Ultra-intense Laser Pulse Research Articles

Cone-guided fast ignition with ponderomotively accelerated carbon ions

Carbon ions are used to heat the precompressed deuterium–tritium (DT) fuel in a cone-guided fast ignitor scheme with an areal mass density of about 2.6 g cm−2. An ultra-intense laser pulse with a focal intensity of 1.45 × 1022 W cm−2 accelerates the carbon ions to an energy of 450 MeV from a homogeneous layer of 0.2 g/cm3 density, which fills the head of the gold cone. Pellet ignition was observed in hybrid numerical simulations for a laser energy of about 65 kJ in a rectangular pulse of 4 ps duration. This corresponds to estimated overall efficiencies of more than 24% for ion acceleration and 17% for core heating. Reducing the laser intensity to the value 5 × 1021 W cm−2, carbon ions with the energy of 175 MeV will be accelerated, and ignition occurred in hydrodynamic simulations for a laser energy of 115 kJ at a reduced heating efficiency of 6%. The comparison with ignition of a large-scale DT pellet, showing similar hydrodynamic characteristics and heated by in situ accelerated DT ions with 10 MeV mean energy, demonstrates the advantage of the carbon ion ignitor beam due to the more effective acceleration and expected higher directionality.

Standoff spectroscopy via remote generation of a backward-propagating laser beam.

In an earlier publication we demonstrated that by using pairs of pulses of different colors (e.g., red and blue) it is possible to excite a dilute ensemble of molecules such that lasing and/or gain-swept superradiance is realized in a direction toward the observer. This approach is a conceptual step toward spectroscopic probing at a distance, also known as standoff spectroscopy. In the present paper, we propose a related but simpler approach on the basis of the backward-directed lasing in optically excited dominant constituents of plain air, N(2) and O(2). This technique relies on the remote generation of a weakly ionized plasma channel through filamentation of an ultraintense femtosecond laser pulse. Subsequent application of an energetic nanosecond pulse or series of pulses boosts the plasma density in the seed channel via avalanche ionization. Depending on the spectral and temporal content of the driving pulses, a transient population inversion is established in either nitrogen- or oxygen-ionized molecules, thus enabling a transient gain for an optical field propagating toward the observer. This technique results in the generation of a strong, coherent, counterpropagating optical probe pulse. Such a probe, combined with a wavelength-tunable laser signal(s) propagating in the forward direction, provides a tool for various remote-sensing applications. The proposed technique can be enhanced by combining it with the gain-swept excitation approach as well as with beam shaping and adaptive optics techniques.

Extensive comparison among Target Normal Sheath Acceleration theoretical models

Abstract During the last 10 years of research it has been experimentally demonstrated that ions can be accelerated up to energies of tens of MeV as a consequence of the ultra-intense laser pulse interaction with a solid target. This process has been shown to take place via the so-called Target Normal Sheath Acceleration (TNSA) mechanism. A fundamental challenge in the investigation of this phenomenon is to understand the dependencies of the ion beam features on the laser and target properties. In order to achieve this goal, several TNSA theoretical models have been proposed. In the present work a collection of maximum ion energies experimentally measured has been compared to the values predicted by six different theoretical descriptions. The models we chose to compare are based either on a fluid approach, on a quasi-static approach, or on a combination of the before-mentioned. On the basis of such a quantitative comparison we aim to provide a reliable test of these theoretical descriptions, in which their strengths, weaknesses, open problems and capability in predicting the ion energy are underlined.

Magnetic collimation of fast electrons in specially engineered targets irradiated by ultraintense laser pulses

The efficient magnetic collimation of fast electron flow transporting in overdense plasmas is investigated with two-dimensional collisional particle-in-cell numerical simulations. It is found that the specially engineered targets exhibiting either high-resistivity-core-low-resistivity-cladding structure or low-density-core-high-density-cladding structure can collimate fast electrons. Two main mechanisms to generate collimating magnetic fields are found. In high-resistivity-core-low-resistivity-cladding structure targets, the magnetic field at the interfaces is generated by the gradients of the resistivity and fast electron current, while in low-density-core-high-density-cladding structure targets, the magnetic field is generated by the rapid changing of the flow velocity of the background electrons in transverse direction (perpendicular to the flow velocity) caused by the density jump. The dependences of the maximal magnetic field on the incident laser intensity and plasma density, which are studied by numerical simulations, are supported by our analytical calculations.

Few femtosecond, few kiloampere electron bunch produced by a laser–plasma accelerator

Particle accelerators driven by the interaction of ultraintense and ultrashort laser pulses with a plasma(1) can generate accelerating electric fields of several hundred gigavolts per metre and deliver high-quality electron beams with low energy spread(2-5), low emittance(6) and up to 1 GeV peak energy(7,8). Moreover, it is expected they may soon be able to produce bursts of electrons shorter than those produced by conventional particle accelerators, down to femtosecond durations and less. Here we present wide-band spectral measurements of coherent transition radiation which we use for temporal characterization. Our analysis shows that the electron beam, produced using controlled optical injection(9), contains a temporal feature that can be identified as a 15 pC, 1.4-1.8 fs electron bunch (root mean square) leading to a peak current of 3-4 kA depending on the bunch shape. We anticipate that these results will have a strong impact on emerging applications such as short-pulse and short-wavelength radiation sources(10,11), and will benefit the realization of laboratory-scale free-electron lasers(12-14).

