Pion production under the action of intense ultrashort laser pulse on a solid target

Two-dimensional “particle-in-cell” modeling was carried out to determine the laser intensity threshold for pion production by protons accelerated by the relativistically strong short laser pulses acting on a solid target. The pion production yield was determined as a function of laser intensity.     

目标红外光谱辐射强度测量技术研究

本文从理论上分析了测量野外目标红外光谱辐射强度方法，提出了实现测量装置的结构形式，并对测量中的关键技术，例如测量速度、系统噪声、分光、信号处理等进行了重点探讨。     

Time-resolved analysis of infrared plasma absorption due to electron-hole-drop in germanium

By way of time-resolved analysis of the infrared plasma absorption, the decay profile of the electron-hole-drop in germanium is examined for the temperature range 1.8-4.2 K with the help of an H₂O laser. In this temperature range, the time decay in absorption intensity is found exponential as long as 80 μsec after photoexcitation. The observed temperature dependence of the decay time can be explained by the evaporation theory. It has been necessary, however, to introduce a temperature dependent term into the work function in the manner like Φ = (14 ± 1) + (0.5 ± 0.2)T² K.     

Control of the intensity of a single harmonic generated in laser plasma by varying the chirp of intense femtosecond pulses

A significant increase, as well as a decrease, in the intensity of single higher harmonics of femtosecond radiation of a Ti:sapphire laser that are generated in a weakly excited laser plasma on the surface of GaAs, Cr, InSb, In, and stainless steel is demonstrated. The intensities of some higher harmonics in different regions of their plateaulike distribution varied from a 23-fold excess over the intensity of the neighboring harmonics to the complete absence in the extreme ultraviolet spectrum. The specific features of a 200-fold increase in the intensity of the 13th harmonic generated in indium plasma are analyzed. These variations in the intensity distribution of harmonics in the plateau region are obtained by varying the chirp of laser pulses.     

Mechanism of generation of high-intensity terahetrz radiation under the action of high-power laser pulsed on a target

The mechanism of generation of terahertz radiation upon irradiation of a target by short (∼ 0.1 ns) high-intensity laser pulses (I ∼ 10¹⁸−10¹⁹ W cm⁻²) is investigated by numerical simulation using the relativistic electromagnetic PIC code. The interaction of such a pulse with the target, a plasma is formed on it. Electrons emitted from the plasma form a virtual cathode whose oscillations are determined not only by their self-field, but also by the field of ions of the plasma. Generation occurs in the terahertz frequency range with the efficiency thrice as high as in the absence of ions (i.e., with traditional reditron generation mechanism). The explanation for this effect is also given.     

Spontaneous Faraday rotation due to strong laser radiation in a plasma

Theoretical studies on intense laser radiation interacting with plasma reveal an induced nonlinear birefringence which turns out to be spontaneous Faraday rotation. Here, this rotation is called nonlinear Faraday rotation (NFR). The expressions for the nonlinear refractive indices of the laser fields are calculated both in relativistic and non-relativistic limits for a two-component plasma. The NFR angles due to nonlinear induced birefringence are derived in the absence of externally imposed magnetic fields. It is shown that, in the relativistic limit, the electron motion plays an important role in producing NFR and so in the generation of induced magnetic fields.     

Generation of harmonics of intense laser radiation in a transparent collisionless plasma

The spectral composition of a relativistically strong uniform nonlinear electromagnetic wave in a transparent collisionless plasma is analyzed. The vortex and potential components of the wave field are shown to contain only odd and even harmonics, respectively; in a transparent plasma, the wave remains quasi-monochromatic, since the intensities of the harmonics decrease exponentially with increasing harmonic number. An equation that includes diffraction effects is derived to describe the propagation of wavepackets. The results obtained are compared with experimental data.     

Theory of the interaction of intense laser radiation with magnetoactive plasma

Analytical expressions for the density of magnetoactive plasma perturbed by circularly polarized relativistically strong laser radiation are obtained.     

X-ray study of microdroplet plasma formation under the action of superintense laser radiation

We have measured the X-ray emission spectra of a plasma generated by laser radiation with an intensity above 10¹⁹W/cm² and a pulse duration of 30 fs acting upon an argon jet target with a large abundance of micron-sized clusters. The time variation of the X-ray yield from ions of various multiplicities, calculated within the framework of a nonstationary kinetic model, shows a good qualitative agreement with the experimental time-integrated spectrum.     

Characterization of the third-harmonic radiation generated by intense laser self-formed filaments propagating in air

We perform laboratory experiments to study ultraviolet radiation generated by intense self-formed laser filaments produced by propagating high-power femtosecond laser pulses in air. The laser used in the experiment is a 0.5 TW Ti:sapphire system with the center wavelength at 800 nm. The observed ultraviolet emission occurs in the form of the third harmonic and frequency-upshifted radiation from the fundamental. We present direct characterization of the generated harmonic and frequency-upshifted radiation, including transverse imaging and spatially resolved spectral measurements.     

激光入射角对靶面方向超热电子发射的影响

实验研究了超短超强激光脉冲与薄膜靶相互作用中产生的超热电子角分布随激光入射角的变化.在靶面方向观测到一束方向性很好的高能超热电子.该高能超热电子束的电子数目随着激光入射角的增大而增大.对结果的分析表明，表面准静态磁场是导致表面电子产生的主要原因. 关键词： 超热电子 表面准静态磁场 超强激光脉冲与等离子体相互作用     

Generation of low-frequency radiation by dense hot plasma under pondermotive action of a short laser pulse

The theory of the generation of low-frequency radiation under the pondermotive action of a femtosecond laser pulse on dense hot plasma is developed. It is shown that, at fairly high plasma temperatures, when electron-electron collisions are rare and the low-frequency field is excited under conditions of the anomalous skin effect, the generation efficiency can be close to maximal. The optimal generation conditions are achieved if the carrier frequency of the laser pulse is close to the plasma frequency and the pulse is tightly focused. Under irradiation by pulses with durations of tens to hundreds femtoseconds, terahertz radiation is generated in a broad angular range.     

Detonation-like mode of the destruction of optical fibers under intense laser radiation

A “fast,” detonation-like mode of the propagation of an optical discharge with velocities up to 3 km/s is observed in a silica optical fiber at the laser radiation intensity in the core up to 40 W/μm².     

Time evolution of the infrared laser ablation plasma plume of SiO

The spatio-temporal evolution of the silicon monoxide SiO plasma produced by a high-power CO₂ pulsed laser has been investigated using optical emission spectroscopy (OES) and imaging methods. The formed plasma was found to be strongly ionized, yielding Si⁺, O⁺, Si²⁺, O²⁺ and Si³⁺ species, rich in neutral silicon and oxygen atoms, and very weak molecular bands of SiO time-integrated and time-resolved two-dimensional OES plasma profiles were recorded as a function of emitted wavelength and distance from the target. The temporal behavior of specific emission lines of Si, Si⁺, O⁺, Si²⁺ and O²⁺ was characterized. The results show a faster decay of O²⁺ and Si²⁺ than that of O⁺, Si⁺ and Si. The Stark broadening of isolated single-ionized silicon emission lines was employed for deducing the electron density during the plasma expansion. The relative intensities of two Si²⁺ lines were used to calculate the time evolution of the plasma temperature.