Anisotropic evaporation of a large nanoplasma

Laser-induced plasma dynamics in $\approx$600~nm sized helium droplets are monitored by femtosecond time-resolved X-ray coherent diffractive imaging at the Linac Coherent Light Source (LCLS). An anisotropic, 20~nm wide softened surface region is established within 300~fs. At longer timescales, the width of this region remains largely constant but the dense plasma core diameter shrinks at average rates of 140~nm/ps along and 70~nm/ps perpendicular to the laser polarization. An anisotropic plasma evaporation model reproduces the key experimental observations and connects the findings of previous studies on small nanoscale systems with the dynamics of self-contained plasmas approaching micron scales. The model is supported by angle-resolved ion kinetic energy distributions recorded in coincidence with the X-ray diffraction patterns.