Advanced targets,diagnostics and applications of laser-generated plasmas

High-intensity sub-nanosecond-pulsed lasers irradiating thin targets in vacuum permit generation of electrons and ion acceleration and high photon yield emission in non-equilibrium plasmas. At intensities higher than 10¹⁵?W/cm² thin foils can be irradiated in the target-normal sheath acceleration regime driving ion acceleration in the forward direction above 1?MeV per charge state. The distributions of emitted ions in terms of energy, charge state and angular emission are controlled by laser parameters, irradiation conditions, target geometry and composition. Advanced targets can be employed to increase the laser absorption in thin foils and to enhance the energy and the yield of the ion acceleration process. Semiconductor detectors, Thomson parabola spectrometer and streak camera can be employed as online plasma diagnostics to monitor the plasma parameters, shot by shot. Some applications in the field of the multiple ion implantation, hadrontherapy and nuclear physics are reported.     

Ponderomotive ion acceleration in dense plasmas at super-high laser intensities

Short laser pulses at super-high intensities such as those proposed in the Extreme Light Infrastructure (ELI) project open new prospects for efficient acceleration of ions in overdense plasmas. A simple analytical model and numerical simulations demonstrate that pulses with intensities exceeding 10²² W/cm² may efficiently accelerate ions to high energies up to a few GeV. Maximum ion energy and the energy spectrum of the accelerated ions can be tuned by an appropriate choice of laser pulse intensity and duration at a given plasma density distribution.     

Observations of MeV ions emitted from a laser produced plasma

Fast-ion emission from tungsten plasmas has been studied with neodymium laser intensities from 5x10¹² to 2x10¹⁵ W/cm². Ion energies into the MeV region have been observed. The variation of ion energy (E_I) with laser intensity (I) was found to be of the form E_I∞I^0.65.     

The Effect of Pre-plasma and Self-focusing on Characteristics of Laser Produced Ions

Generation of various ion groups was studied using lasers of different pulse length and intensities. Up to thirteen ion subgroups have been distinguished when using the PALS iodine laser system in experiments that involve a laser interaction with a preformed plasma. In addition to generally accepted thermal, fast, and slow ion groups, the existence of the second fast (superfast) group was clearly proven. The threshold laser intensity I _L necessary fo appearance of nonlinear effects was confirmed to be × 10¹⁴W/cm² at our experimental conditions. Above this value, there exists an interval of laser focus positions in front of the target in which ions with significantly higher charge states and energies are produced, compared to experiments without pre-plasma interactions, due to nonlinear processes in preformed plasma.     

Tantalum ion acceleration in laser‐generated plasma and dependence on the pulse duration

The laser irradiation of tantalum targets is presented for different pulsed laser intensities ranging from 10¹⁰ up to about 10¹⁸ W/cm² and pulse durations from 9 ns up to 40 fs. The results show that the produced non‐equilibrium plasma accelerates Ta ions in the backward direction from values of the order of keV up to values of about 5 MeV. In thin foils, the forward plasma, developed behind the target along the direction of incoming laser, at intensities of about 10¹⁶ W/cm² and 300 ps pulse duration, accelerates Ta ions at energies of the order of 4.6 MeV and produces charge states up to about 40+. For fs lasers at intensities of the order of 10¹⁸ W/cm², only proton acceleration occurs up to 2.1 MeV while no Ta ions are accelerated, due to the reduced duration of the electric field and to the too high inertial mass of the Ta ions.     

On the role of prepulses during solid target heating by picosecond laser pulses

X-ray emission spectra of the plasma created at the surface of magnesium, aluminum, copper, and zinc targets heated by 1-ps laser pulses with a peak power density of up to 10¹⁶ W/cm² were measured. The effect of a picosecond prepulse on the spectra was studied for various power densities and intensity contrasts of the main laser pulse. It is established that the emission spectra of laser plasmas are weakly affected by a change from 10⁵ to 10⁷ in the main pulse contrast relative to the first prepulse. Variations in the parameters of emission from aluminum and magnesium plasmas were calculated using relative intensities and widths of the resonance lines of H-and He-like ions and their two-electron satellite peaks.     

Microspectral analysis of dentine with femtosecond laser induced plasma

Microspectral analysis of dentine plasma produced by femtosecond laser radiation with intensities of I ～ 10¹³?10¹⁵ W/cm² in ambient atmosphere has been measured. C, O, Ca, Zn, Na, and Cu spectral lines were identified. The X-ray radiation with energies E > 30 keV has been observed upon laser beam intensities of I ～ 5 × 10¹⁵ W/cm².     