Dominant front-side acceleration of energetic proton beams from plastic targets irradiated by an ultraintense laser pulse

An experimental observation has been made by using aluminum-coated Mylar foils, which strongly supports that in the case of plastic target, the energetic part of the proton beam originates from the front-side of the target. When a 30 fs laser pulse with an intensity of 1.6×1019 W/cm2 was irradiated on the 12.5-μm-thick Mylar side of the aluminum-coated Mylar foil, the maximum proton energy was reduced by a factor 5.5 as compared to that of 3.3 MeV observed from the single layer of the Mylar foil. With the help of a two-dimensional particle-in-cell simulation, these observations can be interpreted that in the case of plastic target, the energetic proton beam originates from the front-side of the target. In the case of an aluminum-coated 6-μm-thick Mylar foil, more energetic proton beams of 4.7 MeV were also observed when the laser pulse was irradiated on the aluminum side as compared to those of 3.4 MeV from the single Mylar foil.

Tests of proton laser-acceleration using circular laser polarization, foams and half gas-bag targets

We have analyzed laser-accelerated protons, generated at the rear surface of a solid target irradiated by an ultra-intense (I ∼ 1018 to 5 × 1019 W cm−2) short laser pulse in different conditions: (1) using linear and circular polarization; (2) using ‘foam’ targets, i.e. targets where a density gradient has been artificially generated by adding a foam onto a solid and (3) using ‘half gas-bag’ targets, where the density gradient has been artificially generated by having a gas assembly in front of the solid. In all these varied conditions, and for our laser and target parameters, no enhancement of proton acceleration could be found, compared with the standard set-up using a linearly polarized laser irradiating a flat metal foil. Potential issues and solutions are discussed.

Radiation reaction effects on electron nonlinear dynamics and ion acceleration in laser–solid interaction

Radiation Reaction (RR) effects in the interaction of an ultra-intense laser pulse with a thin plasma foil are investigated analytically and by two-dimensional (2D3P) Particle-In-Cell (PIC) simulations. It is found that the radiation reaction force leads to a significant electron cooling and to an increased spatial bunching of both electrons and ions. A fully relativistic kinetic equation including RR effects is discussed and it is shown that RR leads to a contraction of the available phase space volume. The results of our PIC simulations are in qualitative agreement with the predictions of the kinetic theory.

Generation and optimization of electron currents along the walls of a conical target for fast ignition

The interaction of an ultraintense laser pulse with a conical target is studied by means of numerical particle-in-cell simulations in the context of fast ignition. The divergence of the fast electron beam generated at the tip of the cone has been shown to be a crucial parameter for the efficient coupling of the ignition laser pulse to the precompressed fusion pellet. In this paper, we demonstrate that a focused hot electron beam is produced at the cone tip, provided that electron currents flowing along the surfaces of the cone sidewalls are efficiently generated. The influence of various interaction parameters over the formation of these wall currents is investigated. It is found that the strength of the electron flows is enhanced for high laser intensities, low density targets, and steep density gradients inside the cone. The hot electron energy distribution obeys a power law for energies of up to a few MeV, with the addition of a high-energy Maxwellian tail.

Enhanced Propagation for Relativistic Laser Pulses in Inhomogeneous Plasmas Using Hollow Channels

The influence of long (several millimeters) and hollow channels, bored in inhomogeneous ionized plasma by using a long pulse laser beam, on the propagation of short, ultraintense laser pulses has been studied. Compared to the case without a channel, propagation in channels significantly improves beam transmission and maintains a beam quality close to propagation in vacuum. In addition, the growth of the forward-Raman instability is strongly reduced. These results are beneficial for the direct scheme of the fast ignitor concept of inertial confinement fusion as we demonstrate, in fast-ignition-relevant conditions, that with such channels laser energy can be carried through increasingly dense plasmas close to the fuel core with minimal losses.

Spectral Enhancement in the Double Pulse Regime of Laser Proton Acceleration

The use of two separate ultraintense laser pulses in laser-proton acceleration was compared to the single pulse case employing the same total laser energy. A double pulse profile, with the temporal separation of the pulses varied between 0.75-2.5ps, was shown to result in an increased maximum proton energy and an increase in conversion efficiency to fast protons by up to a factor of 3.3. Particle-in-cell simulations indicate the existence of a two stage acceleration process. The second phase, induced by the main pulse preferentially accelerates slower protons located deeper in the plasma, in contrast to conventional target normal sheath acceleration.

Electron Acceleration in the Bubble Regime with Dense-Plasma Wall Driven by an Ultraintense Laser Pulse

An optimizing scheme for electron acceleration in the wake bubble with dense-plasma wall driven by an ultraintense laser pulse is presented and investigated by particle-in-cell simulation. The wall has an inner diameter matching the expected lateral bubble size. The bubble shape can be transversely controlled and longitudinally shrunk. The accelerated electrons as a bunch have a high quality because the electrons almost stay close to the bottom of the bubble and are accelerated to much high energy with narrow energy spread.

