A new approach is proposed to the production of high-energy ions from a cluster jet irradiated with relativistic (5×10^18 W/cm2) femtosecond laser pulses, based on the formation of mixed clusters with an Xe core surrounded by a Kr shell. The appearance of distinguished charge states of accelerated ions was registered: instead of low-charge Kr2+, Kr3+, Kr4+, Kr5+ for pure Kr clusters, there are three components Kr8+, Kr14+, Kr20+ for mixed KrXe clusters. The energy range of detected ions expands significantly: from 1–6 MeV for Kr clusters to 2–16 MeV for KrXe clusters.

A new approach is proposed to form a jet with submicron aggregates for femtosecond laser neutron generation under nonlinear interaction with relativistically intense laser pulse. Aggregates are formed through the rapid expansion into vacuum of the supercritical mixture of CO 2 + CD 3 OD (3:1). For the first time, fusion neutrons (2.45 MeV) with a peak output of 3 × 10 3 neutron/pulse/4 π and efficiency of 6 × 10 4 neutron J −1 were obtained under interaction of Ti:Sa laser pulse having 3 × 10 18 W cm −2 intensity with submicron aggregates produced from supercritical CO 2 + CD 3 OD mixture.

This paper presents a phenomenological analysis of thermodynamic parameters of a pulsed supersonic gas jet in a conical nozzle. Conditions have been found out for the condensation and formation of large krypton clusters resulting from phase transitions during adiabatic expansion of gas through the nozzle into vacuum. We have identified the phase state of the large clusters (submicron droplets consisting of ∼10 7 monomers) forming in the jet and obtained a high yield of X-rays from a cluster nanoplasma produced by femtosecond laser pulses of relativistic intensity I . It has been shown that, at I ≈ 3 × 10 18 W cm −2 , a nanoplasma consisting of large krypton clusters emits a broadband X-ray spectrum in the range 5 – 100 keV. The integrated laser to X-ray energy conversion efficiency achieved is ∼10 −5 (400 nJ), and the efficiency of characteristic K α line (12.6 keV) generation is ∼1.5 × 10 −6 (68 nJ), which corresponds to a quantum yield of ∼3 × 10 8 photons s −1 (4 π ) −1 , with a contrast of ∼25 relative to bremsstrahlung background.

In comparative experiments, it was found that the maximum efficiency of third harmonic generation (THG) in cluster nanoplasma was ∼9x10−5 and was approximately 4 times less effective than the case of THG in gas under similar experimental conditions. This result is consistent with theoretical calculations and show that THG amplitude in a gas is about an order of magnitude higher than in a cluster medium. The discrepancy between the THG efficiency in gas and clusters obtained theoretically and experimentally in discussed.

In this chapter we introduce our recent studies on terahertz (THz) and X-ray emission generated in an Ar gas-cluster jet under irradiation with high-intense ultrashort laser pulses. We carried out a numerical simulation of cluster formation processes in a supersonic jet produced under adiabatic extension of gaseous Ar into vacuum, which shows that the concentration ratios among non-clusterized Ar monomers, small-size Ar clusters and large-size Ar clusters significantly vary along the jet. The chapter presents experimental results on THz and X-ray emission generated by irradiation of the jet with intense femtosecond laser pulses at various downstream distances measured from the nozzle outlet along the axis of symmetry of the nozzle. It is shown that THz and X-ray emission from the jet is a useful tool for the study of clustering dynamics in the course of free expansion of gas through a nozzle into vacuum.

We present the results of experimental studies of THz emission from argon cluster nanoplasma switched by the intense femtosecond laser pulses. We have performed measurements of the angular distribution of the THz radiation emitted from the clustered nanoplasma with two polarization states of the linearly polarized laser beam. We have found that polarization state of the laser radiation may influence on the efficiency of THz generation in cluster beam.

The acceleration of electrons to MeV energies from a Kr cluster jet irradiated by a relativistic (6 x 10(18) W cm(-2)) femtosecond laser pulse is studied. For the first time, the action of transform-limited laser pulse effects on the generation of hot electrons with an effective temperature (T-hot) of around 150 keV is experimentally demonstrated. The particles were accelerated in the plasma channel formed under self-focusing of the laser beam. Introducing a linear positive chirp together with pulse elongation from 50-similar to 120 fs results in substantially enhanced hot electrons flux with a growth of T-hot up to 300 keV and a maximal energy of over 2 MeV. The observed effect may be related to the strongly nonlinear propagation of the pulse through plasma, accompanied by more favorable conditions for efficient particles energy gain and pulse nonlinear compression.

Rayleigh scattering technique was proposed for diagnostics of mixed Ar/Kr clusters. Clusterization of krypton fraction was observed. Dual energy X-ray source with tunable line amplitudes, based on laser excitation of the mixed clusters has been developed. It was shown that the relative line yield in dual-energy X-rays can be controlled by selecting of proper fraction of initial gas mixture components.

Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text A. V. Balakin, M. S. Dzhidzhoev, V. M. Gordienko, I. A. Zhvanya, I. E. Ivanov, N. A. Kuzechkin, P. M. Solyankin, A. P. Shkurinov, and I. A. Kotelnikov, "Terahertz science of discrete and extreme matter," in The 9th International Symposium on Ultrafast Phenomena and Terahertz Waves, OSA Technical Digest (online) (Optica Publishing Group, 2018), paper WG1. Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article

This paper studies the phenomenon of joint generation of terahertz (THz) and X-ray radiation in the argon nanocluster jet under the action of high-power femtosecond laser pulse in both the single-color and dual-color regimes. It was discovered that in a gas cluster beam the pulse duration affects the properties of THz and X-ray emission differently. For the same given total energy of optical pulse in the dual-color excitation regime of cluster medium, more than a five times increase of THz radiation power was observed in comparison with the single-color regime, while the conversion efficiency to the argon X-ray K-line reached 7 × 10 -6 and remained unchanged. The possibility of separation of contributions of different beam components into the THz signal was demonstrated experimentally, using contributions from clusters and nonclustered gas as an example. We suggest an interpretation of experimental results based on a theoretical model of cluster ionization that self-consistently predicts the level and dynamics of ionization and electron temperature in the clusters.

