Short-pulse Radiation Research Articles

The influence of the state of a compressed thermonuclear substance on the combustion of inertial fusion targets under direct ignition by a short radiation pulse

One-dimensional numerical calculations were performed to study the dependence of conditions for initiating thermonuclear combustion and of the target gain of direct-ignition inertial fusion targets ignited by a short radiation pulse on the initial temperature of a preliminarily compressed fuel and the initial heat energy distribution between plasma electrons and ions in the ignition region (igniter). The igniter parameters at which an effective thermonuclear target explosion with a G ∼ 103 target gain occurred were shown to substantially depend on the initial temperature of the major fuel fraction and the initial heat energy distribution between igniter electrons and ions. The heat energy of the igniter passed a minimum as the size of the igniter decreased. The dependences of these minimum energies on the temperature of the major fuel fraction at various initial energy distributions between igniter electrons and ions were determined. An increase in the temperature of the major fuel fraction was shown to decrease the target gain.

Negative ion emission from ionic crystals upon ultra short laser pulse irradiation

We report on experiments studying the appearance of negative ions during ablation from nominally transparent dielectric crystals (Al 2O 3, BaF 2, NaCl), irradiated by an 800 nm, 100 fs-laser in the multiple pulse regime. A state-of-the-art time-of-flight mass spectrometer was used for detection, in combination with ion trajectory simulations. Based on the investigation of the appearance threshold dependence on laser intensity, the kinetic energy and the angular distribution, we present evidence that the expulsion process for negative ions is different from that for positive ions. It appears to be most likely that gas-phase interactions within the ablation plume lead to the creation of negative ions from neutral precursors.

Single‐cycle terahertz electromagnetic pulses: A new test bed for physical seismic modeling

We describe a new technique for physical modeling, based on the use of short pulses of electromagnetic radiation. We employ radiation at a frequency near 1 THz (1012 Hz), corresponding to a wavelength of ∼0.3 mm. Very recent advances in optoelectronic techniques have enabled the generation and detection of broadband terahertz pulses using compact room‐ temperature transducers. These pulses can be used to form images in a variety of modes, including a multistatic reflection configuration which mimics that of many seismic surveys. We illustrate the capabilities of the modeling technique with a simple experiment which demonstrates accurate surface reconstruction using migration techniques. With respect to conventional modeling techniques using piezoelectric transducers and microphones, the use of terahertz pulses has both benefits and limitations. To our knowledge, this work represents the first use of electromagnetic radiation for modeling acoustic imaging problems.

Measurement Transducer – Digital Voltmeter System for Measuring the Energy of Laser Pulses

The results are given of theoretical investigations and of a simulation modeling of the process of measuring the energy of short-pulse laser radiation. The grounds are given for choosing the principal parameters of the measurement transducers depending on the probability density distribution function of the values of the output signal. Comparative values are given of the relative standard deviation in accordance with a proposed mathematical model and with experimental data obtained using a working standard of the unit of energy created at the All-Russia Scientific-Research Institute of Optophysical Measurements.

Time-domain radiation from large two-dimensional apertures via narrow-waisted gaussian beams

This paper deals with the short-pulse radiation of three-dimensional (3-D) vector electromagnetic fields from arbitrarily polarized large two-dimensional (2-D) truncated aperture distributions, which are parameterized in terms of narrow-waisted ray-like pulsed Gaussian basis beams centered on a discretized Gabor lattice in a four-dimensional (4-D) configuration-spectrum phase space. The study extends our previous Gabor-based investigation of time-domain (TD) short-pulse radiation of 2-D fields from 1-D large truncated apertures with nonphased, linearly phased (delayed) and nonlinearly phased focusing aperture field profiles . We begin with, and summarize, a Gabor-based frequency domain (FD) formulation of the 2-D aperture problem which has been presented and tested elsewhere, but we include additional numerical examples for validation and quality assessment. As done by Galdi, Felsen and Castanon (see ibid., vol 49, p. 1322-32, Sept. 2001) we access the time domain by Fourier inversion from the FD, starting from the initial 3-D space-time Kirchhoff formulation (whose numerical integration furnishes reference solutions), and then passing on to Gabor-parameterized field representations in terms of pulsed beam (PB) wavepackets which are launched by linearly and nonlinearly phase-delayed focusing aperture distributions. Example calculations and comparisons with numerically generated reference data serve to calibrate the Gabor-PB algorithms and assess their domains of validity.

