M. Botton

5.5-7.5 MeV Proton Generation by a Moderate-Intensity Ultrashort-Pulse Laser Interaction with H{sub 2}O Nanowire Targets

We report on the first generation of 5.5-7.5 MeV protons by a moderate-intensity short-pulse laser (∼5×10(17)  W/cm(2), 40 fsec) interacting with frozen H(2)O nanometer-size structure droplets (snow nanowires) deposited on a sapphire substrate. In this setup, the laser intensity is locally enhanced by the snow nanowire, leading to high spatial gradients. Accordingly, the nanoplasma is subject to enhanced ponderomotive potential, and confined charge separation is obtained. Electrostatic fields of extremely high intensities are produced over the short scale length, and protons are accelerated to MeV-level energies.

Extended lifetime of high density plasma filament generated by a dual femtosecond?nanosecond laser pulse in air

A substantially extended lifetime of a high-density plasma channel generated in the wake of an intense femtosecond pulse propagating in air is experimentally demonstrated. Free electron density above 1015 cm−3 in the formed plasma filament is measured to be sustained for more than 30 ns. This high-density plasma lifetime prolongation of more than one order of magnitude is achieved by properly timed irradiation of the filament with a relatively low-intensity nanosecond laser pulse, in comparison with a filament without such irradiation. The experimental results are in good agreement with our theoretical model that follows the evolution of the temperature and density of various molecules, atoms, and ion species. The results point to the possibility of generating extremely long time duration, stable high-density plasma filaments in air.

Temporal evolution of femtosecond laser induced plasma filament in air and N2

We present single shot, high resolution, time-resolved measurements of the relaxation of laser induced plasma filaments in air and in N2 gas. Based on the measurements of the time dependent electromagnetic signal in a waveguide, an accurate and simple derivation of the electron density in the filament is demonstrated. This experimental method does not require prior knowledge of filament dimensions or control over its exact spatial location. The experimental results are compared to numerical simulations of air plasma chemistry. Results reveal the role of various decay mechanisms including the importance of O4+ molecular levels.

Generation of concatenated long high-density plasma channels in air by a single femtosecond laser pulse

We experimentally demonstrate a stable and reproducible generation of long concatenated high-density plasma channels in air by a single femtosecond laser pulse. Each segment of the plasma channel is created by a plasma filament left in the wake of the same single high power laser pulse. Our method enables a control of a few millimeters over the position of each segment as well as exact temporal synchronization between them. The combined plasma channel can extend up to several meters long. The plasma density along the entire concatenated plasma channels is measured to be above 1015 cm−3. The demonstrated approach can be further extrapolated to a higher number of filament segments, thus to much longer high-density plasma channels.

Uniform lifetime prolongation of a high density plasma channel left in the wake of femtosecond filament

We demonstrate uniform lifetime prolongation of an entire plasma filament generated by a high-power femtosecond laser pulse. The entire filament is irradiated by a secondary nano-second laser pulse that co-propagates with the femtosecond laser. The plasma filament partially absorbs the nanosecond laser radiation, and plasma lifetime is extended along the entire channel resulting in a smooth and continuous high-density plasma column with lifetime longer than 30 ns. We present an experimental and theoretical study of the intensity range of the secondary laser required for effective lifetime prolongation and the behavior of the plasma density at the onset of breakdown triggered by the secondary laser. Our study shows that an efficient prolongation of the lifetime of plasma filament occurs in the intensity range of 0.03–0.3 TW/cm2.

Proton acceleration to above 5.5 MeV by interaction of 10[sup]17[/sup] W/cm[sup]2[/sup] laser pulse with H[sub]2[/sub]O nano-wire targets

Compact sources of high energy protons (50-500MeV) are expected to be key technology in a wide range of scientific applications 1-8. Particularly promising is the target normal sheah acceleration (TNSA) scheme 9,10, holding record level of 67MeV protons generated by a peta-Watt laser 11. In general, laser intensity exceeding 1018 W/cm2 is required to produce MeV level protons. Enhancing the energy of generated protons using compact laser sources is very attractive task nowadays. Recently, nano-scale targets were used to accelerate ions 12,13. Here we report on the first generation of 5.5-7.5MeV protons by modest laser intensities (4.5 × 1017 W/cm2) interacting with H2O nano-wires (snow) deposited on a Sapphire substrate. In this setup, the plasma near the tip of the nano-wire is subject to locally enhanced laser intensity with high spatial gradients, and confined charge separation is obtained. Electrostatic fields of extremely high intensities are produced, and protons are accelerated to MeV-level energies. Nano-wire engineered targets will relax the demand of peak energy from laser based sources.

