Petapascal Pressure Driven by Fast Isochoric Heating with a Multipicosecond Intense Laser Pulse

Homogeneous, Micron-Scale High-Energy-Density Matter Generated by Relativistic Laser-Solid Interactions

Short-pulse, laser-solid interactions provide a unique platform for studying complex high-energy-density matter. We present the first demonstration of solid-density, micron-scale keV plasmas uniformly heated by a high-contrast, 400 nm wavelength laser at intensities up to 2×10^{21}  W/cm^{2}. High-resolution spectral analysis of x-ray emission reveals uniform heating up to 3.0 keV over 1  μm depths. Particle-in-cell simulations indicate the production of a uniformly heated keV plasma to depths of 2  μm. The significant bulk heating and presence of highly ionized ions deep within the target are attributed to the few MeV hot electrons that become trapped and undergo refluxing within the target sheath fields. These conditions enabled the differentiation of atomic physics models of ionization potential depression in high-energy-density environments.

Isochoric heating of solid-density plasmas beyond keV temperature by fast thermal diffusion with relativistic picosecond laser light

The interaction of relativistic short-pulse lasers with matter produces fast electrons with over megaampere currents, which supposedly heats a solid target isochorically and forms a hot dense plasma. In a picosecond timescale, however, thermal diffusion from hot preformed plasma turns out to be the dominant process of isochoric heating. We describe a heating process, fast thermal diffusion, launched from the preformed plasma heated resistively by the fast electron current. We demonstrate the fast thermal diffusion in the keV range in a solid density plasma by a series of one-dimensional particle-in-cell simulations. A theoretical model of the fast thermal diffusion is developed and we derive the diffusion speed as a function of the laser amplitude and target density. Under continuous laser irradiation, the diffusion front propagates at a constant speed in uniform plasma. Our model can provide a guideline for fast isochoric heating using future kilojoule petawatt lasers.

Nanoscale Electrostatic Modulation of Mega-Ampere Electron Current in Solid-Density Plasmas

Transport of high-current relativistic electron beams in dense plasmas is of interest in many areas of research. However, so far the mechanism of such beam-plasma interaction is still not well understood due to the appearance of small time- and space-scale effects. Here we identify a new regime of electron beam transport in solid-density plasma, where kinetic effects that develop on small time and space scales play a dominant role. Our three-dimensional particle-in-cell simulations show that in this regime the electron beam can evolve into layered short microelectron bunches when collisions are relatively weak. The phenomenon is attributed to a secondary instability, on the space- and timescales of the electron skin depth (tens of nanometers) and few femtoseconds of strong electrostatic modulation of the microelectron current filaments formed by Weibel-like instability of the original electron beam. Analytical analysis on the amplitude, scale length, and excitation condition of the self-generated electrostatic fields is clearly validated by the simulations.

Pulse duration constraint of whistler waves in magnetized dense plasma

Interactions between large-amplitude laser light and strongly magnetized dense plasma have been investigated by one- and two-dimensional electromagnetic particle-in-cell simulations. Since whistler waves have no critical density, they can propagate through plasmas beyond the critical density in principle. However, we have found the propagation of whistler waves is restricted significantly by the stimulated Brillouin scattering. It is confirmed that the period during which the whistler wave can propagate in overcritical plasmas is proportional to the growth time of the ion-acoustic wave via the Brillouin instability. The allowable pulse duration of the whistler wave has a power-law dependence on the amplitude of the whistler wave and the external magnetic field.

Dynamics of laser-generated magnetic fields using long laser pulses

We report on the experimental investigation of magnetic field generation with a half-loop gold sheet coil driven by long-duration (10 ns) and high-power (0.5 TW) laser pulses. The amplitude of the magnetic field was characterized experimentally using proton deflectometry. The field rises rapidly in the first 1 ns of laser irradiation, and then increases slowly and continuously up to 10 ns during further laser irradiation. The transient dynamics of current shape were investigated with a two-dimensional (2D) numerical simulation that included Ohmic heating of the coil and the resultant change of electrical resistivity determined by the coil material temperature. The numerical simulations show rapid heating at the coil edges by current initially localized at the edges. This current density then diffuses to the central part of the sheet coil in a way that depends both on normal current diffusion as well as temporal changes of the coil resistance induced by the Ohmic heating. The measured temporal evolution of the magnetic field is compared with a model that determines a solution to the coil current and voltage that is consistent with a plasma diode model of the drive region and a 2D simulation of current diffusion and dynamic resistance due to Ohmic heating in the laser coil.

Fast electron transport dynamics and energy deposition in magnetized, imploded cylindrical plasma

Inertial confinement fusion approaches involve the creation of high-energy-density states through compression. High gain scenarios may be enabled by the beneficial heating from fast electrons produced with an intense laser and by energy containment with a high-strength magnetic field. Here, we report experimental measurements from a configuration integrating a magnetized, imploded cylindrical plasma and intense laser-driven electrons as well as multi-stage simulations that show fast electrons transport pathways at different times during the implosion and quantify their energy deposition contribution. The experiment consisted of a CH foam cylinder, inside an external coaxial magnetic field of 5 T, that was imploded using 36 OMEGA laser beams. Two-dimensional (2D) hydrodynamic modelling predicts the CH density reaches [Formula: see text], the temperature reaches 920 eV and the external B-field is amplified at maximum compression to 580 T. At pre-determined times during the compression, the intense OMEGA EP laser irradiated one end of the cylinder to accelerate relativistic electrons into the dense imploded plasma providing additional heating. The relativistic electron beam generation was simulated using a 2D particle-in-cell (PIC) code. Finally, three-dimensional hybrid-PIC simulations calculated the electron propagation and energy deposition inside the target and revealed the roles the compressed and self-generated B-fields play in transport. During a time window before the maximum compression time, the self-generated B-field on the compression front confines the injected electrons inside the target, increasing the temperature through Joule heating. For a stronger B-field seed of 20 T, the electrons are predicted to be guided into the compressed target and provide additional collisional heating. This article is part of a discussion meeting issue 'Prospects for high gain inertial fusion energy (part 2)'.

Thermonuclear fusion triggered by collapsing standing whistler waves in magnetized overdense plasmas

Thermal fusion plasmas initiated by standing whistler waves are investigated numerically by two- and one-dimensional particle-in-cell simulations. When a standing whistler wave collapses due to the wave breaking of ion plasma waves, the energy of the electromagnetic waves transfers directly to the ion kinetic energy. Here we find that ion heating by use of standing whistler waves is operational even in multidimensional simulations of multi-ion species targets, such as deuterium-tritium (DT) ices and solid ammonia borane (H_{6}BN). The energy conversion efficiency to ions becomes as high as 15% of the injected laser energy, which depends significantly on the target thickness and laser pulse duration. The ion temperature could reach a few tens of keV or much higher if appropriate laser-plasma conditions are selected. DT fusion plasmas generated by this method must be useful as efficient neutron sources. Our numerical simulations suggest that the neutron generation efficiency exceeds 10^{9} n/J per steradian, which is beyond the current achievements of the state-of-the-art laser experiments. Standing whistler-wave heating would expand the experimental possibility for an alternative ignition design of magnetically confined laser fusion and also for more difficult fusion reactions, including the aneutronic proton-boron reaction.