S. C. Chiu

Nondimensional transport scaling in DIII‐D: Bohm versus gyro‐Bohm resolved

The scaling of cross‐field heat transport with relative gyroradius ρ* was measured in low (L) and high (H) mode tokamak plasmas using the technique of dimensionally similar discharges. The relative gyroradius scalings of the electron and ion thermal diffusivities were determined separately using a two‐fluid transport analysis. For L‐mode plasmas, the electron diffusivity scaled as χe∝χBρ1.1±0.3* (gyro‐Bohm‐like) while the ion diffusivity scaled as χi∝χBρ−0.5±0.3* (worse than Bohm‐like). The results were independent of the method of auxiliary heating (radio frequency or neutral beam). Since the electron and ion fluids had different gyroradius scalings, the effective diffusivity and global confinement time scalings were found to vary from gyro‐Bohm‐like to Bohm‐like depending upon whether the electron or ion channel dominated the heat flux. This last property can explain the previously disparate results with dimensionally similar discharges on different fusion experiments that have been published. Experimen...

Fokker-Planck simulations mylb of knock-on electron runaway avalanche and bursts in tokamaks

The avalanche of runaway electrons in an ohmic tokamak plasma triggered by knock-on collisions of traces of energetic electrons with the bulk electrons is simulated by the bounce averaged Fokker-Planck code, CQL3D. It is shown that even when the electric field is small for the production of Dreicer runaways, the knock-on collisions can produce significant runaway electrons in a fraction of a second at typical reactor parameters. The energy spectrum of these knock-on runaways has a characteristic temperature. The growth rate and temperature of the runaway distribution are determined and compared with theory. In simulations of pellet injection into high temperature plasmas, it is shown that a burst of Dreicer runaways may also occur depending on the cooling rate due to the pellet injection. Implications of these phenomena on disruption control in reactor plasmas are discussed.

Theory of fast wave current drive for tokamak plasmas

The paper presents calculations of the efficiency of fast wave current drive at reactor-like densities and temperatures, including toroidal effects. Accessibility and competitive absorption mechanisms are estimated. Two bands of frequencies are found to be of interest for reactor applications – one in the ion cyclotron range of frequencies and the other at higher harmonics but below the lower hybrid frequency.

Radio-frequency-driven radial current and plasma rotation in a tokamak

Plasma rotational shear is potentially important for controlling the formation and positioning of internal transport barriers that could stabilize tokamak microturbulence and improve plasma confinement. A new physical mechanism capable of inducing plasma rotation and rotational shear via the ion cyclotron resonance frequency (ICRF) heating of minority ion species in a tokamak has been proposed [Perkins, White, Bonoli, and Chan, Phys. Plasmas 8, 2181 (2001)]. The present work evaluates the validity of this mechanism under the realistic condition when fast ions are continuously heated and slowed down in a driven system. Ion dynamics are calculated with a Monte Carlo code in which wave-induced diffusion in velocity space is accounted for by a quasilinear operator. The code follows the drift trajectories of test particles in a tokamak geometry under the influence of given rf fields and collisions with the background plasma. When the heating geometry is such that no net toroidal wave momentum is injected, the ...

Coupling of fast waves in the ion cyclotron range of frequencies to H-mode plasmas in DIII-D

Measurements of low power ( 1 mW) antenna loading are used to study the coupling of a compact loop antenna structure to plasmas in the divertor configuration in DIII-D heated by neutral beam injection (NBI) or electron cyclotron heating (ECH). When a transition to the H-mode regime occurs during NBI, the antenna loading resistance drops by approximately a factor of two. This coupling decrease is due to a steepening of the edge density profile near the separatrix, accompanied by a reduction in edge density in the scrape-off layer. During edge localized modes, the opposite effects occur, and the antenna coupling increases transiently. The loading measurements are compared with theoretical calculations which take into account the measured density profiles as well as the conducting side-walls of the recessed antenna housing. Absolute agreement between the theoretical and the experimental results is obtained, including the correct dependence on the density, antenna position, RF frequency and antenna geometry. The theoretical interpretation of the results is discussed, together with the technological implications for future high power experiments.

Characteristics of ICRF heating near the second harmonic

Analytical and numerical results of physical processes taking place around the second-harmonic resonance surface in ICRF heating are presented. It is shown that (1) symmetry of transmission coefficients follow from Onsagers reciprocity relation of the dielectric tensor, and (2) direct dissipation around the cyclotron harmonic layer is mostly due to the Bernstein branch and depends on k||, becoming low for low k||. The latter has the consequence that mode conversion and reflection are sensitively reduced by damping, but transmission is not.

