Particle-in-cell simulations of hot electron generation using defocused laser light in cone targets

The effects of defocusing a high intensity pulse of laser light on the generation of hot electrons in a cone are investigated using particle-in-cell simulations. The results indicate that defocused laser light can soften the electron energy spectrum and increase the coupling efficiency compared to the use of a laser in tight focus. It is shown that this is a consequence of the density profile of plasma produced by the laser prepulse, which is less dense in the case of the defocused laser. The relevance of this result to fast ignition inertial confinement fusion is discussed

Producing shock-ignition-like pressures by indirect drive

The shock ignition scheme is an alternative Inertial Confinement Fusion ignition scheme that offers higher gains and a robustness to hydrodynamic instabilities. A desirable aspect of shock ignition is that the required intensities are achievable on existing facilities. Conventional approaches to shock ignition have only considered the use of direct laser drive. This is in part due to concerns that achieving the rapid rise in drive pressure needed in the final pressure spike may not be feasible using the indirect drive approach. The primary advantage of being able to utilise a hohlraum drive for a shock ignition experiment is that experiments could be carried out at existing, or soon to be completed, Mega-Joule scale facilities. Furthermore, this could be done without the need for any major modification to the facility architecture, such as would be required for direct drive experiments. One and two-dimensional radiation hydrodynamic simulations have been performed using the codes HYADES and h2d. The simulations investigated the level of x-ray fluxes that could produce shock ignition scale pressures as well as the laser powers that would be required to generate those pressures in a NIF scale-1 hohlraum. The second aspect of this work was to investigate the x-ray flux rise times that would be necessary to create a large enough shock ignition spike pressure (200-300 Mbar). It was found that pressures of 230 Mbar could be achieved through indirect drive using a laser source with a peak power of 400 TW. In addition, the rate of pressure increase in the final pressure spike is similar to the expected requirements for directly-driven shock ignition

Core electrons and specific heat capacity in the fast electron heating of solids

The accuracy with which the Thomas–Fermi (TF) model can provide electronic specific heat capacities for use in calculations relevant to fast electron transport in laser-irradiated solids is examined. It is argued that the TF model, since it neglects the quantum shell structure, is likely to be significantly inaccurate for low- and intermediate-Z materials. This argument is supported by examining the results of calculations using more sophisticated methods that account for degeneracy, the quantum shell structure, and other non-ideal corrections. It is further shown that the specific heat capacity curve generated by this more advanced treatment leads to substantial (factor of two) changes in fast electron transport simulations relative to similar modelling based upon the TF model

Ignition criteria for x-ray fast ignition inertial confinement fusion

The derivation of the ignition energy for fast ignition inertial confinement fusion is reviewed and one-dimensional simulations are used to produce a revised formula for the ignition energy of an isochoric central hot-spot, which accounts for variation in the radius of the hot-spot r_h as well as the density rho. The required energy may be as low as 1 kJ when rho*r_h ~ 0:36 g cm^-2; T ~ 20 keV, and rho greater or equal to 700 g cm^-2. Although there are many physical challenges to creating these conditions, a possible route to producing such a hot-spot is via a bright source of nonthermal soft x-rays. Further one-dimensional simulations are used to study the non-thermal soft x-ray heating of dense DT and it is found to offer the potential to significantly reduce hydrodynamic losses as compared to particle driven fast ignition due to the hotspot being heated supersonically in a layer-by-layer fashion. A sufficiently powerful soft x-ray source would be difficult to produce, but line emission from laser-produced-plasma is the most promising option

Compact acceleration of energetic neutral atoms using high intensity laser-solid interaction

Recent advances in high-intensity laser-produced plasmas have demonstrated their potential as compact charge particle accelerators. Unlike conventional accelerators, transient quasi-static charge separation acceleration fields in laser produced plasmas are highly localized and orders of magnitude larger. Manipulating these ion accelerators, to convert the fast ions to neutral atoms with little change in momentum, transform these to a bright source of MeV atoms. The emittance of the neutral atom beam would be similar to that expected for an ion beam. Since intense laser-produced plasmas have been demonstrated to produce high-brightness-low-emittance beams, it is possible to envisage generation of high-flux, low-emittance, high energy neutral atom beams in length scales of less than a millimeter. Here, we show a scheme where more than 80% of the fast ions are reduced to energetic neutral atoms and demonstrate the feasibility of a high energy neutral atom accelerator that could significantly impact applications in neutral atom lithography and diagnostics

Controlling X-Ray Flux in Hohlraums Using Burn-through Barriers

A technique for controlling X-ray flux in hohlraums is presented. In Indirect Drive Inertial Confinement Fusion (ICF) the soft X-rays arriving at the spherical fuel capsule are required to have a specific temporal profile and high spatial uniformity in order to adequately compress and ignite the fuel. Conventionally this is achieved by modifying the external driver, the hohlraum geometry, and the sites of interaction between the two. In this study a technique is demonstrated which may have utility in a number of scenarios, both related to ICF and otherwise, in which precise control over the X-ray flux and spatial uniformity are required. X-ray burn-through barriers situated within the hohlraum are shown to enable control of the flux flowing to an X-ray driven target. Control is achieved through the design of the barrier rather than by modification of the external driver. The concept is investigated using the one-dimensional (1-D) radiation hydrodynamics code HYADES in combination with a three-dimensional (3-D) time-dependent viewfactor code

