Measuring correlations with two lasers reveals the kinetic signature of plasma waves

The plasma velocity-space cross-correlation matrix describes collective plasma kinetic fluctuations. Fluid and magnetohydrodynamic descriptions account for fluctuations in space, not particle velocity and its correlation with waves. For the first time, two University of Iowa plasma physicists employed two laser induced fluorescence (LIF) systems to measure the plasma velocity-space cross-correlation matrix. They report on their research in Physics of Plasmas.

In their University of Iowa plasma device, Sean Mattingly and Fred Skiff excited ion sound waves and aimed two LIF systems at the same plasma location to correlate ion oscillations at different velocities. These cross-correlations in the ion velocity distribution function were obtained as a function of frequency. The researchers used “singular value decomposition” to determine the velocity-space wave structure at each frequency and remove the effects of noise.

In addition to techniques for suppressing noise and subtracting background light, Mattingly says that it was important to verify the known symmetries of the matrix (when Fourier transformed in time, the matrix should be self-adjoint with respect to the two velocity indices) and to find a way to separate out mode eigenvectors. These vectors serve as a generalized wave-admittance which can be used to identify modes.

Skiff says that, given the fundamental nonlocal response of plasmas, measuring particle-orbit perturbations at one location provides information on “the environment all around, including sorting out the plasma’s multiple waves.” One application of this technique could be with spacecraft, Skiff says, because spacecraft use localized measurements to probe broader environments. The technique does not require LIF, just a way to measure particle velocity distributions. Both Skiff and Mattingly underscore that this research opens an empirical basis from which theorists can work toward an understanding of plasma phase-space fluctuations.

Source: “Velocity space degrees of freedom of plasma fluctuations,” by Sean Mattingly and Fred Skiff, Physics of Plasmas (2017). The article can be accessed at https://doi.org/10.1063/1.4996012 .