Tz. B. Petrova

Neutron production from 7Li(d,xn) nuclear fusion reactions driven by high-intensity laser–target interactions

Numerical simulations of neutron production from deuterium-lithium nuclear fusion reactions have been performed. A set of differential cross sections for the Li-7(d,xn) reaction for incident deuteron energies of up to 50MeV is assembled. The angular distribution of neutrons from a thick lithium target is simulated and benchmarked against experimental data. Two-stage neutron production from laser-target experiments has been studied as a function of laser intensity and energy. During the first stage a well collimated deuteron beam is generated using a high-intensity ultrashort pulse laser. During the second stage it is transported through a lithium target using a 3D Monte-Carlo ion beam-target deposition model. The neutron yield is estimated to be similar to 10(8) neutrons J(-1) laser energy. Some 10(10) neutrons can be expected from a similar to 100 J petawatt-class laser. For incident deuteron energies above 1 MeV the proposed scheme for neutron production from d-Li reactions is superior to that from d-d reactions, producing a collimated beam of neutrons with higher neutron yield.

The impact of contaminants on laser-driven light ion acceleration

The impact of contaminants on laser-driven ion acceleration is investigated using particle-in-cell simulations. The conventional ion acceleration mechanism, target normal sheath acceleration, has been revisited for targets with proton-rich contaminants in the form of water vapor. The targets considered have a deuterated plastic layer on the rear surface of an aluminum target, and the influence of the contaminant layer on the deuteron acceleration is investigated. In the early stage of ion acceleration, the space-charge electrostatic field on the rear target surface accelerates only the outermost, proton-rich layer of ions, which inhibits the deuteron acceleration by shielding it from the field. When the proton layer is depleted, the deuterons become exposed to the space-charge field and are promptly accelerated. This scenario was tested with a two-dimensional particle-in-cell simulation model by varying the contaminant layer thickness and laser fluence (laser energy per unit area). For laser fluences Flas...

Nonequilibrium dynamics of laser-generated plasma channels

A time-dependent nonequilibrium kinetics model based upon the time-dependent electron Boltzmann equation coupled with an extensive air chemistry model accounting for gas heating and vibrational kinetics is developed. The model is applied to the temporal evolution of femtosecond laser-generated air plasma channels at atmospheric pressure in an external electric field. The plasma channel dynamics depend upon the initial free electron density, the initial electron energy of the plasma, and upon the externally applied electric field strength. The model predicts an electric breakdown field strength of 5–10kV∕cm with a delay time of hundreds of nanoseconds when the electron density drops to the optimum value of ∼1012–1013cm−3. The experimentally observed breakdown field is ∼5.7kV∕cm with a statistical breakdown delay time of ∼200ns. The reduction in the breakdown field strength in natural air from ∼30to5kV∕cm is attributed to a combination of processes such as enhanced ionization due to relaxation of the initia...

Ionization dynamics of high-intensity laser?target interactions

The ionization dynamics of a thin aluminum foil irradiated by an ultrashort high-intensity laser is investigated with a two-dimensional relativistic electromagnetic particle-in-cell model, which includes optical field and collisional ionizations. The spatio-temporal characteristics of the ion charge and electron density have been studied for peak laser intensities between 1022 and 1024 W m−2 and a laser pulse duration of 80 fs. A series of ionization waves, launched near the front target surface, propagate through the target with a velocity of about two tenths the speed of light. In the pre-plasma region the aluminum is almost fully ionized due to optical field ionization, while in the bulk of the target the collisional ionization is more efficient. The ion charge in the bulk is a result of a complex sequence of events, the major role in which is played by the deposition of laser energy in the system and its distribution among the various degrees of freedom.

