Recently achieved high intensities of short laser pulses open new prospects in their application to hole boring in inhomogeneous overdense plasmas and for ignition in precompressed DT fusion targets. A simple analytical model and numerical simulations demonstrate that pulses with intensities exceeding 10;{22} W/cm;{2} may penetrate deeply into the plasma as a result of efficient ponderomotive acceleration of ions in the forward direction. The penetration depth as big as hundreds of microns depends on the laser fluence, which has to exceed a few tens of GJ/cm;{2}. The fast ions, accelerated at the bottom of the channel with an efficiency of more than 20%, show a high directionality and may heat the precompressed target core to fusion conditions.

The interaction of short and intense laser pulses with plasmas is a very efficient source of relativistic electrons with tunable properties. In low-density plasmas, we observed bunches of electrons up to 200 MeV, accelerated in the wakefield of the laser pulse. Less energetic electrons (tens of megaelectronvolt) have been obtained, albeit with a higher efficiency, during the interaction with a pre-exploded foil or a solid target. When these relativistic electrons slow down in a thick tungsten target, they emit very energetic Bremsstrahlung photons which have been diagnosed directly with photoconductors, and indirectly through photonuclear activation measurements. Dose, photoactivation, and photofission measurements are reported. These results are in reasonable agreement, over three orders of magnitude, with a model built on laser–plasma interaction and electron transport numerical simulations.

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