Main Laser Pulse Research Articles

High-contrast, high-intensity laser pulse generation using a nonlinear preamplifier in a Ti:sapphire laser system

We report a high-contrast, high-intensity Ti:sapphire chirped-pulse amplification system that incorporates a nonlinear preamplifier based on optical parametric chirped-pulse amplification (OPCPA). By cooling the Ti:sapphire crystal in the final amplifier down to 77 K, the chirped-pulses are amplified to 2.9 J at a 10 Hz repetition rate without a thermal lensing effect. Pulse compression down to 19 fs duration obtained after amplification indicates a peak power of 80 TW. With the OPCPA, the temporal contrast is significantly improved to better than 7x10(-9) in a few picoseconds interval prior to the main laser pulse.

A plasma channel scheme for the interaction of an intense femtosecond laser pulse with a deuterium cluster jet

We propose a plasma channel scheme to obtain an improved table-top laser driven fusion neutron yield as a result of explosions of large deuterium clusters irradiated by an intense laser pulse. A cylindrical plasma channel is created by two moderate intensity laser prepulses at the edge of a deuterium cluster jet along which an intense main laser pulse propagates several nanoseconds later. With the aid of this plasma channel, the main laser pulse will be allowed to deposit its energy into the central region of the deuterium gas jet where the cluster sizes are larger and the atomic density is higher. The plasma channel formation and its impact on the deuterium ion energy spectrum and the consequent fusion neutron yield have been investigated. The calculated results show that a remarkable increase of the table-top laser driven fusion neutron yield would be expected.

Suppression of suprathermal ions from a colloidal microjet target containing SnO2 nanoparticles by using double laser pulses

We have demonstrated suppression of suprathermal ions from a colloidal microjet target plasma containing tin-dioxide (SnO2) nanoparticles irradiated by double laser pulses. We observed a significant decrease of the tin and oxygen ion signals in the charged-state-separated energy spectra when double laser pulses were irradiated. The peak energy of the singly ionized tin ions decreased from 9to3keV when a preplasma was produced. The decrease in the ion energy, considered as debris suppression, is attributed to the interaction between an expanding low-density preplasma and a main laser pulse.

Linear and nonlinear absolute phase effects in interactions of ulrashort laser pulses with a metal nano-layer or with a thin plasma layer

It has been shown that in the scattered radiation, generated by an ultrashort laser pulse impinging on a metal nano-layer, non-oscillatory wakefields appears with a definite sign. The magnitude of these wakefields is proportional to the incoming field strength, and the definite sign of them is governed by the cosine of the carrier-envelope phase difference of the incoming pulse. When we let such a Wakefield excite the electrons of a secondary target (say an electron beam, a metal surface or a gas jet), we can obtain 100 percent modulation in the electron signal in a given direction. This scheme can serve as a basis for the construction of a robust linear carrier-envelope phase difference meter. At relativistic laser intensities, the target is considered as a plasma layer in vacuum produced from a thin foil by a prepulse, which is followed by the main high-intensity laser pulse. The nonlinearities stemming from the relativistic kinematics lead to the appearance of higher-order harmonics in the scattered spectra. In general, the harmonic peaks are downshifted due to the presence of an intensity-dependent factor. This phenomenon is analogous to the famous intensity-dependent frequency shift in the nonlinear Thomson scattering on a single electron. In our analysis, an attention has also been paid to the role of the carrier-envelope phase difference of the incoming few-cycle laser pulse. It is also shown that the spectrum has a long tail where the heights of the peaks vary practically within one order of magnitude forming a quasi-continuum. Fourier synthesizing the components from this plateau region attosecond pulses has obtained.

X-ray spectroscopy observation of fast ions generation in plasma produced by short low-contrast laser pulse irradiation of solid targets

X-ray spectra of plasma produced by the interaction of Ti:Sa laser pulses (duration from 60 fs to 1 ps, and energy from 15 mJ to 128 mJ) with foil and solid Teflon and AL targets are investigated. It is shown experimentally and theoretically that the use of low contrast (10−2 – 10−4) short laser pulses, essentially promotes the conditions for generation of fast multi-charged ions. This effect is caused by self-focusing of the main laser pulse in a preplasma produced by intense laser prepulses. Modeling of the observed spectral line shape gives evidence of a considerable (about 3%) amount of multi-charged He-like F ions with energy E ∼ 1 MeV at rather low values of laser intensity IL ≈ 6 × 1016 W cm−2.

