The aim of this paper is to describe the newly-opened High Power Laser Laboratory (HPLL) at the Institute of Plasma Physics and Laser Microfusion (IPPLM) and presents its research possibilities in terms of the laser source and the available diagnostics. The interactions of the ultra-short laser pulses of femto to pico second duration and energies of up to 1 J with solid-state targets leads to very distinguish phenomena which can be used for investigation of exotic states of matter and to apply them for numerous technological purposes. in this goal the application of sophisticated and especially designed diagnostic systems is also needed. As the effects of interactions include the broad range of processes as acceleration of fast ions and electrons, x-rays generation and solid material modification in terms of its mechanical, physical and optical properties the research needs to use both online plasma diagnostics and the methods of post-mortem material research characterization methods. The paper presents a brief description of the laser system, the interaction vacuum chambers and the available diagnostics as well as the scientific investigation trends which can be picked up at the HPLL at the IPPLM.
This paper presents the goals and some of the results of experiments conducted within the Working Package 10 (Fusion
Experimental Programme) of the HiPER Project. These experiments concern the study of the physics connected to
"Advanced Ignition Schemes", i.e. the Fast Ignition and the Shock Ignition Approaches to Inertial Fusion. Such schemes
are aimed at achieving a higher gain, as compared to the classical approach which is used in NIF, as required for future
reactors, and making fusion possible with smaller facilities.
In particular, a series of experiments related to Fast Ignition were performed at the RAL (UK) and LULI (France)
Laboratories and were addressed to study the propagation of fast electrons (created by a short-pulse ultra-high-intensity
beam) in compressed matter, created either by cylindrical implosions or by compression of planar targets by (planar)
laser-driven shock waves. A more recent experiment was performed at PALS and investigated the laser-plasma coupling
in the 1016 W/cm2 intensity regime of interest for Shock Ignition.
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.
Interaction of TW-ps laser with plasma results in a skin layer mechanism for nonlinear (ponderomotive) force driven two dimensional plasma blocks (pistons) if a very high contrast ratio is provided for suppression of relativistic self-focusing. This Skin layer acceleration (SLA) [1] results in space charge neutral plasma blocks with ion current densities larger than 1010 Amp/cm2 [1-3]. Using Ions in the MeV range results in 1000 times higher proton or DT current densities [3] than the proton fast igniter [4] is using and may result in better conditions of this fast ignitor scheme. Using ballistic focusing of the generated plasma blocks and a short time thermal expansion of these blocks for increasing their thickness while keeping the high ion current densities, results in conditions favourable for this option of fast ignition of a fusion target. Some details of the interaction processes are still to be analysed but the solutions studies to date are most encouraging.
It is being clarified why the observations of plane wave geometry interaction within the skin depth of a laser irradiated target are very unique exceptions from the broad stream of the usual experiments of laser plasma interaction. This permits a much more simplified description by plane wave interaction theory for laser pulses of about ps or shorter duration and powers above TW and simplifies computations in contrast to the usual cases with relativistic self-focusing. After establishing theoretically and experimentally the generation of highly directed plasma blocks with ion current densities above 1010 A/cm2 moving against the laser light or into the target, applications for laser fusion, and a completely new improvement of ion sources for the next generation of accelerators are discussed.
Plasma emission or ablation from laser-irradiated targets shows very complicated properties. One novelty was observed at irradiation of neodymium glass laser pulses of ps duration and TW power if there was a very strong suppression of prepulses by a contrast ratio of about 108 until 100 ps before the main pulse arrived. The emitted ion maximum energy was more than 50 times below the values observed in all the comparable numerous experiments. The other anomaly is that the number of the fast ions did not change when the laser intensity varied by a factor 30. This permitted a separation of the usual effects of self-focusing and permitted an analysis fully based on simplified plane geometry as a skin layer interaction mechanism. The consequence is that plasma blocks are accelerated by the nonlinear (ponderomotive) force with ion current densities above 1010 A/cm2. This provides basically new aspects for laser fusion using uncompressed solid DT fuel and a new kind of x-ray laser process may be possible.
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 paper presents a review of recent studies of plasmas produced in various experimental conditions performed mainly at the IPPLM and partially at the PALS Joint Research Laboratory ASCR in Prague in an international cooperation. These investigations were directed towards the clarification of the physical processes in such plasmas as well as at the optimization of sources of multi-charged ions for various applications. A 1-ps terawatt Nd:glass laser system (pulse energy up to 1 J, wavelength: 1053 nm, power density up to 1017 W/cm2) was employed for the experiments carried out at the IPPLM in Warsaw. Also, an option of this system operating with the 0.5 ns pulse (power density up to 1014 W/cm2) for comparative studies was used. In common experiments at the PALS JRL in Prague we used the PALS iodine laser system producing up to 1.2 kJ in a 0.4 ns pulse at 1315 nm wavelength or 0.25 kJ at 438 nm (third harmonic) wavelengths. The time-of-flight measuring systems namely: different ion collectors and an electrostatic ion energy analyzer were employed as main diagnostic methods. The properties of ion emission were investigated at various experimental conditions with the use of different massive and thin foil targets.
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.
