Initial Spot Size Research Articles

Ponderomotive self-focusing of Gaussian laser beam in warm collisional plasma

The propagation characteristics of a Gaussian laser beam through warm collisional plasma are investigated by considering the ponderomotive force nonlinearity and the complex eikonal function. By introducing the dielectric permittivity of warm unmagnetized plasma and using the WKB and paraxial ray approximations, the coupled differential equations defining the variations of laser beam parameters are obtained and solved numerically. Effects of laser and plasma parameters such as the collision frequency, the initial laser intensity and its spot size on the beam width parameter and the axis laser intensity distribution are analyzed. It is shown that, self-focusing of the laser beam takes place faster by increasing the collision frequency and initial laser spot size and then after some distance propagation the laser beam abruptly loses its initial diameter and vastly diverges. Furthermore, the modified electron density distribution is obtained and the collision frequency effect on this distribution is studied.

The influence of Monte Carlo source parameters on detector design and dose perturbation in small field dosimetry

To obtain accurate Monte Carlo simulations of small radiation fields, it is important model the initial source parameters (electron energy and spot size) accurately. However recent studies have shown that small field dosimetry correction factors are insensitive to these parameters. The aim of this work is to extend this concept to test if these parameters affect dose perturbations in general, which is important for detector design and calculating perturbation correction factors.The EGSnrc C++ user code cavity was used for all simulations. Varying amounts of air between 0 and 2 mm were deliberately introduced upstream to a diode and the dose perturbation caused by the air was quantified. These simulations were then repeated using a range of initial electron energies (5.5 to 7.0 MeV) and electron spot sizes (0.7 to 2.2 FWHM).The resultant dose perturbations were large. For example 2 mm of air caused a dose reduction of up to 31% when simulated with a 6 mm field size. However these values did not vary by more than 2 % when simulated across the full range of source parameters tested.If a detector is modified by the introduction of air, one can be confident that the response of the detector will be the same across all similar linear accelerators and the Monte Carlo modelling of each machine is not required.

The diffraction propagation properties of double-half inverse Gaussian hollow beams

Abstract In the paper, double-half inverse Gaussian hollow beams, which are a new type of hollow beams, have been proposed. In the paraxial approximation, the propagation model of double-half inverse Gaussian hollow beams in free space is deduced by utilizing the Collins formula. The distribution of the optical field is simulated with different initial spot sizes ( ω 0 =1 mm and 5 mm) and different propagating distances ( z =10 mm, 6×10 2 mm, 1.5×10 3 mm, 1×10 4 mm and z =10 mm, 1.5×10 4 mm, 3×10 4 mm and 10×10 4 mm). And the simulated results demonstrate that the Airy pattern is derived after propagating some distance. Meanwhile, the focusing property appears visibly and the energy is redistributed intricately. In the research, the focusing distance could be controlled by transforming the initial spot size and the intensity inequality is also shown.

Generating the Optimum Self-Focusing in the Relativistic Laser-Plasma Interaction

In this paper, the effect of basic parameters of laser and plasma on controlling self-focusing is investigated, and based on the introduced effective parameter for self-focusing (EPSF), the effects of laser wavelength, the initial spot size of laser beam, the initial intensity of laser, and the plasma electron density on self-focusing are discussed. It is found that the relativistic self-focusing strongly depend on the laser wavelength and the beam intensity. The nonlinear effect of beam intensity on the introduced parameter EPSF indicates that after an increasing region for focusing, the self-focusing effect decreases at ultrarelativistic intensities. The effect of the initial beam spot size on the introduced parameter EPSF indicates that pulses with larger spot sizes can be focused stronger, and the diagram of EPSF versus the electron density shows the direct dependence between self-focusing and the initial plasma density. The nonlinear effect of laser wavelength on self-focusing also is studied at relativistic and ultrarelativistic intensities. The presented model introduces the optimum selection of laser and plasma parameters for achieving the desired focused/unfocused laser beam in the relativistic laser-plasma interaction.

Self-modulated wakefield acceleration in a centimetre self-guiding channel

AbstractSelf-modulated wakefield acceleration was investigated at densities down to ~4 × 1018 cm−3 by propagating the 50 TW 300 fs LULI laser in helium gas jets at lengths up to 1 cm. Long interaction lengths were achieved by closer matching of the initial focal spot size to the matched spot size for these densities. Electrons with energies extending to 180 MeV were observed in broad energy spectra which show some evidence for non-Maxwellian features at high energy. Two-dimensional PIC simulations indicate that the intial laser pulse breaks up into small pulselets that are eventually compressed and focused inside the first few plasma periods, leading to a ‘bubble-like’ acceleration of electron bunches.

