative analysis. The additional vorticity source is expected to considerably enhance the generation of the radial electric field and subsequent E × B flow shear in the off-axis heating case.

An analytic study is conducted on the impact of poloidal E × B flow on geodesic acoustic mode (GAM) in the edge pedestal of a tokamak plasma. A set of coupled nonlinear equations is derived from a reduced magnetohydrodynamic model. Analytic solutions to the set of coupled equations reveal that the non-geodesic component of a poloidally asymmetric pressure perturbation begins to contribute to GAM when the E × B flow is accounted for in the analysis. The full nonlinear solution shows that the sinusoidal GAM oscillation changes into the cnoidal one which is represented by the Jacobi elliptic function. The GAM frequency increases in proportion to the initial radial electric field (Er). The potential impact of this pedestal Er oscillation on the transport process in the externally perturbed edge transport barrier is discussed.

We develop a conservative gyrofluid model that is suitable for global flux-driven simulations of electrostatic tokamak plasma turbulence. On the basis of the general gyrofluid moment equations, we derive energy equations which enable us to manipulate the impact of a gyrofluid closure on energetics. We demonstrate that an artificial manipulation of high order moment contributions to gyrofluid moment equations via a closure model can lead to the violation of the energy conservation. A fluid closure is also found to restrict the maximum attainable order of finite Larmor radius terms, implying the loss of dynamical information by the closure.

Using a statistical correlation analysis, we compute evolution of the magnetic Kubo number during an edge pedestal collapse in nonlinear reduced magnetohydrodynamic simulations. The Kubo number is found not to exceed the unity in spite of performing the simulation with a highly unstable initial pressure profile to the ideal ballooning mode. During the edge pedestal collapse, the Kubo number is within the values of $0.2$ and $0.6$ suggesting that the quasilinear diffusion model is sufficient to explain the energy loss mechanism during the pedestal collapse. Temporal evolution of poloidal correlation lengths of pressure fluctuations resembles with that of the Chirikov parameter and the Kubo number; while radial correlation lengths of the pressure fluctuations are strongly correlated with the radial width of the magnetic stochastic layer., Comment: 10 pages, 13 figures, Submitted to Physics of Plasmas

This work presents a nonlinear gyrokinetic analysis, which addresses one of the long-lived conundrums in Ohmically heated fusion plasmas. The widely observed linear Ohmic confinement (LOC) to saturated Ohmic confinement (SOC) transition of the energy confinement time $\tau_E$ with increasing density is successfully reproduced from nonlinear gyrokinetic simulations for the first time. Suppression of trapped electron turbulence by collisional detrapping due to increasing density is found to be responsible for the transition. Microturbulence transition from TEM to ITG is found to coincide, but not a necessary condition for the LOC-SOC transition. Damping of nonlinearly generated zonal flow by increasing collisionality with density can be responsible for the energy confinement degradation in SOC regime.

From nonlinear flux-driven fluid simulations, we show that the external torque has a profound influence on ion thermal confinement. A key parameter controlling the confinement change is found to be the ratio of the net external torque to the applied power input. For a given value of external torque, the ion temperature profile destiffening and the corresponding confinement enhancement occur when the ratio is below a threshold value. This confinement improvement is shown to originate from the increase in the zonal flow shearing rate due to the conversion of parallel flow compressibility to zonal vorticity. Confinement degradation and the restoration of profile stiffness arise beyond the critical value of the ratio due to the onset of the parallel velocity gradient instability. This result implies the existence of an optimal torque value for the given heating power to maximize the thermal confinement.

We perform a computational study of characteristics of coupled drift wave (DW)-parallel velocity gradient (PVG) driven turbulence. The three-dimensional Hasegawa–Mima equation combined with ion parallel flow dynamics is used for this study. Energetic analyses, both analytic and computational, show the energy equipartition of the parallel Reynolds power (i.e. the free energy due to a gradient in equilibrium parallel velocity) in parallel and perpendicular directions. A considerable portion of the perpendicular energy that is transferred through the parallel coupling generates the perpendicular zonal flow (ZF), implying the increase of the ZF level when an equilibrium PVG is present. A careful analysis of parallel-perpendicular coupling dynamics shows that the vortex stretching-like term in this system plays a significant role in the ZF generation. This is in contrast to the neutral fluid turbulence where it hinders the inverse cascade process. However, the ZF generation from PVG turbulence is found to be less effective in comparison with that of DW turbulence.

