Collisional excitation soft x-ray laser pumped by optical field ionization in a cluster jet

An optical-field-ionization soft x-ray laser using a clustered gas jet was demonstrated. Pd-like xenon lasing at 41.8 nm with 95-nJ pulse energy and 5.2-mrad divergence was achieved, indicating near-saturation amplification. By using a prepulse to control the expansion of ionized clusters, it was found that the microscopic uniformity of the plasma is essential for efficient lasing. In addition, the optimal atom density for maximum lasing reported previously is verified to result from the tradeoff between large gain coefficient and short gain length due to ionization-induced refraction.