The first evidence of x-ray harmonic radiation extending to 3.3 Å , 3.8 keV (order n > 3200) from petawatt class laser-solid interactions is presented, exhibiting relativistic limit efficiency scaling ( n ÿ2:5 -n ÿ3 ) at multi-keV energies. This scaling holds up to a maximum order, n RO 8 1=2 3 , where is the relativistic Lorentz factor, above which the first evidence of an intensity dependent efficiency rollover is observed. The coherent nature of the generated harmonics is demonstrated by the highly directional beamed emission, which for photon energy h > 1 keV is found to be into a cone angle 4 , significantly less than that of the incident laser cone (20 Coherent high order harmonic x-ray generation (HOHG) has the potential to open up the world of physical processes on an attosecond time scale [1][2][3]. The key to this is converting high-power optical laser pulses into broad, phase-locked harmonic spectra extending to multi-keV photon energies-which can be achieved, with unprecedented efficiency and brightness, by reflection off relativistically oscillating plasmas [2,3]. Of particular note is the implication this has for the production of high brightness attosecond pulses [3]. For an attosecond pulse with a fixed fractional bandwidth at a given central frequency n cf ! laser the energy in the pulse scales as [3] att n ÿ1:5 cf ;( 1) where n cf is the harmonic order of the carrier frequency and ! laser the laser frequency. The unique properties of such a source have lead to the investigation of its potential for use in many exciting applications [1,3,4]. The availability of bright attosecond x-ray pulses will allow the probing of the dynamics and properties of atoms and molecules on temporal scales shorter than that of the period of atomic vibrations, i.e., attosecond resolution of bound-free electronic transitions (e.g., from the 4p state of krypton) [5,6].Recently, HOHG pulse production has been cited as a possible route to achieving the huge intensities required for probing the nonlinear quantum electrodynamical properties of the vacuum, providing a significant intensity boost for existing or imminently anticipated laser technology and highlighting the enormous potential of HOHG [4]. These predictions rely on the fact that the focused harmonic radiation can in principle have a substantially higher intensity I max than that of the laser I used to generate them, scaling as I max In 1:5 cf . This is the result of the slow decay of the conversion efficiency for pulse generation ( n ÿ1:5 cf ), coupled with the increased focusability ( n 2 cf ) and temporal compression of the reflected energy ( n cf ). For example, an incident intensity of 10 22 W cm ÿ2 could be refocused to >10 29 W cm ÿ2 corresponding to the critical Schwinger limit [7] electric field of 10 16 V cm ÿ1 for electron-positron pair production from the vacuum [8].In this Letter we show, for the first time, HOHG extending to multi-keV energies and the first experimental evidence for a high frequency rollover of relativistic limit conversion effi...

A beam of multi-MeV helium ions has been observed from the interaction of a short-pulse high-intensity laser pulse with underdense helium plasma. The ion beam was found to have a maximum energy for He2+ of (40(+3)(-8)) MeV and was directional along the laser propagation path, with the highest energy ions being collimated to a cone of less than 10 degrees. 2D particle-in-cell simulations show that the ions are accelerated by a sheath electric field that is produced at the back of the gas target. This electric field is generated by transfer of laser energy to a hot electron beam, which exits the target generating large space-charge fields normal to its boundary.

The acceleration of electrons to '0:8 GeV has been observed in a self-injecting laser wakefield accelerator driven at a plasma density of 5:5 Â 10 18 cm À3 by a 10 J, 55 fs, 800 nm laser pulse in the blowout regime. The laser pulse is found to be self-guided for 1 cm (>10z R), by measurement of a single filament containing >30% of the initial laser energy at this distance. Three-dimensional particle in cell simulations show that the intensity within the guided filament is amplified beyond its initial focused value to a normalized vector potential of a 0 > 6, thus driving a highly nonlinear plasma wave.

Ion acceleration by the interaction of an ultraintense short-pulse laser with an underdense-plasma has been studied at intensities up to 3 x 10(20) W/cm(2). Helium ions having a maximum energy of 13.2+/-1.0 MeV were measured at an angle of 100 degrees from the laser propagation direction. The maximum ion energy scaled with plasma density as n(0.70+/-0.05)(e). Two-dimensional particle-in-cell simulations suggest that multiple collisionless shocks are formed at high density. The interaction of shocks is responsible for the observed plateau structure in the ion spectrum and leads to an enhanced ion acceleration beyond that possible by the ponderomotive potential of the laser alone.

The generation of MeV electron and ion beams using lasers with intensities of up to 10 20 W cm Ϫ2 is reported. Intense ion beams with high energies ͑up to 40 MeV and to 3ϫ10 12 protons Ͼ5 MeV͒ are observed. The properties of these particle beams were measured in considerable detail and the results are compared to current theoretical explanations for their generation.

An experimental investigation of lateral electron transport in thin metallic foil targets irradiated by ultraintense (>or=10(19) W/cm2) laser pulses is reported. Two-dimensional spatially resolved ion emission measurements are used to quantify electric-field generation resulting from electron transport. The measurement of large electric fields ( approximately 0.1 TV/m) millimeters from the laser focus reveals that lateral energy transport continues long after the laser pulse has decayed. Numerical simulations confirm a very strong enhancement of electron density and electric field at the edges of the target.

Proton and heavy ion acceleration in ultrahigh intensity ( approximately 2 x 10(20) W cm(-2) ) laser plasma interactions has been investigated using the new petawatt arm of the VULCAN laser. Nuclear activation techniques have been applied to make the first spatially integrated measurements of both proton and heavy ion acceleration from the same laser shots with heated and unheated Fe foil targets. Fe ions with energies greater than 10 MeV per nucleon have been observed. Effects of target heating on the accelerated ion energy spectra and the laser-to-ion energy conversion efficiencies are discussed. The laser-driven production of the long-lived isotope (57 )Co (271 days) via a heavy ion induced reaction is demonstrated.

Intense laser-plasma interactions produce high brightness beams of gamma rays, neutrons and ions and have the potential to deliver accelerating gradients more than 1000 times higher than conventional accelerator technology, and on a tabletop scale. This paper demonstrates one of the exciting applications of this technology, namely for transmutation studies of long-lived radioactive waste. We report the laser-driven photo-transmutation of long-lived 129 I with a half-life of 15.7 million years to 128 I with a half-life of 25 min. In addition, an integrated cross-section of 97±40 mbarns for the reaction 129 I(γ ,n) 128 I is determined from the measured ratio of the (γ ,n) induced 128 I and 126 I activities. The potential for affordable, easy to shield, tabletop laser technology for nuclear transmutation studies is highlighted.