We have used multiframe picosecond optical interferometry to make direct measurements of the hole boring velocity, $v_{\mathrm{HB}}$, of the density cavity pushed forward by a train of $\mathrm{C}{\mathrm{O}}_2$ laser pulses in a near critical density helium plasma. As the pulse train intensity rises, the increasing radiation pressure of each pulse pushes the density cavity forward and the plasma electrons are strongly heated. After the peak laser intensity, the plasma pressure exerted by the heated electrons strongly impedes the hole boring process and the $v_{\mathrm{HB}}$ falls rapidly as the laser pulse intensity falls at the back of the laser pulse train. A heuristic theory is presented that allows the estimation of the plasma electron temperature from the measurements of the hole boring velocity. The measured values of $v_{\mathrm{HB}}$, and the estimated values of the heated electron temperature as a function of laser intensity are in reasonable agreement with those obtained from two-dimensional numerical simulations.

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• Laser-plasma interactions
• Optical interferometry