Generation of 100 μJ pulses at 82.8 nm by frequency tripling of sub-picosecond KrF laser radiation

. Investigations of the efficient generation of powerful coherent radiation at 82.8 nm by frequency tripling of short-pulse KrF laser radiation are presented. Argon gas is selected as nonlinear medium due to the resonantly enhanced 3rd-order susceptibility χ(3)(-3ω,ω,ω,ω). Pulse energies of 100 μJ at 82.8 nm have been measured for a pump pulse energy of 14 mJ. An upscaling to more than 500 μJ is expected with available more powerful pump lasers. Features of this XUV source and possible applications are discussed.

The secondary response distortion in transient absorption spectroscopy

Transient absorption spectroscopy has been used for many years to record the kinetics and spectra of transient chemical species following a short photolysis or radiation pulse. Linearity of the detector is always important, but cannot always be assumed for experiments on a submicrosecond timescale, and should never be assumed on a subnanosecond timescale. In addition, fast photodiode detectors often exhibit a delayed secondary response, which depends on the wavelength of excitation. In this article we demonstrate the distortions introduced by secondary detector response with some examples from pulse radiolysis. A method for calibration is demonstrated using the broad visible/near-infrared absorption of solvated electrons in water. A standard decay shape is constructed from stroboscopic measurements at short time, and from the (400–800 nm) response of an EG&G FND100Q silicon diode at times greater than 10 ns. The calibration is applied to the FND100Q silicon diode, and to GMP566 germanium and GAP520 InGaAs diodes manufactured by Germanium Power Devices, Inc. Correction of the latter diode is needed to record a visible/near infrared spectrum of solvated electrons in heavy water at elevated temperatures.

Health effects: hearing microwaves: the microwave auditory phenomenon

The possible health risk associated with mobile telecommunication devices, used close to the human head and, in particular, effects on the inner ear and hearing of users have been recommended for further investigation by several groups. Recently, a number of projects have been initiated to investigate the structures and functions of the middle and inner ear following exposure to wireless communication radiation. The microwave auditory phenomenon, or microwave hearing effect, pertains to the hearing of short pulses of modulated microwave radiation at high peak power by humans and laboratory animals. Research into this effect is examined.

Propagation of short pulses in a medium with frequency dispersion

Propagation of short pulses of electromagnetic radiation in a transparent medium with a frequency dispersion is considered for the linear and nonlinear modes. The role of back reflection of radiation in the spectral regions with the negative dielectric constant was clarified. For the nonlinear mode, the exact form of the stationary solitary wave whose profile includes a point with infinite steepness is found.

Chirped-beam two-stage SASE-FEL for high power femtosecond X-ray pulse generation

A method for generating femtosecond duration X-ray pulses using a single-pass free-electron laser (FEL) is presented. This method uses an energy-chirped electron beam propagating through an undulator to produce a frequency-chirped X-ray pulse through self-amplified spontaneous emission (SASE). After the undulator, we consider passing the radiation through a monochromator. The frequency is correlated to the longitudinal position within the pulse; therefore, by selecting a narrow bandwidth, a short temporal pulse will be transmitted. The short pulse radiation is used to seed a second undulator, where the radiation is amplified to saturation. In addition to short pulse generation, this scheme has the ability to control shot-to-shot fluctuations in the central wavelength due to electron beam energy jitter. We present calculations of the radiation characteristics produced by a chirped-beam two-stage SASE–FEL, and consider the performance of the chirped-beam two-stage option for the Linac Coherent Light Source.

