Choong-Seock Chang

An atmospheric pressure plasma source

An atmospheric pressure plasma source operated by radio frequency power has been developed. This source produces a unique discharge that is volumetric and homogeneous at atmospheric pressure with a gas temperature below 300 °C. It also produces a large quantity of oxygen atoms, ∼5×1015 cm−3, which has important value for materials applications. A theoretical model shows electron densities of 0.2–2×1011 cm−3 and characteristic electron energies of 2–4 eV for helium discharges at a power level of 3–30 W cm−3.

Numerical study of neoclassical plasma pedestal in a tokamak geometry

The fundamental properties of steep neoclassical plasma pedestals in a quiescent tokamak plasma have been investigated with a new guiding center particle code XGC: an X-point included Guiding Center code. It is shown that the width of the steepest neoclassical pedestals is similar to an experimentally observed edge pedestal width, and that a steep pedestal must be accompanied by a self-consistent negative radial electric field well. It is also shown that a steep neoclassical pedestal can form naturally at a quiescent diverted edge as the particle source from the neutral penetration (and heat flux from the core plasma) is balanced by the sharply increasing convective ion loss toward the separatrix. The steep neoclassical pedestal and the strong radial electric field well are suppressed by an anomalous diffusion coefficient of a strength appropriate to an L-mode state; nonetheless, the E×B shearing rate increases rapidly with pedestal temperature. Additionally, the present study shows that a steep pedestal ...

Design and construction of the KSTAR tokamak

The extensive design effort for KSTAR has been focused on two major aspects of the KSTAR project mission - steady-state-operation capability and advanced tokamak physics. The steady state aspect of the mission is reflected in the choice of superconducting magnets, provision of actively cooled in-vessel components, and long pulse current drive and heating systems. The advanced tokamak aspect of the mission is incorporated in the design features associated with flexible plasma shaping, double null divertor and passive stabilizers, internal control coils and a comprehensive set of diagnostics. Substantial progress in engineering has been made on superconducting magnets, the vacuum vessel, plasma facing components and power supplies. The new KSTAR experimental facility with cryogenic system and deionized water cooling and main power systems has been designed, and the construction work is under way for completion in 2004.

Full-f gyrokinetic particle simulation of centrally heated global ITG turbulence from magnetic axis to edge pedestal top in a realistic tokamak geometry

Global electrostatic ITG turbulence physics, together with background dynamics, has been simulated in a realistic tokamak core geometry using XGC1, a full-function 5D gyrokinetic particle code. An adiabatic electron model has been used. Some verification exercises of XGC1 have been presented. The simulation volume extends from the magnetic axis to the pedestal top inside the magnetic separatrix. Central heating is applied, and a number, momentum and energy conserving linearized Monte Carlo Coulomb collision is used. In the turbulent region, the ion temperature gradient profile self-organizes globally around R/LT = (Rd logT/dr = major radius on the magnetic axis/temperature gradient length) 6.5–7, which is somewhat above the conventional nonlinear criticality of 6. The self-organized ion temperature gradient profile is approximately stiff against variation of heat source magnitude. Results indicate that the relaxation to a self-organized state proceeds in two phases, namely, a transient phase of excessively bursty transport followed by a 1/f avalanching phase. The bursty types of behaviour are allowed by the quasi-periodic collapse of local E × B shearing barriers.

An overview of intrinsic torque and momentum transport bifurcations in toroidal plasmas

An overview of the physics of intrinsic torque is presented, with special emphasis on the phenomenology of intrinsic toroidal rotation in tokamaks, its theoretical understanding, and the variety of momentum transport bifurcation dynamics. Ohmic reversals and electron cyclotron heating-driven counter torque are discussed in some detail. Symmetry breaking by lower single null versus upper single null asymmetry is related to the origin of intrinsic torque at the separatrix. (Some figures may appear in colour only in the online journal)

