Alpha-particle spectra have been measured with a time-of-flight detector in laser fusion experiments on the symmetrical illumination, six-beam ZETA laser system (Nd-doped phosphate glass, λ = 1.054 μm). For a series of fast imploding targets, upward mean energy shifts of alpha particles were observed. These energy shifts were attributed to a time-varying electric field which charged reaction products experience as they pass through the corona of the imploding microballoon. A one-dimensional, two-temperature hydrodynamics code was used to simulate both the expanding corona and the imploding glass. The corona simulation gave rise to an electric field with a decay time of the order of the laser pulse width. The acceleration of alpha particles by this field satisfactorily accounted for their observed alpha-energy shifts.