We evaluate electromagnetic (EM) loads on the main systems of the ITER machine using a single finite element model. The 20° sector of the full ITER machine includes the main in-vessel components as well as the vacuum vessel. Narrow slits of the in-vessel components are effectively modeled by using the element splitting method without significant increase of computation memory and time as well as without sacrificing the accuracy. Furthermore, the halo current is taken into account at the same time together with the plasma current. To apply both currents concurrently, dedicated conversion codes are utilized to transfer the plasma simulation results by DINA to the electromagnetic analysis by ANSYS-EMAG used here. The electromagnetic loads on the ITER machine are calculated for various disruption scenarios. Investigation on the analysis results is made to find the worst plasma disruption case and the design-driving load component for each system as well as to compare load contribution from eddy and halo currents. The effect of the narrow slits on load reduction is also examined.

A simple and efficient modeling technique is presented for a proper analysis of complicated eddy current flows in conducting structures with fine air gaps. It is based on the idea of replacing a slit with the decoupled boundary of finite elements. The viability and efficacy of the technique is demonstrated in a simple problem. Application of the method to electromagnetic load analyses during plasma disruptions in ITER has been successfully carried out without sacrificing computational resources and speed. This shows the proposed method is applicable to a practical system with complicated geometrical structures.

Electromagnetic (EM) loads due to eddy current and halo current during plasma disruptions are evaluated for the ITER diagnostic upper port plug. To reduce strong EM loads acting on the port plug fixed to the vacuum vessel like a cantilever beam, three design options have been considered: removal of the diagnostic first wall, slitting of the diagnostic shield module and recess of the port plug. The main focus of the present study is to examine the efficacy of these options in terms of EM loads on the upper port plug. It is found that making slits is more effective than removing the first wall. It is also shown that the upper port plug needs to be recessed to reduce the EM load induced by halo current.

An analysis is carried out on the three-dimensional modeling and computation of the magnetic field in ITER. The commercial finite element code ANSYS-EM is employed for this study. In particular, an emphasis is put on the analysis of the characteristics of non-axisymmetric magnetic fields produced by ferromagnetic materials, including ferromagnetic inserts (FIs) and helium cooled solid breeder test blanket modules (TBMs). It is found that the ITER design requirement for toroidal field ripple is violated by the presence of TBMs, even in the presence of FIs. Calculations of TBM-produced error fields also show that TBM produces a significant error field at q = 2 surface exceeding the ITER design requirement. Discussions are made of the potential implication of the TBM-produced non-axisymmetric fields on plasma performance and the design of a TBM emulation system.

From global gyrokinetic simulations of toroidal ion temperature gradient-driven turbulence, we show that the ion parallel compression has a strong influence on the generation and radial profile formation of zonal flows. The kinetic potential vorticity (PV) flux and its fluid expression are used to elucidate the zonal flow generation mechanism. In the absence of sheared equilibrium flow, the dominant contributions to the net PV flux are shown to come from the parallel compression and the grad-B drift, and to largely cancel out each other. With a finite parallel rotation shear, however, the parallel compression-driven flux becomes dominant over the grad-B drift-driven one, leading to a change in the radial zonal flow profile. The imbalance between the parallel compression and the grad-B drifts results in a considerable amplification of the zonal flow and a reduction of turbulence fluctuation levels as compared to the non-rotating plasma. These findings demonstrate an essential role of the parallel compression in the zonal flow generation and confinement improvement for rotating tokamak plasmas.