X-ray emission from short-pulse laser plasmas

Highly intense picosecond and subpicosecond laser pulses interacting with solids can create hot and dense plasmas which emit x-ray pulses in a broad spectral range from 100 eV up to MeV. The duration of these x-ray pulses depends on the transient behaviour of the relaxation and recombination mechanisms, as well as on the lifetime of energetic electrons produced via nonlinear processes in the plasma. This paper reports experiments using a 1.5-ps laser pulse with high constrast ratio (up to 10¹⁰) and intensities up to 10¹⁸ W cm^-2 irradiating solid targets. Both the line spectrum characteristics of a magnesium plasma, recorded using crystal spectrometers with high spectral resolution, and kinetic calculations have allowed the deduction of plasma parameters in the process of plasma evolution. In addition, hard x-ray pulses from a tantalum plasma were measured and their scaling was explained as bremsstrahlung emission from energetic electrons. Absolute dose values of x-ray pulses are given.     

Hot electrons produced from long scale-length laser-produced droplet plasmas

Microplasmas produced from 15 μm methanol droplets irradiated by 100 fs laser pulses in the intensity range 10¹⁴–10¹⁶ W cm^?2 are investigated via measurements of the hot electron temperature and x-ray yields under different conditions of intensity, polarization state, and plasma scale-length. The scale length of the drop-let plasma is increased with an intentional prepulse that is 10 ns ahead of the main pulse. Hot electron temperatures up to 48 keV have been measured at intensities of 2.5 × 10¹⁵W cm^?2 and the scaling of temperature as a function of intensity is determined for a long scale-length droplet plasma. The polarization and ellipticity dependence of the hard x-ray yield from the microdroplet plasmas are used to probe the shape of the droplet after irradiation by a prepulse.     

Correlation of highly charged ion and X-ray emissions from the laser-produced plasma in the presence of non-linear phenomena

Flux of X-ray radiation emitted from the Ta plasma, produced by the fundamental (1ω) and the third harmonic (3ω) frequencies of the high-power iodine laser PALS, was studied in dependence on the laser focus position. One or two (three) maxima, corresponding to the hard or soft component of the emitted spectrum, can appear, according to the experimental conditions. These dependencies are compared with those published by other authors, and also with our results concerning the highly charged ion generation. At laser intensities above I _L～ 10¹⁴ W/cm², the participation of non-linear processes in the pre-formed plasma was confirmed.     

Coherent acceleration by laser pulse echelons in periodic plasma structures

We consider a possibilty to use an echelon of mutually coherent laser pulses generated by the emerging CAN (Coherent Amplification Network) technology for direct particle acceleration in periodic plasma structures. We discuss resonant and free streaming configurations. The resonant plasma structures can trap energy of longer laser pulses but are limited to moderate laser intensities of about 10¹⁴?W/cm² and are very sensitive to the structure quality. The free streaming configurations can survive laser intensities above 10¹⁸?W/cm² for several tens of femtoseconds so that sustained accelerating rates well above TeV/m are feasible. In our full electromagnetic relativistic particle-in-cell (PIC) simulations we show a test electron bunch gaining up to 200?GeV over a distance of 10.2?cm only.     

Investigations on laser plasma soft X-ray sources generated with low energy laser systems

Spectra of laser-induced plasmas at low laser energies and intensities (around 100 mJ and 10¹⁰ W cm^–2 respectively) have been recorded in the spectral range of 20 to 100 nm for different target materials, laser intensities at the target and laser wavelengths. For heavy target materials, a broadband spectrum with a spectral maximum typically around 30 nm is obtained. This broadband radiation source is well suited for photoionization processes and the generation of short wavelength inner-shell photoionization lasers. For the cadmium-photoionization laser, the influence of different soft X-ray spectra on the laser energy has been investigated. The potential of laser plasma soft X-ray sources for scientific and technical applications is briefly reviewed.Dedicated to Prof. Dr. Herbert Welling on the occasion of his 60th birthday     

Decay and recondensation of electron-hole liquid in germanium under CO2 laser pulse irradiation: Investigation by microwave conductivity measurements

In microwave conductivity investigations of photoexcited germanium at low temperatures under CO₂ laser pulse irradiation the evaporation of EHL and e-h plasma formation have been observed. This plasma irreversibly vanishes at high CO₂ laser intensities I_CO2 >4 × 10⁵ W cm^?2 but recondenses at low intensities. It was found that complete and irreversible disappearance of EHL is due to the e-h plasma throw out to the crystal boundaries by phonon wind, generated in 10.6 μm radiation absorption whereas at I_CO2 > 10⁶ W cm^?2 it is connected with the crystal lattice heating over the condensation critical temperature. A theoretical analysis of the CO₂ laser produced phonon wind interaction with e-h plasma is briefly presented. By comparing with experimental data on recondensation process the phonon wind efficiency is estimated.     