Measurement of the Electron Density Produced by the Prepulse in an Experiment of High Energy Proton Beam Generation

The characteristics of the preplasma, such as the electron density and its profile, were investigated from a measurement of the phase shift due to the preplasma in an experiment on high energy proton beam generation. The electron density and its profile were calculated from the measured phase shifts, and their dependences on the prepulse intensity and the time delay were investigated. Although the size of the electron density depended on the prepulse intensity, the maximum electron density ranged from 1.5 ◊ 10 19 to 2.1 ◊ 10 19 cm 3 , irrespective of the prepulse intensity and the time delay in the picosecond range. The analysis showed that a preplasma having an electron density of 10 19 cm 3 existed on a target before the main laser pulse arrived at the target for the

Preplasma effects on the emission directions of energetic electrons in relativistic laser–solid interactions

Motivated by recent experimental observations of fast electron jets emitted along the target surface in intense laser–solid interactions with a large incident angle of the laser pulse, we simulate electron emissions by two-dimensional particle-in-cell simulations. When there is no preplasma in advance of the main laser pulse, electrons are emitted dominantly along the target surface. However, when there is preplasma, electron emission changes to the target normal. This difference originates from the different absorption mechanisms and different Coulomb electrostatic fields and self-generated magnetic fields induced in front of the target for the two cases.

Generation of fast highly charged ions in laser–plasma interaction

The nonthermal and nonlinear coupling of strong laser light wave with plasma transfers a part of laser energy into hot electrons and fast ions. The efficiency of these effects depends on the characteristics of a laser pulse, target properties and irradiation geometry.The reported studies were performed with the use of a high-power and high-energy iodine PALS laser system (energy up to 1 kJ in a 0.4 ns pulse at wavelength of 1315 nm and energy up to 250 J at wavelength of 438 nm). The properties of the laser-produced ion streams were determined with the use of ion diagnostics based on the time-of-flight method. The characteristics of x-rays were measured using various semiconductor detectors.The main ion stream characteristics as well as the ion acceleration processes in plasmas of different Z numbers were studied in dependence on laser pulse parameters. The parameters of a fast ion group depend evidently on Z number of the ions. The influence of the electron density scale length on fast ion generation was investigated using a low intensity laser pre-pulses to generate preformed plasmas (pre-plasmas) with which the main laser pulse interacted. The obtained results suggest that ion acceleration processes were most effective at a specific electron density gradient scale length of pre-plasma determined by the pre-pulse parameters.

Generation of fast ions in femto-and picosecond laser plasmas at low intensities of the heating radiation

X-ray spectra from Teflon targets irradiated by laser pulses with a duration of 60 fs to 1 ps have been investigated experimentally. It is shown that, when the contrast of the laser pulse is sufficiently low, the effect of self-focusing of the main laser pulse in the plasma produced by the prepulse can significantly enhance the generation efficiency of fast particles. In this case, ions with energies as high as ∼1 MeV are observed at relatively low laser intensities, qlas ≈ (4–6) × 1016 W/cm2.

Effect of External Static Magnetic Field on the Emittance and Total Charge of Electron Beams Generated by Laser-Wakefield Acceleration

Significant enhancement of emittance and an increase of the total charge of femtosecond electron beams produced by a 12 TW, 40 fs laser pulse, tightly focused in a He gas jet, are observed after applying a static magnetic field, B> or =0.2 T, directed along the axis of laser pulse propagation. The effect appears when plasma produced by a laser prepulse becomes magnetized in the vicinity of the focus point: the electron Larmor frequency exceeds the collisional frequency, while periphery of the plasma remains unmagnetized. The entailed change in the shape of the plasma suppresses the diffraction of the main laser pulse that results in a much higher charge of electrons self-injected during the longitudinal wave breaking of the laser wake as well as the excellent stability of the beams.

