Abstract
Presented here are the first kinetic two-dimensional Vlasov– Fokker–Planck calculations of inertial confinement fusion-related laser–plasma interactions, to include self-consistent magnetic fields, hydrodynamic plasma expansion and anisotropic electron pressure. An underdense plasma, reminiscent of the gas fill of a hohlraum, is heated by a laser speckle with Iλ2=1.0×1015 W cm− 2μm2 and radius w0=5 μm. Inverse bremsstrahlung absorption of the laser and non-local electron transport lead to the development of a collisional analogue of the Weibel electromagnetic instability. The instability is seeded by magnetic fields, generated in an initial period of linear growth due to the anisotropic electron distribution arising in a laser speckle. Using the circular polarization does not generate significant fields. For linear polarization, the field generally saturates when the magnetization is ωτei> 1, and the effective growth rate is similar to the coherence time of typical laser speckles. The presence of these magnetic fluctuations significantly affects the heat fluxes and hydrodynamics in the plasma.
Highlights
Magnetic field generation in nanosecond timescale, laser generated, collisional plasmas occurs due to a number of mechanisms, each described by a term in Ohm’s law
The fields were generated in quadrupole structures around laser speckles occurring in the laser profiles
The laser is smoothed over the hydrodynamic timescale, the rapidity of the growth of these fields could, very feasibly, sufficiently magnetize the plasma within the coherence time of a laser speckle
Summary
Magnetic field generation in nanosecond timescale, laser generated, collisional plasmas occurs due to a number of mechanisms, each described by a term in Ohm’s law. Understanding the role magnetic fields take in suppressing heat flow is important in laser fusion scenarios, such as hohlraums [18] Such magnetic fields are significant, in that their presence affects the magnitude and direction of the particle fluxes, e.g. electron heat flux, and the long-time evolution of the system. Since the source of magnetic field generation is proportional to the gradients in electron pressure and laser intensity, these small scale-length speckles can be significant sources of magnetic field, and the resulting fields can affect the hydrodynamics over longer timescales Under such conditions of small temperature scale lengths due to speckles, non-local effects can play a significant role in affecting the transport properties of the plasma [22]. Heat flux and ion flux across a large number of similar magnetic perturbations are expected to be modulated, leading to a modification of the transport properties
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