Generation of harmonics of femtosecond radiation from the surface of aluminum targets

The results of a study of the generation of harmonics from a laser plasma resulting from the interaction of radiation of femtosecond duration (λ=1.06 μm, t=475 fs, and I～2×10¹⁷ W cm^?2) with aluminum targets are presented. The observed frequency shift of harmonics to the short-wavelength region (1.6 and 5.1 nm for the second and fifth harmonics, respectively) is determined by a collisionless absorption resulting from an anomalous skin effect. The efficiencies of conversion into the second and fifth harmonics in an s-polarized pumping field were lower than the conversion efficiencies in a p-polarized pumping field by a factor of eight and a factor of two, respectively (for intensities I<10¹⁷ W cm^?2). With a further increase in the pumping intensity, these values decreased to 0.8 and 0.5, respectively. The mechanisms of such behavior of the conversion process are considered.     

X-ray spectroscopy diagnostics of a recombining plasma in laboratory astrophysics studies

The investigation of a recombining laser plasma is topical primarily because it can be used to simulate the interaction between plasma jets in astrophysical objects. It has been shown that the relative intensities of transitions of a resonance series of He-like multicharged ions can be used for the diagnostics of the recombining plasma. It has been found that the intensities of the indicated transitions for ions with the nuclear charge number Z _n ~ 10 are sensitive to the plasma density in the range N _e ~ 10¹⁶–10²⁰ cm^–3 at temperatures of 10–100 eV. The calculations performed for the F VIII ion have determined the parameters of plasma jets created at the ELFIE nanosecond laser facility (Ecole Polytechnique, France) in order to simulate astrophysical phenomena. The resulting universal calculation dependences can be used to diagnose different recombining plasmas containing helium-like fluorine ions.     

Generation of MeV photons and protons in laser picosecond plasmas

The results of experiments devoted to studying the generation of MeV photons and protons in picosecond laser plasmas at a laser beam intensity of 10¹⁸ W/cm² on Be, Ta, LiF and H⁷Li targets are presented. Nuclear reactions (γ, n) and (p, n) were used to detect MeV photons and protons. The number of MeV photons and protons generated in laser plasmas was found from the measured neutron yield. Possibilities of particle acceleration due to the formation of pinch structures in laser plasmas are discussed. Calculated and experimental results are compared.     

超短超强激光与稀薄等离子体相互作用中后孤立子的观测

开展了超短超强激光与稀薄等离子体相互作用实验,在实验中采用等离子体单色成像法观测等离子体发光图像,捕捉到了近乎对称的环形等离子体发光结构. 在对实验结果进行分析并与理论预言进行比较后确认这是由激光-等离子体相互作用形成的后孤立子云外围的高密度等离子体壳层发光所致. 同时通过对等离子体通道的观测还发现,孤立子的形成对超短超强激光在稀薄等离子体中的传输产生了非常大的影响. 关键词： 超短超强激光 稀薄等离子体 单色成像 后孤立子     

Proton and light-ion acceleration to relativistic GeV energies by the superstrong laser radiation interacting with a structured plasma target

A new scheme is proposed for proton and light-ion acceleration to relativistic energies by superstrong laser radiation interacting with a structured plasma target. The proposal consists in the use of two-component targets consisting of heavy and light ions, where an ambipolar field is formed under the action of the ponderomotive force of incident radiation, and, in contrast to the traditional schemes, acceleration starts from the front boundary of the layer. It is shown that, for the optimized target parameters, monoenergetic GeV ion beams can be produced for radiation pulse intensities on the order of 10²¹−10²² W/cm².     

Characterization of compact bright soft X-ray source based on picosecond laser plasma

Radiation emission of silicon and aluminum plasmas produced by 40-ps laser pulses with peak intensity above 10¹⁴ W/cm² was studied. High-resolution soft X-rayspectra of H-like and He-like ions were analyzed to determine plasma parameters. We compared the line shape of resonance transitions and their intensity ratios to corresponding dielectronic satellites and the intensities of the intercombination lines of He-like ions with the results of model calculations. Such comparisons gave average values of the electron number density N_e=(1-1.9)×10²¹ cm^-3 and the electron temperature T_e=460–560 eV for Si plasmas and about 560 eV for Al plasmas produced by the first and the second laser harmonics. The plasma size is about 100 μm. According to our estimations, more than 10¹² photons were produced within the resonance line spectral width and in the solid angle 2π during the total decay period. PACS 41.50.+h; 52.25.Os; 52.50.Jm     

Ion acceleration from aluminium plasma generated by a femtosecond laser in different conditions

Non-equilibrium plasma was obtained by irradiating Al foils in vacuum with a femtosecond (fs) laser at intensities of the order of 10¹⁸ W/cm². Protons and other light ions were accelerated in the forward direction by using the target-normal-sheath acceleration regime. Time-of-flight technique was employed to measure the ions' kinetic energy using SiC detectors placed at known distances and angles. The ion acceleration was monitored under different conditions of laser focal position, laser pulse energy, and laser contrast. The target was irradiated using different thicknesses and anti-reflecting graphene films. By optimizing the laser parameters, irradiation conditions, and target properties, it was possible to accelerate up to 2.3 MeV per charge state, as will be presented and discussed.