Preformed Plasma Research Articles

Laser wakefield bubble regime acceleration of electrons in a preformed non uniform plasma channel

AbstractA model of bubble regime electron acceleration by an intense laser pulse in non uniform plasma channel is developed. The plasma electrons at the front of the pulse and slightly off the laser axis in the plasma channel, experience axial and radial ponderomotive and space charge forces, creating an electron evacuated non uniform ion bubble. The expelled electrons travel along the surface of the bubble and reach the stagnation point, forming an electron sphere of radius re. The electrons of this sphere are pulled into the ion bubble and are accelerated to high energies. The Lorentz boosted frame enabled us to calculate energy gain of a test electron inside the bubble.

Theoretical aspect of enhancement and saturation in emission from laser produced plasma

AbstractThis paper presents a simplified theoretical model for the study of emission from laser produced plasma to better understand the processes and the factors involved in the onset of saturation in plasma emission as well as in increasing emission due to plasma confinement. This model considers that plasma emission is directly proportional to the square of plasma density, its volume, and the fraction of laser pulse absorbed through inverse Bremsstrahlung in the pre-formed plasma plume produced by the initial part of the laser. This shows that plasma density and temperature (that means the electron-ion collision frequency νei) decide the threshold for saturation in emission, which occurs for νei ≥ 1013 s−1, beyond which plasma shielding effects become dominant. Any decrease in plasma sound (expansion) velocity shows drastic enhancement in emission supporting the results obtained by magnetic as well as spatial confinement of laser produced plasma. The temporal evolution of plasma emission in the absence and presence of plasma confinement along with the effect of laser pulse duration are also discussed in the light of this model.

Stimulated Raman forward scattering of laser in a pre-formed plasma channel

AbstractStimulated Raman forward scattering (SRFS) of an intense short pulse laser in a plasma channel formed by two pre-laser pulses is investigated. The density nonuniformity of a plasma channel increases the focusing of main laser pulse. Main laser pulse excites a plasma wave and two electromagnetic sideband waves. Laser and the sidebands exert an axial ponderomotive force on electrons driving the plasma wave. The nonlinear currents arise at sideband frequencies. The density perturbation due to plasma wave beats with the oscillatory velocity due to pump to drive the sidebands. The normalized growth rate of SRFS increases with the density nonuniformity of a plasma channel. However, in the presence of a deep plasma channel the focusing is ineffective to laser intensity, but the growth rate increases with the intensity of main laser pulse.

X-ray backlight measurement of preformed plasma by kJ-class petawatt LFEX laser

Foot and pedestal pulses that precede the main pulse from a high-intensity laser greatly affect laser–plasma interactions. Especially in fast ignition schemes, preceding pulses generate a plasma prior to irradiation by the main pulse. This results in a too energetic and divergent electron beam being generated in the preformed plasma, which reduces the energy coupling efficiency from the heating laser to the dense fuel core. A preformed plasma with a density scale length of 40–60 μm was observed by a time- and space-resolved x-ray backlight technique using the LFEX laser system at the Institute of Laser Engineering, Osaka University. Preceding pulses (i.e., the foot and pedestal) of the LFEX were characterized by comparing observations with calculations results obtained using a two-dimension (2D) radiation-hydrodynamic simulation code. In a separate experiment, the 2D code was benchmarked with the experimentally observed hydrodynamic behavior of a gold plasma produced by a nanosecond laser pulse that mimicked foot and pedestal pulses (intensity: 1 ×1011−1×1012W/cm2). The preceding pulses were estimated to have an intensity of 1 ×1012−1013W/cm2, a duration of 2.0 ns, and a spot diameter at the target of 200–600 μm by comparing the measured hydrodynamics of the preformed plasma with that calculated by the 2D hydrodynamic simulation code.