Comparison between four-level model and six-temperature model on the description of a simple mechanical Q-switched CO2 laser

We investigate both the four-level model and the six-temperature model to describe the dynamical processes of a simple mechanical Q-switched CO2 laser that is realized by a fast chopper. The Q-switched process and the effect of the dynamic parameters on the output intensity of the Q-switched CO2 laser are reported using the two models. Theoretical comparisons between the two models on the effect of dynamical parameters, and the experimental results are presented. Both the models are well consistent with the experimental one for the main output laser pulse.

TH‐C‐230A‐06: High‐Energy Proton Acceleration Driven by Ultra‐Intense Ultra‐Clean Laser Pulses

Purpose: To improve the contrast ratio of the multi‐terawatt Chirped‐Pulse Amplification (CPA) Ti:Sapphire laser to 1011 to allow Coulomb explosion regime of ion acceleration in the interaction of ultra‐short high‐intensity laser pulses with ultra‐thin (< 1 micron) foils. Method and Materials: The cross‐polarized wave generation (XPW) technique in BaF2 crystals was implemented. This technique improves contrast by rejecting the low‐intensity amplified spontaneous emission (ASE) preceding the main laser pulse. Particle‐in‐cell (PIC) simulations were conducted under the anticipated experimental conditions: 225 TW in a 6.75 J, 30 fs laser pulse with no prepulse, focused to a spot size of 2.4 microns (FWHM) on thin foils of varying thickness. Results: Implementation of the crosspolarized wave generation technique resulted in a contrast improvement of three orders of magnitude to approximately 1011. The performed PIC simulations show that for a 0.2 μm thick hydrogen foil, protons with energy of about 200 MeV can be generated. In the case of the two‐layer aluminum‐hydrogen foil the maximum energy of accelerated protons is about 150 MeV, but the proton spectrum has a flatter distribution, which may be more advantageous for therapy applications. Conclusion: We demonstrated that pulse cleaning based on cross‐polarized wave generation (XPW) using two BaF2 crystals yields a 1011 contrast ratio for a 50 TW laser system. Such contrast may be sufficient for a preplasma‐free interaction of sub‐Petawatt laser pulses with a sub‐micron thickness foils at intensity of ∼1022 W/cm2. Modeling of this interaction with PIC simulations demonstrated proton energies that are of interest for the radiation therapy.This study was supported by the National Science Foundation through the Frontiers in Optical and Coherent Ultrafast Science Center at the University of Michigan and the National Institute of Health.

Simulations of a grazing-incidence pumped x-ray laser at14.7nm

Numerical simulations of the grazing-incidence pumped Ni-like Pd x-ray laser at $14.7\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ ($4d\ensuremath{\rightarrow}4p$, $J=0\ensuremath{\rightarrow}1$) is performed using a modified one-dimensional hydrodynamic code MEDUSA. The effective absorption of the main laser pulse in the gain region is greatly increased due to the lengthened propagation path. Results predict that a saturated output of the x-ray laser can be achieved with only subjoules driver energy on a $4\text{\ensuremath{-}}\mathrm{mm}$-long Pd target.

Diagnostics of picosecond laser pulse absorption in preformed plasma

Abstract We present results where highly supersonic plasma jets and accelerated plasma fragments are generated by interaction of an intense picosecond laser pulse with a metallic target (Al, Cu, W, and Ta) in gas atmosphere. The formation of jets and well-localized massive plasma fragments occurs when a strong forward shock from a main laser pulse and a reverse shock from a pre-pulse meet to. Interferometric and shadow graphic measurements with high temporal (100 ps) and spatial (1 μm) resolution yield information about the formation and evolution of plasma jets and plasma fragments. The excitation of the electric and self-generated magnetic field by ponderomotive force during propagation of the laser pulse in a gas atmosphere was investigated as well. It had been shown previously that under certain conditions a hollow current channel can be generated in laser-produced plasma. The azimuthal magnetic field in such a micro-channel was determined by Faraday rotation of a probing laser beam to be 7.6 MGauss (MG). Ion acceleration in a pinched annular current channel up to 8 MeV analogous to micro-“plasma focus” conditions, may be realized at lengths of 100 μm. Self-generated magnetic fields of 4–7 MG have also been measured in thin skin layers in front of shock waves, where well-collimated plasma blocks were separated and accelerated away from the plasma body. The velocity of dense plasma blocks reaches values of order of 3 × 10 8 cm/s and they are stable during acceleration and propagation in gas.

