Min-Gu Yoo

Suppression of edge localized mode crashes by multi-spectral non-axisymmetric fields in KSTAR

Among various edge localized mode (ELM) crash control methods, only non-axisymmetric magnetic perturbations (NAMPs) yield complete suppression of ELM crashes beyond their mitigation, and thus attract more attention than others. No other devices except KSTAR, DIII-D, and recently EAST have successfully achieved complete suppression with NAMPs. The underlying physics mechanisms of these successful ELM crash suppressions in a non-axisymmetric field environment, however, still remain uncertain. In this work, we investigate the ELM crash suppression characteristics of the KSTAR ELMy H-mode discharges in a controlled multi-spectral field environment, created by both middle reference and top/bottom proxy in-vessel control coils. Interestingly, the attempts have produced a set of contradictory findings, one expected (ELM crash suppression enhancement with the addition of n = 1 to the n = 2 field at relatively low heating discharges) and another unexpected (ELM crash suppression degradation at relatively high heating discharges) from the earlier findings in DIII-D. This contradiction indicates the dependence of the ELM crash suppression characteristics on the heating level and the associated kink-like plasma responses. Preliminary linear resistive MHD plasma response simulation shows the unexpected suppression performance degradation to be likely caused by the dominance of kink-like plasma responses over the island gap-filling effects.

Improvement of neutral beam injection heating efficiency with magnetic field well structures in a tokamak with a low magnetic field

Magnetic well structures are introduced as an effective means to reduce the prompt loss of fast ions, the so-called first orbit loss from neutral beam injection (NBI), which is beneficial to tokamaks with a low magnetic field strength such as small spherical torus devices. It is found by single-particle analysis that this additional field structure can modify the gradient of the magnetic field to reduce the shift of the guiding center trajectory of the fast ion. This result is verified by a numerical calculation of following the fast ions trajectory. We apply this concept to the Versatile Experiment Spherical Torus [1], where NBI is under design for the purpose of achieving high-performance plasma, to evaluate the effect of the magnetic well structure on NBI efficiency. A 1D NBI analysis code and the NUBEAM code are employed for detailed NBI calculations. The simulation results show that the orbit loss can be reduced by 70%–80%, thereby improving the beam efficiency twofold compared with the reference case without the well structure. The well-shaped magnetic field structure in the low-field side can significantly decrease orbit loss by broadening the non-orbit loss region and widening the range of the velocity direction, thus improving the heating efficiency. It is found that this magnetic well can also improve orbit loss during the slowing down process.

Development of 2D implicit particle simulation code for ohmic breakdown physics in a tokamak

Abstract A physical mechanism of an ohmic breakdown in a tokamak has not been clearly understood due to its complexity in physics and geometry especially for a role of space charge in the plasma. We have developed a 2D implicit particle simulation code BREAK, to study the ohmic breakdown physics under a realistic complicated situation considering the space charge and kinetic effects consistently. The ohmic breakdown phenomena span a broad range of spatio-temporal scales, from picoseconds order of the electron gyromotion to milliseconds order of the plasma transport. It is impossible to employ a typical explicit particle simulation method to see the slow plasma transport phenomena of our interest, because a time step size is restricted to be smaller than a period of the electron gyromotion in the explicit scheme. Hence, we adopt several physical and numerical models, such as a toroidally symmetric model and a direct-implicit method, to relax or remove the spatio-temporal restrictions. In addition, coalescence strategies are introduced to control the number of numerical super particles within acceptable ranges to handle the exponentially growing plasma density during the ohmic breakdown. The performance of BREAK is verified with several test cases so that BREAK is expected to be applicable to investigate the ohmic breakdown physics in the tokamak by considering 2-dimensional plasma physics in the RZ plane, self-consistently.

Development of vector following mesh generator for analysis of two-dimensional tokamak plasma transport

Abstract A field-based new adaptive mesh generator, VEGA (VEctor-following Grid generator for Adaptive mesh), is developed for 2-D core–edge coupled tokamak plasma transport simulations. VEGA can generate time-varying and spatially non-uniform grids by using a stretching function. It provides two operation modes for generating non-uniform radial distributions. One is so-called ion mode where the grid is automatically generated by considering the ion temperature gradient which plays an important role in the ion and the momentum transport mechanism of a tokamak plasma. The other is so-called high-gradient mode where the grid is produced by considering the locality of plasma profiles which appears particularly in transport barriers. VEGA is benchmarked with a conventional code for a reference double null (DN) KSTAR divertor configuration. Three factors are newly introduced in this work to evaluate the quality of a grid. It is found that VEGA is particularly suitable for delicate integrated simulations of the plasma edge and the scrape off layer (SOL) due to its high cell orthogonality and low radial flux deviation. Quality of non-uniform grids generated by the two operation modes of VEGA, the ion mode and the high-gradient mode is examined. A more refined grid is found near the edge region characterized with steeper gradients whereas coarser one in the core region. Such fine grids at the edge region can result in highly reduced radial flux deviation, which is indeed important for analysis of edge–SOL physics with time-varying simulations.

Evidence of a turbulent ExB mixing avalanche mechanism of gas breakdown in strongly magnetized systems

Although gas breakdown phenomena have been intensively studied over 100 years, the breakdown mechanism in a strongly magnetized system, such as tokamak, has been still obscured due to complex electromagnetic topologies. There has been a widespread misconception that the conventional breakdown model of the unmagnetized system can be directly applied to the strongly magnetized system. However, we found clear evidence that existing theories cannot explain the experimental results. Here, we demonstrate the underlying mechanism of gas breakdown in tokamaks, a turbulent ExB mixing avalanche, which systematically considers multi-dimensional plasma dynamics in the complex electromagnetic topology. This mechanism clearly elucidates the experiments by identifying crucial roles of self-electric fields produced by space-charge that decrease the plasma density growth rate and cause a dominant transport via ExB drifts. A comprehensive understanding of plasma dynamics in complex electromagnetic topology provides general design strategy for robust breakdown scenarios in a tokamak fusion reactor.Gas breakdown mechanism in plasma under the influence of complex electromagnetic field topology is still debatable. Here the authors present the evidence of the E×B mixing avalanche for gas breakdown in magnetized plasmas in fusion devices as tokamak.

The effect of electron cyclotron resonance heating on breakdown for start-up of a tokamak

Summary form only given. Although various experiments have been conducted routinely in many tokamak devices, the detailed physics background of so-called, start-up including processes of the plasma breakdown, burn-through, and current ramp-up has not been identified because too many parameters are involved together non-linearly and diagnostic tools are restricted in this phase. Plasma breakdown using electron cyclotron resonance heating (ECRH) has been proposed in the Korean superconducting tokamak advanced research (KSTAR) device because the loop voltage is limited due to thick vacuum liners and engineering limits of superconducting poloidal field coils. Particle-in-cell (PIC) simulation is a very effective tool to research the plasma phenomena but difficult to be applied to three dimensional, whole-size tokamak devices because of its large computation time. We developed a three-dimensional PIC code for investigating the breakdown of the tokamak start-up with ECRH. A simplified toroidal geometry is adopted for easy calculations with a prescribed magnetic field profile calculated for the KSTAR device. This PIC code includes Monte Carlo collision (MCC) routine for hydrogen atoms. In this work, we investigated the discharge characteristics and the effect of ECRH when the breakdown occurs in a tokamak. The influence of the magnitude of the toroidal magnetic field, loop voltage, gas pressure, and incident angle and intensity of ECRH is studied for breakdown in a tokamak.