Pareto-optimality study into the comparison of the separation potential of comprehensive two-dimensional liquid chromatography in the column and spatial modes

The expected performance of spatial (“flat-bed”) two-dimensional liquid chromatography (xLC×xLC) has been calculated using the Pareto-optimality strategy. This approach allowed different objectives (total peak capacity, total analysis time, and total dilution) to be considered simultaneously and to establish optimal parameters (pressure drop, particle size, bed length, and initial spot size). The performance of spatial two-dimensional chromatographic systems was compared with that of conventional on-line, real-time two-dimensional column-liquid-chromatography systems (tLC×tLC). The potential gain in peak capacity and/or analysis time of the spatial configuration was confirmed. By restricting the spatial parameters to realistic chromatographic conditions (limiting the stress, as counterbalance for the pressure drop through the sorbent bed, to 2500kg) it was found that xLC×xLC is attractive for very fast analysis of complex samples, rather than for extremely efficient separations. For example, a peak capacity of 780 may be achieved in only 2.7min using a 100×100mm sorbent bed of a quality currently encountered thin-layer chromatography. Furthermore, if beds can be packed as efficiently as contemporary columns, the predicted peak capacity increases to around 1000, corresponding to a peak-production rate of about 6.3peaks/s. Possibilities to boost the performance of xLC×xLC further are briefly discussed. Unless we can overcome the severe stress requirements of high-performance xLC×xLC, conventional tLC×tLC may be more amenable to very complex separations, thanks to the very high peak capacities. However, tLC×tLC separations will require long analysis times (e.g. 10,000 peaks in 37h, corresponding to 0.075peaks/s at a pressure drop of 40MPa). The best trade-off between total peak capacity, total analysis time, and total dilution under restricted (realistic) conditions was obtained using high pressures, small chromatographic beds, small particles, and relatively large sample spots.

Interaction physics for the shock ignition scheme of inertial confinement fusion targets

This paper presents an analysis of laser–plasma interaction risks of the shock ignition (SI) scheme and experimental results under conditions relevant to the corona of a compressed target. Experiments are performed on the LIL facility at the 10 kJ level, on the LULI 2000 facility with two beams at the kJ level and on the LULI 6-beam facility with 100 J in each beam. Different aspects of the interaction of the SI pulse are studied exploiting either the flexibility of the LULI 6-beam facility to produce a very high intensity pulse or the high energy of the LIL to produce long and hot plasmas. A continuity is found allowing us to draw some conclusions regarding the coupling quality and efficiency of the SI spike pulse. It is shown that the propagation of the SI beams in the underdense plasma present in the corona of inertial confinement fusion targets could strongly modify the initial spot size of the beam through filamentation. Detailed experimental studies of the growth and saturation of backscattering instabilities in these plasmas indicate that significant levels of stimulated scattering reflectivities (larger than 40%) may be reached at least for some time during the SI pulse.

Uncertainties and correction methods when modeling passive scattering proton therapy treatment heads with Monte Carlo

Nowadays, Monte Carlo models of proton therapy treatment heads are being used to improve beam delivery systems and to calculate the radiation field for patient dose calculations. The achievable accuracy of the model depends on the exact knowledge of the treatment head geometry and time structure, the material characteristics, and the underlying physics. This work aimed at studying the uncertainties in treatment head simulations for passive scattering proton therapy. The sensitivities of spread-out Bragg peak (SOBP) dose distributions on material densities, mean ionization potentials, initial proton beam energy spread and spot size were investigated. An improved understanding of the nature of these parameters may help to improve agreement between calculated and measured SOBP dose distributions and to ensure that the range, modulation width, and uniformity are within clinical tolerance levels. Furthermore, we present a method to make small corrections to the uniformity of spread-out Bragg peaks by utilizing the time structure of the beam delivery. In addition, we re-commissioned the models of the two proton treatment heads located at our facility using the aforementioned correction methods presented in this paper.

