The conditions of the plasma formed by the X-rays from the hohlraum and the material inside it are crucial to achieving ignition.
For ignition to occur, accurate measurements of the density and temperature of the plasma inside the hohlraum, as well as the conditions of the fusion capsule itself, are essential. At NIF, plasma physicists have developed novel diagnostic techniques based on active probing by optical light or X-ray scattering (see Diagnostics). Knowledge of the hohlraum plasma temperatures and density is then used to predict and model the interaction of the laser beams with the hohlraum plasma and the radiation production where the laser energy is deposited at the hohlraum wall. Plasmas are known to interact with lasers through acoustic and electrodynamic waves that can be driven to instabilities, reflecting large fractions of the light. In addition, the laser beams can filament – that is, break up, refract, diffract and transfer energy among each other (see Laser-Plasma Interactions). The goal is to determine the onset for these processes through experiments and modeling, and to choose hohlraums for NIF that efficiently achieve the required radiation conditions (see Target Fabrication).
Data from first experiments on the NIF, done early on in the construction of the facility with just four of the 192 beams, successfully demonstrated that beam propagation can be achieved over the plasma-scale length of an ignition-size hohlraum (see NIF Early Light). This was achieved by applying laser beam smoothing techniques that remove high-intensity speckles, or hot spots, in the laser beam where instabilities first develop. Extensive supercomputer modeling has been subsequently applied, and the propagation of the beams and the observed threshold behavior for the onset of instabilities and laser beam break-up is now understood (see Target Physics). These studies will be continued on NIF when half of the beams are available for experiments to optimize the laser beam intensities and hohlraum materials for ignition experiments.