The nonlinear Compton scattering by a linearly polarized laser pulse of finite duration is analyzed, with a focus on the spin effects of target electrons. We show that, although the Compton scattering accompanied by the electron no-spin flip is dominant, for some energy regions of Compton photons their emission is dominated by the process leading to the electron spin flip. This feature is observed for different pulse durations, and can be treated as a signature of quantum behavior. Similar conclusions are reached when analyzing the scattered electron energy spectra. This time, the sensitivity of spin effects to the carrier-envelope phase of the driving pulse is demonstrated. The possibility of electron acceleration by means of Compton scattering is also discussed.

The X-ray emission from planar targets made of aluminum, copper, or gold irradiated by a frequency-doubled Nd laser (530-nm wavelength and 1012–1014-W/cm2 laser intensity) was measured at two pulse durations: 3 ns and 30 ps. We absolutely measured the X-ray emission with spectral, temporal, and spatial resolution in the wavelength range 3 Å < λ < 250 Å by using filtered bolometers, a transmission grating spectrometer, X-ray diodes, and an X-ray streak camera as diagnostics. In addition, the absorption of laser light was measured. For the short, 30-ps laser pulse the conversion of incident laser energy into X rays was considerably less than that with the long, 3-ns pulse. This is caused by less absorption of laser light and, in addition, by less conversion of absorbed laser energy into X rays in the case of the short pulse. The results are compared with numerical simulations performed with the MULTI hydrocode.

The optical emission spectra of the plasma generated by a 1.06-μm Nd:YAG laser irradiation of Al target in air was recorded and analyzed in a spatially resolved manner. Electron temperatures and densities in the plasma were obtained using the relative emission intensities and the Stark-broadened linewidths of Al(I) emission lines, respectively. The dependence of the electron density and temperature on the distance from the target surface and on the laser irradiance were manifested. We also discussed how the air takes part in the plasma evolution process and confirmed that the ignition of the air plasma was by the collisions between the energetic electrons and the nitrogen atoms through a cascade avalanche process.

Measurement of extremely new phenomena during the interaction of laser pulses with terawatt and higher power and picoseconds with plasmas arrived at drastically different anomalies in contrast to the usual observations if the laser pulses were very clean with a contrast ratio higher than 108. This was guaranteed by the suppression of prepulses during less than dozens of ps before the arrival of the main pulse resulting in the suppression of relativistic self-focusing. This anomaly was confirmed in many experimental details, and explained and numerically reproduced as a nonlinear force acceleration of skin layers generating quasi-neutral plasma blocks with ion current densities above 1011 A/cm2. This may support the requirement to produce a fast ignition deuterium tritium fusion at densities not much higher than the solid state by a single shot PW-ps laser pulse. With the aim to achieve separately studied ignition conditions, we are studying numerically how the necessary nonlinear force accelerated plasma blocks may reach the highest possible thickness by using optimized dielectric properties of the irradiated plasma. The use of double Rayleigh initial density profiles results in many wavelength thick low reflectivity directed plasma blocks of modest temperatures. Results of computations with the genuine two-fluid model are presented.

Electron acceleration in the plasma wakefield driven by asymmetric laser pulses is investigated analytically. It is found that the asymmetric laser pulse can significantly modify the phase portrait of the electron dynamics and enhance the maximum energy of the accelerated electrons. There exists an optimum ratio of the lengths of the rising and falling segments of the asymmetric laser-pulse. A linear scaling law relating the accelerated electrons' energy and the plasma density is obtained. This result differs from the power-law dependence often associated with symmetric laser pulses.

The plume images of the laser produced silver plasma in the absence and presence of 0.45 T transverse magnetic field has been investigated under vacuum ~10−4 torr and in air. An Nd:YAG laser (1.064 µm, 1.1 MW, 9 ns) with intensity ~1012 Wcm−2 was used to generate plasma. A CCD image capture system was used for plasma imaging to explore the plume. A magnetic probe was employed to measure the variation in internal magnetic field of plasma with as well as without 0.45 T external transverse magnetic field. The X-ray emission from plasma in both the cases (with and without B field) was also monitored using two PIN photodiodes filtered with 24 µm Cu and 24 µm Al. The plume images in both the cases were then correlated with the time resolved soft X-ray emission. It was found that the self generated magnetic field of the plasma increases in the presence of magnetic field. Plume images reveal that the confinement of the plume takes place in the presence of magnetic field both in the cases of air and vacuum. Jet and spikes like structures were also observed due to plasma instabilities. Lobe formation in the plume at latter stages of plasma evolution was more prominent in air than under vacuum. X-ray emission signals exhibited an enhancement in the emission under transverse magnetic field. An increased rate of recombination due to high density as a result of plasma confinement across the applied magnetic field was found to be the main reason behind emission enhancement.

