When a terawatt-peak-power laser beam is focused into a gas jet, an electron plasma wave, driven by forward Raman scattering, is observed to accelerate a naturally collimated beam of electrons to relativistic energies (up to 10(9) total electrons, with an energy distribution maximizing at 2 megaelectron volts, a transverse emittance as low as 1 millimeter-milliradian, and a field gradient of up to 2 gigaelectron volts per centimeter). Electron acceleration and the appearance of high-frequency modulations in the transmitted light spectrum were both found to have sharp thresholds in laser power and plasma density. A hole in the center of the electron beam may indicate that plasma electrons were expelled radially.

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Sarkisov, G.S.; Bychenkov, B. Yu.; Novikov, V.N.; Tikhonchuk, V.T.; Maksimchuk, Anatoly; Chen, Shouyuan; Wagner, R.; Mourou, G.; and Umstadter, Donald, "Self-focusing, channel formation, and high-energy ion generation in interaction of an intense short laser pulse with a He jet" (1999 Using interferometry, we investigate the dynamics of interaction of a relativistically 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 m in diameter, with a radial gradient of electron density ϳ5ϫ10 22 cm Ϫ4 and with an on-axis electron density approximately ten times less than its maximum value of 8ϫ10 19 cm Ϫ3 . A high radial velocity of the surrounding gas ionization of ϳ3.8ϫ10 8 cm/s has been observed after the channel formation, and it is attributed to the fast ions expelled from the laser channel and propagating radially outward. We developed a kinetic model which describes the plasma channel formation and the subsequent ambient gas excitation and ionization. Comparing the model predictions with the interferometric 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 energy. ͓S1063-651X͑99͒07505-4͔ PACS number͑s͒: 52.40.Nk, 52.75. Di, 52.35.Mw