A. R. Bell

A study of picosecond laser–solid interactions up to 1019 W cm−2

The interaction of a 1053 nm picosecond laser pulse with a solid target has been studied for focused intensities of up to 1019 W cm−2. The maximum ion energy cutoff Emax (which is related to the hot electron temperature) is in the range 1.0–12.0 MeV and is shown to scale as Emax≈I1/3. The hot electron temperatures were in the range 70–400 keV for intensities up to 5×1018 W cm−2 with an indication of a high absorption of laser energy. Measurements of x-ray/γ-ray bremsstrahlung emission suggest the existence of at least two electron temperatures. Collimation of the plasma flow has been observed by optical probing techniques.

Fast-electron transport in high-intensity short-pulse laser-solid experiments

The interaction of short-pulse high-intensity lasers with solid targets generates large numbers of energetic electrons. The energetic electrons can only penetrate into the solid if the solid can supply an equivalent charge-neutralizing return current. We develop a simple model which shows that in many cases the solid cannot support the required return current and the fast electrons are confined by electric fields to the surface of the target. The target response to laser irradiation depends strongly on the electrical conductivity of the solid.

Neutron production from picosecond laser irradiation of deuterated targets at intensities of

Neutron fluxes of up to were measured when planar deuterated targets were irradiated with 1.3 ps FWHM (full width at half maximum) laser pulses at a wavelength of 1054 nm and focused intensities up to . The neutron energy spectra are consistent with an angularly dispersed beam target interaction, whereas a thermonuclear source is considered unlikely.

Calibration of one-dimensional boosted kinetic codes for modeling high-intensity laser–solid interactions

An efficient means of performing kinetic simulations of oblique-incidence laser–plasma interaction via a relativistic Lorentz transformation is described in detail. Comparisons are made between one-dimensional boost codes and previous two-dimensional particle-in-cell (PIC) simulations in an effort to define benchmarks for modeling high-intensity, short-pulse interactions. Apparent discrepancies between results obtained using PIC and Vlasov codes are resolved, and some pitfalls involved with both techniques are identified.

Two-dimensional magneto-hydrodynamic modeling of carbon fiber Z-pinch experiments

A two-dimensional magneto-hydrodynamic simulation incorporating cold start conditions is used to explain the early phase of carbon fiber Z-pinch experiments. The rapid development of large scale, nonlinear m=0 perturbations in the plasma corona is reproduced. X-ray bright spot formation in the necks of the instability is followed by bright spot bifurcation and fast axial motion. Bright spot bifurcation is found to be due to axial components of the j×B force and occurs off-axis due to the presence of a residual core of unionized carbon. Artificial diagnostic images are generated from the simulations data to allow direct comparison with experimental x-ray imaging and laser probing diagnostics. The accurate reproduction of the experimental images provides confirmation that the experimentally observed features are a repercussion of the non-linear development of the m=0 instability in an ionizing medium.

Smoothing and instability with magnetic field in a nonuniformly laser-irradiated planar target (plasma)

Calculations are presented of the magneto-hydrodynamic response of a planar target to nonuniformities in energy deposition by a laser. The amplitude of the nonuniformities are assumed small and the equations are linearised in small perturbations about the solution for steady planar ablation driven by uniform laser energy deposition. The grad(n)xgrad(T) magnetic field source is included, along with Nernst convection and the Righi-Leduc heat flow. The magnetic field is shown to give a small increase in smoothing. A source term for magnetic field is induced to simulate the effects of the Weibel instability. The instability is not strong enough to overcome the smoothing processes under the present assumptions.

ELECTRON NON-LINEARITIES IN LANGMUIR-WAVES WITH APPLICATION TO BEAT-WAVE EXPERIMENTS

Non-linear Langmuir waves are examined in the context of the heat-wave accelerator. With a background of immobile ions the waves in one dimension are subject to the relativistic non-linearity of Rosenbluth and Liu (Phys. Rev. Lett vol.29, p.701 1972). In two or three dimensions, other electron nonlinearities occur which involve electric and magnetic fields. The quasi-linear equations for these nonlinearities are developed and solved numerically in a geometry representative of laser-driven beat waves.

The nature of the inverse skin effect

A one-dimensional Lagrangian code is used to study a simple example of the development of the inverse skin effect in a Z-pinch. The pinch expands although plasma at the surface does not detach itself. The code is then used to model the falling current phase of an experimental Z-pinch at Imperial College. The maximum rate of cooling of the pinch is deduced by comparing the simulated and the observed dynamical behaviour. Using this result the compressional pinch is shown to be anomalously stable.

Non-Spitzer return currents in intense laser-plasma interactions

The propagation of an intense (1019Wcm−2) laser-produced hot electron distribution through overdense, long scale-length plasma is modeled using the one-dimensional relativistic electron Vlasov-Fokker-Planck code KALOS [Bell et al., Plasma Phys. Control. Fusion 48, R37 (2006)]. The initial density profile is chosen to be relevant to the coronal region in fast ignition fusion scenarios in which the density rises from around 10ncr to solid density over 35μm. Under these conditions, the return current transport is intermediate between that of collisional Spitzer transport characterized by strong resistivity and that of collisionless transport characterized by electron inertia. It is shown that the isotropic component of the distribution function of the return-current-carrying electrons becomes doubly peaked. Strong anisotropic pressure inhomogeneities can drive additional contributions to the return current not considered in Spitzer’s transport equations. As a result the electric field can become an order-of-...

The effect of radial dynamics on the stability of diffuse profile Z pinches

The effect of radial dynamics on the ideal magnetohydrodynamic (MHD) linear m=0 instability in diffuse profile Z pinches is investigated with a two-dimensional (2D) MHD code. The radial dynamics impose a periodic behavior within which three distinct phases of instability development can be distinguished. The comparison with the Rayleigh–Taylor instability is presented. The behavior is best characterized as MHD instability enhanced by the radial dynamics.