Effect of the laser wavelength: A long story of laser-plasma interaction physics for Inertial Confinement Fusion Teller Medal Lecture

Laser-driven Inertial Confinement Fusion (ICF) relies on the use of high-energy laser beams to compress and ignite a thermonuclear fuel with the ultimate goal of producing energy. Fusion is the holy grail of energy sources-combining abundant fuel with no greenhouse gas emissions, minimal waste products and a scale that can meet mankind's long-term energy demands. The quality and the efficiency of the coupling of the laser beams with the target are an essential step towards the success of laser fusion. A long-term program on laser-plasma interaction physics has been pursued to understand the propagation and the coupling of laser pulses in plasmas for a wide range of parameters. Soon after the laser discovery, the idea to build high-power lasers to drive fusion reactions in a small pellet to produce energy was proposed by Jean Robieux (1). Fusion energy is an attractive, environmentally clean, power source with no greenhouse gases or long-lived radioactive waste materials. An essential step towards ignition is the efficient coupling of the laser beams with the target. During the first picoseconds of the laser pulse irradiating a material, a plasma is produced. The physics of laser- plasma interaction includes the propagation of the beams in this plasma and the processes by which the energy of the electromagnetic wave is given to the plasma. Because of the high energy that must be transferred to the target during the laser pulse, laser intensities are such that non-linear processes are expected. One important parameter for the laser propagation in a plasma is the critical density, which is the maximum electron density in which the electromagnetic wave can propagate. The critical density is inversely proportional to the square of the laser wavelength (0): (nc (cm −3 ) = 1.1 × 10 21 /0(m) 2 ). The energy deposited in the corona is then transferred by conduction in the denser plasma. The main processes of energy deposition or reflection were identified in the seventies. Among them, it was soon demonstrated that collisional absorption, which transfers the laser energy to the bulk of the plasma, was the only useful mechanism for Inertial Confinement Fusion (ICF). Other laser-plasma coupling mechanisms, also named anomalous absorption processes, were shown to be deleterious for inertial fusion because the energy was given to hot electrons. Suprathermal electrons can preheat the fuel and thus be detrimental to efficient compression. Among the non-linear processes are the resonant couplings of the incident laser electromagnetic (EM) wave with the plasma modes, which generate scattered electromagnetic waves (2). So are stimulated Brillouin (SBS) and Raman scattering (SRS), which can be described as the decay of the incident EM wave into a scattered EM wave and an ion-acoustic wave (IAW) or an electron plasma