Recent enormous progress in the technology of ultrashort-pulse lasers has resulted in the generation of extremely high light intensities approacing 1021 W/cm2. Even at a hundred times lower intensity, electrons oscillate in the laser field with velocities close to the velocity of light and the interaction of laser light with matter is relativistic. The great non-linear forces produced by a laser pulse duing the interaction can be used to accelerate electrons and ions to energies of megaelectronvolts in distances of only microns. This creates a prospect for the construction of compact laser-based particle accelerators and their unique applications in material science, medicine and nuclear physics on a tabletop. In this paper recent achievements in the laser-based acceleration of electrons, protons and heavy ions are briefly reviewed. Some results obtained in this fascinating field in Poland are presented.
The results of investigation of ion emission from various solid targets, irradiated with an intense 1-ps laser pulse are reported. The ion measurements were based on the time- of-flight method and were performed with the use of ion collectors and electrostatic ion-energy analyzer. The emission of highly-charged heavy ions is demonstrated for the first time for a plasma produced with an ultrashort laser pulse. Ion collector signals show two groups of ions existence of which corresponds to different mechanisms of their generation. Ion yield depends essentially on the laser beam focusing conditions. A production of intense streams of highly charged, high energy ions was observed.
The specific properties of the interaction of a high-power ultrashort laser pulse with a solid target related to the processes of target ionization, absorption and reflection of laser radiation as well as generation of ion streams and X- rays are discussed. The results of the experimental investigations concerning the above problems are presented. Some possible applications are discussed briefly as well.
A high-power all-Nd:glass laser using fiberless chirped-pulse- amplification technique is presented. The laser is capable of producing 1.2-ps, 2-TW pulses while the effective divergence of the beam does not exceed 10-4 rad.
A novel, efficient method for generation of ultrashort pulses in short-gain-duration lasers is described and investigated numerically. The method employs fast mode locking accomplished by means of an electro-optic deflector. The possibilities of the method are demonstrated taking XeCl and KrF excimer lasers as an example. It is proved that, in contradistinction to conventional active mode locking (e.g., with an acousto-optic modulator), even at the gain duration as short as 50 - 100 ns, it is possible to obtain pulses shorter than 10-11 s with this method using a sine-wave driven electro-optic deflector.
A possibility of generation and compression of ultrashort pulses in excimer systems with saturable absorbers was investigated numerically. It is shown that using fast pulse periodic Q- switching millijoule pulses of duration in the 1 - 10 ps range are attainable from KrF oscillator with acridine dye as an absorber. Then the pulses can be compressed to subpicosecond range in an excimer amplifying system containing saturable absorbers. The proposed technique makes it possible to construct all-excimer pico- and subpicosecond high-power systems considerably less complex and less expensive than hybrid systems used at present.
A method for the generation of short, single-mode CO2 laser pulses by applying two voltage gates to an electro-optic Q-switch placed in a three-mirror cavity is proposed. The results of experimental and numerical tests of the method for a TEA CO2 laser are presented. It is shown that the method enables the user to obtain single megawatt pulses of duration in the 1 - 10 ns range.
Ultrashort-pulse lasers inspire increasing researcher interest because of their wide possibilities of application in various branches of science and technology. Rapid development of such lasers during the last decade made it possible to obtain pulses of duration in the femtosecond range and to increase their energy and power by several orders of magnitude. Considerable development was attained in improvement of operation stability, increase of repetition frequency, and enhancement of tunability of femtosecond lasers, which made them indispensable research tools applied in many laboratories. The paper describes principal ideas concerning generation of femtosecond pulses and reviews important achievements in femtosecond laser investigations. Ultrashort-pulse formation mechanisms in passively mode- locked lasers and the role of dispersion and self-phase modulation in techniques for generation of such pulses are discussed. Principal methods for generation of femtosecond pulses in dye lasers and solid-state lasers are characterized, techniques for ultrashort-pulse generation within the ultraviolet with the use of excimer lasers are described, and typical construction layouts of femtosecond lasers are presented.
The paper presents investigations of the influence of parameters of the voltage pulse applied to the electrodes of electron gun on the current density spatial distribution of large aperture electron beam. It was shown that uniformity of current density distribution essentialy increases with the increase of steepness of the voltage pulse front and with the increase of the voltage amplitude.
There were carried out numerical investigations of the possibility of applying the self-injection technique for generation of short pulses jil rare-gas halide excimer lasers. We investigated a case in which the electrooptical switch is placed in a resonator and is contrulled by two half-wave voltage gates and active medium has the parameters typical for the KrF laser. The model regarded vibrational relaxation and collisional mixing of the B states of excimer molekule. The influence of parameters characterizing the course of voltage applied to the Q-switch upon energy power temporal shape and contrast of generated pulses was analysed. It was stated that there is a possibility of a controlled generation of pulses of time duration from a few hundreds of picoseconds to a few nanoseconds and power of on order higher than in case of free oscillation.
The influence of pressure and composition of Ar+Xe+HCl mixture on the pulse duration of electron-beam-controlled discharge XeCl laser was experimentally investigated. It was found, that the variation of total pressure of the mixture of fixed composition can be the effective and simple way of controlling the pulse duration in the range of one order of magnitude without significant changes of the pulse power
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