arXiv subjects

P. T. Bonoli

Publications and source records attributed to P. T. Bonoli.

6 recordsLinked to original sources

arXiv2026

Parametric Decay of Broadband Lower Hybrid Waves

We derive a nonlinear dispersion relation for the parametric decay of an arbitrary lower hybrid pump spectrum, generalizing the standard three-wave treatment to the broadband case. Finite pump bandwidth is found to reorganize the decay spectrum, collapsing the nonresonant continuum predicted by monochromatic theory into narrow bands localized at ion cyclotron harmonics. This reorganization is distinct from the gradual suppression usually associated with finite pump coherence: it occurs at modest bandwidth, while growth rates remain large. Accounting for finite bandwidth restores agreement between theoretical growth rate spectra and sideband measurements from lower hybrid current drive experiments on Alcator C-Mod. More broadly, these results suggest that pump coherence can qualitatively determine the accessible decay channels in magnetized plasmas with kinetic resonances.

physics.plasm-ph↗

arXiv2022

A quasi-local inhomogeneous dielectric tensor for arbitrary distribution functions

Treatments of plasma waves usually assume homogeneity, but the parallel gradients ubiquitous in plasmas can modify wave propagation and absorption. We derive a quasilocal inhomogeneous correction to the plasma dielectric for arbitrary distributions by expanding the phase correlation integral and develop a novel integration technique that allows our correction to be applied in many situations and has greater accuracy than other inhomogeneous dielectric formulas found in the literature. We apply this dielectric tensor to the lower-hybrid current drive problem and demonstrate that inhomogeneous wave damping does not affect the lower-hybrid wave's linear damping condition, and in the non-Maxwellian problem damping and propagation should remain unchanged except in the case of waves with very large phase velocities.

physics.plasm-ph↗

arXiv2022

Verifying raytracing/Fokker-Planck lower-hybrid current drive predictions with self-consistent full-wave/Fokker-Planck simulations

Raytracing/Fokker-Planck (FP) simulations used to model lower-hybrid current drive (LHCD) often fail to reproduce experimental results, particularly when LHCD is weakly damped. A proposed reason for this discrepancy is the lack of "full-wave" effects, such as diffraction and interference, in raytracing simulations and the breakdown of raytracing approximation. Previous studies of LHCD using non-Maxwellian full-wave/FP simulations have been performed, but these simulations were not self-consistent and enforced power conservation between the FP and full-wave code using a numerical rescaling factor. Here we have created a fully-self consistent full-wave/FP model for LHCD that is automatically power conserving. This was accomplished by coupling an overhauled version of the non-Maxwellian TORLH full-wave solver and the CQL3D FP code using the Integrated Plasma Simulator. We performed converged full-wave/FP simulations of Alcator C-Mod discharges and compared them to raytracing. We found that excellent agreement in the power deposition profiles from raytracing and TORLH could be obtained, however, TORLH had somewhat lower current drive efficiency and broader power deposition profiles in some cases. This discrepancy appears to be a result of numerical limitations present in the TORLH model and a small amount of diffractional broadening of the TORLH wave spectrum. Our results suggest full-wave simulation of LHCD is likely not necessary as diffraction and interference represented only a small correction that could not account for the differences between simulations and experiment.

physics.plasm-ph↗

arXiv2020

Disruption Avoidance via RF Current Condensation in Magnetic Islands Produced by Off-Normal Events

As tokamaks are designed and built with increasing levels of stored energy in the plasma, disruptions become increasingly dangerous. It has been reported that 95% of the disruptions in the Joint European Torus (JET) tokamak with the ITER-like wall are preceded by the growth of large locked islands, and these large islands are mostly produced by off-normal events other than neoclassical tearing modes. This paper discusses the use of RF current drive to stabilize large islands, focusing on nonlinear effects that appear when relatively high powers are used to stabilize large islands. An RF current condensation effect can concentrate the RF driven current near the center of the island, increasing the efficiency of the stabilization. A nonlinear shadowing effect can hinder the stabilization of islands if the aiming of the ray trajectories does not properly consider the nonlinear effects.

physics.plasm-ph↗

arXiv2020

Generation of Localized Lower-Hybrid Current Drive By Temperature Perturbations

Despite high demonstrated efficiency, lower-hybrid current drive (LHCD) has not been considered localized enough for neoclassical tearing mode (NTM) stabilization in tokamaks. This assessment must be reconsidered in view of the RF current condensation effect. We show that an island with a central hot spot induces significant localization of LHCD. Furthermore, in steady state tokamaks where a significant amount of current is provided by LHCD, passive stabilization of NTMs may occur automatically, particularly as islands become large, without requiring precise aiming of the wave power.

physics.plasm-ph↗

arXiv2010

The lower hybrid wave cutoff: A case study in eikonal methods

Eikonal, or ray tracing, methods are commonly used to estimate the propagation of radio frequency fields in plasmas. While the information gained from the rays is quite useful, an approximate solution for the fields would also be valuable, e.g., for comparison to full wave simulations. Such approximations are often difficult to perform numerically because of the special care which must be taken to correctly reconstruct the fields near reflection and focusing caustics. In this paper, we compare the standard eikonal method for approximating fields to a method based on the dynamics of wave packets. We compare the approximations resulting from these two methods to the analytical solution for a lower hybrid wave reflecting from a cutoff. The algorithm based on wave packets has the advantage that it can correctly deal with caustics, without any special treatment.

physics.plasm-ph↗