We report a numerical study for the impact of resonant magnetic perturbation (RMP) on fast ion prompt loss in KSTAR using full orbit following simulation. Experimental observation in KSTAR indicates a sudden increase in RMP-induced fast ion prompt loss when the applied RMP field exceeds a threshold amplitude. Full orbit simulation with 3D perturbed equilibrium computed by the ideal plasma response reproduces the experimentally observed feature of RMP-induced prompt loss of fast ions and the existence of threshold RMP amplitude. Simulation with vacuum fields only shows a small increase in the fast ion loss that is insufficient to explain the observation. We show that the conservation of canonical angular momentum is broken due to the RMP during the fast ion transit, which is consistent with the enhanced fast ion loss above the threshold RMP amplitude. Phase space analysis suggests that trapped and intermediate pitch passing particles modified from initial high pitch passing ones are responsible for the threshold behavior of the fast ion loss in the simulation, while the phase space distribution of lost particles depends on the RMP field configuration.

According to recent updates of ITER shield blanket design, electromagnetic loads during the plasma disruption are being evaluated to verify the mechanical confidence and reliability. As a course of such evaluations, a benchmark activity for the electromagnetic analysis, coordinated by ITER Organization, is underway between ITER parties to compare the calculation results for disruption loads on the blankets. In this paper, we present calculation results for the electromagnetic loads on the simplified but practical model of ITER shield blankets with respect to six representative disruption scenarios of which ITER distributes simulation results based on the DINA code as a reference of the design and analysis. Commercial finite element method software, ANSYS/Emag™, was employed to evaluate the eddy current on the blanket modules with the 40° sector model for major conducting structure of the tokamak including double-walled vacuum vessel, triangular support, and vertical targets of divertors. An interface between ANSYS/Emag™ and plasma simulator was implemented with a conversion tool assigning the plasma current density on the ANSYS elements corresponding to the current filaments in DINA outputs. Discussions are made of the possible improvement of the blanket model taking more realistic blanket configuration into account at the cost of the moderate increase in computational time. A final remark is given of the possibility of incorporating halo currents into ANSYS disruption simulations, which are major sources of electromagnetic loads on in-vessel components including blankets.

The ideal ballooning mode model in the presence of the externally applied resonant magnetic perturbation (RMP), developed by Bird and Hegna [Nucl. Fusion 53, 013004 (2013)], is extended to include the non-ideal effects, i.e., the finite resistivity and diamagnetic drift effects. Using the eigenvalue equation with the non-ideal effects, a comprehensive analysis is carried out to elucidate the impact of the RMP on ballooning mode stability. Finite resistivity is shown to reduce the line bending stabilization in low to intermediate toroidal mode number (n), resulting in the more peaked growth rate spectrum with respect to n. It is shown that the combination of finite resistivity and the RMP-induced local shear modulation have a strong influence on line bending stabilization, leading to an interesting threshold behavior of the ballooning mode stability. This signifies the importance of the RMP to the line bending stabilization coupled with finite resistivity, as well as the local shear modulation.

We show that the threshold condition for the toroidal ion temperature gradient (ITG) mode with an inverted density profile can be derived from a simple physics argument. The key in this picture is that the density inversion reduces the ion compression due to the ITG mode and the electron drift motion mitigates the poloidal potential build-up. This condition reproduces the same result that has been reported from a linear gyrokinetic calculation [T. S. Hahm and W. M. Tang, Phys. Fluids B 1, 1185 (1989)]. The destabilizing role of trapped electrons in toroidal geometry is easily captured in this picture.

Two-step mode coupling analyses for nonlinear excitation of the ballooning mode (BM) in pedestal plasma by external helical magnetic field perturbation [Resonant Magnetic Perturbations (RMP)] are presented. This technique allows calculating the effect of higher harmonic sidebands generated by interaction of long scale RMP pump and BM. It is shown that RMP field perturbations can modify the BM growth rate and frequency through nonlinear Reynolds stress and magnetic stress. In particular, it is shown that both stresses can efficiently excite high wavenumber BM fluctuations which, in turn, can enhance the transport in the pedestal. Another notable feature of this analysis is the existence of short scale (high- k y) nonlinear instability at Alfven time scale near the ideal BM threshold boundary.

In cold pulse experiments in J-TEXT, not only are rapid electron temperature increases in the core observed, but also steep rises in the inner density are found. Moreover, some evidence of acceleration of the core toroidal rotation is also observed during the non-local transport process of electron temperature. These new findings of cold pulse experiments in J-TEXT suggest that turbulence spreading is a possible mechanism for the non-local transport dynamics.