Laser-Accelerated Protons with Energy-Dependent Beam Direction

The spatial distribution of protons, accelerated by intense femtosecond laser pulses interacting with thin target foils under oblique irradiation are investigated. Under certain conditions, the proton beams are directed away from the target normal. This deviation is towards the laser forward direction, with an angle that increases with the level and duration of the amplified spontaneous emission pedestal before the main laser pulse. In addition, for a given laser pulse, this beam deviation increases with proton energy. The observations are discussed in terms of different electron acceleration mechanisms and target normal sheath acceleration, in combination with a laser-controllable shock wave locally deforming the target rear surface.

Measurements of low-level prepulse on Nike KrF laser

The krypton fluoride (KrF) laser is a leading candidate driver for inertial fusion energy. Some of the current fusion target designs call for targets with thin metallic coatings. These targets could be particularly susceptible to preheat by a low-level laser prepulse. Knowledge of the prepulse can be important in understanding and modeling the behavior of such targets. This paper presents measurements of low-level prepulse on target with the Nike KrF laser. Sources of prepulse are discussed and measurements are performed under several specific laser conditions in order to evaluate the relative contribution of these sources to the overall prepulse. Prepulse is found to be ∼2×10−7 from peak intensity for approximately 120ns prior to the main laser pulse. Prepulse energy density on target is ∼2J∕cm2. The first laser amplifier in the time- and angle-multiplexed section of the laser is found to be the dominant source of prepulse.

Optimum Target Position of Ion Production in Ultrashort-Pulse Laser Interaction with Foil Targets under the Existence of Prepulses

It was found that ion production during ultrashort-pulse laser interaction with foil targets is most efficient when the target is positioned behind the waist of the laser, and if the main laser pulse is accompanied by prepulses that evaporate the target. The maximum proton energy was 1.2 MeV when the target was positioned 0.3 mm behind the waist, while it was 0.25 MeV at the waist. A model is proposed in which a prepulse ablates the surface of the target. The resultant neutral gas guides the head of the main pulse caused by the Kerr effect while it is ionized, so the ion generation is more efficient than that without a prepulse.

Theory of laser-induced adiabat shaping in inertial fusion implosions: The relaxation method

The theory of the adiabat shaping induced by a strong shock propagating through a relaxed density profile is carried out for inertial confinement fusion (ICF) capsules. The relaxed profile is produced through a laser prepulse, while the adiabat-shaping shock is driven by the foot of the main laser pulse. The theoretical adiabat profiles accurately reproduce the simulation results. ICF capsules with a shaped adiabat are expected to benefit from improved hydrodynamic stability while maintaining the same one-dimensional performances as flat-adiabat shells.

Characterization of preformed plasmas with an interferometer for ultra-short high-intensity laser-plasma interactions

The evolution of an Al preformed plasma produced by a prepulse was observed before and after the arrival of the main pulse by an interferometer using a femtosecond probe pulse. A central density depression due to the ponderomotive force of the main laser pulse in the preformed plasma with a ∼100 μm scale length was clearly visible after the main pulse irradiation at an intensity of ∼5×1016 W/cm2. The temporal profiles of the prepulse, characterized by a cross-correlation in conjunction with a precise density profile measurement by an interferometer, contribute to the better understanding of femtosecond laser-matter interactions.

Effect of pulse slippage on resonant second harmonic generation of a short pulse laser in a plasma

The process of second harmonic generation of an intense short pulse laser in a plasma is resonantly enhanced by the application of a magnetic wiggler. The wiggler of suitable wave number provides necessary momentum to second harmonic photons to make harmonic generation a resonant process. The laser imparts an oscillatory velocity to electrons and exerts a longitudinal ponderomotive force on them at , where ω1 and are the frequency and the wave number of the laser, respectively. As the electrons acquire oscillatory velocity at the second harmonic, the wiggler magnetic field beats with it to produce a transverse second harmonic current at , driving the second harmonic electromagnetic radiation. However, the group velocity of the second harmonic wave is greater than that of the fundamental wave, hence, the generated pulse slips out of the main laser pulse and its amplitude saturates.