D. Adams

Diagnostic of laser contrast using target reflectivity

Using three different laser systems, we demonstrate a convenient and simple plasma based diagnostic of the contrast of high-power short-pulse lasers. The technique is based on measuring the specular reflectivity from a solid target. The reflectivity remains high even at relativistic intensities above 10(19) W/cm(2) in the case of a high-contrast prepulse-free laser. On the contrary, the specular reflectivity drops with increasing intensities in the case of systems with insufficient contrast due to beam breakup and increased absorption caused by preplasma.

High contrast plasma mirror: spatial filtering and second harmonic generation at 1019 W cm-2

Recently, the use of plasma optics to improve temporal pulse contrast has had a remarkable impact on the field of high-power laser-solid density interaction physics. Opening an avenue to previously unachievable plasma density gradients in the high intensity focus, this advance has enabled researchers to investigate new regimes of harmonic generation and ion acceleration. Until now, however, plasma optics for fundamental laser reflection have been used in the sub-relativistic intensity regime (10 15 -10 16 Wcm 2 ) showing high reflectivity ( 70%) and good focusability. Therefore, the question remains as to whether plasma optics can be used for such applications in the relativistic intensity regime (>10 18 Wcm 2 ). Previous studies of plasma mirrors (PMs) indicate that, for 40fs laser pulses, the reflectivity fluctuates by an order of magnitude and that focusability of the beam is lost as the intensity is increased above 5◊10 16 Wcm 2 . However, these experiments were performed using laser pulses with a contrast ratio of 10 7 to generate the reflecting surface. Here, we present results for PM operation using high contrast laser pulses resulting 5 Author to whom any correspondence should be addressed.

Spectral modification of laser-accelerated proton beams by self-generated magnetic fields

Target normal measurements of proton energy spectra from ultrathin (50-200 nm) planar foil targets irradiated by 10(19) W cm(-2) 40 fs laser pulses exhibit broad maxima that are not present in the energy spectra from micron thickness targets (6 mu m). The proton flux in the peak is considerably greater than the proton flux observed in the same energy range in thicker targets. Numerical modelling of the experiment indicates that this spectral modification in thin targets is caused by magnetic fields that grow at the rear of the target during the laser-target interaction.

New developments in energy transfer and transport studies in relativistic laser?plasma interactions

Two critical issues related to the success of fast ignition inertial fusion have been vigorously investigated in a co-ordinated campaign in the European Union and the United States. These are the divergence of the fast electron beam generated in intense, PW laser–plasma interactions and the fast electron energy transport with the use of high intensity contrast ratio laser pulses. Proof is presented that resistivity gradient-induced magnetic fields can guide fast electrons over significant distances in (initially) cold metallic targets. Comparison of experiments undertaken in both France and the United States suggests that an important factor in obtaining efficient coupling into dense plasma is the irradiation with high intensity contrast ratio laser pulses, rather than the colour of the laser pulse itself.

Magnetic collimation of petawatt driven fast electron beam for prospective fast ignition studies

Collimated transport of fast electron beam through solid density matter is one of the key issues behind the success of the fast ignition scheme by means of which the required amount of ignition energy can be delivered to the hot spot region of the compressed fuel. Here we report on a hot electron beam collimation scheme in solids by tactfully using the strong magnetic fields generated by an electrical resistivity gradient according to Faradays law. This was accomplished by appropriately fabricating the targets in such a way that the electron beam is directed to flow in a metal which is embedded in a much lower resistivity and atomic number metal. Experimental results showed guided transport of hot electron beam over hundreds of microns length inside solid density plasma, which were obtained from two experiments examining the scheme for petawatt laser driven hot electron beam while employing various target configurations.

Diffraction Limited Harmonic Emission from Laser Produced Plasmas

High order harmonic generation (HOHG) from intense laser — solid density interactions has emerged as a promising route to the generation of attosecond pulses extending to keV energies. With efficiency of the nth harmonic, η n , scaling as ~ n −2.5− n −3 in the relativistic limit up to a maximum order n max~ 81/2γ3 (where γ is the maximum relativistic Lorentz factor of the oscillating plasma surface), the potential for a bright solution to attosecond science is a distinct possibility. Some of the recent work performed in the field verifies for the first time that the exceptional coherence properties of the driving laser can be transferred directly to the high harmonic orders produced when plasma surfaces are driven to relativistic velocities under the action of intense laser pulses.