Transition from isentropic to isothermal expansion in laser produced plasmas

The transition that the expansion flow of laser-produced plasmas experiences when one moves from long, low intensity pulses (temperature vanishing at the isentropic plasma-vacuum front, lying at finite distance) to short, intense ones (non-zero, uniform temperature at the plasma-vacuum front, lying at infinity) is studied. For planar geometry and large ion number Z_i the transition occurs for d phi /dt approximately=0.14(27/8)k⁷2/Z_i³2/n_c²/m_i³2/K; phi , n_c, m_i, and K are laser intensity, critical density, ion mass, and Spitzer's heat conduction coefficient (1962). This result remains valid for finite Z_i, though the numerical factor in d phi dt is different. Shorter wavelength lasers and higher Z_i plasmas allow faster rising pulses below transition.