Fast wave and electron cyclotron current drive in the DIII-D tokamak

The non-inductive current drive from directional fast Alfven and electron cyclotron waves was measured in the DIII-D tokamak in order to demonstrate these forms of radiofrequency (RF) current drive and to compare the measured efficiencies with theoretical expectations. The fast wave frequency was 8 times the deuterium cyclotron frequency at the plasma centre, while the electron cyclotron wave was at twice the electron cyclotron frequency. Complete non-inductive current drive was achieved using a combination of fast wave current drive (FWCD) and electron cyclotron current drive (ECCD) in discharges for which the total plasma current was inductively ramped down from 400 to 170 kA. For steady current discharges, an analysis of the loop voltage revealed up to 195 kA of non-inductive current (out of 310 kA) during combined electron cyclotron and fast wave injection, with a maximum of 110 kA of FWCD and 80 kA of ECCD achieved (not simultaneously). The peakedness of the current profile increased with RF current drive, indicating that the driven current was centrally localized. The FWCD efficiency increased linearly with the central electron temperature as expected; however, the FWCD was severely degraded in low current discharges owing to incomplete fast wave absorption. The measured FWCD agreed with the predictions of a ray tracing code only when a parasitic loss of 4% per pass was included in the modelling along with multiple pass absorption. Enhancement of the second harmonic ECCD efficiency by the toroidal electric field was observed experimentally. The measured ECCD was in good agreement with Fokker-Planck code predictions

Monte Carlo orbit/full wave simulation of ion cyclotron resonance frequency wave damping on resonant ions in tokamaks

To investigate the experimentally observed interaction between beam ion species and fast Alfven wave (FW), a Monte Carlo code, ORBIT-RF [V. S. Chan, S. C. Chiu, and Y. A. Omelchenko, Phys. Plasmas 9, 501 (2002)], which solves the time-dependent Hamiltonian guiding center drift equations, has been upgraded to incorporate a steady-state neutral beam ion slowing-down distribution, a quasilinear high harmonic radio frequency diffusion operator and the wave fields from the two-dimensional ion cyclotron resonance frequency full wave code (TORIC4) [M. Brambilla, Plasma Phys. Controlled Fusion 41, 1 (1999)]. Comparison of ORBIT-RF simulation of power absorption with fixed amplitudes of FW fields from TORIC4 power absorption calculation, which assumes Maxwellian plasma distributions, attains agreement within a factor of two. The experimentally measured enhanced neutron rate is reproduced to within 30% from ORBIT-RF simulation using a single dominant toroidal and poloidal wave number.

Toroidal mode conversion in the ICRF

Mode conversion is studied in the ion cyclotron range of frequencies (ICRF), taking into account the toroidal geometry relevant for tokamaks. The global wavefields obtained using the gyrokinetic toroidal PENN code illustrate how the fast wave propagates to the neighbourhood of the ion-ion hybrid resonance, where it is converted to a slow wave that deposits the wave energy through resonant Landau and cyclotron interactions with the particles. The power deposition profiles obtained are dramatically different from the toroidal resonance absorption, showing that Buddens fluid model is not a good approximation in the torus. Radially and poloidally localized wavefield structures characteristic of slow wave eigenmodes are predicted, which could be used in experiments to form transport barriers and to interact with fast particles.

Ion Bernstein wave antenna loading measurements on the DIII-D tokamak

Antenna loading measurements carried out during high power ion Bernstein wave (IBW) heating experiments on the DIII-D tokamak indicate that efficient, direct coupling to the IBW at ω 2ωci as predicted by linear coupling theory did not occur. Whereas strong peaking in the loading resistance near cyclotron harmonics is predicted for high edge densities (ω < ωLH|edge), the observed loading resistance is nearly independent of the toroidal magnetic field. The loading anomaly can be explained by invoking the ponderomotive force which decreases the edge density immediately in front of the antenna, thus allowing coupling to the cold plasma lower hybrid wave (LHW). A linear LHW coupling code including the modified density profile due to the ponderomotive force reproduces the measured dependence of antenna loading on toroidal magnetic field, edge density, antenna frequency and antenna phasing. Further evidence for the ponderomotive force is obtained from reactive loading measurements which indicate that the plasma is pushed away from the antenna as the radiofrequency power level is increased. The results indicate that the lack of central ion heating observed during DIII-D IBW experiments may be due to a lack of efficient mode transformation from the coupled LHW to a centrally propagating IBW, possibly as a result of nonlinear mechanism(s)