  An advance in experimental plasma diagnostics is presented and used to make
the first measurement of a plasma velocity-space cross-correlation matrix. The
velocity space correlation function can detect collective fluctuations of
plasmas through a localized measurement. An empirical decomposition, singular
value decomposition, is applied to this Hermitian matrix in order to obtain the
plasma fluctuation eigenmode structure on the ion distribution function. A
basic theory is introduced and compared to the modes obtained by the
experiment. A full characterization of these modes is left for future work, but
an outline of this endeavor is provided. Finally, the requirements for this
experimental technique in other plasma regimes are discussed.
