Search

Intense beams of heavy ions, envisaged for the near future at the Institute for Theoretical and Experimental Physics (Moscow) and Gesellschaft fur Schwerionenforschung (Darmstadt), will be well suited for conducting implosion experiments in cylindrical geometry. In such implosions, the initial pressure generated by the direct beam heating can be enhanced by more than a factor of 10. If, in addition, an external magnetic field is introduced, the effect of magnetothermal insulation may allow to reach kilovolt temperatures and significant thermonuclear neutron yields in magnetized implosions driven by the beam heating intensities as low as e≃1 TW/g. It is shown how the combined effect of the electrical resistivity and thermal conductivity sets a lower limit on the product UR (R is the radius, and U is the velocity of an imploding plasma volume) as a necessary condition for the regime of self-sustained magnetized implosion (SSMI). The optimal plasma parameters required for initiation of this regime are evalu...

In connection with fundamental issues relevant to magnetized target fusion, the distribution function of thermonuclear alpha particles produced in situ in a dense, hot, and strongly magnetized hydrogenic plasma considered fully ionized in a cylindrical geometry is investigated. The latter is assumed in local thermodynamic equilibrium with Maxwellian charged particles. The approach is based on the Fokker–Planck equation with isotropic source S and loss s terms, which may be taken arbitrarily under the proviso that they remain compatible with a steady state. A novel and general expression is then proposed for the isotropic and stationary distribution f(v). Its time-dependent extension is worked out numerically. The solutions are valid for any particle velocity v and plasma temperature T. Higher order magnetic and collisional corrections are also obtained for electron gyroradius larger than Debye length. f(v) moments provide particle diffusion coefficient and heat thermal conductivity. Their scaling on colli...

A self-similar solution of the gas dynamics equations with heat conduction, which describes homologous contraction and expansion of gaseous masses with a free external boundary, is investigated in detail. As a primary application, implosion of deuterium–tritium (DT) fuel in inertial confinement fusion targets is considered. For strongly non-adiabatic implosions the self-similar solution predicts that the flow pattern should approach an asymptotical regime in which it ceases to depend on the initial entropy. For DT masses relevant to inertial confinement fusion (ICF) this regime begins to dominate at αUim≳6×108 cm/s, where α=p/pdeg is the fuel isentrope parameter, and Uim is its implosion velocity. The solution has also a branch which describes an asymptotical regime of explosive expansion after an ultra-fast initial heating to a strongly radiating state.

A modified version of the view factor equations is proposed which improves the accuracy of the description of temporal effects in energy redistribution by thermal radiation in cavities driven by power pulses typical for inertial confinement fusion (ICF). The method is applied to analyze the process of radiative symmetrization in the simplest type of closed cylindrical hohlraums heated by two x‐ray rings on the sidewall of the hohlraum case. Such hohlraums may be used in certain types of ICF targets driven by ion beams.

Theoretical investigation has been performed on the conversion efficiency (CE) into the 13.5-nm extreme ultraviolet (EUV) radiation in a scheme where spherical microspheres of tin (Sn) are simultaneously irradiated by two laser pulses with substantially different wavelengths. The low-intensity short-wavelength pulse is used to control the rate of mass ablation and the size of the EUV source, while the high-intensity long-wavelength pulse provides efficient generation of the EUV light at λ=13.5 nm. The problem of full optimization for maximizing the CE is formulated and solved numerically by performing two-dimensional radiation-hydrodynamics simulations with the RALEF-2D code under the conditions of steady-state laser illumination. It is shown that, within the implemented theoretical model, steady-state CE values approaching 9% are feasible; in a transient peak, the maximum instantaneous CE of 11.5% was calculated for the optimized laser-target configuration. The physical factors, bringing down the fully o...

In the long‐wavelength limit, many aspects of the Rayleigh–Taylor (RT) instability of accelerated fluid shells can be explored by using the thin sheet approximation. For two‐dimensional (2‐D) planar eigenmodes, analytic nonlinear solutions [E. Ott, Phys. Rev. Lett. 29, 1429 (1972)] are available. Comparing the simplest of them for the nonconstant acceleration, g∝t−2, with Ott’s solution for constant g, the applicability of nonlinear results obtained for constant g to situations with variable acceleration is analyzed. Nonlinear three‐dimensional (3‐D) effects are investigated by comparing the numerical solutions for axisymmetric Bessel eigenmodes with Ott’s solution for 2‐D modes. It is shown that there is a qualitative difference between 2‐D and 3‐D bubbles in the way they rupture a RT unstable fluid shell: In contrast to the exponential thinning of 2‐D bubbles, mass is fully eroded from the top of an axisymmetric 3‐D bubble within a finite time of (1.1–1.2)γ−1 after the onset of the free‐fall stage; γ is...

The effect of strong thermal radiation on the structure of quasi-stationary laser ablation fronts is investigated under the assumption that all the laser flux is absorbed at the critical surface. Special attention is paid to adequate formulation of the boundary-value problem for a steady-state planar ablation flow. The dependence of the laser-to-x-ray conversion efficiency ϕr on the laser intensity IL and wavelength λL is analyzed within the non-equilibrium diffusion approximation for radiation transfer. The scaling of the main ablation parameters with IL and λL in the strongly radiative regime 1−ϕr≪1 is derived. It is demonstrated that strongly radiating ablation fronts develop a characteristic extended cushion of “radiation-soaked” plasma between the condensed ablated material and the critical surface, which can efficiently suppress perturbations from the instabilities at the critical surface.

A new self-similar solution is presented which describes nonrelativistic expansion of a finite plasma mass into vacuum with a full account of charge separation effects. The solution exists only when the ratio Λ=R∕λD of the plasma scale length R to the Debye length λD is invariant, i.e., under the condition Te(t)∝[ne(t)]1−2∕ν, where ν=1, 2, and 3 corresponds, respectively, to the planar, cylindrical, and spherical geometries. For Λ⪢1 the position of the ion front and the maximum energy Ei,max of accelerated ions are calculated analytically: in particular, for ν=3 one finds Ei,max=2ZTe0W(Λ2∕2), where Te0 is the initial electron temperature, Z is the ion charge, and W is the Lambert W function. It is argued that, when properly formulated, the results for Ei,max can be applied more generally than the self-similar solution itself. Generalization to a two-temperature electron system reveals the conditions under which the high-energy tail of accelerated ions is determined solely by the hot-electron population.