The question of laser wavelength for filling magnetic confinement machines with laser produced plasmas

Radiation absorption and bulk ionization efficiencies are considered in a one-dimensional hydrodynamic approximation by taking finite rate ionization and recombination processes into account. The magnetic field effect on the plasma motion is taken into account by assuming that the plasma remains diamagnetic during its spherical expansion phase. At the moderate laser flux intensities and associated corona temperatures considered classical absorption is the dominating energy transfer mechanism, and the shorter the wavelength the better are the attainable absorption and ionization efficiencies. However, even at anomalous absorption rates manifested by complete energy damping at the critical density, pellet thermalization and bulk ionization are found to be more efficient at shorter wavelengths. Neglecting the finite ionization and recombination rates may lead to errors in predicting the location of the cut-off electron density layer and of the obtainable bulk ionization degree. The formation of a high-density high-temperature plasma sheath with appreciable polarization across the sheath is shown to take place at the plasma cloud magnetic field interface. Methods for avoiding the [V x B]-induced drift motion associated with non-symmetric pellet irradiation are outlined. (orig.)