Thomas G. Jenkins

Time-domain simulation of nonlinear radiofrequency phenomena

Nonlinear effects associated with the physics of radiofrequency wave propagation through a plasma are investigated numerically in the time domain, using both fluid and particle-in-cell (PIC) methods. We find favorable comparisons between parametric decay instability scenarios observed on the Alcator C-MOD experiment [J. C. Rost, M. Porkolab, and R. L. Boivin, Phys. Plasmas 9, 1262 (2002)] and PIC models. The capability of fluid models to capture important nonlinear effects characteristic of wave-plasma interaction (frequency doubling, cyclotron resonant absorption) is also demonstrated.

High-performance finite-difference time-domain simulations of C-Mod and ITER RF antennas

Finite-difference time-domain methods have, in recent years, developed powerful capabilities for modeling realistic ICRF behavior in fusion plasmas [1, 2, 3, 4]. When coupled with the power of modern high-performance computing platforms, such techniques allow the behavior of antenna near and far fields, and the flow of RF power, to be studied in realistic experimental scenarios at previously inaccessible levels of resolution. In this talk, we present results and 3D animations from high-performance FDTD simulations on the Titan Cray XK7 supercomputer, modeling both Alcator C-Mod’s field-aligned ICRF antenna and the ITER antenna module. Much of this work focuses on scans over edge density, and tailored edge density profiles, to study dispersion and the physics of slow wave excitation in the immediate vicinity of the antenna hardware and SOL. An understanding of the role of the lower-hybrid resonance in low-density scenarios is emerging, and possible implications of this for the NSTX launcher and power balan...

Vibrational modes of thin oblate clouds of charge

A numerical method is presented for finding the eigenfunctions (normal modes) and mode frequencies of azimuthally symmetric non-neutral plasmas confined in a Penning trap whose axial thickness is much smaller than their radial size. The plasma may be approximated as a charged disk in this limit; the normal modes and frequencies can be found if the surface charge density profile σ(r) of the disk and the trap bounce frequency profile ωz(r) are known. The dependence of the eigenfunctions and equilibrium plasma shapes on nonideal components of the confining Penning trap fields is discussed. The results of the calculation are compared with the experimental data of Weimer et al. [Phys. Rev. A 49, 3842 (1994)] and it is shown that the plasma in this experiment was probably hollow and had mode displacement functions that were concentrated near the center of the plasma.

Improvements to the ICRH antenna time-domain 3D plasma simulation model

We present a summary of ongoing improvements to the 3D time-domain plasma modeling software that has been used to look at ICRH antennas on Alcator C-Mod, NSTX, and ITER [1]. Our past investigations have shown that in low density cases where the slow wave is propagating, strong amplitude lower hybrid resonant fields can occur. Such a scenario could result in significant parasitic power loss in the SOL. The primary resonance broadening in this case is likely collisions with neutral gas, and thus we are upgrading the model to include realistic neutral gas in the SOL, in order to provide a better understanding of energy balance in these situations. Related to this, we are adding a temporal variation capability to the local plasma density in front of the antenna in order to investigate whether the near fields of the antenna could modify the local density sufficiently to initiate a low density situation. We will start with a simple scalar ponderomotive potential density expulsion model [2] for the density evolu...

Fluid equations in the presence of electron cyclotron current drive

Two-fluid equations, which include the physics imparted by an externally applied radiofrequency source near electron cyclotron resonance, are derived in their extended magnetohydrodynamic forms using the formalism of Hegna and Callen [Phys. Plasmas 16, 112501 (2009)]. The equations are compatible with the closed fluid/drift-kinetic model developed by Ramos [Phys. Plasmas 17, 082502 (2010); 18, 102506 (2011)] for fusion-relevant regimes with low collisionality and slow dynamics, and they facilitate the development of advanced computational models for electron cyclotron current drive-induced suppression of neoclassical tearing modes.

Developing chemistry, visualization, and RF sheath modeling tools for fusion and low-temperature plasma simulations

Summary form only given. We present an overview of ongoing enhancements to VSim, a plasma modeling code capable of both PIC/MCC and fluid FDTD representations. A new sub-grid kinetic sheath boundary condition1 enables the physical effects of DC and RF sheath physics to be included in macroscopic-scale plasma simulations that need not explicitly resolve sheath scale lengths. We demonstrate that evolution of sheath potentials, together with the ensuing particle fluxes and sputtering effects, can thus be simulated on complex plasma-facing components such as the ICRF antenna in the Alcator C-Mod fusion device. Generalizations of the sheath boundary condition to include multiple ion species and other relevant physical effects (e.g. secondary electron emission) will also be presented.Complex chemistry scenarios arising in low-temperature plasmas can be modeled in VSim via an interface with MUNCHKIN, a standalone python/C++/SQL code that identifies reaction paths for a given set of input species, solves 1D rate equations to analyze the time-dependent chemical evolution of the system, and generates corresponding VSim input blocks with appropriate cross-sections or reaction rates. These features, together with others such as calculation of reaction rates from user-specified distribution functions, principal path analysis (for reducing the number of simulated chemical reactions while retaining accuracy), and parallelization for high-performance analysis, will also be discussed. In addition, we will demonstrate the reconstruction of time-varying species energy and angular distribution functions from PIC data, enabling key physics processes in existing experimental and industrially relevant plasmas to be explored. A variety of techniques, including singular-value decomposition and binning, can be used for this reconstruction, and a comparison of techniques will be shown. We will also discuss how the VisIt2 visualization software can be used to generate complex physics animations from VSim data.

RF models for plasma-surface interactions: Sheath boundary conditions with dielectrics

Computational models for DC and oscillatory (RF-driven) sheath potentials, arising at metal or dielectric-coated surfaces in contact with plasma, are developed from first principles using particle-in-cell modeling in the VSim FDTD code. These results are used to formulate a subgrid kinetic sheath boundary condition [1], applicable in both fluid [2] and particle VSim modeling scenarios, wherein sub-grid models for sheaths or dielectric-modified sheaths retain self-consistency in modeling salient physical properties (e.g. sheath-modified particle and heat fluxes) at material surfaces. In this manner, sheath potentials, EEDF evolution, and sputtering physics associated with sheath formation can be included in macroscopic simulations which need not resolve the spatial scales of the sheath explicitly. Ultimately, the developed models will be used to simulate plasma-facing ICRF antenna structures in existing and future magnetic fusion experiments (e.g. Alcator C-Mod, ITER), assessing the efficacy of dielectric-coated antenna surfaces in reducing sputtering-induced high-Z impurity contamination of the fusion reaction. They may also have applications for industrial plasma modeling scenarios.