Mechanism of heating of pre-formed plasma electrons in relativistic laser-matter interaction

The role of the longitudinal ambipolar electric field, present inside a pre-formed plasma, in electron heating and beam generation is investigated by analyzing single electron motion in the presence of one electromagnetic plane wave and “V” shaped potential well (constant electric field) in a one dimensional slab approximation. It is shown that for the electron confined in an infinite potential well, its motion becomes stochastic when the ratio of normalized laser electric field a0, to normalized longitudinal electric field Ez, exceeds unity, i.e., a0/Ez≳1. For a more realistic potential well of finite depth, present inside the pre-formed plasma, the condition for stochastic heating of electrons gets modified to 1≲a0/Ez≲L, where L is the normalized length of the potential well. The energy of electron beam leaving such a potential well and entering the solid scales ∼a02/Ez, which can exceed the laser ponderomotive energy (∼a0) in the stochastic regime.

Programmably structured plasma waveguide for development of table-top photon and particle sources

Programmable fabrication of longitudinal spatial structures in an optically preformed plasma waveguide in a gas jet was achieved, by using laser machining with a liquid-crystal spatial light modulator as the pattern mask. Fabrication of periodic structures with a minimal period of 200 μm and density-ramp structures with a minimal slope length of 100 μm was attained. The technique is useful for the optimization of various laser-plasma-based photon and particle sources.

Propagation of an ultrashort, high-intensity laser pulse in gas-target plasma

AbstractFor high-energy gain of electron acceleration by a laser wakefield, a stable or guiding propagation of an ultrashort, high-intensity laser pulse in a gas-target plasma is of fundamental importance. Preliminary experiments were carried out for the propagation of 30-fs, ~100-TW laser pulses of intensities ~1019W/cm2in plasma of densities ~1019/cm3. Self-guiding length of nearly 1.4 mm was observed in a gas jet and 15 mm in a hydrogen-filled capillary. Fluid-dynamics simulations are used to characterize the two types of gas targets. Particle-in-cell simulations indicate that in the plasma, after the pulse's evolution of self-focusing and over-focusing, the high-intensity pulse could be stably guided with a beam radius close to the plasma wavelength. At lower plasma densities, a preformed plasma channel of a parabolic density profile matched to the laser spot size would be efficient for guiding the pulse.

Quasi-matched propagation of ultra-short, intense laser pulses in plasma channels

The propagation of an ultrashort and relativistically intense laser pulse in a preformed plasma channel is investigated. The nonlinear paraxial wave equation describing the laser propagation in the plasma is solved both analytically and numerically. For any arbitrary temporal laser pulse profile with a given power (less then the critical power for self-focusing) and any prescribed transverse density profile in the channel, we determine the laser intensity distribution along the pulse ensuring quasi-matched propagation, neglecting non-paraxial effects. For the case of a Gaussian laser with an initially uniform spot throughout the pulse, we determine the optimal channel depth that minimizes laser evolution (e.g., minimizes spot size oscillations). The analytical and semi-analytical results obtained for both cases in the weakly relativistic regime are presented and validated through comparison with numerical simulations.

Characteristics of argon plasma waveguide produced by alumina capillary discharge for short wavelength laser application

We have reported the argon (Ar) plasma waveguide produced in an alumina (Al2O3) capillary discharge and used to guide ultrashort laser pulses at intensities of the order of 1016 W/cm2. The electron density in the plasma waveguide was measured to be 1×1018 cm−3, in agreement with one-dimensional magnetrohydrodynamic (MHD) simulations. The MHD code was also used to evaluate the degree of ionization of argon (Ar) in the preformed plasma waveguide. The maximum ion charge state of Ar3+ in capillary discharge was measured and obtained in the MHD simulations. The spectrum of the propagated laser pulse in the Ar plasma waveguide was not modified and was well reproduced by a particle-in-cell simulations under initial ion charge state of Ar3+ in the preformed plasma waveguide. The optimum timing for the laser pulse injection was around 150 ns after initiation of a discharge with a peak current of 200 A.

Effects of CH foam preplasma on fast ignition

AbstractCreation of a preformed plasma (preplasma) by heating laser prepulse is crucial to fast ignition. Because it is difficult to control the prepulse and preplasma, control of fast electron beam generation by low-density foam was recently reported. However, this simulation study ignored the foam preplasma. Therefore, we calculated foam preplasma formation using a hydrodynamic code and investigated the effects of the preplasma on fast ignition by using integrated simulations, including radiation hydrodynamic, Particle-In-Cell (PIC) and Fokker–Planck simulations. We conclude that the average core temperature decreased by approximately 10% in the integrated simulations of the foam preplasma case.

