Parametric instabilities of plasma channels

Summary form only given. Plasma waveguides for guiding intense laser pulses have applications in particle acceleration and x-ray generation schemes. Waveguides can be formed using a variety of methods. One method is to create a plasma channel by breaking down a gas with a laser pulse focused through an axicon. Ideally, the plasma channel will be axially symmetric and allow for guided single mode propagation of short laser pulses. However, for certain experimental conditions the channel develops periodic axial modulations. The heating of the guided mode and the axicon field leads to modulations in the heating rate and a ponderomotive force which reinforce the modulations in the expansion rate, in other words, there is a parametric instability. A simple model of this process will be presented. This model predicts both the onset of this instability and the modulation period for the channel as related to the background gas pressure.