Measuring the Alfvén wave Parametric Decay Instability Growth Rate in the Laboratory*

Alfvén waves, a fundamental mode of magnetized plasmas, are ubiquitous in lab and space. The non-linear behavior of these modes is thought to play a key role in important problems in space plasma such as the heating of the solar corona and solar wind turbulence. In particular, theoretical predictions show that these Alfvén waves may be unstable to various parametric instabilities, but space observations of these processes are limited. We present an experiment on the Large Plasma Device at UCLA aimed at measuring the Parametric Decay Instability (PDI) growth rate in the laboratory. In these experiments, a high amplitude δB/B₀~0.7% pump Alfvén wave is launched from one end of the device and a smaller seed Alfvén wave is launched from the other side. When the frequency of the seed wave is chosen to match the backward wave expected from PDI, damping of the seed wave is reduced. This reduction in damping is well-matched within errorbar to the theoretically expected PDI growth rate and scales with the pump wave amplitude.  Accompanying numerical simulations show that this measurement does not work at all axial locations because the seed wave amplitude is modulated at about half the seed wavelength when the pump is on; this modulation was recently verified experimentally and work is underway to determine the underlying physical mechanism for incorporation into our growth rate measurement technique.  Results may help validate PDI theories and establish signatures for future space observations.