Lifting shell structures in the dynamically assisted Schwinger effect in   periodic fields
  The dynamically assisted pair creation (Schwinger effect) is considered for the superposition of two periodic electric fields acting a finite time interval. We find a strong enhancement by orders of magnitude caused by a weak field with a frequency being a multitude of the strong-field frequency. The strong low-frequency field leads to shell structures which are lifted by the weaker high-frequency field. The resonance type amplification refers to a new, monotonously increasing mode, often hidden in some strong oscillatory transient background which disappears during the smoothly switching off the background fields, thus leaving a pronounced residual shell structure in phase space. 
Andreas Otto, Daniel Seipt, David Blaschke, Burkhard Kampfer,   Stanislav Alexandrovich Smolyansky
hep-ph