A long-wavelength hosing instability in laser-plasma interactions

The nonlinear final state of short-pulse lasers is examined using fully exp licit particle-in-cell simulations. A new long-wavelength hosing instabilit y is found to be dominant after a few Rayleigh lengths of propagation. This instability causes self-trapped electrons to be displaced off axis; we fin d that ion motion is important at the highest densities studied. A possible explanation for this instability is given based on a new variational princ iple analysis for short-pulse lasers propagating in underdense plasma.