in Kim

δf Monte Carlo calculation of neoclassical transport in perturbed tokamaks

Non-axisymmetric magnetic perturbations can fundamentally change neoclassical transport in tokamaks by distorting particle orbits on deformed or broken flux surfaces. This so-called non-ambipolar transport is highly complex, and eventually a numerical simulation is required to achieve its precise description and understanding. A new delta#14;f particle code (POCA) has been developed for this purpose using a modi ed pitch angle collision operator preserving momentum conservation. POCA was successfully benchmarked for neoclassical transport and momentum conservation in axisymmetric con guration. Non-ambipolar particle flux is calculated in the non-axisymmetric case, and results show a clear resonant nature of non-ambipolar transport and magnetic braking. Neoclassical toroidal viscosity (NTV) torque is calculated using anisotropic pressures and magnetic fi eld spectrum, and compared with the generalized NTV theory. Calculations indicate a clear #14;B2 dependence of NTV, and good agreements with theory on NTV torque pro les and amplitudes depending on collisionality.

Virtual bone drilling for dental implant surgery training

Mechanical removal of bone material is the most critical procedure during dental implant surgery because it can jeopardize patient safety in several ways such as damage in the mandibular canal and piercing of the maxillary sinus. With recognition of the effectiveness in virtual training, many simulators with haptic feedback have been proposed. Although there are many varieties in drill bits, most of previously developed simulators consider only a spherically shaped tool due to its simplicity in tool-bone interaction. In this paper, we propose a new simulation method that can handle any arbitrarily shaped tools with multiple contacts between the tool and the bone. The tool is represented by a signed-distance field, and the bone is represented as voxels surrounded by a point shell. Upon chipping away bone elements, the point shell is updated reflecting the deformation of bone in real-time, while the collision detection and the reflected force is efficiently and accurately computed from the distance field encoded in the tool. We also present the experimental results with 12 dental implantologists to evaluate realism of the proposed simulator.

Neoclassical toroidal viscosity in perturbed equilibria with general tokamak geometry

This paper presents a calculation of neoclassical toroidal viscous torque independent of large-aspect-ratio expansions across kinetic regimes. The Perturbed Equilibrium Nonambipolar Transport (PENT) code was developed for this purpose, and is compared to previous combined regime models as well as regime specific limits and a drift kinetic δf guiding center code. It is shown that retaining general expressions, without circular large-aspect-ratio or other orbit approximations, can be important at experimentally relevant aspect ratio and shaping. The superbanana plateau, a kinetic resonance effect recently recognized for its relevance to ITER, is recovered by the PENT calculations and shown to require highly accurate treatment of geometric effects.

Theory comparison and numerical benchmarking on neoclassical toroidal viscosity torque

Systematic comparison and numerical benchmarking have been successfully carried out among three different approaches of neoclassical toroidal viscosity (NTV) theory and the corresponding codes: IPEC-PENT is developed based on the combined NTV theory but without geometric simplifications [Park et al., Phys. Rev. Lett. 102, 065002 (2009)]; MARS-Q includes smoothly connected NTV formula [Shaing et al., Nucl. Fusion 50, 025022 (2010)] based on Shaings analytic formulation in various collisionality regimes; MARS-K, originally computing the drift kinetic energy, is upgraded to compute the NTV torque based on the equivalence between drift kinetic energy and NTV torque [J.-K. Park, Phys. Plasma 18, 110702 (2011)]. The derivation and numerical results both indicate that the imaginary part of drift kinetic energy computed by MARS-K is equivalent to the NTV torque in IPEC-PENT. In the benchmark of precession resonance between MARS-Q and MARS-K/IPEC-PENT, the agreement and correlation between the connected NTV formu...

Volume-based haptic model for bone-drilling

For bone surgery haptic simulation, we need to consider both volume cutting deformation and stable force feedback. In this paper, we propose a volume-based haptic model where both bone and a tool are represented by voxel-based models so that we can handle shape deformation of bone efficiently. For fast and stable haptic rendering, we propose that the tool is represented by a signed-distance field and the bone surface is tracked by a set of points, called a point shell. In order to handle shape deformation of bone while drilling, fast reconstruction of point shell points are performed locally based on the voxel properties of bone and the tool.

