No abstract

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...

Kalpha x-ray emission, extreme ultraviolet emission, and plasma imaging techniques have been used to diagnose energy transport patterns in copper foils ranging in thickness from 5 to 75 microm for intensities up to 5x10(20) W cm-2. The Kalpha emission and shadowgrams both indicate a larger divergence angle than that reported in the literature at lower intensities [R. Stephens, Phys. Rev. E 69, 066414 (2004)]. Foils 5 microm thick show triple-humped plasma expansion patterns at the back and front surfaces. Hybrid code modeling shows that this can be attributed to an increase in the mean energy of the fast electrons emitted at large radii, which only have sufficient energy to form a plasma in such thin targets.

The emission characteristics of intense laser driven protons are controlled using ultrastrong (of the order of 10(9) V/m) electrostatic fields varying on a few ps time scale. The field structures are achieved by exploiting the high potential of the target (reaching multi-MV during the laser interaction). Suitably shaped targets result in a reduction in the proton beam divergence, and hence an increase in proton flux while preserving the high beam quality. The peak focusing power and its temporal variation are shown to depend on the target characteristics, allowing for the collimation of the inherently highly divergent beam and the design of achromatic electrostatic lenses.

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.