Investigation of the metrological characteristics of calorimetric transducers of laser radiation energy

The results are given of experimental investigations of calorimetric cavity transducers for measuring the energy of short-pulse laser radiation used in secondary and working standards and also in high accuracy measuring instruments. The sources of the basic error components of the transducer are analyzed and their values are estimated. An averaged value of the basic error of the transducer is obtained.

Preparation and De-excitation Measurements of 177mLu Using Fast Neutrons

The t1/2=160.4 d isomer 177mLu has many attractive features related to inducing gamma-ray decay to the ground state of the t1/2=6.4 d 177Lu. Also, as opposed to other isomers it maybe possible to produce it in quantities without the need for extravagant or costly methods. An experimental approach is outlined for both studies using fast neutron inelastic scattering and short-pulse bremsstrahlung radiation to produce the rapid release of gamma rays from this isomer. Furthermore, cross sections for production of the isomer using fast neutrons with energies between 10–500 keV will also be studied.

Photon kinetic theory of self-phase modulation

We derive the universal features of self-phase modulation (SPM) of short pulses of radiation, using a photon kinetic theory, equivalent to propagating the Wigner function of the electric field in the short-wavelength limit. The signatures of SPM are discussed within this extended model. We interpret our results in terms of the quasi-classical dynamics of the photons and the concept of photon acceleration.

Microwave-Induced Quantum Beats in Micellized Radical Pairs under Spin-Locking Conditions

The effect of short microwave pulse irradiation on the spin correlated radical pairs generated by the hydrogen abstraction reaction of triplet excited xanthone with tert-butyl-substituted phenols in a micellar sodium dodecyl sulfate solution is investigated by a time-resolved transient optical absorption technique. An increase in the microwave pulse duration under the electron spin resonant conditions leads to the alternation of the radical pair concentration. It is explained by the periodical change of the radical pair spin state population with an angular frequency of ω1 = γeB1 due to the pumping with resonant microwave field B1. The double frequency (2ω1) quantum beats are observed under spin-locking conditions. The behavior of quantum beats in this case is rationalized by the effect of microwave pulse on the electron spins of both radicals in the radical pair and reflects the spin dynamics in the triplet manifold of radical pairs that is isolated from that of the singlet during the microwave pulse by ...

RADIATION ELEMENT METHOD FOR TRANSIENT HYPERBOLIC RADIATIVE TRANSFER IN PLANE-PARALLEL INHOMOGENEOUS MEDIA

In this study the radiation element method is formulated to solve transient radiative transfer with light radiation propagation effect in scattering, absorbing, and emitting media with inhomogeneous property. The accuracy of the method is verified by good agreement between the present calculations and Monte Carlo simulations. The sensitivity of the method against element size, ray emission number, and time increment size is examined. The transient effect of radiation propagation is essential in short-pulse laser radiation transport when the input pulse width is not considerably larger than the system radiation propagation time. The transient characteristics of radiative transfer are investigated in the media subject to collimated laser irradiation and/or diffuse irradiation withtemporal Gaussian and/or square profiles. The inhomogeneous profile of extinction coefficient of the medium affects strongly the transient radiative flux divergence inside the medium.

Photoionization of atoms described by Fermi Molecular Dynamics: toward a firmer theoretical basis

The application of Fermi Molecular Dynamics (FMD) to the modeling of the photoionization of atoms by a short pulse of long wavelength laser radiation is examined in detail. Depression of the single ionization threshold to values of the electric field strength below the classical over-the-barrier threshold, a common occurrence in FMD, is shown to arise from the preexcitation of bound electrons into a continuum of unphysical low-lying excited states. A connection is made to analogous calculations performed with the quantum Hamilton-Jacobi equation, in which the time-dependent quantum potential ($Q$) plays a role similar to that played in FMD by the so-called Heisenberg potential (V H). Replacement of (V H) by Q in the FMD equations of motion results in a large reduction in the number of excitations to unphysical bound states, while producing no essential change in the photoionization probability.

