R. J. Mason

Ignition and high gain with ultrapowerful lasers

Ultrahigh intensity lasers can potentially be used in conjunction with conventional fusion lasers to ignite inertial confinement fusion (ICF) capsules with a total energy of a few tens of kilojoules of laser light, and can possibly lead to high gain with as little as 100 kJ. A scheme is proposed with three phases. First, a capsule is imploded as in the conventional approach to inertial fusion to assemble a high‐density fuel configuration. Second, a hole is bored through the capsule corona composed of ablated material, as the critical density is pushed close to the high‐density core of the capsule by the ponderomotive force associated with high‐intensity laser light. Finally, the fuel is ignited by suprathermal electrons, produced in the high‐intensity laser–plasma interactions, which then propagate from critical density to this high‐density core. This new scheme also drastically reduces the difficulty of the implosion, and thereby allows lower quality fabrication and less stringent beam quality and symmet...

Thermonuclear burn characteristics of compressed deuterium‐tritium microspheres

The phenomenology of thermonuclear burn in deuterium‐tritium microspheres at high densities is described, and numerical results characterizing the burn for a broad range of initial conditions are given. The fractional burnup, bootstrap‐heating, and depletion of the DT fuel, its expansive disassembly, and thermonuclear ignition by propagating burn from central hot spots in the microspheres are discussed. Extensive numerical results from a 3 T Lagrangian simulation code are presented. The yields Y0 from uniform 10, 1, and 0.1 μg microspheres with densities ρ = 1 to 4 × 104 g/cm3 and temperatures Te = Ti = 1.8 to 100 keV are given. It is shown that Y0 ∼ ρR, ρR < 0.3 (R is the microsphere radius) or, equivalently, Y0 ∼ ρ2/3 for spheres of fixed mass m. The gain‐factor G0 ≡ Y0/mI0 (I0 is the internal energy) is shown to measure burn efficiency in uniform microspheres. More than a four‐fold increment in the gain factor is shown to derive from apportionment of the internal energy in a central hot spot. The limit...

Implicit moment particle simulation of plasmas

It is shown that an implicit E field can be obtained from Poissons equation with the aid of the lower two fluid moment equations, permitting stable particle simulations for ..omega../sub p/..delta..t>>1 and ..delta..x/lambda/sub D/>>1, where ..omega../sub p/ and lambda/sub D/ are the plasma frequency and Debye length, respectively. In the quasi-neutral limit the effect of this E is to provide just the predicted current required to drive all present deviations in the total charge density to zero in the next cycle. In near vacuum, or with ..omega../sub p/..delta..t>>1, the field expression reduces to the standard form used with conventional leapfrog schemes. Sample applications are discussed.

An electromagnetic field algorithm for 2d implicit plasma simulation

Abstract A new, robust algorithm is presented for the implicit calculation of the electromagnetic fields used in the full-particle and hybrid modeling of 2D simulation plasmas. The algorithm allows for calculations at time steps Δt well in excess of the plasma period and for mesh scales Δξ far exceeding the Debye length-with electron inertial terms retained. The implicit fields suppress the numerical instability associated with plasma waves. Still, the At remain constrained by an electron Courant limit. The algorithm is considerably simpler than earlier implicit schemes, and more complete in its treatment of field errors. In its present form the algorithm is limited to plasmas moving and accelerating in a plane across a single component of magnetic field. An extension to include all the field components is suggested, however. In accordance with the implicit moment method, estimated electric and magnetic fields are obtained by solving Maxwells equations self-consistently for a set of implicit sources, estimated with the aid of an auxiliary set of lower fluid moment equations (for component fluxes and density). The fluid pressure terms are treated explicitly, and the spatial differencing of the auxiliary moments is centered to facilitate the solution of the resultant field equations. Solution for the single magnetic field component is obtained by one elliptic equation .inversion, readily managed by a vectorized solver package. A subsequent irrotational old E-field correction is found to be crucial for the maintainence of anticipated quasi-neutrality. A concomitant rotational correction is needed for physical solutions in steep density gradient problems. We show that both corrections can be obtained simultaneously by referencing the deviations between the true currents flowing, and the currents predicted to flow in the plasma at the end of a cycle. The current correction is shown to be equivalent to the first (and usually sufficient) step of an iterative procedure leading to an exact solution for the fields. In addition, we demonstrate that electrostatic solutions can be obtained from the implicit algorithm by setting the speed of light to very large multiples of its physical value. Comparisons are made with earlier moment and direct method approaches, and the scheme is related to previous classical hybrid models. Demonstrative applications are discussed.

Hydrodynamics and burn of optimally imploded deuterium‐tritium spheres

The phenomenology of optimized laser‐driven DT sphere implosions leading to efficient thermonuclear burn is reviewed. The optimal laser deposition profile for spheres is heuristically derived. The performance of a 7.5 μg sphere, exposed to its optimal 5.3 kJ pulse, is scrutinized in detail. The timing requirements for efficient central ignition of propagating burn in the sphere are carefully explored. The difficulties stemming from superthermal electron production and thermal flux limitation are discussed. The hydro‐burn performance of spheres is characterized as a function of the pulse energy, peak power, time scale, pulse exponent, wavelength, and on the degree of flux limitation. The optimal pulse parameters are determined for spheres with masses ranging from 40 ng to 250 μg, requiring from 50 J to 150 kJ of input energy, and the corresponding optimal performance levels are calculated. Discussion is given to the hydro‐burn performance of new structured fusion targets, in which the DT is contained as a ...