The effect of the transverse magnetic field on laser-induced breakdown spectroscopy and surface modifications of germanium (Ge) has been investigated at various fluences. Ge targets were exposed to Nd: YAG laser pulses (1064 nm, 10 ns, 1 Hz) at different fluences ranging from 3 to 25.6 J/cm2 to generate Ge plasma under argon environment at a pressure of 50 Torr. The magnetic field of strength 0.45 Tesla perpendicular to the direction of plasma expansion was employed by using two permanent magnets. The emission spectra of laser-induced Ge plasma was detected by the laser-induced breakdown spectroscopy system. The electron temperature and number density of Ge plasma are evaluated by using the Boltzmann plot and stark broadening methods, respectively. The variations in emission intensity, electron temperature (Te), and number density (ne) of Germanium plasma are explored at various fluences, with and without employment of the magnetic field. It is observed that the magnetic field is responsible for significant enhancement of both excitation temperature and number density at all fluences. It is revealed that an excitation temperature increases from Te,max,without B = 16,190 to Te,max,with B = 20,123 K. Similarly, the two times enhancement in the electron density is observed from ne,max,without B = 2 × 1018 to ne,max,with B = 4 × 1018 cm−3. The overall enhancement in Ge plasma parameters in the presence of the magnetic field is attributed to the Joule heating effect and adiabatic compression. With increasing fluence both plasma parameters increase and achieve their maxima at a fluence of 12.8 J/cm2 and then decrease. In order to correlate the plasma parameters with surface modification, scanning electron microscope analysis of irradiated Ge was performed. Droplets and cones are formed for both cases. However, the growth of ridges and distinctness of features is more pronounced in case of the absence of the magnetic field; whereas surface structures become more diffusive in the presence of the magnetic field.

Short pulse laser plasma interaction experiments using diffraction limited beams provide an excellent platform to investigate the fundamental physics of stimulated Raman scattering. Detailed understanding of these laser plasma instabilities impacts the current inertial confinement fusion ignition designs and could potentially impact fast ignition when higher energy lasers are used with longer pulse durations (>1 kJ and >1 ps). Using short laser pulses, experiments can be modeled over the entire interaction time of the laser using particle-in-cell codes to validate our understanding quantitatively. Experiments have been conducted at the Trident laser facility and the Laboratoire pour l'Utilisation des Lasers Intenses (LULI) to investigate stimulated Raman scattering near the threshold of the instability using 527 nm and 1059 nm laser light, respectively, with 1.5–3.0 ps pulses. In both experiments, the interaction beam was focused into pre-ionized helium gas-jet plasma. Measurements of the reflectivity as a function of intensity and kλD were completed at the Trident laser facility, where k is the electron plasma wave number and λD is the plasma Debye length. At LULI, a 300 fs Thomson scattering probe is used to directly measure the density fluctuations of the driven electron plasma and ion acoustic waves. Work is currently underway comparing the results of the experiments with simulations using the VPIC particle-in-cell code. Details of the experimental results are presented in this manuscript.

The quantitative analysis of the collisional line broadening and the ionization state distributions for determining optical properties of aluminum plasmas are the main goal of this paper. In a preliminary analysis, results from an average atom model are compared with those from detailed configurations, assuming LTE conditions. The sensitivity to these physical aspects in the extinction coefficient and in the Rosseland and Planck mean opacities are carefully studied.