Central impurity toroidal rotation in ICRF heated Alcator C-Mod plasmas

Central impurity toroidal rotation has been observed in Alcator C-Mod ICRF heated plasmas, from the Doppler shifts of argon X ray lines. Rotation velocities of up to 1.3 × 105 m/s in the co-current direction have been observed in H mode discharges with no direct momentum input. There is a strong correlation between the increase in the central impurity rotation velocity and the increase in the plasma stored energy, induced by ICRF heating, although other factors may be involved. This implies a close association between energy and momentum confinement. Co-current rotation is also observed during purely ohmic H modes. In otherwise similar discharges with the same stored energy increase, plasmas with lower current rotate faster. For hydrogen minority (D(H)) heating, plasmas with the highest rotation have an H/D ratio between 5 and 10% and have the resonance location in the inner half of the plasma, i.e. in the same conditions that are conducive to the best ICRF absorption and heating. Comparisons with neoclassical theory indicate that the ion pressure gradient is an unimportant contributor to the central impurity rotation and the presence of a substantial core radial electric field is inferred during the ICRF pulse. An inward shift of ions induced by ICRF waves could give rise to a non-ambipolar electric field in the plasma core.

Compressed ion temperature gradient turbulence in diverted tokamak edge

It is found from a heat-flux-driven full-f gyrokinetic particle simulation that there is ion temperature gradient (ITG) turbulence across an entire L-mode-like edge density pedestal in a diverted tokamak plasma in which the ion temperature gradient is mild without a pedestal structure, hence the normalized ion temperature gradient parameter ηi=(d log Ti/dr)/(d log n/dr) varies strongly from high (>4 at density pedestal top/shoulder) to low (<2 in the density slope) values. Variation of density and ηi is in the same scale as the turbulence correlation length, compressing the turbulence in the density slope region. The resulting ion thermal flux is on the order of experimentally inferred values. The present study strongly suggests that a localized estimate of the ITG-driven χi will not be valid due to the nonlocal dynamics of the compressed turbulence in an L-mode-type density slope. While the thermal transport and the temperature profile saturate quickly, the E×B rotation shows a longer time damping during...

Spontaneous rotation sources in a quiescent tokamak edge plasma

Spontaneous rotation sources in a quiescent tokamak edge plasma are studied without an external momentum source, such as, beam injected or wall-born neutrals. Discussions are based upon example neoclassical solutions from an edge gyrokinetic particle code. The main study is performed in a DIII-D plasma [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] with the ion Grad-B drift directed toward the single-null divertor. Comparison with a reversed Grad-B drift case is also shown. It is found that there is a robust spontaneous co-current toroidal plasma rotation source in the far scrape-off plasma due to the wall sheath effect. As the edge pedestal width becomes narrower, the co-current rotation in the far scrape-off becomes weaker, but there appears a stronger co-current rotation in the pedestal top/shoulder from the X-point orbit loss effect, possibly providing a co-rotation boundary condition to the core plasma. Reversal of the magnetic field and plasma current brings down the overall co-rotation, especially in t...

Plasma transport in stochastic magnetic field caused by vacuum resonant magnetic perturbations at diverted tokamak edge

A kinetic transport simulation for the first 4 ms of the vacuum resonant magnetic perturbations (RMPs) application has been performed for the first time in realistic diverted DIII-D tokamak geometry [J. Luxon, Nucl. Fusion 42, 614 (2002)], with the self-consistent evaluation of the radial electric field and the plasma rotation. It is found that, due to the kinetic effects, the stochastic parallel thermal transport is significantly reduced when compared to the standard analytic model [A. B. Rechester and M. N. Rosenbluth, Phys. Rev. Lett. 40, 38 (1978)] and the nonaxisymmetric perpendicular radial particle transport is significantly enhanced from the axisymmetric level. These trends agree with recent experimental result trends [T. E. Evans, R. A. Moyer, K. H. Burrell et al., Nat. Phys. 2, 419 (2006)]. It is also found, as a side product, that an artificial local reduction of the vacuum RMP fields in the vicinity of the magnetic separatrix can bring the kinetic simulation results to a more detailed agreement with experimental plasma profiles.

Electron temperature control with grid bias in inductively coupled argon plasma

The mechanism of controlling electron temperature with grid-biased voltage is studied experimentally and the relevant physics is discussed in an inductively coupled Ar discharge. To obtain the electron density and electron temperature, the electron energy distribution functions (EEDFs) are measured with a Langmuir probe. As the grid voltage decreases negatively, the effective electron temperature is controlled from 2.0 to 0.6 eV and the electron density changes from 3×1010 to 2×1010 cm−3 in the diffusion region, while the effective electron temperature and electron density are not changed in the source region. The dependence of such various parameters, as electron density, electron temperature, plasma potential in each region, and so on, on the applied voltage, is presented. The functional relations between the measured physical quantities are well explained based on a global particle and energy balance relations.