It is shown that inertial confinement fusion targets designed with low implosion velocities can be shock-ignited using laser-plasma interaction generated hot electrons (hot-e's) to obtain high energy gains. These designs are robust to multimode asymmetries and are predicted to ignite even for significantly distorted implosions. Electron shock ignition requires tens of kilojoules of hot-e's which can be produced only at a large laser facility like the National Ignition Facility, with the laser-to-hot-e conversion efficiency greater than 10% at laser intensities ∼10^{16} W/cm^{2}.
In atmospheric radio frequency discharges at 13.56 MHz, with the electrode gap reduced, the sheath region eventually occupies a main portion of the electrode spacing and the bulk plasma region is significantly compressed. The computational results in this letter based on a one-dimensional fluid model show that by increasing the excitation frequency over 13.56 MHz, the traditional glow-plasma structure could gradually recover even at very small sizes with a well defined quasineutral plasma region, and the electron density is improved but the electric fields in sheath region are reduced. This study indicates that the excitation frequency can be used to modulate the discharge structure and then tailor the plasma-surface interaction in atmospheric microplasmas.Atmospheric microplasmas have commanded much attention in recent years due to the considerable scientific depth and potential applications. 1-4 For atmospheric capacitively radio-frequency (rf) discharges, when the electrode gaps are confined to submillimeter dimensions, the generated microplasmas show many unique properties compared to the large-scale atmospheric plasmas, 3 such as the high energetic electrons, 5 different discharge structures, 6,7 and nonequilibrium characters. 8 The experimental 6 and computational studies 5,7 have demonstrated that in atmospheric rf discharges at 13.56 MHz, with the electrode gap reduced, the traditional glow-plasma (GP) structure will eventually transit to a new sheath-dominated-plasma (SDP) structure, where the sheath region occupies a large portion of the electrode gap and almost no distinct bulk plasma region develops. In a SDP structure, three electron groups with different electron temperatures have been revealed by the Particle-In-Cell (PIC) simulation and the high energetic electrons present a new way to interact with the electrodes or the given surfaces, moreover these electrons maybe contribute to the reactivity of atmospheric microplasmas. 5 The discharge modes in rf microplasmas have been discussed but still need to be further clarified by experiments and simulations. 5-7 Increasing frequency is widely accepted as a way to enhance the discharge stability and reduce the electron temperature at a constant power density. 10-12 Nevertheless, the frequency effects on the structure transition in atmospheric rf discharges are largely unexplored. Although the microplasmas in SDP regime show many advantages and the GP structure with quasineutral plasmas may still be very useful in many applications at very small sizes. The experimental diagnosis of atmospheric microplasmas, however, is facing many new challenges due to the small dimension and high gas pressure, 3,4 and numerical simulations provide a valuable alternative for investigating the properties of microplasmas. 2In this letter a one-dimensional fluid model is explored to study the transition of discharge structures in capacitively rf discharges at atmospheric pressure. 11,12 Briefly, the continuity equations with the drift-diffusion approximation are used to des...
X-ray emission from laser-plasma interaction is an important x-ray source, and improving laser to x-ray conversion is imperative for various applications. The laser to x-ray conversion efficiency (CE) was simulated for gold targets with different initial densities. Using a 0.1 g/cm3 Au layer target, an x-ray conversion efficiency of 50.8% was obtained, which was 1.34 times of the 37.9% for the solid density target. It has been shown that the enhancement of the x-ray conversion efficiency is caused by the increase of absorption from the incident laser and reduction of ion kinetic energy due to the initial low density of the gold target.
Experiments were performed with CH, Be, C, and SiO2 ablators interacting with high-intensity UV laser radiation (5 × 1015 W/cm2, λ = 351 nm) to determine the optimum material for hot-electron production and strong-shock generation. Significantly more hot electrons are produced in CH (up to ∼13% instantaneous conversion efficiency), while the amount is a factor of ∼2 to 3 lower in the other ablators. A larger hot-electron fraction is correlated with a higher effective ablation pressure. The higher conversion efficiency in CH is attributed to stronger damping of ion-acoustic waves because of the presence of light H ions.
As an important x-ray source, enhancement of x-ray emissions from laser-produced plasmas is significant for various applications. Due to less expanding kinetic loss, gold foam with low initial density can have an enhanced x-ray conversion efficiency compared with solid-density gold. However, low-Z impurities within gold foam targets will diminish the enhancement remarkably, and should be tightly controlled. This paper presents an experimental study of a high brightness laser plasma soft x-ray source, based on a 0.36 g cm −3 gold foam target with negligible impurities irradiated by nanosecond laser pulses with power density around 3 × 10 14 W cm −2 at the Shenguang II laser facility. A conversion efficiency, from multi-eV to multi-keV, of 51.2% is achieved in the x-ray emissions-about 21% relative enhancement compared with a solid-density gold target, and the highest conversion efficiency for Au foam planar targets yet. Good agreement has been achieved between the semi-analytical model prediction and the experimental results.
The enhancement of laser to x-ray conversion efficiencies using low density gold targets [W. L. Shang, J. M. Yang, and Y. S. Dong, Appl. Phys. Lett. 102, 094105 (2013)] is demonstrated. Laser to x-ray conversion efficiencies with 6.3% and 12% increases are achieved with target densities of 1 and 0.25 g/cm3, when compared with that of a solid gold target (19.3 g/cm3). Experimental data and numerical simulations are in good agreement. The enhancement is caused by larger x-ray emission zone lengths formed in low density targets, which is in agreement with the simulation results.
The discharge in pure helium and the influence of small nitrogen impurities at atmospheric pressure are investigated based on a one-dimensional self-consistent fluid model controlled by a dielectric barrier between two coaxial electrodes. The simulation of the radiofrequency (rf) discharge is based on the one-dimensional continuity equations for electrons, ions, metastable atoms, and molecules, with the much simpler current conservation law replacing the Poisson equation for electric field. Through a computational study of rf atmospheric glow discharges over a wide range of current density, this paper presents evidence of at least two glow discharge modes, namely the α mode and the γ mode. The simulation results show the asymmetry of the discharge set exercises great influence on the discharge mechanisms compared to that with parallel-plane electrodes. It is shown that the particle densities are not uniform in the discharge region but increase gradually from the outer to the inner electrode in both modes. The contrasting dynamic behaviors of the two glow modes are studied. Secondary electron emission strongly influences gas ionization in the γ mode yet matters little in the α mode.
Foam gold was proposed to increase the laser to x-ray conversion efficiency due to its important applications. To understand the mechanism of x-ray enhancement, the detailed energy distributions and plasma profiles for laser-irradiated solid gold and foam gold targets were studied comparatively by hydrodynamic simulations using the code Multi-1D. It is confirmed that the radiation heat wave is subsonic for the normal solid gold target, while supersonic for the foam gold target. The shock wave, which is behind the supersonic radiation heat wave for the foam gold target, generates a plasma temperature gradient with high temperature near the shock wave front to produce an additional net outward radiation for enhancement of the x-ray emission. Much larger inward plasma velocity is also driven by the shock wave as an initial plasma velocity for the laser deposition and electron thermal conduct zone, which decreases the expanding plasma kinetic energy loss and helps to increase the x-ray radiation.
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