Study of electron avalanches produced by multiphoton ionisation of O$_2$ using the Pulsed Townsend Technique*

We report the observation of electron swarms using the Pulsed Townsend Technique (PTT), using a 266 nm laser pulse (3 ns) traversing the interelectrode volume and ionising O$_2$ by multiphoton ionization, thereby generating a cathodeless electron avalanche. The resulting measurements of the electron drift velocity, W, and the density-reduced effective ionisation coefficient, $\alpha _{eff} = (\alpha - \eta)/N$, derived from the measured current are compared those obtained with a ''formal'' pulsed Townsend experiment in which the initial photoelectrons are produced by a pulse of UV light on the cathode. The W data tie in fairly well with those measured by the ''formal'' PTT method. Implications of this effect for laser assisted plasma diagnostics and special electron or ion sources will be discussed.