Search

Copper activation was used to characterize high-energy proton beam acceleration from near-critical density plasma targets. An enhancement was observed when decreasing the target density, which is indicative for an increased laser-accelerated hot electron density at the rear target-vacuum boundary. This is due to channel formation and collimation of the hot electrons inside the target. Particle-in-cell simulations support the experimental observations and show the correlation between channel depth and longitudinal electric field strength is directly correlated with the proton acceleration.

Anomalous reduction of the fusion yields by 50% and anomalous scaling of the burn-averaged ion temperatures with the ion-species fraction has been observed for the first time in D[superscript 3]He-filled shock-driven inertial confinement fusion implosions. Two ion kinetic mechanisms are used to explain the anomalous observations: thermal decoupling of the D and [superscript 3]He populations and diffusive species separation. The observed insensitivity of ion temperature to a varying deuterium fraction is shown to be a signature of ion thermal decoupling in shock-heated plasmas. The burn-averaged deuterium fraction calculated from the experimental data demonstrates a reduction in the average core deuterium density, as predicted by simulations that use a diffusion model. Accounting for each of these effects in simulations reproduces the observed yield trends., United States. National Nuclear Security Administration (Grant DE-NA0001857), University of Rochester. Fusion Science Center (Grant 415023-G), National Laser User’s Facility (Grant DE-NA0002035), University of Rochester. Laboratory for Laser Energetics (Grant 415935-G), Lawrence Livermore National Laboratory (Grant B600100)

Clear evidence of the transition from hydrodynamiclike to strongly kinetic shock-driven implosions is, for the first time, revealed and quantitatively assessed. Implosions with a range of initial equimolar D[superscript 3]He gas densities show that as the density is decreased, hydrodynamic simulations strongly diverge from and increasingly overpredict the observed nuclear yields, from a factor of ∼2 at 3.1 mg/cm[superscript 3] to a factor of 100 at 0.14 mg/cm[superscript 3]. (The corresponding Knudsen number, the ratio of ion mean-free path to minimum shell radius, varied from 0.3 to 9; similarly, the ratio of fusion burn duration to ion diffusion time, another figure of merit of kinetic effects, varied from 0.3 to 14.) This result is shown to be unrelated to the effects of hydrodynamic mix. As a first step to garner insight into this transition, a reduced ion kinetic (RIK) model that includes gradient-diffusion and loss-term approximations to several transport processes was implemented within the framework of a one-dimensional radiation-transport code. After empirical calibration, the RIK simulations reproduce the observed yield trends, largely as a result of ion diffusion and the depletion of the reacting tail ions., United States. Dept. of Energy (Grant DE-NA0001857), United States. Dept. of Energy (Grant DE-FC52-08NA28752), University of Rochester. Fusion Science Center (5-24431), National Laser User’s Facility (DE-NA0002035), University of Rochester. Laboratory for Laser Energetics (415935-G), Lawrence Livermore National Laboratory (B597367)

A strong nonhydrodynamic mechanism generating atomic fuel-shell mix has been observed in strongly shocked inertial confinement fusion implosions of thin deuterated-plastic shells filled with [superscript 3]He gas. These implosions were found to produce D[superscript 3]He-proton shock yields comparable to implosions of identical shells filled with a hydroequivalent 50∶50 D[superscript 3]He gas mixture. Standard hydrodynamic mixing cannot explain this observation, as hydrodynamic modeling including mix predicts a yield an order of magnitude lower than was observed. Instead, these results can be attributed to ion diffusive mix at the fuel-shell interface., United States. Dept. of Energy (Grant DE-NA0001857), University of Rochester. Fusion Science Center (Grant 5-24431), National Laser User’s Facility (Grant DE-NA0002035), University of Rochester. Laboratory for Laser Energetics (Grant 415935-G), Lawrence Livermore National Laboratory (Grant B597367), United States. National Nuclear Security Administration (Stewardship Science Graduate Fellowship DE-FC52-08NA28752)

We present experimental results of laser-to-proton conversion efficiency as a function of lateral confinement of the refluxing electrons. Experiments were carried out using the T-Cubed laser at the Center for Ultrafast Optical Science, University of Michigan. We demonstrate that the laser-to-proton conversion efficiency increases by 50% with increased confinement of the target from surroundings with respect to a flat target of the same thickness. Three-dimensional hybrid particle-in-cell simulations using LSP code agree with the experimental data. The adopted target design is suitable for high repetition rate operation as well as for Inertial Confinement Fusion applications. (C) 2013 AIP Publishing LLC.

Recent progress in generating high-energy (>50 MeV) protons from intense laser–matter interactions (1018–1021Wcm2; refs 1–7) has opened up new areas of research, with applications in radiography8, oncology9, astrophysics10, medical imaging11, high-energy-density physics12–14, and ion-proton beam fast ignition15–19. With the discovery of proton focusing with curved surfaces20,21, rapid advances in these areas will be driven by improved focusing technologies. Here we report on the first investigation of the generation and focusing of a proton beam using a cone-shaped target.We clearly show that the focusing is strongly affected by the electric fields in the beam in both open and enclosed (cone) geometries, bending the trajectories near the axis. Also in the cone geometry, a sheath electric field effectively ‘channels’ the proton beam through the cone tip, substantially improving the beam focusing properties. These results agree well with particle simulations and provide the physics basis for many future applications.