CO2 Laser acceleration of forward directed MeV proton beams in a gas target at critical plasma density

AbstractThe generation of 1–5 MeV protons from the interaction of a 3 ps TW CO2 laser pulse with a gas target with a peak density around the critical plasma density has been studied by 2D particle-in-cell simulations. The proton acceleration in the preformed plasma with a symmetric, linearly ramped density distribution occurs via formation of sheath of the hot electrons on the back surface of the target. The maximum energy of the hot electrons and, hence, net acceleration of protons is mainly defined by Forward Raman scattering instability in the underdense part of the plasma. Forward directed ion beams from a debris free gaseous target can find an application as a high-brightness ion source-injector to a conventional accelerator operating up to kHz pulse repetition frequency.

The effect of preformed plasma strings on the propagation of ultra-short laser pulse in air

In this investigation, propagation of collinear sequence of twin intense laser pulses is considered in air, numerically. Two sets of twin successive laser pulses were investigated. The first set includes two successive LG01 laser modes and the second one includes LG01 mode followed by a TEM00 mode. The physical effects that are considered include diffraction, nonlinear self-focusing, optical field ionization and associated energy depletion, plasma absorption and defocusing. The first laser pulse produces a ring plasma filament with inhomogeneous electron density in transverse direction. The numerical investigations show that the second laser pulse LG01 mode/TEM00 mode undergoes self-focusing producing plasma filament until joining to the head of the first filament. At this point, it diverges due to the plasma divergence effect and could not penetrate into the preformed plasma ring. The possibility of optical guiding of the second laser pulse through the ring plasma filament is considered, too.

Guiding of a laser beam in collisional magnetoplasma channel

AbstractLaser guiding through an axially non-uniform collisional magnetoplasma channel formed by ionizing laser prepulse has been investigated. Self-defocusing of the ionizing prepulse leads to an axial non-uniform plasma channel. Due to the propagation of second laser beam through such preformed plasma channel, non-uniform heating of electrons takes place on account of non-uniform intensity distribution of laser beam. Non-uniform heating diffuses the electrons away from the axis and thereby enhances the plasma channel. Due to the competition between diffraction and refraction phenomenon through such an axial non-uniform collisional magnetoplasma channel, there is a periodic beam width variation with the distance of propagation. Second order ordinary differential equations for the beam width parameter of prepulse and the guided beam have been set up using the moment theory approach. Effect of axial non-uniformity, intensity of guided beam and magnetic field has been seen on the propagation of the second guided beam in the plasma channel.

Directional transport of fast electrons at the front target surface irradiated by intense femtosecond laser pulses with preformed plasma

AbstractThe effects of laser incidence angle on lateral fast electron transport at front target surface, when a plasma is preformed, irradiated by intense (>1018 W/cm2) laser pulses, are studied by Kα imaging technique and electron spectrometer. A horizontally asymmetric Kα halo, resulting from directional lateral electron transport and energy deposition, is observed for a large incidence angle (70°). Moreover, a group of MeV high energy electrons is emitted along target surface. It is believed that the deformed preplasma and the asymmetrical distribution of self-generated magnetic field, at large incidence angle, play an important role in the directional lateral electron transport.

Generation of pre-formed plasma and its reduction for fast-ignition

AbstractWe investigated generation of pre-formed plasma on plates and inside cone targets due to a pre-pulse before the arrival of the main ultra-intense laser pulse in the fast-ignition scheme of the inertial confinement fusion. We estimated the pre-pulse level to be 130 mJ for LFEX laser used in the 2009 FIREX experiment, and the density gradient scale length of the pre-formed plasma inside the cone target to be 27–47 microns between the critical and 1/10 of the critical density, based on the two-dimensional radiation hydrodynamic simulations. In order to reduce the generation of pre-formed plasma, we investigated a thin CH foil pre-pulse absorber, and proposed using a cone target with a pointed tip. We simulated CH plasma expansion to show that the CH foils works as a pre-pulse absorber. We also show the aluminum pointed tip of the cone target can delay the shock arrival time by 20 ps, much longer than the delay for the 10 micron thickness gold tip used in the typical implosion of GXII at Osaka University.