In-vessel control coils (IVCCs) are to be used for the fast plasma position control, field error correction (FEC), and resistive wall mode (RWM) stabilization for the Korea Superconducting Tokamak Advanced Research (KSTAR) device. The IVCC system comprises 16 segments to be unified into a single set to achieve following remarkable engineering advantages; (1) enhancement of the coil system reliability with no welding or brazing works inside the vacuum vessel, (2) simplification in fabrication and installation owing to coils being fabricated outside the vacuum vessel and installed after device assembly, and (3) easy repair and maintenance of the coil system. Each segment is designed in 8 turns coil of 32 mm × 15 mm rectangular oxygen free high conductive copper with a 7 mm diameter internal coolant hole. The conductors are enclosed in 2 mm thick Inconel 625 rectangular welded vacuum jacket with epoxy/glass insulation. Structural analyses were implemented to evaluate structural safety against electromagnetic loads acting on the IVCC for the various operation scenarios using finite element analysis. This paper describes the design features and structural analysis results of the KSTAR in-vessel control coils.

We present a general approach and procedures for developing a Grid-based remote monitoring system when the local control system employs Experimental Physics and Industrial Control System (EPICS). A description is given of the prototype KSTAR remote monitoring system that has been implemented on the basis of the general concept. It is demonstrated that the Grid computing technology combined with Globus toolkit software tools, can be successfully utilized for the realization of a remote monitoring system for tokamak experiments. Discussions are made on requirements and a plan to build a complete remote participation system for future tokamak experiments.

The requirements of the DIII-D physics program have led to the development of many operational control results with direct relevance to ITER. These include new algorithms for robust and sustained stabilization of neoclassical tearing modes with electron cyclotron current drive, model-based controllers for stabilization of the resistive wall mode in the presence of ELMs, coupled linear–nonlinear algorithms to provide good dynamic axisymmetric control while avoiding coil current limits, and adaptation of the DIII-D plasma control system (PCS) to operate next-generation superconducting tokamaks. Development of integrated plasma control (IPC), a systematic approach to modelbased design and controller verification, has enabled successful experimental application of high reliability control algorithms requiring a minimum of machine operations time for testing and tuning. The DIII-D PCS hardware and software and its versions adapted for other devices can be connected to IPC simulations to confirm control function prior to experimental use. This capability has been important in control system implementation for tokamaks under construction and is expected to be critical for ITER.

A study is carried out on the real-time plasma shape identification in the KSTAR device. An improved form of the finite current element (FCE) method is utilized in this study. Results are shown that the plasma boundary can be reproduced in 7 mm accuracy for any plasma configuration in ideal cases without invoking measurement errors. A design guideline for magnetic diagnostics (MD) is established when the measurement signals are subject to Gaussian noise. It is found that the measurement errors in poloidal field (PF) coil currents have substantial influence on the determination of the plasma shape.

A study is conducted on the influence of radio frequency (rf) waves upon interchange stability in HANBIT mirror plasmas. An emphasis is put on the interchange stability near the resonance region, ?0 ?i, where ?0 (?i) is the angular frequency of the applied rf wave (ion cyclotron frequency). A strong nonlinear interaction between the rf wave and the interchange mode has been observed with the generation of sideband waves. A theory of rf-interchange mode interaction keeping effects of both the equilibrium ponderomotive force and the nonlinear sideband wave coupling is applied to interpret HANBIT experimental results, resulting in good agreement. Results have shown that the nonlinear coupling process is responsible for the rf stabilization of interchange modes near the resonance region.

We present design concepts and features of the plasma control system (PCS) in Korea Superconducting Tokamak Advanced Research (KSTAR). A design structure of the PCS is proposed as an effort to achieve research objectives of the KSTAR project. The PCS architecture is characterized by the real-time data communication using reflective-memory network, the hierarchical organization of dedicated controllers, and the integrated generation of actuator signals. Discussions are made of the functions and present design choices of the KSTAR PCS.

