Fast Implosions Research Articles

Influence of crystal dimension on performance of spherical crystal self-emission imager

Abstract The spherical crystal imaging system, noted for its high energy spectral resolution (monochromaticity) and spatial resolution, is extensively applied in high energy density physics and inertial confinement fusion research. This system supports studies on fast electron transport, hydrodynamic instabilities, and implosion dynamics. The X-ray source, produced through laser-plasma interaction, emits a limited number of photons within short time scales, resulting in predominantly photon-starved images. Through ray-tracing simulations, we investigated the impact of varying crystal dimensions on the performance of a spherical crystal self-emission imager. We observed that increasing the crystal dimension leads to higher imaging efficiency but at the expense of monochromaticity, causing broader spectral acceptance and reduced spatial resolution. Furthermore, we presented a theoretical model to estimate the spatial resolution of the imaging system within a specific energy spectrum range, detailing the expressions for the effective size of the crystal. The spatial resolution derived from the model closely matches the numerical simulations.

Magnetic implosion of thin aluminum foil liners

Metal liner implosions driven by a pulsed current generator are used in research on inertial confinement fusion, generation of soft x-ray pulses and megagauss magnetic fields. The energy density in the plasma column formed during the liner stagnation is largely determined by the initial radius and the radial convergence of the liner. When the liner implosion time is close to the current rise time, the liner initial radius is proportional to the current rise time. The paper presents the results of experiments on the fast implosion of cylindrical liners on the MIG generator (2 MA, 80 ns) at the Institute of High Current Electronics, Tomsk, Russia. Liners 0.7–1.0 mm in diameter were made from a 1.8 to 2.5 μm thick aluminum foil. A low-density plasma was pre-injected into the liner area. As the Lorentz force sweeps away the pre-injected plasma, the current switches to the liner in 1–3 ns. Then the liner is imploded in 3–7 ns by a current close to the peak generator current. The stagnation radius measured via time-integrated pinhole camera images was found to be not more than 25 µm. Such a radius of the stagnated plasma assumes that the plasma mass density is several times higher than the solid aluminum density.

Development and Testing of Rescue Destruction Charges for the Demolition of Statically Unstable Buildings

Due to the industrial accidents (the effects of explosion of improvised explosive devices or gas) or other unexpected events, heavily damaged buildings represent threat to environment. Generally, their damage is so serious that their reconstruction is not considered and the only solution is a demolition. Advantageously emergency shaped explosive charges can be used in these risk situations of buildings that are beyond repair. With such shaped charges is possible to execute a fast and effective implosion of an unstable building without the posing any threathening effects on surrounding, mainly in urban areas. This papers is focused on the design and development of mentioned shaped explosive charges, their testing in the field test and practical applications.

A simple method to prevent hard X-ray-induced preheating effects inside the cone tip in indirect-drive fast ignition implosions

During fast-ignition implosions, preheating of inside the cone tip caused by hard X-rays can strongly affect the generation and transport of hot electrons in the cone. Although indirect-drive implosions have a higher implosion symmetry, they cause stronger preheating effects than direct-drive implosions. To control the preheating of the cone tip, we propose the use of indirect-drive fast-ignition targets with thicker tips. Experiments carried out at the ShenGuang-III prototype laser facility confirmed that thicker tips are effective for controlling preheating. Moreover, these results were consistent with those of 1D radiation hydrodynamic simulations.

Analytic model for the dynamic Z-pinch

A model is presented for describing the cylindrical implosion of a shock wave driven by an accelerated piston. It is based in the identification of the acceleration of the shocked mass with the acceleration of the piston. The model yields the separate paths of the piston and the shock. In addition, by considering that the shocked region evolves isentropically, the approximate profiles of all the magnitudes in the shocked region are obtained. The application to the dynamic Z-pinch is presented and the results are compared with the well known snowplow and slug models which are also derived as limiting cases of the present model. The snowplow model is seen to yield a trajectory in between those of the shock and the piston. Instead, the neglect of the inertial effects in the slug model is seen to produce a too fast implosion, and the pressure uniformity is shown to lead to an unphysical instantaneous piston stopping when the shock arrives to the axis.

Present Status of Fast Ignition Research and Prospects of FIREX Project

This is the review on the laser fusion research at Institute of Laser Engineering of Osaka University. Since 1996, we have concentrated our efforts on fast ignition laser fusion research. By constructing 100 TW and 1Peta watt lasers, experiments on relativistic laser plasma interactions related to fast ignition and pellet implosion and heating have been carried out. The results indicate that imploded core plasma is heated with relatively high coupling efficiency. According to the above results, we started the FIREX (Fast Ignition Realization Experiment) project for demonstrating ignition and burn with a multi 10kJ short pulse laser. The future prospects of the project are presented in this paper.

The physics of fastZpinches

The spectacular progress made during the last few years in reaching high energy densities in fast implosions of annular current sheaths (fast Z pinches) opens new possibilities for a broad spectrum of experiments, from x-ray generation to controlled thermonuclear fusion and astrophysics. Presently Z pinches are the most intense laboratory X ray sources (1.8 MJ in 5 ns from a volume 2 mm in diameter and 2 cm tall). Powers in excess of 200 TW have been obtained. This warrants summarizing the present knowledge of physics that governs the behavior of radiating current-carrying plasma in fast Z pinches. This survey covers essentially all aspects of the physics of fast Z pinches: initiation, instabilities of the early stage, magnetic Rayleigh-Taylor instability in the implosion phase, formation of a transient quasi-equilibrium near the stagnation point, and rebound. Considerable attention is paid to the analysis of hydrodynamic instabilities governing the implosion symmetry. Possible ways of mitigating these instabilities are discussed. Non-magnetohydrodynamic effects (anomalous resistivity, generation of particle beams, etc.) are summarized. Various applications of fast Z pinches are briefly described. Scaling laws governing development of more powerful Z pinches are presented. The survey contains 36 figures and more than 300 references.

