Dense plasma microfield nonuniformity

Plasma microfield-constrained-average quantities arise in a variety of applications including the ion quadrupole effect, the ion motion problem, and autoionizing processes. We critically compare several methods for computing such quantities in the context of microfield-constrained field gradients, which are used for describing the ion quadrupole effect, a source of spectral line asymmetry. Specifically, we show that one adjustable parameter exponential approximation (APEX)-like theory is divergent and compare existing nearest-neighbor models, APEX, and Monte Carlo results. Moreover, we have performed molecular-dynamics simulations to assess the accuracy of these previous results. Our results indicate that APEX calculations of this particular constrained average are quite accurate except at large field values, which are unimportant for line shapes. Interestingly, nearest-neighbor results are quite accurate for certain field gradients and not for others. copyright 1997 The American Physical Society