Generation of high-energy (>15 MeV) neutrons using short pulse high intensity lasers

A roadmap is suggested and demonstrated experimentally for the production of high-energy (>15 MeV) neutrons using short pulse lasers. Investigation with a 3D Monte Carlo model has been employed to quantify the production of energetic neutrons. Numerical simulations have been performed for three nuclear reactions, d(d,n)3He, 7Li(d,n)8Be, and 7Li(p,n)7Be, driven by monoenergetic ion beams. Quantitative estimates for the driver ion beam energy and number have been made and the neutron spectra and yield in the ion propagation direction have been evaluated for various incident ion energies. In order to generate neutron fluence above a detection limit of 106 neutrons/sr, either ∼1010 protons with energy 20–30 MeV or comparable amount of deuterons with energy 5–10 MeV are required. Experimental verification of the concept with deuterons driven by the Titan laser (peak intensity 2 × 1019 W/cm2, pulse duration of 9 ps, wavelength 1.05 μm, and energy of 360 J) is provided with the generation of neutrons with energy...

Electron beam-generated Ar/N2 plasmas: The effect of nitrogen addition on the brightest argon emission lines

The effect of nitrogen addition on the emission intensities of the brightest argon lines produced in a low pressure argon/nitrogen electron beam-generated plasmas is characterized using optical emission spectroscopy. In particular, a decrease in the intensities of the 811.5 nm and 763.5 nm lines is observed, while the intensity of the 750.4 nm line remains unchanged as nitrogen is added. To explain this phenomenon, a non-equilibrium collisional-radiative model is developed and used to compute the population of argon excited states and line intensities as a function of gas composition. The results show that the addition of nitrogen to argon modifies the electron energy distribution function, reduces the electron temperature, and depopulates Ar metastables in exchange reactions with electrons and N2 molecules, all of which lead to changes in argon excited states population and thus the emission originating from the Ar 4p levels.

The influence of magnetic field on electron beam generated plasmas

Magnetically confined argon plasma in a long cylindrical tube driven by an electron beam is studied experimentally and theoretically. Langmuir probes are used to measure the electron energy distribution function, electron density and temperature in plasmas generated by 2 keV, 10 mA electron beams in a 25 mTorr argon background for magnetic field strengths of up to 200 Gauss. The experimental results agree with simulations done using a spatially averaged Boltzmann model adapted to treat an electron beam generated plasma immersed in a constant magnetic field. The confining effect of the magnetic field is studied theoretically using fluid and kinetic approaches. The fluid approach leads to two regimes of operation: weakly and strongly magnetized. The former is similar to the magnetic field-free case, while in the latter the ambipolar diffusion coefficient and electron density depend quadratically on the magnetic field strength. Finally, a more rigorous kinetic treatment, which accounts for the impact of the magnetic field over the whole distribution of electrons, is used for accurate description of the plasma.

A gain model for x-ray lasing at ∼2.8 Å in an intense laser irradiated gas of xenon clusters

A variety of experiments have been carried out (Borisov et al 2008 J. Phys. B: At. Mol. Opt. Phys. 41 105602) in which a gas of xenon clusters containing between 5 and 20 atoms/cluster (Schroeder et al 2001 J. Phys. B: At. Mol. Opt. Phys. 34 297) was irradiated with a ~230 fs pulse of 248 nm laser radiation focused to an intensity level of ~1.6 × 1019 W cm−2. In these experiments, a channel was formed as the laser beam self-focused. Amplified x-ray emissions at ~2.8 A were observed along the direction of the channel with measured gain coefficients of between 27 and 104 cm−1 being cited. In this paper, a xenon gain model is assembled, built from FAC generated data (Gu 2008 Can. J. Phys. 86 675), which allows gains comparable in magnitude to be calculated under a set of restrictive assumptions about the laser beam–cluster dynamics. The model includes a detailed description of the ionization and excitation dynamics of the Ni- and Co-like ionization stages into which a description of hole state generation in Co-like xenon is made through the photoionization (or collisional ionization) of 2s and 2p electrons. Our calculations show that, under specialized initial conditions and sufficiently high pumping rates, gains larger than 50 cm−1 are achievable in one of the seven radiative decays of the Co-like hole states. The calculated gains are sensitive to the ion density, the risetime of the photoionization rates and the early time heating rate of the cluster plasma.