For the first time, we defined the range of krypton fraction (C-Kr) in an initial binary Ar-Kr gas mixture that provides the production of large (N similar to 10(6) atoms/cluster) mixed Ar/Kr clusters under co-expansion of the mixture at room temperature and the stagnant pressure of 25 bar. Mixed Ar/Kr clusters exist at the krypton fraction of 3-15%. The presence of mixed clusters is detected by the simultaneous generation of both x-ray K alpha lines of argon (E = 3.1 keV) and krypton (E = 12.7 keV) from nanoplasma originating as a result of femtosecond nonlinear laser excitation with intensity about 5.10(17) W cm(-2). It was shown that the amplitude of lines in dual-energy x-ray spectrum can be controlled by proper selection of the fraction of initial gas mixture components. Maximal laser energy conversion efficiency to krypton x-ray line is achieved for pure krypton clusters (i.e. C-Kr = 100%) and reaches 2.10(-7) at laser intensity of 5.10(17) W cm(-2). The laser energy conversion efficiency to argon x-ray line reaches the maximal value of 3.10(-6) at C-Kr = 0%.

The evolution of the Rayleigh scattering signal from mixed Ar/Kr clusters with variation in the partial Kr concentration in the initial gas mixture is investigated. An addition of krypton in small amounts to argon is found to cause an anomalously rapid increase in the Rayleigh scattering signal amplitude, which is due to a sharp increase in the size of clusters as a result of their saturation with krypton atoms. At a partial krypton concentration above 25% in the initial mixture, the scattering signal amplitude is stabilised because of the condensation saturation during the cluster formation.

We present the results of experimental study of terahertz (THz) generation in gas cluster beam excited by intense femtosecond laser pulses. Cluster beam was produced by partial condensation of pure Ar and mixtures CF2Cl2+He, Ar+He during their expansion through a conical nozzle into vacuum. There were used two excitation schemes in our experiments: single color and two color (fundamental frequency mixed with its second harmonic). We have studied how THz signal scales with various control parameters such as laser pulse duration, gas backing pressure and laser pulse energy. Simultaneously we measured intensity of X-Ray emission which originates from laser-cluster interaction. We found that in a single color scheme energy of THz pulses from Ar cluster beam strongly decreases in the region of minimum laser pulse duration while X-Ray power is maximal under these conditions. Both in single- and two color excitation regimes THz signal demonstrated growth without saturation with increasing of optical pulse energy up to its peak value of 25 mJ.

The present work looks at the properties of powerful THz and X-ray radiation generated in gas nanocluster targets. Combining in one experiment the generation of pulsed THz and X-ray radiation using multifrequency femtosecond radiation has made it possible to carry out the study of temporal dynamics of ionization and electron movement in gas clusters. The same experimental approach was used to study hydrodynamic regularities of formation and movement of clusters of different sizes in supersonic gas streams. It is shown that the future use of such combined sources of electromagnetic radiation is THz excitation and X-ray probe of the structure change in the matter.

The present paper analyses the main mechanisms and properties of terahertz radiation generated by intense femtosecond laser pulses from gas and nanosize gas clusters. The possibilities of simultaneous generation terahertz and X-ray radiations are discussed and demonstrated experimentally. It was shown that optimal conditions for effective generation of terahertz and X-ray radiation are different. That makes possible to control the magnitudes of terahertz and X-ray signals simultaneously generated from gas nano-cluster jet. Controlling could be provided by means: by varying of time delay between laser pulse and cluster jet formation moment, or by varying of chirp parameter of laser pulses.

We report on effective x-ray generation under excitation of clusters of polyatomic CF2Cl2 molecules by femtosecond laser radiation of moderate intensity I ≈ 5 × 1015 W cm−2 (E ≈ 5 mJ). The maximal laser energy conversion efficiency to chlorine x-ray lines (E = 2.6–2.8 keV) reached 2 × 10−5 when the CF2Cl2 clusters were formed in the presence of the carrier gas helium. It was demonstrated that the spectrum of laser-induced x-rays can be used to detect mixed clusters, appearing in a 3% mixture of CF2Cl2 with Ar. We proposed adding a light thermalizing gas to the mixture, i.e., using a ternary mixture CF2Cl2–Ar–He, to control the fraction of components that the mixed clusters are composed of. We have given the first demonstration of a dual-energy x-ray source with a tunable line amplitude that is controlled by the helium fraction in the mixture.

We have demonstrated that the efficiency of hard X-ray generation (with energy 2 − 4 keV) from the femtosecond laser excitation (intensity I ∼ 1016 W/cm2, pulse energy E ∼ 5 mJ) of large molecular clusters (SF6, CF3I) can reach ∼ 10−6 (with yield ∼ 109 photons/J). It was shown that at maximal X-ray yield, the plasma filament is essentially non-uniform and has a multifocal structure. It was observed that the amplitude of the central and trailing spectral components of the chirped laser pulse decreases after its interaction with the cluster target as a result of efficient laser energy absorption under self-focusing conditions.