A theoretical study is made of the effects of radio frequency (rf) waves in the range of ion cyclotron frequency on interchange modes in mirror plasmas. A two-fluid cold plasma model including particle collisions is utilized for the investigation. Both the equilibrium ponderomotive force produced by the gradient of rf wave amplitude and the sideband wave coupling mechanism originated from the nonlinear beating between high frequency waves are taken into account. A tractable form of the dispersion relation for the interchange mode is obtained and applied to carrying out a parametric study. Key parameters affecting the stability boundary are identified and the consequences of the parameter variation on the stability boundary change are analyzed near ion cyclotron resonance region.

We calculate eddy current induced vertical forces on poloidal field (PF) coils and plasma facing conductors during a plasma disruption in Korea Superconducting Tokamak Advanced Research (KSTAR). A system of axisymmetric circuit equations, taking into account a saddle-connected three-dimensional configuration of KSTAR passive plates, combined with a prescribed plasma disruption scenario, is solved using a symmetric and anti-symmetric decomposition technique. The maximum vertical force on each independent conductor, which is designed to have its own vertical support, is calculated. It is shown that the effect of initial equilibrium magnetic fields upon maximum vertical forces is significant, and it should be included in the design analysis of vertical supports for tokamak conducting structures.

A brief overview is presented on initial study results of plasma transport and confinement in HANBIT mirror device. The parallel confinement is calculated using a generalized Pastukhov's formula, a...

We perform a computational study of the effects of finite plasma β(= plasma thermal energy/magnetic energy) on the residual stress in the context of the quasi-linear theory. The five-field electromagnetic (EM) toroidal ion temperature gradient (ITG) model is considered in the realistic tokamak geometry. Analyses show that the residual stress significantly increases and its radial profile changes from a dipolar to a unipolar shape with increasing β. The change in the mode parity is found to be responsible for the enhancement and the profile change of the residual stress driven by the global EM toroidal ITG mode. This finding highlights the importance of the global eigenmode structure in determining the radial profile of the residual stress.

Simulation studies on magnetics control for the proposed Korea Superconducting Tokamak Advanced Research (KSTAR) plasmas are presented. Here, the magnetics control includes the fast time scale (∼10 ms) plasma position control, the slow time scale (∼1 s) plasma shape and current control, and the control of resistive wall modes (RWM). The plasma position control is realized by using two pairs of inner control coils inside KSTAR vacuum vessel. The plasma shape and current control is achieved by using superconducting poloidal field (PF) coils in KSTAR. The voltage limitations in PF coil power supplies are taken into account in the simulation. A set of 12 in-vessel control coils, which are mounted on the inner wall of KSTAR vacuum vessel, are used for the control of RWMs. The required feedback currents in in-vessel control coils for the stabilization of RWMs are estimated.

We perform a numerical study of linear stability of the ion temperature gradient (ITG) mode and the trapped electron mode (TEM) in tokamak plasmas with inverted density profiles. A local gyrokinetic integral equation is applied for this study. From comprehensive parametric scans, we obtain stability diagrams for ITG modes and TEMs in terms of density and temperature gradient scale lengths. The results show that, for the inverted density profile, there exists a normalized threshold temperature gradient above which the ITG mode and the TEM are either separately or simultaneously unstable. The instability threshold of the TEM for the inverted density profile is substantially different from that for normal and flat density profiles. In addition, deviations are found on the ITG threshold from an early analytic theory in sheared slab geometry with the adiabatic electron response [T. S. Hahm and W. M. Tang, Phys. Fluids B 1, 1185 (1989)]. A possible implication of this work on particle transport in pellet fueled tokamak plasmas is discussed.

From the nonlinear simulations of the two-dimensional forced-damped Hasegawa–Mima equation, we show that a Reynolds number-like parameter, Rew, can represent a power law exponent of the energy spectrum in Hasegawa–Mima turbulence. Rew is defined as the ratio of nonlinear convection to dissipation. For a same value of Rew, the power law exponent in the inertial range is shown to be uniquely determined regardless of the forcing and dissipation conditions. At high Rew, the power law exponent asymptotically converges to −7.2, consistent with a recent theoretical prediction based on the shell model [Gurcan et al., Plasma Phys. Controlled Fusion 52, 045002 (2010)].

