Energy Extraction and Achievement of the Saturation Limit in a Discharge-Pumped Table-Top Soft X-Ray Amplifier.

A major goal in ultrashort wavelength laser research is the development of practical laser sources that can impact applications. Of particular interest is the demonstration of compact “table-top” amplifiers capable of generating soft x-ray pulses of substantial energy. Such development motivates the demonstration of gain media generated by compact devices that can be successfully scaled in length to reach gain saturation. At this condition, which occurs when the laser intensity reaches the saturation intensity, a large fraction of the energy stored in the laser upper level can be extracted. To date, gain saturation had only been achieved in a few soft x-ray laser transitions in plasmas generated by some of the world’s largest laser facilities [1–4]. While gain in soft x-ray lines has been reported in several compact systems [5–13] only in three cases the gain-length product was measured to be greater than 5 [5–7]. These include the observation of amplification at 46.9 nm in Ne-like Ar by our group in a plasma column generated by a fast capillary discharge [5,14], and the demonstration of gain at 41.8 nm in Pd-like Xe and at 32.6 nm in Ne-like Ti in plasmas generated by relatively compact terawatt class femtosecond and picosecond laser facilities, respectively [6,7]. In all cases, however, the laser energy extraction from table-top amplifiers has been small and detection of the laser lines was performed with high sensitivity detectors such as microchannel plate (MCP) intensifiers. A next major step in the development of compact ultrashort wavelength lasers consists in advancing from gain observations to the demonstration of substantial laser output energies. Depending on the specific amplifier characteristics, this amounts to overcoming barriers that are imposed by small gain volumes and short plasma lengths, by short duration of the gain, or by axial plasma inhomogeneities and limiting refraction effects. Recent optimization of the capillary discharge scheme, which has the advantages of a relatively large gain volume and long gain duration, allowed amplification at 46.9 nm to reach 14 gain-length products [15], but no demonstration of substantial laser energy extraction was realized. In this Letter we report the generation of laser pulse en-