Efficient, high-brightness soft-x-ray laser at 21.2 nm

In this paper we review the development of a soft-x-ray laser at 21.2 nm, recently carried out at the Laboratoire de Spectroscopie Atomique et Ionique. Amplification is generated by electron collisional pumping on a 2s²{2p}⁵$3p-${2s}²2p⁵3s, J=0--1 transition in neonlike zinc (Zn²⁰⁺). The lasing medium is a {~}150 {μ}m×{}2 cm line plasma produced by irradiating slab Zn targets at a net intensity of {~}1.4×{}10¹³ W cm^-2, using a 1.06-{μ}m, 600-ps-long pulsed laser delivering {~}350 J of net energy on the target. Accompanying the driving pulse by a 100-mJ-level prepulse train through deliberately imperfect isolation of the mode-locked laser oscillator, the J=0--1 gain coefficient {~}5 cm^-1 is generated. The emission appears in an {~}100-ps burst and precedes the lasing on both the much weaker J=2--1 lines and the x-ray continuum peak. The J=0--1 output source region, the beam spatial profile, and its time history have been measured and their relevance to the kinetics of this lasing system is discussed. A half cavity was implemented and a strong double-pass amplification at 21.2 nm with a 2-cm-long plasma obtained. With the gain-length product achieving {~}17.3 according to the particular configuration, saturation was demonstrated, which is confirmed through a schematic model of the level kinetics. To our knowledge, it is the first neonlike soft-x-ray laser where saturation on the J=0--1 line was achieved by using a half-cavity arrangement. Absolute energy measurements indicate {~}400 {μ}J in the half-cavity-generated beam and a peak output power of {~}5 MW, which makes this laser one of the most efficient soft-x-ray lasing systems demonstrated to date.