Each successive generation of X-ray machines has opened up new frontiers in science, such as the first radiographs and the determination of the structure of DNA. State-of-the-art X-ray sources can now produce coherent high-brightness Xrays of greater than kiloelectronvolt energy and promise a new revolution in imaging complex systems on nanometre and femtosecond scales. Despite the demand, only a few dedicated synchrotron facilities exist worldwide, in part because of the size and cost of conventional (accelerator) technology(1). Here we demonstrate the use of a new generation of laser-driven plasma accelerators(2), which accelerate high-charge electron beams to high energy in short distances(3-5), to produce directional, spatially coherent, intrinsically ultrafast beams of hard X-rays. This reduces the size of the synchrotron source from the tens of metres to the centimetre scale, simultaneously accelerating and wiggling the electron beam. The resulting X-ray source is 1,000 times brighter than previously reported plasma wigglers(6,7) and thus has the potential to facilitate a myriad of uses across the whole spectrum of light-source applications.

Experiments were performed to investigate the propagation of a high intensity (I similar to 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.

X-ray harmonic radiation extending to 3.3 angstrom, 3.8 keV from Petawatt class laser-solid interactions is presented. The harmonic spectra display a relativistic limit scaling up to similar to 3000th order, above which an intensity dependent scaling roll-over is observed. Highly directional beamed emission for harmonic photon energy h nu > 1 keV is found to be into a cone angle

As the state of the art for high power laser systems increases from terawatt to petawatt level and beyond, a crucial parameter for routinely monitoring high intensity performance is laser spot size on a solid target during an intense interaction in the tight focus regime (<10 mu m). Here we present a novel, simple technique for characterizing the spatial profile of such a laser focal spot by imaging the interaction region in third harmonic order (3 omega(laser)). Nearly linear intensity dependence of 3 omega(laser) generation for interactions > 10(19) Wcm(-2) is demonstrated experimentally and shown to provide the basis for an effective focus diagnostic. Importantly, this technique is also shown to allow in-situ diagnosis of focal spot quality achieved after reflection from a double plasma mirror setup for very intense high contrast interactions (> 10(20) Wcm(-2)) an important application for the field of high laser contrast interaction science.

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 converted into x rays.

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.

Significant reduction of inherent large divergence of the laser driven MeV proton beams is achieved by strong (of the order of 10(9) V/m) electrostatic focussing field generated in the confined region of a suitably shaped structure attached to the proton generating foil. The scheme exploits the positively charging of the target following an intense laser interaction. Reduction in the proton beam divergence, and commensurate increase in proton flux is observed while preserving the beam laminarity. The underlying mechanism has been established by the help of particle tracing simulations. Dynamic focussing power of the lens, mainly due to the target discharging, can also be exploited in order to bring up the desired chromaticity of the lens for the proton beams of broad energy range.

Proton imaging has become a common diagnostic technique for use in laser-plasma research experiments due to their ability to diagnose electric field effects and to resolve small density differences caused through shock effects. These interactions are highly dependent on the use of radiochromic film (RCF) as a detection system for the particle probe, and produces very high-resolution images. However, saturation effects, and in many cases, damage to the film limits the usefulness of this technique for high-flux particle probing. This paper outlines the use of a new technique using contact radiography of (p,n)-generated isotopes in activation samples to produce high dynamic range 2D images with high spatial resolution and extremely high dynamic range, whilst maintaining both energy resolution and absolute flux measurements. (C)007 Elsevier B.V. All rights reserved.

An intense laser pulse interacting with a near discontinuous plasma vacuum interface causes the plasma surface to perform relativistic oscillations. The reflected laser radiation then contains very high order harmonics of fundamental frequency and-according to current theory-must be bunched in radiation bursts of a few attoseconds duration. Recent experimental results have demonstrated x-ray harmonic radiation extending to 3.3 angstrom (3.8 keV, order n > 3200) with the harmonic conversion efficiency scaling as eta(n) n(-2.5) over the entire observed spectrum ranging from 17 nm to 3.3 angstrom. This scaling holds up to a maximum order, n(RO) 81 8(1/2)gamma(3), where gamma is the peak value of the Lorentz factor, above which the harmonic efficiency decreases more rapidly. The coherent nature of the generated harmonics is demonstrated by the highly directional beamed emission, which for photon energy h nu > 1 keV is found to be into a cone angle similar to 4 degrees, significantly less than that of the incident laser cone (20 degrees).

The first evidence of x-ray harmonic radiation extending to 3.3 A, 3.8 keV (order n > 3200) from petawatt class laser-solid interactions is presented, exhibiting relativistic limit efficiency scaling (eta similar to n(-2.5)-n(-3)) at multi-keV energies. This scaling holds up to a maximum order, n(RO)similar to 8(1/2)gamma(3), where gamma 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 nu > 1 keV is found to be into a cone angle similar to 4 degrees, significantly less than that of the incident laser cone (20 degrees).