Laser Guiding Through an Axially Nonuniform Collisionless Plasma Channel

This paper presents an investigation of laser guiding through an axially nonuniform collisionless plasma channel formed by the ionizing laser prepulse. When a delayed second laser beam is allowed to propagate through such preformed plasma channel, on account of nonuniform intensity distribution of laser beam, ponderomotive force comes into play, which further enhances the plasma channel. Unbalanced diffraction and refraction phenomenon through such an axially nonuniform collisionless plasma channel results in periodic beam width variation with the distance of propagation. Wave equations governing the propagation characteristics of the ionizing pre-pulse and delayed pulse through axially nonuniform collisionless plasma channel have been solved by moment theory approach. Effects of the axial nonuniformity and second delayed pulse intensity on the laser guidance have been studied.

Effects of long rarefied plasma on fast electron generation for FIREX-I targets

AbstractLong-scale preformed plasmas are generated inside the cone by the pre-pulse of the heating laser in the cone-guided fast ignition scheme and it is found that coupling efficiency from the heating laser to fast electrons especially suitable for core heating is drastically reduced by the preformed plasmas. To mitigate this serious problem, an extremely thin film is suggested to cover the entrance of the cone. This method, however, introduces long rarefied plasmas around the entrance of the cone and the main pulse must propagate through these plasmas. Therefore, fast electron characteristics produced by the main pulse could be affected, and effects of long rarefied plasmas on fast electron generation are investigated. It is found that the electron beam intensity becomes larger than that without the rarefied plasma, but the energy coupling rate from the heating laser to the core decreases due to lack of appropriate electrons for core heating. To achieve less than 10% degradation of the core electron temperature, the thin film must be expanded by irradiation of the pre-pulse so that the length and the density of rarefied plasmas become less than 500 µm and one-tenth of the critical density. A thickness of the thin film can be determined by these criteria and the intensity of the pre-pulse.

High-brightness optical-field-ionization collisional-excitation extreme-ultraviolet lasing pumped by a 100-TW laser system in an optically preformed plasma waveguide

Recent study on optical-field-ionization collisional-excitation extreme-ultraviolet lasing of Ni-like krypton at 32.8 nm pumped by a 100-TW laser system with an optically preformed plasma waveguide is reported. By using a 9-mm-long pure krypton plasma waveguide fabricated with the axicon-ignitor-heater scheme, the 32.8-nm extreme-ultraviolet laser provided an average output of 1012 photons/pulse at pump energy of 1 J, more than one order of magnitude enhancement relative to the previous results with the same scheme at pump energy of 235 mJ. It is also found the far-field pattern of laser beams varies from a single peak profile at low pump energy to an annular profile at high pump energy due to over-ionization of krypton ions at the center of the plasma channel.

Properties of a capillary discharge-produced argon plasma waveguide for shorter wavelength source application

We report the operation of a discharge-produced argon (Ar) plasma waveguide in an alumina (Al(2)O(3)) capillary to guide a 10(16)-W/cm(2) ultrashort laser pulse for shorter wavelength light sources at high repetition rate operation. The electron density in the plasma channel was measured to be 1 × 10(18) cm(-3). Modeling with a one-dimensional magnetrohydrodynamic code was used to evaluate the degree of ionization of Ar in the preformed plasma channel. The observed spectrum of the laser pulse after propagation in the argon plasma waveguide was not modified and was well reproduced by a particle in cell simulation.

Wakefield effects and solitary waves of an intense short laser pulse propagation in a plasma channel

In the presence of relativistic and channel-coupling nonlinearity and wakefield effects, the propagation characteristics and solitary waves of an intense short laser pulse in a preformed plasma channel are investigated. The evolution equation of the laser spot size is derived by using variational technique, the initial laser and plasma parameters for propagation with constant spot size, periodic defocusing and focusing oscillations, and solitary waves are identified. For illustration, some numerical results are also presented. It is found that the laser focusing is enhanced by the wakefield effects that result in a significant reduced focusing power.