Implicit Collisional Three-Fluid Simulation of the Plasma Erosion Opening Switch

The plasma erosion opening switch (PEOS) has been studied with the aid of the ANTHEM implicit simulation code. This switch consists of fill plasma injected into a transmission line. The plasma is ultimately removed by self-electrical forces, permitting energy delivery to a load. Here, ANTHEM treats the ions and electrons of the fill plasma and the electrons emitted from the transmission-line cathode as three distinct Eulerian fluids-with electron inertia retained. This permits analysis of charge separation effects, and avoids the singularities that plague conventional MHD codes at low density. E and B fields are computed by the implicit moment method, allowing for time steps well in excess of the electron plasma period ?t >> ?p-1, and cells much wider than a Debye length, ?x >> ?D. Switch dynamics are modeled as a function of the driving electrical pulse characteristics, the fill plasma parameters, and the emission properties of the transmission line walls-for both collisionless and anomalously collisional electrons. Our low-fill-density (ne ? 4 × 1012 electrons/cm3) collisionless calculations are in accord with earlier particle code results. Our high-density computations (ne ? 2 × 1013 electrons/cm3) show the opening of the switch proceeding through both ion erosion and magnetic pressure effects. The addition of anomalous electron collisions is found to diffuse the driving B field into the fill plasma, producing broad current channels and reduced magnetic pressure effects, in some agreement with NRL experimental measurements.

Collisional effects on the Weibel instability

The effects of electron Rutherford scattering on the Weibel instability are investigated using the implicit plasma simulation code venus [J. Comput. Phys. 46, 271 (1982); 63, 434 (1986)]. Collisions decrease the Weibel growth rate below the collisionless value, in agreement initially with simple linear theory. Likewise, collisions decrease the saturation level, in agreement with a magnetic trapping mechanism. When the collision rate exceeds the collisionless Weibel growth rate, the instability is suppressed altogether. The results provide benchmarks for analysis of proposed short‐wavelength laser‐fusion schemes.

Monte Carlo hybrid modeling of electron transport in laser produced plasmas

A new Monte Carlo hybrid scheme for the transport of electrons in laser produced plamas is described in detail. Comparison is made with alternate schemes. The hot electrons are followed as collisional particles‐in‐cell. The cold electrons are a donor‐cell fluid. The self‐consistent E field is computed approximately by either a moment method which seeks to establish quasi‐neutrality, or by plasma‐period dilation that stretches the local plasma period up to the time step. Application is made to the transport of laser driven electrons in foil‐like geometries. Results from a large number of computer simulations are used to confirm the quasi‐neutrality of the scheme, to demonstrate important aspects of the hot electron collisionality, to explore the influence of return current E fields from classical and ion‐acoustic resistivity, and finally, to examine the efficiency of transport inhibition from thermal convection in the presence of density dips. Attention is drawn to the need for an even more fundamental particle description for the thermals.

Implicit moment PIC-hybrid simulation of collisional plasmas

A self-consistent scheme was developed to model electron transport in evolving plasmas of arbitrary classial collisionality. The electrons and ions are treated as either ultiple Eulerian fluids or collisional particles-in-cell. Particle suprathermal electrons scatter off ions, and drag against fluid background thermal electrons. The background electros undergo ion friction, thermal coupling, and bremsstrahlung. The components accelerate in electric field obtained by the Implicit Moment Method, which permits ..delta..t>>..omega../sup -1//sub p/ and ..delta..x>>lambda/sub p/-allowing the treatment of problems 10/sup 2/-10/sup 3/ times more complex than those accessible with older explicit methods. The fluid description for the background plasma components permits the modeling of transport in systems spanning more than a 10/sup 7/-fold change in density, and encompassing contiguous collisional and collisionless regions. Results are presented from application of the scheme to the modeling of CO/sub 2/ laser-generated suprathermal electron transport in expanding thin foils, and in multi-foil target configurations.

Computational study of laser imprint mitigation in foam-buffered inertial confinement fusion targets

Recent experiments have shown that low density foam layers can significantly mitigate the perturbing effects of beam nonuniformities affecting the acceleration of thin shells. This problem is studied parametrically with two-dimensional LASNEX [G. B. Zimmerman and W. L. Kruer, Comments Plasma Phys. Controlled Fusion 2, 51 (1975)]. Foam-buffered targets are employed, consisting typically of 250 A of gold, and 50 μm of 50 mg/cm3 C10H8O4 foam attached to a 10 μm foil. In simulation these were characteristically exposed to 1.2 ns, flat-topped green light pulses at 1.4×1014 W/cm2 intensity, bearing 30 μm lateral perturbations of up to 60% variation in intensity. Without the buffer layers the foils were severely disrupted by 1 ns. With buffering only minimal distortion was manifest at 3 ns. The smoothing is shown to derive principally from the high thermal conductivity of the heated foam. The simulation results imply that (1) the foam thickness should exceed the disturbance wavelength; (2) intensities exceeding ...