Nonlinear Wave-Particle Interactions in Plasmas (Theoretical)

Robert (Bob) Dewar, Department of Theoretical Physics/Research School of Physical Sciences & Engineering
robert.dewar@anu.edu.au, 02 6125 2949, 0407 240 515

Objectives

To better understand the complex interactions between waves and particles in plasmas, both from a general point and view and in connection with experiments on the H-1NF heliac, and in the Space Plasma, Power and Propulsion (SP3) group.

Description

Because a plasma system combines fluid, particle and electromagnetic properties, there are many types of waves that can propagate in it. Sometimes these waves are deliberately excited to heat the plasma and sometimes they arise spontaneously as instabilities. The latter are often unwelcome, but are a fact of life and we need to understand how to predict and control them if the plasma is to be successfully confined.

Theory seeks to find relatively simple models that can be analysed using rigorous mathematical techniques. Although it is often possible to estimate the real part of the frequency w of a plasma wave using a fluid model (combined with electrodynamics), the imaginary part often depends on resonant interactions between waves and particles.

In a hot plasma, the collisions between particles can often be neglected, so we can use Hamiltonian dynamics to describe the interaction between particles and waves; the interactions typically being nonlinear, and often partially chaotic [1]. The simplest model exhibiting wave-particle interactions is a one-dimensional wave with sinusoidal electrostatic potential, wave vector k, and phase velocity w /k, resonant particles being those with velocity v ≈ w /k. Transforming to a frame moving at w /k, the particle dynamics becomes completely analogous to that of the physical pendulum.

Resonant wave-particle interactions in which energetic alpha particles produced by fusion reactions give up energy to Alfvén waves, rather than to hydrogen ions, are thought to pose a serious threat to the achievement of fusion power. The H-1NF heliac is being used to understand the physics of such phenomena experimentally.

A possible theoretical Honours project is the extension of the models in [2–3] and application to understanding experimental results.

References

1. Y. Elskens and D. Escande, Microscopic Dynamics of Particles and Chaos (Institute of Physics Publishing, Bristol, 2003).
   
2. R.L. Dewar, Saturation of Kinetic Plasma Instabilities by Particle Trapping, Phys. Fluids 16, 431 (1973).
   
3. H.L. Berk, B.N. Breizman and M. Pekker, Nonlinear Dynamics of a Driven Mode near Marginal Stability, Phys. Rev. Letters 76, 1256 (1996).