Achieving inertial confinement fusion using a light-ion-beam driver requires continued improvement in understanding ion diode physics. The power delivered to a light-ion beam target is strongly influenced by the evolution of the charge-particle distributions across the ion beam acceleration gap. Our strategy is to determine this evolution from time- and space-resolved measurements of the electric field using Stark-shifted line emission. In addition to diode physics, the unique high-field (∼10 MV/cm, ∼6T) conditions in present experiments offer the possibility to advance basic atomic physics, for example by measuring field ionization rates for tightly bound low-principal-quantum-number levels. In fact, extension of atomic physics into the high-field regime is required for accurate interpretation of diode physics measurements. This paper describes progress in ion diode physics and basic atomic physics, obtained with visible-light atomic spectroscopy measurements in the ∼20 TW Particle Beam Fusion Accelerator II ion diode.