Peculiar behavior of Si cluster ions in a high-energy-density solid Al plasma

Peculiar behavior is found in a Si cluster ion, moving with a speed of ∼0.22c (c: speed of light) in a solid Al plasma in the context of cluster-ion beam driven inertial confinement fusion: The Si ion, moving closely behind the forward-moving Si ion for a distance of several Å in the cluster, feels the wake field generated by the forward Si. The acceleration interaction force on the rear Si ion by the forward-moving ion may balance with the deceleration backward force in the longitudinal-moving direction. The forward-moving Si ion would be decelerated, as an isolated ion is decelerated without correlation. However, the deceleration of the rear Si ion, moving behind closely, would be reduced significantly. The rear Si ion may catch up and overtake the forward-moving Si ion in the cluster during the Si cluster-ion interaction with the high-density Al plasma. This peculiar behavior appears when the ions are aligned well longitudinally. The wake field is confined around the Si ion in the forward and transverse directions for a distance smaller than the Si cluster interionic distance l_{c}. However, the tail of the wake field extends beyond l_{c} due to the Si ion high speed of ∼0.22c. Therefore, the peculiar behavior shown above appears only for the ions in one cluster aligned well longitudinally.