Self-focusing, channel formation, and high-energy ion generation in interaction of an intense short laser pulse with a He jet

Using interferometry, we investigate the dynamics of interaction of a relat ivistically intense 4-TW, 400-fs laser pulse with a He gas jet. We observe a stable plasma channel 1 mm long and less than 30 mu m in diameter, with a radial gradient of electron density similar to 5 x 10(22) cm(-4) and with an on-axis electron density approximately ten times less than its maximum v alue of 8 x 10(19) cm(-3). A high radial velocity of the surrounding gas io nization of similar to 3.8 x 10(8) cm/s has been observed after the channel formation, and it is attributed to the fast ions expelled from the laser c hannel and propagating radially outward. We developed a kinetic model which describes the plasma channel formation and the subsequent ambient gas exci tation and ionization, Comparing the model predictions with the interferome tric data, we reconstructed the axial profile of laser channel and on-axis laser intensity. The estimated maximum energy of accelerated ions is about 500 keV, and the total energy of the fast ions is 5% of the laser pulse ene rgy.