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

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

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

Direct electron attachment to fast hydrogen in 10^-9 contrast 10^18 Wcm^-2 intense laser solid target interaction

The interaction of an ultra-short (10^18 Wcm^-2) laser pulse with a solid target is not generally known to produce and accelerate negative ions. The transient accelerating electrostatic-fields are so strong that they ionize any atom or negative ion at the target surface. In spite of what may appear to be unfavourable conditions, here it is reported that H- ions extending up to 80 keV are measured from such an interaction. The H- ion flux is about 0.1 % that of the H+ ions at 20 keV. These measurements employ a recently developed temporally-gated Thomson parabola ion spectrometry diagnostic which significantly improves signal-to-noise ratios. Electrons that co-propagate with the fast protons cause a two-step charge-reduction reaction. The gas phase three-body attachment of electrons to fast neutral hydrogen atoms accounts for the measured H- yield. It is intriguing that such a fundamental gas-phase reaction, involving the attachment of an electron to a hydrogen atom, has not been observed in laboratory experiments previously. Laser-produced plasma offers an alternative environment to the conventional charged particle beam experiments, in which such atomic physics processes can be investigated

Quasi mono-energetic heavy ion acceleration from layered targets

In the present work, we demonstrate acceleration of quasi monoenergetic heavy ions during the interaction of a high-intensity short-pulse laser with multi-layer targets. The targets, consisting of layers of high-Z (gold) and low-Z (carbon) species a few nm thick, have been used to tailor the energy spectra of the high-Z ion species. Au-ion bunches of energy around 500 keV with an energy spread of less than 20% are observed. Particle-in-cell simulations provide explanation for a number of features of the experimental observations. Several behaviors, in addition to the expected sheath-field acceleration, were found to be involved. It is found that the Au layer is pistoned outward by the underlying Si substrate whilst simultaneously being tamped at its leading edge by the carbon overlay. The simulations show best agreement with the experiments when the carbon layer is first rarefied by the laser prepulse. In these cases, the simulations reproduce the double-humped spectra found in the experiment. Ion-electrostatic instabilities rapidly lead to the formation of a single trapping-like structure in phase space of relatively long wavelength. This long-lived structure dominates the ion acceleration and produces a double-peaked energy spectrum. It is suggested that the instability responsible may be of the Pierce-type

Recombination of Protons Accelerated by a High Intensity High Contrast Laser

Short pulse, high contrast, intense laser pulses incident onto a solid target are not known to generate fast neutral atoms. Experiments carried out to study the recombination of accelerated protons show a 200 times higher neutralization than expected. Fast neutral atoms can contribute to 80% of the fast particles at 10 keV, falling rapidly for higher energy. Conventional charge transfer and electron-ion recombination in a high density plasma plume near the target is unable to explain the neutralization. We present a model based on the copropagation of electrons and ions wherein recombination far away from the target surface accounts for the experimental measurements. A novel experimental verification of the model is also presented. This study provides insights into the closely linked dynamics of ions and electrons by which neutral atom formation is enhanced