Enhanced hard x-ray emission from microdroplet preplasma

We perform a comparative study of hard x-ray emission from femtosecond laser plasmas in 15 μm methanol microdroplets and Perspex target. The hard x-ray yield from droplet plasmas is ≃68 times more than that obtained from solid plasmas at 2× 1015 W cm-2. A 10 ns prepulse at about 5% of the main pulse appears to be essential for hard x-ray generation from droplets. Hot electron temperature of 36 keV is measured from the droplets at 8× 1014 W cm-2, whereas a three times higher intensity is needed to obtain similar hot electron temperatures from Perspex plasmas. Particle-in-cell simulations with very long scale-length density profiles support experimental observations.