Relative phase interactions of two copropagating laser beams in underdense plasmas at different intensities and spot sizes

The mutual interactions of two copropagating laser beams at a relative phase are studied using a two-dimensional fluid code. The interactions are investigated in underdense plasma at selected beam configurations and beam parameters for two separate nonlinearities, i.e., the ponderomotive and the relativistic nonlinearity. The selected beam configurations are introduced by different initial transverse spot size perturbations (finite and infinite) and different initial transversal intensity distributions (nonuniform and uniform) over those spot sizes and the selected beam parameters are given by different initial beam intensities relevant to each nonlinearity. In the ponderomotive nonlinearity, simulation results show that no mutual interactions are demonstrated between the copropagating beams regardless of the initial beam configurations and parameters. In nonlinear relativistic simulations, the mutual interactions between the beams are clearly observed, a mutual repulsion is formed in the presence of initial intensities that are nonuniformly distributed over finite spot sizes, and an effective strongly modulated mutual attraction takes places in the presence of initial intensities that are uniformly distributed over infinite spot sizes. Moreover, it is found in these simulations that increasing the initial beam intensities improves the attraction properties between the copropagationg beams.

Self-Guiding of Ultrashort, Relativistically Intense Laser Pulses through Underdense Plasmas in the Blowout Regime

The self-guiding of relativistically intense but ultrashort laser pulses has been experimentally investigated as a function of laser power, plasma density, and plasma length in the blowout regime. The extent of self-guiding, observed by imaging the plasma exit, is shown to be limited by nonlinear pump depletion with observed self-guiding of over tens of Rayleigh lengths. Spectrally resolved images of the plasma exit show evidence consistent with self-guiding in the plasma wake. Minimal losses of the self-guided pulse resulted when the initial spot size was matched to the blowout radius.

Nanoconcentration of terahertz radiation in plasmonic waveguides

We establish the principal limits for the nanoconcentration of the THz radiation in metal/dielectric waveguides and determine their optimum shapes required for this nanoconcentration. We predict that the adiabatic compression of THz radiation from the initial spot size of R(0) approximately lambda(0) to the final size of R = 100- 250 nm can be achieved, while the THz radiation intensity is increased by a factor of x10 to x250. This THz energy nanoconcentration will not only improve the spatial resolution and increase the signal/noise ratio for the THz imaging and spectroscopy, but in combination with the recently developed sources of powerful THz pulses will allow the observation of nonlinear THz effects and a variety of nonlinear spectroscopies (such as two-dimensional spectroscopy), which are highly informative. This will find a wide spectrum of applications in science, engineering, biomedical research, environmental monitoring, and defense.

Growth criterion for small dry spots in the falling liquid films

In thin films of liquid falling over an inclined surface random (or non-random) small dry spots shut and disappear or expand into the large-scale dry spots depending on the initial spot sizes and parameters of the liquid film. On consideration of the gravity forces, inertial forces of the film and surface tension, affecting the elements of a roller surrounding the dry spot, we have formulated a criterion, which helped us to answer the following question: if this spot shuts or not? This criterion includes the numbers of Bond and Reynolds and the contact wetting angle as the determining parameters. It is generalization of the known Hartley — Murgatroid criterion, which allows determination of the zone of falling film metastability, i. e., determination of the critical film thickness, below whose value the formation of stable dry spots is possible. It is shown that the critical values of parameters of the initial dry spot depend on the shape and sizes of a roller surrounding this spot.

Results with an Apparatus for Pressurized Planar Electrochromatography

Pressurized planar electrochromatography (PPEC) is a fast and efficient planar chromatographic technique. The mobile phase is driven by electroosmotic flow, while the system is pressurized in a manner that allows heat to flow between the sorbent layer and the pressurizing medium. The reproducibility of solute retention was not satisfactory in the initial report describing PPEC. In the current report, this reproducibility is improved by better control of several experimental variables. The pressure at which PPEC is performed is now free of drift, and the temperature at which the layer is preconditioned is maintained to within +/-1 degrees C. The best reproducibility of retention is obtained when the plate is soaked in the mobile phase for a defined time before each run. In the original prototype, the temperature of the sorbent layer was not controlled. In the present apparatus, water, at a constant temperature between 3 and 60 degrees C, is circulated through channels in the two die blocks that pressurize the layer. The highest efficiency is obtained at an intermediate temperature. This behavior is ascribed to high resistance to mass transfer at the lower temperatures and increased diffusion at higher temperatures. Efficiency, as measured by the number of theoretical plates, increases with increasing migration distance. The height equivalent of a theoretical plate diminishes with increasing migration distance, and values as low as 0.0106 mm are obtained under appropriate conditions. This extrapolates to 94 000 plates/m. Manual spotting was used in this report. Evidence is presented that substantially better efficiency would be obtained if the initial spot size were smaller. The efficiency of PPEC in its current form is illustrated by a chromatogram showing the separation of nine solutes in 2 min. PPEC was also performed with TLC plates in a back-to-back configuration, and this doubles the number of samples that can be simultaneously separated.