In this work, a laser-produced plasma extreme ultraviolet source and a free electron laser were used to create Ne photo-ionized plasmas. In both cases, a radiation beam was focused onto a gas stream injected into a vacuum chamber synchronously with the radiation pulse. Extreme ultraviolet radiation from the plasma spanned a wide spectral range with pronounced maximum centered at λ = 11 ± 1 nm while the free electron laser pulses were emitted at a wavelength of 32 nm. The power density of the focused plasma radiation was approximately 2 × 107 W/cm2 and was seven orders of magnitude lower compared with the focused free electron laser beam. Radiation fluences in both experimental conditions were comparable. Despite quite different spectral characteristics and extremely different power densities, emission spectra of both photo-ionized plasmas consist of the same spectral lines within a wavelength range of 20 to 50 nm, however, with different relative intensities of the corresponding lines. The dominating spectral lines originated from singly charged ions (Ne II); however, Ne III lines were also detected. Additionally, computer simulations of the emission spectra, obtained for photo-ionized plasmas, driven by the plasma extreme ultraviolet source, were performed. The corresponding measured and calculated spectra are presented. An electron temperature and ionic composition were estimated. Differences between the experimental spectra, obtained for both irradiation conditions, were analyzed. The differences were attributed mainly to different energies of driving photons.

A new approach to spectroscopy of laser induced proton beams using radiochromic film (RCF) is presented. This approach allows primary standards of absorbed dose-to-water as used in radiotherapy to be transferred to the calibration of GafChromic HD-810 and EBT in a 29 MeV proton beam from the Birmingham cyclotron. These films were then irradiated in a common stack configuration using the TARANIS Nd:Glass multi-terawatt laser at Queens University Belfast, which can accelerate protons to 10–12 MeV, and a depth-dose curve was measured from a collimated beam. Previous work characterizing the relative effectiveness (RE) of GafChromic film as a function of energy was implemented into Monte Carlo depth-dose curves using FLUKA. A Bragg peak (BP) “library” for proton energies 0–15 MeV was generated, both with and without the RE function. These depth-response curves were iteratively summed in a FORTRAN routine to solve for the measured RCF depth-dose using a simple direct search algorithm. By comparing resultant spectra with both BP libraries, it was found that the effect of including the RE function accounted for an increase in the total number of protons by about 50%. To account for the energy loss due to a 20 µm aluminum filter in front of the film stack, FLUKA was used to create a matrix containing the energy loss transformations for each individual energy bin. Multiplication by the pseudo-inverse of this matrix resulted in “up-shifting” protons to higher energies. Applying this correction to two laser shots gave further increases in the total number of protons, N of 31% and 56%. Failure to consider the relative response of RCF to lower proton energies and neglecting energy losses in a stack filter foil can potentially lead to significant underestimates of the total number of protons in RCF spectroscopy of the low energy protons produced by laser ablation of thin targets.

Preliminary experimental results are shown on the production of plasmas by low frequency helicon waves. At constant rf frequency and power, the results show a maximum in density as the dc magnetic field is varied. The measurements confirm the theoretical expectation that the plasma is ionized by Landau damping of the radiofrequency waves.

A camera has been developed that directly measures the deuterium-tritium burn region of laser-driven inertial confinement fusion targets. Images are formed by 14-MeV thermonuclear neutrons emitted from the targets. Our demonstration instrument is based on a coded-aperture imaging technique known as penumbral imaging, and has produced images of high-yield (> 1012 neutrons) direct-drive targets with resolutions of 80 μm. The camera consists of four major components: the penumbral aperture, alignment hardware, detector system, and image analysis software.

The field distribution and the restraint effect of multipactor and plasma discharge on the periodic triangular surface have been theoretically and experimentally analyzed. It has been found by computational and simulative analysis that the periodic profile can quickly restrain or weaken multipactor and plasma discharge in low pressure within several microwave periods. Considering the field enhancement, increasing the slope angle, advancing the electric field, and lowering the frequency can enhance the multipactor suppression. X-band giga-watt high power microwave experiment with 20 ns short pulse was conducted. It was demonstrated that the periodic profile can effectively improve the breakdown threshold and slower the speed of tail erosion.

The possibilities of electron acceleration by ponderomotive force of a circularly polarized laser pulse in magnetized quantum plasma have been explored. The basic mechanism involves acceleration of electron by the axial gradient in the ponderomotive potential of the laser. The quantum effects have been taken into account for a high-density plasma. The ponderomotive force of the laser is resonantly enhanced when Doppler up-shifted laser frequency equals the cyclotron frequency.