5.5: A new complex envelope ADI-FDTD algorithm for 3D simulation of slow wave structures

We present a new time-domain algorithm designed for self-consistent 3D simulation of the nonlinear interaction between an electron beam and RF fields in planar slow wave structures, based on a modified alternating direction implicit (ADI) finite difference time domain (FDTD) time advance scheme. The algorithm forms the basis of a new simulation code, NEPTUNE.

Microwave diagnostics of plasma filaments left in the wake of high power femtosecond laser pulse

Summary form only given. We present a simple non-intrusive experimental method allowing a complete single shot time resolved measurement of the relaxation of laser induced plasma filaments. The method is based on filament interaction with low intensity microwave radiation in a rectangular single mode waveguide. The suggested diagnostics allow a complete single shot temporal analysis of filament plasma decay with temporal resolution better than 0.3 ns and high spatial resolution along the filament. This experimental method does not require prior knowledge of filament dimensions or control over its exact spatial location. The experimental results are compared to numerical simulations of air plasma chemistry. Results reveal the role of various decay mechanisms including the importance of O4+ molecular levels. Microwave diagnostics was used to study plasma lifetime prolongation by properly timed irradiation of the filament with a relatively love intensity nanosecond laser pulse. Lifetime prolongation of high density plasma filament by more than an order of magnitude has been demonstrated. The experimental results are in good agreement with our theoretical model that follows the evolution of the temperature and density of various molecules, atoms and ion species. The results point to the possibility of generating extremely long time duration, stable high density plasma filaments in air.

Microstructured snow targets for high energy quasi-monoenergetic proton acceleration

Compact size sources of high energy protons (50-200MeV) are expected to be key technology in a wide range of scientific applications 1-8. One promising approach is the Target Normal Sheath Acceleration (TNSA) scheme 9,10, holding record level of 67MeV protons generated by a peta-Watt laser 11. In general, laser intensity exceeding 1018 W/cm2 is required to produce MeV level protons. Another approach is the Break-Out Afterburner (BOA) scheme which is a more efficient acceleration scheme but requires an extremely clean pulse with contrast ratio of above 10-10. Increasing the energy of the accelerated protons using modest energy laser sources is a very attractive task nowadays. Recently, nano-scale targets were used to accelerate ions 12,13 but no significant enhancement of the accelerated proton energy was measured. Here we report on the generation of up to 20MeV by a modest (5TW) laser system interacting with a microstructured snow target deposited on a Sapphire substrate. This scheme relax also the requirement of high contrast ratio between the pulse and the pre-pulse, where the latter produces the highly structured plasma essential for the interaction process. The plasma near the tip of the snow target is subject to locally enhanced laser intensity with high spatial gradients, and enhanced charge separation is obtained. Electrostatic fields of extremely high intensities are produced, and protons are accelerated to MeV-level energies. PIC simulations of this targets reproduce the experimentally measured energy scaling and predict the generation of 150 MeV protons from laser power of 100TW laser system18.

Fast implicit time-domain simulation of complex 3D slow-wave structures

The Courant—Friedrichs—Lewy (CFL) stability condition limits the size of time step that may be used by conventional explicit finite-difference time-domain particle-in-cell (FDTD-PIC) codes in proportion to the grid cell size. In slow-wave vacuum electronic devices with non-relativistic electron beams, the typical scale length for fields in the beam region is L ≅ βλ << λ, where β v<inf>b</inf>/c is the normalized beam velocity and λ is the vacuum wavelength. This dictates a very small cell size h << L << λ for PIC simulation which via the CFL condition imposes extremely small time steps compared to the RF period. Consequently explicit methods require long simulation times to model slow-wave devices.