The Electron Trajectory in a Relativistic Femtosecond Laser Pulse

In this report, we start from Lagrange equation and analyze theoretically the electron dynamics in electromagnetic field. By solving the relativistic government equations of electron, the trajectories of an electron in plane laser pulse, focused laser pulse have been given for different initial conditions. The electron trajectory is determined by its initial momentum, the amplitude, spot size and polarization of the laser pulse. The optimum initial momentum of the electron for LSS (laser synchrotron source) is obtained. Linear polarized laser is more advantaged than circular polarized laser for generating harmonic radiation.

Gouy phase shift of single-cycle picosecond acoustic pulses

Ultrafast laser pulses are used to generate single-cycle picosecond acoustic pulses in thin metal films on silicon. For small initial excitation spot sizes, propagation of the acoustic pulses across a 485-μm Si crystal leads to significant diffraction effects. The temporal reshaping of the acoustic wave form due to diffraction is investigated, and we demonstrate that the acoustic far field can be reached.

Mitigation of thermal blooming and diffraction effects with high-power laser beams

The three-dimensional (3-D) pulse propagation code, pulsed ultrashort laser simulation emulator (PULSE), has been used to model the interaction of thermal blooming, diffraction, and nonlinear self-focusing in the atmosphere. PULSE is a derivative of the free-electron laser (FEL) simulation code, free-electron laser experiment (FELEX) [Nucl. Instrum. Methods Phys. Res. A250, 449 (1986)] that was used for many years to model the 3-D interaction of optical pulses with electron beams. This code has now been modified to model nonlinear material interactions. In its current configuration, an electromagnetic pulse is modeled on a Cartesian grid, thereby allowing for arbitrary amplitude and phase modulations in the transverse (x, y) and longitudinal (z) directions. The code includes models for thermal blooming (in a laminar transverse wind field), diffraction, Kerr self-focusing and self-phase modulation, plasma defocusing and absorption, and multiphoton and avalanche ionization. Simulation of picosecond length pulses indicate that the defocusing effects of thermal blooming and diffraction can be effectively eliminated by Kerr self-focusing. Simulation results show that the beam arrives at the target with a spot size comparable to (or less than) its initial spot size. For propagation distances of many kilometers, this can result in order-of-magnitude increases of the on-target intensity at range, which can significantly reduce the average power requirement of the laser required to provide a given irradiance level.

Backlighter predictive capability

Correctly predicting the intensity and spatial extent of an area backlighter is important in optimizing the design and analysis of a laser-based experiment. In this work, the spatial extent of an area backlighter is calculated using a view factor code to obtain the laser illumination pattern and then converting to x rays using the measured x-ray conversion efficiency. The view factor model can also be compared to a simple illumination calculation. The models were validated with experiments where five 1-ns-square OMEGA [Boehly et al., Opt. Commun. 133, 495 (1997)] laser beams containing a total of 1.85 kJ were directed onto Fe or Ti foils. The predicted emission size was compared to time-gated two-dimensional images of the Fe emission region or to time-integrated images from both Fe and Ti. The models correctly predict the spatial extent of the emitting region for the first hundred picoseconds. The emission region grows logarithmically with time during the laser pulse; eventually reaching a diameter that is 1.6 times the initial laser spot size. Folding the x-ray conversion efficiency into the calculated intensities allows prediction of backlighter brightness and structure that is useful in optimizing the experimental design.

Coupling protocol for thin layer chromatography/matrix-assisted laser desorption ionization

This paper reports the investigation and optimization of the thin layer chromatography/matrix-assisted laser desorption/ionization (TLC-MALDI) coupling protocol. The fundamental coupling parameters which influence sensitivity and lateral analyte spreading are extraction solvent selection, extraction time, and pressure. Selection of the solvent is dependent upon its extraction efficiency, which has been correlated with extraction solventR f value. Maximum extraction efficiency occurs after two minutes for the analyte/solvent system studied. High extraction efficiency solvents cause significant lateral spreading of analyte; up to a three-fold increase in initial analyte spot size. Analyte recovery was found to be limited by the silica gel inter-partice porosity and the solvent extraction efficiency. For maintaining chromatographic resolution and maximizing signal intensity, extraction solvents withR f values between 0.4 to 0.6 are optimal. The upper analyte recovery limit, using extraction solvents within thisR f range, is estimated at 22%.