Kinetic effects on the electron thermal transport in ignition target design

Preheating is one of the most critical issues in laser fusion, because of significant reduction of volume compression. The nonlocal heat transport in an ablative plasma is found to play an important role in the preheating under high intensity laser irradiation. Namely, the electron heat transport should be described by the Fokker–Planck (FP) equation in the fluid implosion code. The Spitzer–Härm (SH) thermal conduction model is not applicable because the electron mean free path is comparable to the temperature scale length. The numerical simulations of the implosion with the FP heat transport have been carried out for the fast (high entropy) implosion mode in which the implosion velocity reaches as high as 6×107 cm/s. In the fast implosion, the required laser energy for ignition can be reduced. It is found in the simulation that the isentrope in the FP simulation code is higher by two to four times than that in the flux limited SH simulation.

The dense radiating deuterium Z-pinch plasma

The radiative emission from a frozen deuterium Z-pinch driven by a slowly rising current (∼3 kA/ns) is investigated. This approach is in marked contrast to the standard radiating Z-pinch plasma which is driven by a rapid-current rise leading to a fast implosion. The slow pinch is modeled as a quasistatic contraction subject to Bennett-pressure equilibrium and driven by a constant generator voltage of 200 kV. Electron degeneracy is approximately accounted for in the pressure, however, the resistivity is taken as purely classical. The bremsstrahlung spectrum transitions from thin to nearly thick during the collapse phase. The predictions from this simple model suggests that this approach may have interesting applications as a radiation source in the kilovolt regime. In particular, we find that a deuterium plasma of 1018 cm−1 line density yields ∼60 kJ of radiation above 3 keV while undergoing a radiative collapse.

Sodium-fluoride discharge for fast Z-pinch experiments

A capillary-discharge plasma source has been developed to produce a sodium-bearing plasma for fast Z-pinch implosion experiments. Peak currents of 40–50 kA from a 0.5-kJ capacitor bank were driven through a 0.5-mm-diam, few cm long capillary drilled in packed sodium-fluoride powder to form the source. A nozzle was used to collimate plasma ejected from one end of the capillary to produce a 1–2-cm-diam, several cm long cylindrical plasma. Ions with velocities of 2.2–3.4 cm/μs and densities of up to 5×1015 cm−3 were measured with biased charge collectors located at least 5 cm from the nozzle. Measurements of visible light from neutrals near the nozzle exit gave velocities of 1.5–1.7 cm/μs. Indications of axial and radial nonuniformities of the plasma were observed in framing photographs of visible-light emission and in spatially resolved spectral measurements. Neutral-sodium and neutral-fluorine lines were identified in the spectral range from 2300 to 6700 Å. Also, impurity lines of carbon, copper, and hydrogen were identified and used to characterize the plasma. Stark broadening of the Balmer alpha line of hydrogen was used to deduce a peak electron density of 8×1016 cm−3 at the exit of a 2-cm-diam nozzle. Electron temperatures of 1.4–1.6 eV at the nozzle exit were inferred from relative intensities of the C i and C ii lines. At this density and temperature, Saha-equilibrium-model calculations indicate that the plasma consists primarily of singly ionized sodium and neutral fluorine. A total mass per unit length (sodium and fluorine) of at least 15 μg/cm is deduced from this analysis of the plasma constituents. This capillary discharge has been used to produce 50–100 GW of sodium K-shell x rays in fast Z-pinch experiments.

Experimental Results from SHIVA Star Vacuum Inductive Store/Plasma Flow Switch Drven Implosions

Using a 1313-μF, 3-nH, 120-kV, 9.4-MJ SHIVA Star capacitor bank, we have performed vacuum inductive store/plasma flow switch (PFS) driven implosions of low mass (200-400 μg/cm2) cylindrical foil liners of 2-cm height and 5-cm radius. This technique employs a coaxial discharge through a plasma armature, which stores magnetic energy over 3-4 μs and rapidly switches it to an imploding load as the plasma armature exits the coaxial gun muzzle. The current transferred to the load by the PFS has a rise time of less than 0.2 μs. With 5-MJ stored energy, we have driven fast liner implosions with a current of over 9 MA, obtaining an isotropic equivalent 2.7-TW 0.5-MJ X-ray yield.

Increased nuclear energy yields from the fast implosion of cold shells driven by nonlinear laser plasma interactions

The nonlinear interaction force between intense laser fields and cold plasma shells efficiently transforms radiant energy into mechanical energy of implosion. This transfer of energy has been considered before in numerical experiments and it is treated here analytically in a didactic example starting with an inhomogeneous Rayleigh density profile. Up to 50% of the laser energy can be transformed into the energy of compression if a single untailored pulse of 2.5×1O16 W/cm2 intensity and of only a few picosecond duration is used for spherical illumination of a shell. If the pulse is short enough to reduce collisiona) thermalization, then the collapse and compression of the plasma can remain at the threshold of Fermi degeneracy and still be adiabatic. This results in nuclear reaction gains G, based on the deposited energy, E0, and without a-particle reheating, of G = 400 for E0 = 2.25 kJ (D–T reaction), 900 kJ (D–D), 13 MJ (H–B). About 1000 times less laser energy is necessary than in the case of gas dynamic ablation resulting in the same nuclear reaction yields.