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.
A novel technique for ion implantation of electronics materials by means of a laser ion source emitting multi-energetic ion streams was investigated. A UV pulsed laser beam, at intensities of the order of 108 W/cm2, was employed to produce plasma in a vacuum from a Ge target. The apparatus utilized was very versatile and able to contain an expansion chamber in order to allow the plasma to be diluted before the application of an accelerating voltage. The mean ion energy increased with the laser pulse energy and the ion charge state, and ranged between about 100 eV and 1 keV. To increase the ion energy a post-acceleration up to 50 kV was employed, which resulted in ion energies from about 50 keV to about 150 keV, depending on the charge state. The multi-energetic ion beam, with current density of the order of 10 mA/cm2, was employed to irradiate silicon substrates and to obtain surface implantations up to a depth of about 150 nm. During the implantation process the ion beams were generated with a repetition rate of the laser pulse of 1 Hz. The depth profiles of the ion implants were investigated by Rutherford backscattering spectrometry and laser ablation - inductively coupled plasma - mass spectrometry.
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.
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.
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 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 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 experimental results of investigations on influence of external magnetic and electric fields on characteristics of ion stream emitted from a plasma produced by the Nd:glass performed at IPPLM, Warsaw are presented. A negatively biased target up to -15 kV and a magnetic field up to 0.45 T were used in the experiment. A set of ion collectors and an electrostatic cylindrical ion energy analyzer located at small angles with respect to the laser beam axis and at large distances from the target were applied for ion measurements. The effect of an external magnetic field is essential to plasma expansion but the effect of the retarding potential of the target is very weak in our experimental conditions. The presented results relate only to tungsten plasma. The aim of the studies was to prove a possibility of optimization of ion beam parameters from laser-produced plasma for particular application as a Laser Ion Source coupled with the Electron Cyclotron Resonance ion source for particle accelerators.
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.
Karel Masek, Josef Krasa, Leos Laska, Miroslav Pfeifer, Karel Rohlena, Bozena Kralikova, Jiri Skala, Eugeniusz Woryna, J. Farny, Piotr Parys, Jerzy Wolowski, W. Mraz, H. Haseroth, B. Sharkov, G. Korschinek
Ions in different charge state and with different energy distribution are generated in the process of interaction of intense laser radiation with solid targets. Multiply charged ions of medium- and high-Z elements (Al, Co, Ni, Cu, Sn, Ta, W, Pt, Au, Pb, Bi), produced by photodissociation iodine laser system PERUN ((lambda) equals 1.315 micrometer, EL approximately 40 J, (tau) approximately 500 ps) are reported. Corpuscular diagnostics based on time-of-flight method (ion collectors and a cylindrical electrostatic ion energy analyzer) as well as Thomson parabola spectrometer were used in the experiments. The ions in maximum charge state up to about 55+ and with energies of several MeV were registered at a distance of about 2 m from the plasma plume. Measured ion current densities higher than 10 mA/cm2 in about 1 m from the target demonstrate the performance of laser ion source. A theoretical interpretation of ion spectra is attempted.
Leos Laska, Josef Krasa, Karel Masek, Bozena Kralikova, Tomas Mocek, Miroslav Pfeifer, Jiri Skala, Petr Straka, Karel Rohlena, Eugeniusz Woryna, Jerzy Wolowski
Laser beam focus position was varied with respect to the target surface to obtain the maximum current of highly charged Ta ions from laser-produced plasma in the far expansion zone. The experiments prove that the optimum position must be settled with the accuracy plus or minus 125 micrometers.
Thermoluminescent dosimeters LiF:Mg,Ti (TLD100), LiF:Mg,Cu,P (GR200A) and CaF2:Dy (TLD200) as well as Si commercial photodiodes, Polish production, BPYP44 type, were used for time-integrated absolute measurements of x-ray emission from laser-produced plasmas generated by a Nd:glass laser or an iodine photodissociation laser. All the detectors were calibrated by application of radioactive standard sources. Calibrated responses to x-ray radiation emitted from a laser-produced Al plasma are compared and a discrepancy in the results is discussed.
A dense jet of a plasma consisting of multiply charged ions was generated in the interaction of a laser plasma with a strong external axial magnetic field. It is shown that use of high- luminosity x-ray specroheliograph technique allows us to measure plasma emission spectra with 2-dimensional spatial resolution even in the cases when these spectra have a small intensity. The x-ray spectroscopy and interferometry methods are used to measure plasma parameters distributions. The dependencies of Ne(z) and Te(z) measured in this paper can be used to calculate the evolution of plasma ionization state during plasma expansion. The quasihomogeneous laser jet, which appears when a laser plasma interacts with an external magnetic field, can be used not only to form an active medium of a shortwavelength laser, but probably also to tackle the urgent problem of transport in a laser ion injector.
The new design of three-frame interferometer is proposed. The optical system of the interferometer, though very simple, has some advantages in comparison with classical interferometers (e.g., Mach-Zehnder interferometer). For registration and analysis of interferograms the CCD cameras and multichannel image acquisition system are used. The interferometer has been tested in a laser-matter experiment. Some results of plasma investigation are presented.
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