Experimental investigation of the effects of plasma nonideality on ionization potential lowering and plasma absorption

At high plasma densities (above 1018 electrons/cm -3), plasma behaviour descriptions based on the Debye screening radius break down due to incomplete screening of an ion from Coulomb interactions outside this radius. Alternative models predict a larger, electron mobility-based screening radius and a smaller ionization potential lowering than that of the Debye theory. Our goal is to compare predictions of plasma characteristics resulting from these models with experimentally determined quantities.; A weakly nonideal plasma with electron densities up to 4 x 10 20 cm-3 and temperatures up to 60 eV is produced for this purpose by focusing a Q-switched Nd:glass laser onto an aluminum target in vacuum. The range of nonideality parameter thereby achieved is 0.05 < Gamma < 0.13. Total laser energies of 2.3, 2.9, 3.5 and 4.0 J are used. Attenuation of the incident laser beam and its second harmonic by the plasma are measured. The energy absorbed into the plasma and the mass of the aluminum atoms and ions present in the plasma are independently measured.; Time-, and space-resolved, continuum emission luminosity of the plasma is measured by means of streak photography. A new algorithm is developed to, invert the integrated line-of-sight luminosity into local specific intensities of the continuum plasma emissions. Due to the dependence of the inversion on plasma absorption, the inversion algorithm incorporates the scaling relations: T = cTIalpha and p = cpIalpha + beta , where T and p denote the plasma temperature and pressure, respectively. Equilibrium plasma calculations are carried out to self-consistently determine the scaling exponents (alpha and beta) and scaling constants (cT and cp). The plasma calculations include Saha's equations for up to 13-fold ionizations, the lowering of ionization potentials and modifications to collision frequencies in the plasma. The optimal values of the constants cp = 340, cT = 0.19, alpha = 0.45 and beta = 1.0 are found by minimizing the discrepancy between the calculated and measured plasma mass, energy and attenuation of the laser beams by absorption. We have also required that the time evolution of the plasma energy scale with the incident laser energy. The analysis shows that the plasma nonideality modifies the classical electron-ion collision frequency calculation, which in turn significantly affects the plasma absorption coefficient.