L-shell spectroscopy of neon and fluorine like copper ions from laser produced plasma

Ne, F, and O-like Rydberg resonance lines along with some of the inner shell satellite lines of Copper plasma, in the wavelength range of 7.9-9.5 Å, are experimentally observed using a thallium acid phthalate crystal spectrometer. The plasma is produced by the irradiation of a Cu target with a 15 J, 500 ps Nd: Glass laser with a focusable intensity up to 5 × 10 14 W/cm 2 . The observed lines result from the transitions among 2p-nd, 2p-ns, and 2s-nd (n = 4-6) levels. Transition wavelengths, transition probabilities, and oscillator strengths of these lines are calculated using the Multi-Configuration Dirac-Fock method. In this computation, the contribution of relativistic corrections such as two-body Breit corrections and QED corrections due to vacuum polarization and self-energy has also been considered. FLYCHK simulations are used to analyze the distribution of the various charge states of the Copper ions and to find the temperature and density of plasma. Moreover, the effect of self-absorption of the plasma (opacity), as well as of suprathermal electrons on charge state distribution of ions, is also studied. The synthetic spectrum provides a best-match with the experimental spectrum at a laser intensity of 1.3 × 10 14 W/cm 2 for T c = 150 eV, T h = 1000 eV, f = 0.008, and density 4.5 × 10 20 cm −3 .The temperature and density ranges are also calculated using a radiative hydrodynamic code. The calculated temperature and density range are in agreement with the experimentally determined values. The effect of the change in laser intensity on the L-shell spectrum of Cu is studied which indicates the switching between lower (Cu XX) and higher charge states (Cu XXI and Cu XXII) at higher laser intensities

Investigation of the performance of mid-Z hohlraum wall liners for producing X-ray drive

M-band transitions (n = 4 to n = 3) in Gold are responsible for a population of X-rays with energy > 1.8 keV in indirect drive inertial fusion. These X-rays can preheat the fuel, cause the ablator-fuel interface to become unstable to Rayleigh-Taylor instabilities, and introduce radiation non-uniformity to the X-ray drive. This work investigates the performance of mid-Z lined hohlraums for producing an efficient drive spectrum absent of M-band X-rays using the two-dimensional lagrangian radiation hydrodynamics code h2d. The removal of the M-band transitions is observed in the Cu-lined hohlraum reducing the total X-ray energy above 1.8 keV to 58% that of the un-lined hohlraum. Total radiation energy in the Cu-lined hohlraum is 93% that of the energy in the pure Au hohlraum for a 1 ns pulse. However, the soft X-ray drive energy (below 1.8 keV) for the lined hohlraum is 98% that of the pure Au hohlraum

Hydrodynamic motion of guiding elements within a magnetic switchyard in fast ignition conditions

Magnetic collimation via resistivity gradients is an innovative approach to electron beam control for the cone-guided fast ignition variant of inertial confinement fusion. This technique uses a resistivity gradient induced magnetic field to collimate the electron beam produced by the high-intensity laser–plasma interaction within a cone-guided fast ignition cone-tip. A variant of the resistive guiding approach, known as the “magnetic switchyard,” has been proposed which uses shaped guiding elements to direct the electrons toward the compressed fuel. Here, the 1D radiation-hydrodynamics code HYADES is used to investigate and quantify the gross hydrodynamic motion of these magnetic switchyard guiding elements in conditions relevant to their use in fast ignition. Movement of the layers was assessed for a range of two-layer material combinations. Based upon the results of the simulations, a scaling law is found that enables the relative extent of hydrodynamic motion to be predicted based upon the material properties of the switchyard, thereby enabling optimization of material-combination choice on the basis of reducing hydrodynamic motion. A multi-layered configuration, more representative of an actual switchyard, was also simulated in which an outer Au layer is employed to tamp the motion of the outermost guiding element of the switchyard

Mass selection in laser-plasma ion accelerator on nanostructured surfaces

When an intense laser pulse interacts with a solid surface, ions get accelerated in the laser-plasma due to the formation of transient longitudinal electric field along the target normal direction. However, the acceleration is not mass-selective. The possibility of manipulating such ion acceleration scheme to enhance the energy of one ionic species (either proton or carbon) selectively over the other species is investigated experimentally using nanopore targets. For an incident laser intensity of approximately 5×1017 W/cm2, we show that the acceleration is optimal for protons when the pore diameter is about 15-20 nm, while carbon ions are optimally accelerated when the pore diameter is close to 40-50 nm. The observed effect is due to tailoring targetry by the pulse pedestal of the laser prior to the arrival of the main pulse