Interferometric determination of the sound speed in magnetized plasma using time-delayed correlation techniques

Time-delayed correlation measurements with a CO2 laser, multichord plasma interferometer show density perturbations propagating along the local magnetic field in the edge region of the ZT-40M reversed field pinch. The observed speed is consistent with ion-acoustic propagation. Such measurements of field-parallel waves with long wavelength (λ∥ ≳several centimeters) may provide a quasilocal, quasi-cw diagnostic of the sound speed in many low β magnetically confined plasmas, useful in estimating plasma temperatures.