Charge resolved electrostatic diagnostic of colliding copper laser plasma plumes

The collision of two laser generated plasma plumes can result, under appropriate conditions, in the formation of a “stagnation layer.” The processes underlying this phenomenon are complex and time dependent. The majority of experiments over the last few decades have focused upon spectroscopic diagnostic of colliding plasmas. We have performed electrostatic diagnosis of multiply charged copper ions (Cu⁺ to Cu⁵⁺) generated via Q-switched pulsed laser (λ = 1.06 μm, τ = 6 ns, and E_L = 52−525 mJ) generation of copper plasma plumes from a planar target. Time dependent current traces, charge yields, and kinetic energy (K_e) distributions are obtained for single plasma plumes (S_p) and colliding plasma plumes (C_p). The charge yield from a C_p relative to twice that from a S_p is characterized by a charge yield ratio (CYR) parameter. Superior ion yields for all charge states occur for a discrete range of fluences (F) from colliding plasma plumes leading to a CYR parameter exceeding unity. The kinetic energy distributions from colliding plasma plumes display well defined energy compression via narrowing of the distributions for all fluences and charge states. The extent of this energy compression is charge dependent. Space charge forces within the stagnation layer and the resulting charge dependent acceleration of ions are proposed to account for the transfer of ion kinetic energy in favour of collisional ionization mechanisms.