NERSC Initiative for Scientific Exploration (NISE) 2011 Awards

Numerical Study of Mode Conversion and Ion Heating in Laboratory-Relevant Plasmas

Paul Cassak, West Virginia University

Associated NERSC Project: Three Dimensional and Diamagnetic Effects on Magnetic Reconnection (m866)

One effect of sound waves propagating through air is the transport of energy from one location to another. Waves in plasmas (gases that are so hot they are ionized) also transport energy. However, there are additional waves that can be excited in plasmas compared to waves in the air (because of the importance of electromagnetic fields), including so-called Alfven waves. When a wave propagating through a plasma reaches a region with a different plasma density, the wave can “mode convert” into a different type of wave. This mode conversion can lead to nonlinear interactions between modes, which can stimulate wave-particle interactions and ultimately transfer energy from the wave into the ionized particles making up the plasma. This behavior is ubiquitous in naturally occurring and experimental plasmas. Mode conversion and ion heating occur in laboratory studies of fusion in toroidal confinement devices (tokamaks), which makes this topic important for renewable energy production. It is also known or thought to occur in the Earth’s magnetosphere between adjacent ionized layers of differing density, and this type of energy transport has been cited as a possible mechanism for heating the solar atmosphere (the corona) to 200 times the temperature of the solar surface.

Mode conversion and ion heating is currently being studied in laboratory experiments at West Virginia University, the PI's home institution. A high density helicon source, which maintains a strong density gradient, excites waves in a magnetized plasma. Previous experiments identified that mode conversion and ion heating occurs. The proposed simulation study will support these ongoing experimental investigations. We will simulate mode conversion and heating properties of plasma waves, such as Alfven waves, by simulating their launch into an inhomogeneous plasma. Using the simulations, we will answer the following questions: How does the nature of the mode conversion and the energization of ions depend on the density inhomogeneity? How do they depend on the amplitude and wavelength of the mode launched from the antenna? What is the effect of the geometry of the inhomogeneity?

The experimental device in question is only about 10 times bigger than the radius at which ions gyrate around the device’s magnetic field. For this reason, and because a focus of the present study is on the kinetic heating of ions in the plasma, the magnetohydrodynamic description which describes a plasma as a fluid is inadequate. In preliminary simulations with a hybrid code (with electrons described as a fluid and ions described with the particle-in-cell model), it was determined that kinetic electron physics cannot be ignored if an accurate depiction of the ion heating process is desired, which necessitates the use of fully particle-in-cell simulations for the present study. We will use the P3D code, an electromagnetic particle-in-cell which is already in working order on NERSC computers, to carry out these simulations.