The Beating Wave
Experiment (BWX) investigates what is potentially a very
efficient method of energizing (or heating) plasma with electrostatic
(ES) waves. One area of application for such a heating mechanism is in
plasma propulsion. However, before any specific application is found, it
is essential to investigate and understand the fundamentals of the
wave-particle interaction.
Electromagnetic waves and plasma interact in a complex manner. To
simplify the analysis, it is often assumed that the wave amplitude is
small – linearity approximation. Unfortunately, beating electrostatic
wave – ion heating is fundamentally a non-linear mechanism. One can’t
obtain an exact solution to the ion motion during ES heating.
Furthermore, most (but not all) studies focus on resonant types of wave – plasma interaction. A good analogy to the resonant interactions in plasma is the famous example how soldiers marching in step over a bridge caused the bridge to collapse. Another analogy is a surfer trying to catch waves. If he simply floats in the water it will not work. However if he pedals a little and matches his speed to that of the wave, he will ride the wave.
In plasma, one
example of resonant interaction is a phenomenon known as Landau damping
where charged particles with velocities close to the phase velocity of
the propagating wave, vp = w/k,
are accelerated/slowed until their velocity becomes equal to the wave's
phase velocity.
An important application of resonant plasma heating is called Ion Cyclotron Resonance Heating (ICRH). Here, an electromagnetic circularly polarized wave is launched into the plasma such that some ions are rotating nearly in resonance with the wave. Once the resonance ions are accelerated by the wave, they transfer the energy they gained from the wave to other ions in the plasma via collisions.
As opposed to resonant phenomena, the interaction that is being studied in the Beating Wave Experiment is a non-resonant type of wave – plasma interaction. During this type of interaction an electrostatic beating wave “creates” resonance broadening – there is no single resonance. Many more ions are affected by the beating wave. This type of interaction has been observed to take place in the ionosphere [1], and is suspected to cause some instabilities in fusion devices.