An imaging proton spectrometer for short-pulse laser plasma experiments

The ultraintense short pulse laser pulses incident on solid targets can generate energetic protons. In addition to their potentially important applications such as in cancer treatments and proton fast ignition, these protons are essential to understand the complex physics of intense laser plasma interaction. To better characterize these laser-produced protons, we designed and constructed a novel spectrometer that will not only measure proton energy distribution with high resolution but also provide its angular characteristics. The information obtained from this spectrometer compliments those from commonly used diagnostics including radiochromic film packs, CR39 nuclear track detectors, and nonimaging magnetic spectrometers. The basic characterizations and sample data from this instrument are presented.

High-Energy Ions from Near-Critical Density Plasmas via Magnetic Vortex Acceleration

Ultraintense laser pulses propagating in near-critical density plasmas generate magnetic dipole vortex structures. In the region of decreasing plasma density, the vortex expands both in forward and lateral directions. The magnetic field pressure pushes electrons and ions to form a density jump along the vortex axis and induces a longitudinal electric field. This structure moves together with the expanding dipole vortex. The background ions located ahead of the electric field are accelerated to high energies. The energy scaling of ions generated by this magnetic vortex acceleration mechanism is derived and corroborated using particle-in-cell simulations.

Slow wave plasma structures for direct electron acceleration

Two highly versatile experimental techniques are demonstrated for making preformed plasma waveguides with a periodic structure capable of supporting the propagation of ultra-intense femtosecond laser pulses up to 2×1017 W cm−2, limited by the available laser energy. These waveguides are made in hydrogen, nitrogen and argon plasmas with a length of 15 mm and a modulation period as short as 35 μm. Simulations show that these guides allow direct laser acceleration of electrons, achieving gradients of 80 MV cm−1 and 10 MV cm−1 for laser pulse powers of 1.9 TW and 30 GW, respectively. It is also shown that the periodic structure in these waveguides supresses the Raman forward instability, which could otherwise interfere with the direct acceleration scheme proposed.

Extreme Nonlinear Optics with Ultra-Intense Self-Bending Airy Beams

Recent research in nonlinear optics with ultra-intense and ultra-short laser pulses has opened the door to applications in remote spectroscopy, communications and atmospheric science. This article reviews studies into the propagation of ultra-intense self-bending Airy beams in transparent dielectric media.

Directional Bremsstrahlung from a Ti Laser-Produced X-Ray Source at Relativistic Intensities in the 3–12 keV Range

Front and rear side x-ray emission from thin titanium foils irradiated by ultraintense laser pulses at intensities up to ≈5 × 10(19) W/cm2 was measured using a high-resolution imaging system. Significant differences in intensity, dimension, and spectrum between front and rear side emission intensity in the 3-12 keV photon energy range was found even for 5 μm thin Ti foils. Simulations and analysis of space-resolved spectra explain this behavior in terms of directional bremsstrahlung emission from fast electrons generated during the interaction process.

Relativistic hole boring and fast ion ignition with ultra-intense laser pulses

An analytical model and numerical simulations demonstrate that pulses with intensities exceeding 1022 W/cm2 may penetrate deeply into the plasma and accelerate efficiently ions in the forward direction. We propose a new scheme for fast ignition of precompressed DT fusion targets by using two laser pulses. The first pulse (or a sequence of several pulses) creates a channel with diameter ∼ 30μm through the plasma corona up to the fuel density ∼ 1 g/cm3. The second pulse with a higher intensity accelerates the deuterium and tritium ions at the head of this channel. The overall ignition energy is ∼ 100 kJ.

Geometrical optimization of an ellipsoidal plasma mirror toward tight focusing of ultra-intense laser pulse

We developed for the first time, very compact (<1 cm3) extremely low f-number (f/# = 0.4) confocal ellipsoid focusing systems. Direct measurement of the laser focal spot using a low-energy laser beam indicates 1/5 reduction of the spot size compared to standard focusing (using a f/2.7 optics). Such mirror is thus able to achieve significant enhancement of the focused laser intensity without modifying the laser system itself. The mirror is then used under plasma mirror regime which enables us to compactify the size, to liberate us from the anxiety of protecting the optics from target debris after shots, and to enhance the temporal contrast. In this paper, we focus our attention to designing and optimizing the geometry of such innovative plasma optics.

Effects of substrate-ion density gradients on light-ion acceleration from ultraintense laser pulse irradiated thin-foils

A general solution of the electrostatic potential that determines the maximum light-ion energy is derived for the test-particle acceleration model by taking into account the influence of the substrate-ion density gradient. It is shown that the substrate-ion density structure is also dependent on laser pulse duration. In the picosecond or sub-picosecond regime, the decreasing density gradient of the substrate-ions leads to an evident reduction in the acceleration efficiency of the light-ions. However, this kind of influence is negligible in the ultrashort regime.