Due to their particular properties, the beams of the multi-MeV protons generated during the interaction of ultraintense (I>1019 W/cm2) short pulses with thin solid targets are most suited for use as a particle probe in laser-plasma experiments. The recently developed proton imaging technique employs the beams in a point-projection imaging scheme as a diagnostic tool for the detection of electric fields in laser-plasma interaction experiments. In recent investigations carried out at the Rutherford Appleton Laboratory (RAL, UK), a wide range of laser-plasma interaction conditions of relevance for inertial confinement fusion (ICF)/fast ignition has been explored. Among the results obtained will be discussed: the electric field distribution in laser-produced long-scale plasmas of ICF interest; the measurement of highly transient electric fields related to the generation and dynamics of hot electron currents following ultra-intense laser irradiation of targets; the observation in underdense plasmas, after the ...

Electric field detection in laser-plasma interaction experiments via the proton imaging technique

Proton imaging is a recently proposed technique for diagnosis of dense plasmas, which favourably exploits the properties of protons produced by high-intensity laser-matter interaction. The technique allows the distribution of electric fields in plasmas and around laser-irradiated targets to be explored for the first time with high temporal and spatial resolution. This leads to the possibility of investigating as yet unexplored physical issues. In particular we will present measurements of transient electric fields in laser-plasmas and around laser-irradiated targets under various interaction conditions. Complex electric field structures have been observed in long-scale laser-produced plasmas, while global target charge-up and growth of electromagnetic instabilities have been detected following ultraintense interactions with solid targets.

http://iopscience.iop.org/article/10.1088/0741-3335/43/12A/320/pdf

Proton imaging: a diagnostic for inertial confinement fusion/fast ignitor studies

An iterative algorithm for constructing fully continuous phase screens for tailoring far-field intensity profiles is presented. The algorithm is robust, stable, and, if run properly, maintains the continuous nature of the phase throughout the iterative process. The iterative procedure is applied to generate continuous phase screens to produce a 12th-power super-Gaussian far-field intensity profile.

Designing fully continuous phase screens for tailoring focal-plane irradiance profiles

The optical reflectance of a strong shock front in water increases continuously with pressure above 100 GPa and saturates at ∼45% reflectance above 250 GPa. This is the first evidence of electronic conduction in high pressure water. In addition, the water Hugoniot equation of state up to 790 GPa (7.9 Mbar) is determined from shock velocity measurements made by detecting the Doppler shift of reflected light. From a fit to the reflectance data we find that an electronic mobility gap ∼2.5 eV controls thermal activation of electronic carriers at pressures in the range of 100–150 GPa. This suggests that electronic conduction contributes significantly to the total conductivity along the Neptune isentrope above 150 GPa.

/pdf/electronic-conduction-in-shock-compressed-water-2i3m68q5hn.pdf

Electronic conduction in shock-compressed water

The energy deposition of ions in dense plasmas is a key process in inertial confinement fusion that determines the α-particle heating expected to trigger a burn wave in the hydrogen pellet and resulting in high thermonuclear gain However, measurements of ion stopping in plasmas are scarce and mostly restricted to high ion velocities where theory agrees with the data Here, we report experimental data at low projectile velocities near the Bragg peak, where the stopping force reaches its maximum This parameter range features the largest theoretical uncertainties and conclusive data are missing until today The precision of our measurements, combined with a reliable knowledge of the plasma parameters, allows to disprove several standard models for the stopping power for beam velocities typically encountered in inertial fusion On the other hand, our data support theories that include a detailed treatment of strong ion-electron collisions

/pdf/experimental-discrimination-of-ion-stopping-models-near-the-4yzungw0vf.pdf

Experimental discrimination of ion stopping models near the Bragg peak in highly ionized matter.

The generation of uniform focal intensity profiles is important for a number of applications, including laser–plasma interaction experiments. We report on a focusing system that uses a novel binary-phase optic capable of producing efficient two-dimensional uniform top-hat intensity optical and x-ray profiles.

Binary-phase zone plate arrays for the generation of uniform focal profiles

We report on the theory and development of a diffractive element composed of a binary phase zone-plate array. This component conditions the intensity distribution in the focal plane of a conventional refractive lens to generate efficiently (82%) a flattop intensity envelope on target. Analysis of the design indicates that manufacturing tolerances are not critical. Experimental performances on target from x-ray emission and shock-breakout measurements are also presented.

Binary phase zone-plate arrays for laser-beam spatial-intensity distribution conversion

This paper reports on the development of several diffractive optical elements (DOE) to fulfill applications on high power Nd glass laser systems The measured performance for those components realized is discussed These are focusing beam samplers, beam shapers, and harmonic separation filters (HSF) Designs of more demanding components operating in the resonance domain are also presented These are linear polarizing elements and beam deflectors

Diffractive optics development for application on high-power solid state lasers

Binary-phase optics have been used by a number of high-power laser laboratories in order to achieve relatively smooth focal spots. However, the intensity envelopes have in general been of a sinc form rather than `top-hat.' This paper presents work on the production of uniform `top-hat' intensity focal spot profiles obtained from Fresnel binary phase zone plate (PZP) arrays of various designs. Phase plates are used to generate large area smooth focal spots and both theoretical and experimental focal spots are presented. These demonstrate the flexibility of this technique which provides a simple method of generating both uniform `top-hat' intensity profiles and spatially shaped foci, for use with high-power lasers.

Binary-phase Fresnel zone plate arrays for high-power laser beam smoothing

The smoothing of focal plane distributions of high-power lasers is of particular importance to laser-plasma interaction studies There are many techniques used to achieve the smoothing and this paper presents an overview of the methods as applied on the VULCAN high power Nd:glass laser at the Central Laser Facility, Rutherford Appleton Laboratory These have played an important part in high-power laser-plasma interaction experiments into fusion studies, energy transport, filamentation etc These smoothing techniques all involve placing an optical component in the laser beam to produce either a spatial, temporal, or spectral modification of the laser pulse and these are discussed under the topics of Random Phase Plates (RPP), Induced Spatial Incoherence (ISI), and Smoothing by Spectral Dispersion (SSD) We also describe techniques for tailoring the focal profile using RPPs© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering Downloading of the abstract is permitted for personal use only

R. M. Stevenson

Papers

Binary-phase zone plate arrays for the generation of uniform focal profiles

Binary phase zone-plate arrays for laser-beam spatial-intensity distribution conversion

Diffractive optics development for application on high-power solid state lasers

Binary-phase Fresnel zone plate arrays for high-power laser beam smoothing

Focal spot smoothing and tailoring for high-power laser applications