Direct Evidence of Ponderomotive Filamentation in a Laser-Produced Plasma

We present an experiment in which the ponderomotive filamentation instability is driven in a preformed, underdense plasma. The spatial profile of the driving beam is intentionally modulated to produce a striated intensity distribution of known periodicity. Dark-field imaging is used to observe the density perturbations produced by the instability. The observed threshold intensity for perturbation wavelengths of 42 μm is approximately 3×${10}^{13}$ W/${\mathrm{cm}}^2$ which compares favorably with homogeneous instability theory.