International audience; Dedicated experiments have been performed in KSTAR Ohmic plasmas to investigate the detailed physics of the rotation reversal phenomena. Here we adapt the more general definition of rotation reversal, a large change of the intrinsic toroidal rotation gradient produced by minor changes in the control parameters (Camenen et al 2017 Plasma Phys. Control. Fusion 59 034001), which is commonly observed in KSTAR regardless of the operating conditions. The two main phenomenological features of the rotation reversal are the normalized toroidal rotation gradient (u') change in the gradient region and the existence of an anchor point. For the KSTAR Ohmic plasma database including the experiment results up to the 2016 experimental campaign, both features were investigated. First, the observations show that the locations of the gradient and the anchor point region are dependent on q(95). Second, a strong dependence of u' on nu(eff) is clearly observed in the gradient region, whereas the dependence on R/L-Ti, R/L-Te, and R/L-ne is unclear considering the usual variation of the normalized gradient length in KSTAR. The experimental observations were compared against several theoretical models. The rotation reversal might not occur due to the transition of the dominant turbulence from the trapped electron mode to the ion temperature gradient mode or the neoclassical equilibrium effect in KSTAR. Instead, it seems that the profile shearing effects associated with a finite ballooning tilting well reproduce the experimental observations of both the gradient region and the anchor point; the difference seems to be related to the magnetic shear and the q value. Further analysis implies that the increase of u' in the gradient region with the increase of the collisionality would occur when the reduction of the momentum diffusivity is comparatively larger than the reduction of the residual stress. It is supported by the perturbative analysis of the experiments and the nonlinear gyrokinetic simulations. The absence of the sign change of u' even when a much lower collisionality is produced by additional electron cyclotron heating brings further experimental support to this interpretation.

A study is conducted on the impact of plasma compressibility on the stability of drift ballooning modes. The two-fluid and four-field model developed by Hazeltine et al. [Phys. Fluids 28, 2466 (1985)] is employed in this study. Results of linear numerical simulations show that finite compressibility destabilizes ballooning modes which are otherwise stable due to the ion diamagnetic drift effect. A systematic study reveals that the parallel compressibility originating from the two-fluid effect, rather than the drift-acoustic wave coupling suggested by Hastie et al. [Phys. Plasmas 10, 4405 (2003)], plays the most important role in destabilizing the ballooning modes. An analytic evaluation of the dispersion relation underpins the strong sensitivity of this parallel compressibility term in the ballooning mode stability. The potential impact of this new understanding on the physics of small edge localized modes is discussed.

This report summarizes the contributions to, and discussions at, the 6th Asia-Pacific Transport Working Group (APTWG) meeting, held on 21–25 June 2016 at Korea University, Seoul, Republic of Korea. The objective of the meeting was to develop an integrated understanding of transport phenomena in magnetically confined plasmas. To accomplish this objective, four technical working groups were organized under the headings: (1) turbulence suppression and transport bifurcation, (2) effect of magnetic topology on transport and magnetohydrodynamics-turbulence interaction, (3) nonlocality and non-diffusive transport, and (4) Energetic particles and particle/impurity transport. A summary is also given of the three plenary review talks on impurity transport, the magnetohydrodynamics relaxation process in reversed field pinch, and recent experimental and modelling results on the quiescent H-mode operation.

A study on the feedback stabilization of resistive wall modes in a cylindrical geometry is presented. The effect of radial separation of feedback coils from sensor loops is investigated. It is shown that there is an optimum radial position for feedback coils where sideband mode coupling parameter is minimized, hence enhancing the efficacy of a system of coils for feedback stabilization of resistive wall modes.

Simulation studies of plasma shape identification and shape control for the proposed Korea superconducting tokamak advanced research (KSTAR) are described. It is shown that the total number of magnetic measurements can be effectively reduced by considering the patterns of magnetic flux and fields, generated by plasma, along a prescribed measurement contour. The effect of eddy currents on shape identification is investigated in dynamic simulations. The isoflux control scheme and a standard PID control law are adopted for the development of a model shape control system. It is shown that appropriate weighting factors of poloidal field coils, incorporating the efficacy of each coil to a shape control point, can significantly reduce the total feedback power required for a shape control action. Finally, a shape control simulation using calculated flux errors, which correspond to a more realistic experimental situation, is presented.