Dependence of the L?H transition on X-point geometry and divertor recycling on NSTX

The edge electron (Te) and ion temperature (Ti) at the time of the L?H transition increase when the X-point radius (RX) is reduced to a high-triangularity shape while maintaining constant edge density. Consequently the L?H power threshold (PLH) is larger for the high-triangularity shape. This supports the prediction that a single-particle loss hole, whose properties are strongly linked to RX and Ti, influences the edge radial electric field (Er) and Er???B flow-shearing rate available for turbulence suppression. Simulations using XGC0, a full-f drift-kinetic neoclassical code, indicate that maintaining a constant Er???B flow-shearing rate does require a larger heat flux and edge Ti as RX decreases. NSTX also observes a decrease in PLH when the divertor recycling is decreased using lithium coatings. However, the edge Te and Ti at the L?H transition appear independent of the divertor recycling for a constant shape. XGC0 calculations demonstrate that more heat flux is needed to maintain the edge Ti and the Er???B flow-shearing rate as the contribution of divertor recycling to the overall neutral fuelling rate increases.

Ideal plasma response to vacuum magnetic fields with resonant magnetic perturbations in non-axisymmetric tokamaks

Ideal plasma shielding and amplification of resonant magnetic perturbations in non-axisymmetric tokamak is presented by field line tracing simulation with full ideal plasma response, compared to measurements of divertor lobe structures. Magnetic field line tracing simulations in NSTX with toroidal non-axisymmetry indicate the ideal plasma response can significantly shield/amplify and phase shift the vacuum resonant magnetic perturbations. Ideal plasma shielding for n = 3 mode is found to prevent magnetic islands from opening as consistently shown in the field line connection length profile and magnetic footprints on the divertor target. It is also found that the ideal plasma shielding modifies the degree of stochasticity but does not change the overall helical lobe structures of the vacuum field for n = 3. Furthermore, amplification of vacuum fields by the ideal plasma response is predicted for low toroidal mode n = 1, better reproducing measurements of strong striation of the field lines on the divertor plate in NSTX.

Comparison of divertor heat flux splitting by 3D fields with field line tracing simulation in KSTAR

We present the experimental and numerical analysis of divertor heat flux spitting by 3D magnetic fields in KSTAR. A dedicated experiment to measure the modification of the divertor heat flux profile in the n = 2 resonant and non-resonant field configurations has been conducted. The measured heat flux profiles were directly compared to the magnetic field line tracing (FLT) simulation using the POCA-FLT code, which fully considers vacuum and ideal plasma response models. Measurements indicate that the 3D fields induce strong splitting of heat flux on the divertor target due to the formation of stochastic field layers. Splitting characteristics largely depend on the applied 3D field structure and its alignment to the axisymmetric magnetic pitch. The modeling of the field line connection length shows good agreement with measurements, reproducing well the pattern of heat flux splitting. Simulations with the ideal plasma response better reproduce measured heat flux profiles by excitation of non-resonant field c...

Variation of magnetic braking by non-axisymmetric magnetic fields depending on the perturbed field structure in the KSTAR tokamak

The variation of a magnetic braking profile by non-axisymmetric magnetic fields has been experimentally demonstrated and numerically validated in the KSTAR tokamak. Two types of n = 2 non-resonant magnetic fields were applied by changing the relative phase of non-axisymmetric field coils. One is even parity, of which non-resonant fields deeply penetrate into the plasma core, and the other is odd parity localized at the plasma edge. The even and odd parity produced significantly different perturbed magnetic field structures, and thereby drove global and edge-dominant toroidal rotation damping, respectively. These distinct braking profiles are consistently reproduced by drift-kinetic particle simulations, indicating the possibility of the predictive utilization of non-resonant magnetic fields for rotation profile control.

A dental simulator for training of prevalent interventions: tooth restoration and ultrasonic scaling

This paper presents a haptic-based dental simulator for training of two prevalent interventions: tooth restoration and ultrasonic scaling. The voxel-based collision model is applied to deal with various interactions between the sharp instrument and the tooth. In addition, the spray effect based on a particle system and the adhesion attenuation model for a calculus simulation are introduced. Our training program provides the removal ratio of tooth, caries, and cavity, and also the excessive removal ratio of a tooth for the skills assessment. To develop both manual dexterity and surgical judgment, the haptic training program is associated with 3D animation and put into a scenario of procedural steps of each intervention.