The possibilities of producing ultrahigh-current-density ps ion fluxes by the skin-layer interaction of a short (≤ 1ps) laser pulse with plasma were studied using two-fluid hydrodynamic simulations, and the time-of-flight measurements. Backward-emitted ion fluxes from a massive (Au) target as well as forward-emitted fluxes from various thin foil targets irradiated by a 1-ps laser pulse of intensity up to 2 × 1017 W/cm2 were recorded. Both the simulations and the measurements confirmed that using the short-pulse skin-layer interaction of a laser pulse with a thin pre-plasma layer in front of a solid target, a high-density collimated ion flux of extremely high ion current density (∼ 1010 A/cm2 close to the target), can be generated at laser intensity only ∼ 1017 W/cm2. The ion current densities produced by this way were found to be comparable to (or even higher than) those estimated from recent short-pulse experiments using a target normal sheath acceleration mechanism at relativistic laser intensities. The effect of the target structure on the current densities and energies of forward-emitted ions is demonstrated.

Fast ignition for fusion energy by using petawatt-picosecond (PW-ps) laser pulses was modified due to an anomaly based on extremely clean suppression of prepulses. The resulting plasma blocks with space charge neutral ion current densities above 1011 Amp/cm2 may be used to ignite deuterium-tritium at densities at or little above solid state density. The difficulty is to produce extremely high energy flux densities of the blocks. Results are reported how the threshold can be reduced by a factor up to fife if the inhibition factor for thermal conductivity due to electric double layers is included in the hydrodynamic analysis.

In the present article, the experimental investigation of optical breakdown induced by ns/mJ pulses at two wavelengths, 1064 nm and 532 nm, in air of atmospheric pressure is reported and discussed. The obtained breakdown thresholds were compared with theory and are in good agreement. The generated plasmas have been characterized by their amount of scattered laser light, energy transmission, and change of the transmitted temporal shape. Laser-induced plasma formation in a gas, in air, also generates an acoustic pressure wave. The acoustic energy is compared to the laser pulse energy and is found to be linearly dependent. Moreover, the frequency distribution of the characteristic acoustic pressure wave was analyzed. The experiments described were accomplished in order to optimize a laser ignition system with regard to efficiency and costs. The laser system employed for these investigations is a compact high peak power, passively Q-switched, longitudinally diode-pumped solid-state laser. Such a “laser spark plug” should replace conventional spark plugs in internal combustion engines because conventional ignition has reached its limits in terms of efficiency and durability. Thereby, a reduction of pollutant emission should also be feasible.

In this paper, the effect of a relativistically intense Gaussian laser pulse, on the propagation of electron plasma wave is studied. The nonlinear effects considered here are the relativistic decrease of the plasma frequency and the ponderomotive expelling of the electrons. Modified coupled equations for laser and electron plasma wave are derived from fluid equations. These coupled equations are solved analytically and numerically to study the laser intensity in the plasma and the variation of amplitude of the excited electron plasma wave. It is seen that the effect of including the ponderomotive nonlinearity is significant on the excitation of plasma wave. This should affect the number of energetic electrons and their energy range on account of wave particle interaction.

Due to their particular properties (low emittance, short duration, and large number density), the beams of multi-MeV protons generated during the interaction of ultraintense (I > 1019 W/cm2) short pulses with thin solid targets are suited for use as a particle probe in laser-plasma experiments. When traversing a sample, the proton density distribution is, in general, affected by collisional stopping, scattering and deflections via electromagnetic fields, and each of these effects can be used for diagnostic purposes. In particular, in the limit of very thin targets, the proton beams represent a valuable diagnostic tool for the detection of quasi-static electromagnetic fields. The proton imaging and deflectometry techniques employ these beams, in a point-projection imaging scheme, as an easily synchronizable diagnostic tool in laser- plasma interactions, with high temporal and spatial resolution. By providing diagnostic access to electro-magnetic field distributions in dense plasmas, this novel diagnostics opens up to investigation a whole new range of unexplored phenomena. Several transient processes were investigated employing this technique, via the detection of the associated electric fields. Examples provided in this paper include the detection of pressure-gradient electric field in extended plasmas, and the study of the electrostatic fields associated to the emission of MeV proton beams in high-intensity laser-foil interactions.