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, EL ≤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.
For conventional wavelength (UV-vis-IR) lasers delivering radiation energy to the surface of materials, ablation thresholds, ablation (etch) rates, and the quality of ablated structures often differ dramatically between short (typically nanosecond) and ultrashort (typically femtosecond) pulses. Various short-wavelength (<100 nm) lasers emitting pulses with durations ranging from ~10 fs to ~1 ns have recently been put into routine operation. This makes it possible to investigate how ablation characteristics depend on pulse duration in the XUV spectral region. Four sources of intense short-wavelength radiation available in the authors' laboratories, including XUV and soft x-ray lasers, are used for the ablation experiments. Based on the results of the experiments, the etch rates for three different pulse durations are compared using the XUV-ABLATOR code to compensate for the wavelength difference. Comparing the values of etch rates calculated for nanosecond pulses with those measured for shorter pulses, we can study the influence of pulse duration on XUV ablation efficiency. The results of the experiments also show that the ablation rate increases while the wavelength decreases from the XUV spectral region toward x-rays, mainly due to increase of attenuation lengths at short wavelengths.
For conventional wavelength (UV-Vis-IR) lasers delivering radiation energy to the surface of materials, ablation thresholds, ablation (etch) rates, and the quality of ablated structures often differ dramatically between short (typically nanosecond) and ultrashort (typically femtosecond) pulses. Various short-wavelength (l < 100 nm) lasers emitting pulses with durations ranging from ~ 10 fs to ~ 1 ns have recently been put into a routine operation. This makes it possible to investigate how the ablation characteristics depend on the pulse duration in the XUV spectral region. 1.2-ns pulses of 46.9-nm radiation delivered from a capillary-discharge Ne-like Ar laser (Colorado State University, Fort Collins), focused by a spherical Sc/Si multilayer-coated mirror were used for an ablation of organic polymers and silicon. Various materials were irradiated with ellipsoidal-mirror-focused XUV radiation (λ = 86 nm, τ = 30-100 fs) generated by the free-electron laser (FEL) operated at the TESLA Test Facility (TTF1 FEL) in Hamburg. The beam of the Ne-like Zn XUV laser (λ = 21.2 nm, τ < 100 ps) driven by the Prague Asterix Laser System (PALS) was also successfully focused by a spherical Si/Mo multilayer-coated mirror to ablate various materials. Based on the results of the experiments, the etch rates for three different pulse durations are compared using the XUV-ABLATOR code to compensate for the wavelength difference. Comparing the values of etch rates calculated for short pulses with those measured for ultrashort pulses, we can study the influence of pulse duration on XUV ablation efficiency. Ablation efficiencies measured with short pulses at various wavelengths (i.e. 86/46.9/21.2 nm from the above-mentioned lasers and ~ 1 nm from the double stream gas-puff Xe plasma source driven by PALS) show that the wavelength influences the etch rate mainly through the different attenuation lengths.
Ablation thresholds, etch rates, and quality of ablated structures often differ dramatically if a conventional, UV-Vis-IR laser delivers radiation energy onto a material surface in a short (nanosecond) or ultra-short (picosecond/femtosecond) pulses. Various short-wavelength (λ < 100 nm) lasers emitting pulses with durations ranging from ~ 10 fs to ~ 1 ns have recently been put into a routine operation. This makes possible to investigate how the ablation characteristics depends on the pulse duration in the XUV spectral region. 1.2-ns pulses of 46.9-nm radiation delivered from a capillary-discharge Ne-like Ar laser, focused by a spherical Sc/Si multilayer-coated mirror were used for an ablation of organic polymers and silicon. Various materials were irradiated with an ellipsoidal-mirror-focused XUV radiation (λ = 86 nm, τ = 30-100 fs) generated by the free-electron laser (FEL) operated at the TESLA Test Facility (TTF1 FEL) in Hamburg. The beam of the Ne-like Zn XUV laser (λ = 21.2 nm, τ < 100 ps) driven by the Prague Asterix Laser System (PALS) was also successfully focused by a spherical Si/Mo multilayer-coated mirror to ablate various materials. Based on the results of the experiment the etch rates for three different pulse durations are compared using the XUV-ABLATOR code to compensate for the wavelength difference. Comparing the values of etch rates calculated for short pulses with the measured ones for ultrashort pulses we may study the influence of pulse duration on the XUV ablation efficiency.
The investigations of nonthermal processes in laser-produced plasmas are not yet complete, especially with regard to the ion acceleration in the plasma generated by high-energy short-wavelengths lasers. This contribution presents the results of studies of fast ion emission from plasma generated using a short wavelength (438 nm), high-energy (up to 250 J in 400 p5 pulse) iodine laser PALS at the Joint Research Laboratory PALS ASCR in Prague, Czech Republic. The properties of highly charged ion streams were investigated by ion diagnostic methods: ion collectors and solid state track detectors as well as a cylindrical electrostatic energy analyzer. Attention was paid to the determination of ion energy and comparison of the energies and abundance of different ion groups. The presented results shown the existence of highly charged ions with z <40 (measured z, =57 forTa) and with energies higher then 20 MeV in a far expansion zone. Ion current densities up to tens of mA/cm2 at a distance of 1 m from the target were obtained. On the basis of the ion diagnostic investigations the existence of nonthermal and nonlinear accelerating processes was demonstrated for the plasma produced by a high-energy short-wavelength laser pulse.
The experiment of Badziak et al has shown that irradiation of copper by 1.5 ps laser pulses produced 50 times lower maximum ion energies than the 22 MeV expected after relativistic self focusing from laser pulses of about ns duration. This discrepancy was confirmed in the following reported experiments specifically designed for this clarification, where MeV Au+30 maximum ion energies needed 400 times higher intensity with ps pulses than with 0.5 ns pulses. Comparing the theory for generating the fastest ions by relativistic self focusing and of the second fastest group by a quiver-collision model, we arrived at the conclusion that the mentioned ps-TW-generated ions are not following these usual theories but that a skin depth model with exclusion of relativistic self focusing explains the experiments. The essential importance is the suppression of the prepulse. We conclude how the experiment by Norreys et al. with the highest ever reported fusion gains may be increased to fusion reactor conditions if our results of prepulse control and suppression of relativistic self focusing would be applied following our skin layer interaction model. This extends the fast ignitor to the nonlinear-force block ignition without plasma precompression.
The efficiency and threshold of ablation of polymethylmethacrylate (PMMA), polytetrafluoroethylene (PTFE), and monocrystalline silicon by single pulses of soft x-rays emitted from Z-pinch, plasma-focus, and laser-produced plasmas were investigated. The Z-pinch was driven by the S-300 pulsed-power machine (Kurchatov Institute, Moscow) and the plasma focus was realized in the PF-1000 machine (Institute of Plasma Physics and Laser Microfusion, Warsaw). Higher temperature plasma than with the discharge plasmas was obtained by focusing the near-infrared beam from the PALS high-power iodine laser system (Czech Academy of Sciences, Prague) on the surface of a metallic slab target. The role of nonthermal processes in x-ray ablation was evaluated. Possible ways to use x-ray ablation for micromachining are discussed.
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