S. Ku

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.

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...

Gyrokinetic particle simulation of neoclassical transport in the pedestal/scrape-off region of a tokamak plasma

A gyrokinetic neoclassical solution for a diverted tokamak edge plasma has been obtained for the first time using the massively parallel Jaguar XT3 computer at Oak Ridge National Laboratory. The solutions show similar characteristics to the experimental observations: electric potential is positive in the scrape-off layer and negative in the H-mode layer, and the parallel rotation is positive in the scrape-off layer and at the inside boundary of the H-mode layer. However, the solution also makes a new physical discovery that there is a strong ExB convective flow in the scrape-off plasma. A general introduction to the edge simulation problem is also presented.

Neoclassical physics in full distribution function gyrokinetics

Treatment of binary Coulomb collisions when the full gyrokinetic distribution function is evolved is discussed here. A spectrum of different collision operators is presented, differing through both the physics that can be addressed and the numerics they are based on. Eulerian-like (semi-Lagrangian) and particle in cell (PIC) (Monte-Carlo) schemes are successfully cross-compared, and a detailed confrontation to neoclassical theory is shown.

Suppression of Landau damping via electron band gap

The pondermotive potential in the x-ray Raman compression can generate an electron band gap, which suppresses the Landau damping. The regime is identified where a Langmuir wave can be driven without damping in the stimulated Raman compression. It is shown that the partial wave breaking and the frequency detuning due to the trapped particles would be greatly reduced.

Backward Raman compression of x-rays in metals and warm dense matters

Experimentally observed decay rate of the long wavelength Langmuir wave in metals and dense plasmas is orders of magnitude larger than the prediction of the prevalent Landau damping theory. The discrepancy is explored, and the existence of a regime where the forward Raman scattering is stable and the backward Raman scattering is unstable is examined. The amplification of a x-ray pulse in this regime, via the backward Raman compression, is computationally demonstrated, and the optimal pulse duration and intensity is estimated.

Theory of plasmon decay in dense plasmas and warm dense matters

The prevalent Landau damping theory for classical plasmas does not fully explain the Langmuir wave decay in dense plasmas. A dielectric function theory adapted from the condensed matter physics is extended to be applicable for the dense plasmas and warm dense matters. This theory, accounting for the Umklapp process, predicts much higher decay rates than the Landau damping theory, which results in better agreement with the available experimental data obtained from the metals. The detailed calculations are presented for the following four cases: classical plasmas, Maxwellian plasmas, degenerate quantum plasmas, and partially degenerate plasmas.

Photonic band gap and x-ray optics in warm dense matter

Photonic band gaps for the soft x-rays, formed in the periodic structures of solids or dense plasmas, are theoretically investigated. Optical manipulation mechanisms for the soft x-rays, which are based on these band gaps, are computationally demonstrated. The reflection and amplification of the soft x-rays, and the compression and stretching of chirped soft x-ray pulses are discussed. A scheme for lasing with atoms with two energy levels, utilizing the band gap, is also studied.

Plasmon band gap generated by intense ion acoustic waves

In the presence of an intense ion acoustic wave, the energy-momentum dispersion relation of plasmons is strongly modified to exhibit a band gap structure. The intensity of an ion acoustic wave might be measured from the band gap width. The plasmon band gap can be used to block the nonlinear cascading channel of the Langmuir wave decay.