Waves in plasmas: Highlights from the past and present

To illustrate the development of some fundamental concepts in plasma waves, a number of experimental observations, going back over half a century, are reviewed. Particular attention is paid to the phenomena of dispersion, collision-free damping, ray trajectories, amplitude transport, plasma wave echoes, finite-Larmor-radius and cyclotron and cyclotron-harmonic effects, nonlocal response, and mode conversion; to the straight-trajectory approximation and two-level phase mixing; and to quasilinear diffusion and its relation to radio-frequency heating, current drive, and induced neoclassical transport, and to stochasticity and superadiabaticity. Not only is the constructive interplay between experiment and theory noted, but also that major advances have come from each of the many disciplines that invoke plasma physics as a tool, including radio communication, astrophysics, controlled fusion, space physics, and basic research.