We present measurements of a magnetic reconnection in a plasma created by two laser beams (1 ns pulse duration, 1 x 10(15) W cm(-2)) focused in close proximity on a planar solid target. Simultaneous optical probing and proton grid deflectometry reveal two high velocity, collimated outflowing jets and 0.7-1.3 MG magnetic fields at the focal spot edges. Thomson scattering measurements from the reconnection layer are consistent with high electron temperatures in this region.

The dynamics of plasma electrons in the focus of a petawatt laser beam are studied via measurements of their x-ray synchrotron radiation. With increasing laser intensity, a forward directed beam of x-rays extending to 50 keV is observed. The measured x-rays are well described in the synchrotron asymptotic limit of electrons oscillating in a plasma channel. The critical energy of the measured synchrotron spectrum is found to scale as the maxwellian temperature of the simultaneously measured electron spectra. At low laser intensity transverse oscillations are negligible as the electrons are predominantly accelerated axially by the laser generated wakefield. At high laser intensity, electrons are directly accelerated by the laser and enter a highly radiative regime with up to 5% of their energy turned into x-rays. PACS numbers: Valid PACS appear hereThe advent of high power lasers has led to rapid progress in the field of plasma based particle acceleration [1]. In particular, the measurement of monoenergetic electron beams from wakefields generated by short lasers [2] has stimulated great interest in producing such beams and understanding their dynamics. One potential use for these compact sources of energetic particles is as a driver for novel light sources. Laser-accelerated electrons could be injected into a magnetic undulator realizing a compact tunable-energy femtosecond x-ray source synchronized to the laser. A laser-based x-ray source could be downsized further, using the self-generated magnetic and electrostatic fields of the plasma channel as a miniature undulator [3]. For electron beams of sufficiently high quality, an ion channel laser analogous to conventional free electron lasers may be feasible [4]. X-rays can also be produced in intense laser-plasma interactions by nonlinear Thomson scattering [5].Relativistic electron beams have also been measured from interactions at very high laser intensities, where electrons gain energy directly from the laser [6]. At high intensity, the ponderomotive force of the laser can expel plasma electrons leaving a positively charged ion channel. Electrons inside the channel experience a net focusing force due to the space charge and undergo oscillation at the betatron frequency ω β = ω p / √ 2γ z0 , where ω p is the plasma frequency and γ z0 is the Lorentz factor associated with the electrons motion along the plasma channel. Electrons resonant with the laser frequency can gain energy from the transverse electric field of the laser, which can be directed into longitudinal momentum through the v × B force [7]. Accelerating charges radiate electromagnetic radiation. For small betatron strength parameters a β = γ z0 r β ω β /c 1 (undulator limit), the spectrum of the radiation will be narrowly peaked about the resonant fre-is the Doppler factor and α is the angle between the direction of observation and the direction of γ z0 [8]. This highlights the interdependency of spectral and angular distributions. As a β → 1, emitted radiation also appears at harmonics of the resonant...

The propagation of ultraintense laser pulses through matter is connected with the generation of strong moving magnetic fields in the propagation channel as well as the formation of a thin ion filament along the axis of the channel. Upon exiting the plasma the magnetic field displaces the electrons at the back of the target, generating a quasistatic electric field that accelerates and collimates ions from the filament. Two dimensional particle-in-cell simulations show that a 1 PW laser pulse tightly focused on a near-critical density target is able to accelerate protons up to an energy of 1.3 GeV. Scaling laws and optimal conditions for proton acceleration are established considering the energy depletion of the laser pulse.

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...

We examine a regime in which a linearly-polarized laser pulse with relativistic intensity irradiates a subcritical plasma for much longer than the characteristic electron response time. A steady-state channel is formed in the plasma in this case with quasi-static transverse and longitudinal electric fields. These relatively weak fields significantly alter the electron dynamics. The longitudinal electric field reduces the longitudinal dephasing between the electron and the wave, leading to an enhancement of the electron energy gain from the pulse. The energy gain in this regime is ultimately limited by the superluminosity of the wave fronts induced by the plasma in the channel. The transverse electric field alters the oscillations of the transverse electron velocity, allowing it to remain anti-parallel to laser electric field and leading to a significant energy gain. The energy enhancement is accompanied by development of significant oscillations perpendicular to the plane of the driven motion, making trajectories of energetic electrons three-dimensional. Proper electron injection into the laser beam can further boost the electron energy gain.

Experiments were performed to investigate the propagation of a high intensity (I approximately 10(21) W cm(-2)) laser in foam targets with densities ranging from 0.9n(c) to 30n(c). Proton acceleration was used to diagnose the interaction. An improvement in proton beam energy and efficiency is observed for the lowest density foam (n(e)=0.9n(c)), compared to higher density foams. Simulations show that the laser beam penetrates deeper into the target due to its relativistic propagation and results in greater collimation of the ensuing hot electrons. This results in the rear surface accelerating electric field being larger, increasing the efficiency of the acceleration. Enhanced collimation of the ions is seen to be due to the self-generated azimuthal magnetic and electric fields at the rear of the target.

Short pulse laser interactions at intensities of 2×10(21) W cm(-2) with ultrahigh contrast (10(-15)) on submicrometer silicon nitride foils were studied experimentally by using linear and circular polarizations at normal incidence. It was observed that, as the target decreases in thickness, electron heating by the laser begins to occur for circular polarization leading to target normal sheath acceleration of contaminant ions, while at thicker targets no acceleration or electron heating is observed. For linear polarization, all targets showed exponential energy spreads with similar electron temperatures. Particle-in-cell simulations demonstrate that the heating is due to the rapid deformation of the target that occurs early in the interaction. These experiments demonstrate that finite spot size effects can severely restrict the regime suitable for radiation pressure acceleration.

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.