A comparative study of photoionized plasmas created by soft X-ray (SXR) and extreme ultraviolet (EUV) laser plasma sources was performed. The sources, employing high or low energy laser systems, utilized double-stream Xe/He gas-puff targets irradiated with laser pulses of different parameters. The SXR/EUV beams were used for irradiation of a gas stream, injected into a vacuum chamber synchronously with the radiation pulse. Photoionized plasmas produced this way in Ne gas emitted radiation in the SXR/EUV range. The corresponding spectra were dominated by emission lines originating from singly charged ions. Significant differences between spectra obtained in different experimental conditions concern specific transitions in Ne II ions. Creation of photoionized plasmas by SXR or EUV irradiation resulted in K-shell or L-shell emissions respectively. In case of the low energy system absorption spectra were measured additionally. In case of the high energy system, the electron density measurements were performed by laser interferometry, employing a femtosecond laser system. A maximum electron density reached the value of 2·10<sup>18</sup>cm<sup>-3</sup>. For the low energy system, a detection limit was too high for the interferometric measurements, thus only an upper estimation for electron density could be made.
In this work photoionized plasmas were created by irradiation of atomic and molecular gases by soft X-ray and extreme ultraviolet intense radiation pulses. Two different laser-produced plasma sources, employing a low energy Nd:YAG laser system (NL 129) and a high energy iodine laser system (PALS), were used for creation of photoionized plasmas. In both cases the SXR/EUV beam irradiated the gas stream, injected into a vacuum chamber synchronously with the radiation pulse. Radiation spectra, measured for photoionized plasmas produced in Ne and Ar gases, are dominated by L-shell emission lines except the Ne plasma produced using the high energy system where K-shell emission dominates. Additionally electron density measurements were performed by laser interferometry employing a femtosecond laser system synchronized with the irradiating system. Maximum electron density for Ne plasma, induced using the high energy system, reached 2·10<sup>18</sup>cm<sup>-3</sup>. In case of employing the low energy system a detection limit was too high for interferometric measurements, thus only an upper estimation for electron density could be made.
Since the early 1970s ablative laser propulsion (ALP) has promised to revolutionize space travel by reducing the 30:1 propellant/payload ratio needed for near-earth orbit by up to a factor of 50, by leaving the power source on the ground. But the necessary sub-ns high average power lasers were not available. Dramatic recent progress in laser diodes for pumping solid-state lasers is changing that. Recent results from military laser weapons R&D programs, combined with progress on ceramic disk lasers, suddenly promise lasers powerful enough for automobile-size, if not space shuttle-size payloads, not only the 4 - 10 kg "microsatellites" foreseen just a few years ago. For ALP, the 1.6-μm Er:YAG laser resonantly pumped by InP diode lasers is especially promising. Prior coupling experiments have demonstrated adequate coupling coefficients and specific impulses, but were done with too long pulses and too low pulse energies. The properties of ions produced and the ablated surface were generally not measured but are necessary for understanding and modeling propulsion properties. ALP-PALS will realistically measure ALP parameters using the Prague Asterix Laser System (PALS) high power photodissociation iodine laser (λ = 1.315 μm, E<sub>L</sub> ≤1 kJ, τ ~ 400 ps, beam diameter ~29 cm, flat beam profile) whose parameters match those required for application. PALS' 1.3-μm λ is a little short (vs. 1.53-1.72 μm) but is the closest available and PALS' 2ω / 3ω capability allows wavelength dependence to be studied.