Compact Gain-Saturated Plasma-Based X-Ray Lasers Down to 6.85 Nm and Amplification Down to 5.85 NM

Plasma-based x-ray lasers allow single-shot nano-scale imaging and other experiments requiring a large number of photons per pulse to be conducted in compact facilities. However, compact repetitively fired gain-saturated x-ray lasers have been limited to wavelengths above
$\lambda=8.85$
nm 1. Here we extend their range to
$\lambda=6.85$
nm by transient traveling wave excitation of Ni-like Gd ions in a plasma created with an optimized pre-pulse followed by rapid heating with an intense sub-ps pump pulse, both pulses from a
$\lambda=800$
nm Ti: Sa laser2. Model simulations show that refraction shifts the maximum gain to the lower electron density region of
$\mathrm{n}_{\mathrm{e}}=4-5\times 10^{20}$
cm-3, Isoelectronic scaling also produced strong lasing at 6.67 nm and 6.11 nm in Ni-like Tb, and amplification at 6.41 nm and 5.85 nm in Ni-like Dy. This scaling to shorter wavelengths was obtained progressively increasing the plasma density by increasing the grazing incidence angle at which the sub-ps pulse impinges onto the target. We experimentally demonstrate that the optimum grazing incidence angle increases linearly with atomic number from 17 degrees for Z=42 (Mo) to 43 degrees for Z=66 (Dy), corresponding to densities of 1.5 to 8 x 10
²⁰
cm-3, but as mentioned above for the high Z elements refraction shifts the maxim gain region to lower values.