Hearing microwaves: the microwave auditory phenomenon

The microwave auditory phenomenon, or the microwave hearing effect, pertains to the hearing of short pulses of modulated microwave radiation at high peak power by humans and laboratory animals. Anecdotal and journalistic reports of the hearing of microwave pulses persisted throughout the 1940s; and 1950s. The first scientific report of the phenomenon appeared in 1961. The effect has been observed for RF exposures across a wide range of frequencies (450-3000 MHz). It can arise, for example, at an incident energy-density threshold of 400 mJ/m2 for a single 10-microsecond-wide pulse of 2450 MHz microwave energy, incident on the head of a human subject. And it has been shown to occur at an SAR threshold of 1.6 kW/kg for a single 10-microsecond-wide pulse of 2450 MHz microwave energy, impinging on the head. A single microwave pulse can be perceived as an acoustic click or knocking sound, and a train of microwave pulses to the head can be sensed as an audible tune, with a pitch corresponding to the pulse-repetition rate (a buzz or chirp). Note that the SAR threshold of 1.6 kW/kg is about 1000 times higher than that allowable by FCC rules for cellular mobile telephones.

Narrow-waisted Gaussian beam discretization for short-pulse radiation from one-dimensional large apertures

We develop a Gabor-based Gaussian beam (GB) algorithm for representing two-dimensional (2-D) radiation from finite aperture distributions with short-pulse excitation in the time domain (TD). The work extends previous results using 2-D frequency-domain (FD) narrow-waisted Gaussian beams. The FD algorithm evolves from the rigorous Kirchhoff integration over the aperture distribution, which is then parameterized via the discrete Gabor basis and evaluated asymptotically for high frequencies to furnish the GB propagators to the observer. The TD results are obtained by Fourier inversion from the FD and yield pulsed beams (PB). We describe the resulting TD algorithm for several aperture distributions, ranging from simple linearly phased (linear delay) to arbitrary time delay profiles; the latter accommodate the important case of focusing TD aperture fields. For modulated pulses with Gaussian envelopes, we compute accurate closed form analytic solutions, which have been calibrated against numerical reference data. Our results confirm that the previously established utility of the Gabor-based narrow-waisted FD-GB algorithm for radiation from distributed apertures remains intact in the TD.

Equivalent isotropic scattering formulation for transient short-pulse radiative transfer in anisotropic scattering planar media

An isotropic scaling formulation is evaluated for transient radiative transfer in a one-dimensional planar slab subject to collimated and/or diffuse irradiation. The Monte Carlo method is used to implement the equivalent scattering and exact simulations of the transient short-pulse radiation transport through forward and backward anisotropic scattering planar media. The scaled equivalent isotropic scattering results are compared with predictions of anisotropic scattering in various problems. It is found that the equivalent isotropic scaling law is not appropriate for backward-scattering media in transient radiative transfer. Even for an optically diffuse medium, the differences in temporal transmittance and reflectance profiles between predictions of backward anisotropic scattering and equivalent isotropic scattering are large. Additionally, for both forward and backward anisotropic scattering media, the transient equivalent isotropic results are strongly affected by the change of photon flight time, owing to the change of flight direction associated with the isotropic scaling technique.

Fast laser heating of electrons as a source of ion-acoustic oscillations

It is shown that heating of electrons due to the inverse bremsstrahlung absorption of high-power short-pulse laser radiation results in parametric generation of ion-acoustic waves. Dependences of the generation efficiency on the laser pulse duration and intensity are derived. The range of wave numbers where the amplitude of the ion-acoustic oscillations increases by more than an order of magnitude is determined.