Electron kinetics of the e-beam pumped Ar-Xe laser

Extensive electron collision data for Ar and Xe are assembled and employed to study the e-beam deposition in this noble gas mixture for application to the Ar–Xe laser. Nineteen states of Xe and thirteen of Ar are used for the atomic model. The electron energy distribution function is calculated from the Boltzmann equation using atomic cross sections for excitations from the ground states computed from atomic physics codes. The resulting electron distribution is then used to investigate the electron temperature, energy per electron–ion pair, ionization rates and excitation-to-ionization ratios for both species as a function of three parameters: the Xe mole fraction, the fractional ionization, and the power deposition per Ar atom. While the Ar ionization and excitation are fairly insensitive to these parameters, the rates for Xe, especially total excitation-to-ionization ratio, can vary with all three. Cross sections for excited to excited transitions among all the considered upper states in Xe are also calculated and transition rates, fitted to an Arrhenius form, are presented in tabular form. The assembled atomic data and calculated rates are useful for further detailed electron kinetics modelling of the Ar–Xe laser.

X-ray amplification dynamics at ∼2.8 Å in intense laser irradiated xenon clusters: multiple hole dynamics

In a series of experiments (Borisov et al 2008 J. Phys. B: At. Mol. Opt. Phys. 41 105602, and references 1 through 7 cited in this paper), the amplification of 2.71–2.88 A x-rays was observed, gain coefficients between 27 and 104 cm−1 were measured, and a number of conjectures were made concerning the ionization stages that were involved in the x-ray amplification. It was conjectured, for example, that x-ray emissions from hole states in Cl-, K-, Ca-, and Ti-like xenon were being amplified. In this paper, our earlier xenon gain model (Petrova et al 2010 J. Phys. B: At. Mol. Opt. Phys. 43 025601) is extended to include single- and double-hole state production within the Fe-like ionization stage in addition to single-hole state production within the Co-like ionization stage in order to assess these conjectures. The gain model, based on flexible atomic code generated data (Gu 2008 Can. J. Phys. 86 675), includes Co-like and Fe-like hole-state generation in xenon through photoionization of 2s and 2p electrons. The hole-state dynamics is self-consistently coupled to an extensive collisional-radiative dynamics of the Ni-, Co-, and Fe-like ionization stages of xenon. In addition, the model includes tunnelling ionization rates that confirm the initial condition assumptions that were made in our earlier paper, and they are needed to support the interpretations of the measured broadband x-ray data. With the use of tunnelling ionization rates, we demonstrate how all of the N-shell, n = 4, electrons are striped from a xenon atom in less than a femtosecond at laser intensities larger than 1019 W cm−2. Our calculations also show that, under these initial conditions and with sufficiently high pumping rates (≥1014 s−1), a range of gains larger than 50 cm−1 are achievable under select conditions from both Co- and Fe-like xenon single hole radiative decays, in conformity with experimental observations. However, the calculated gain coefficients are sensitive to the laser intensity, laser pulse risetime, the magnitude of the hole-state pumping rates, ion density, and electron and ion heating rates, and, in general, Co-like holes are found to have much higher gains than Fe-like hole states. These model calculations are also capable of producing gains from the double-hole states in Fe-like xenon, but they are much smaller than those generated in the Fe-like single-hole states in the cases included in this paper. Thus, our model calculations do not support the experimental data interpretation in which the measured gains were attributed to double holes in much higher ionization stages of xenon (Xe32+, Xe34+, Xe35+, and Xe37+). Our calculations suggest that these ionization stages can be reached either early in time at much higher laser intensities (in excess of 1.5 × 1020 W cm−2 for a 248 nm, ~230 fs pulse) or later in time, and only because of tunnelling ionization. In this latter case, however, the measured gains cannot be achieved since cluster densities have fallen by several orders of magnitude from their initial values and ion population have been spread over a much wider range of states.