Preformed Plasma Research Articles

Relativistic laser channeling in plasmas for fast ignition

We report an experimental observation suggesting plasma channel formation by focusing a relativistic laser pulse into a long-scale-length preformed plasma. The channel direction coincides with the laser axis. Laser light transmittance measurement indicates laser channeling into the high-density plasma with relativistic self-focusing. A three-dimensional particle-in-cell simulation reproduces the plasma channel and reveals that the collimated hot-electron beam is generated along the laser axis in the laser channeling. These findings hold the promising possibility of fast heating a dense fuel plasma with a relativistic laser pulse.

Simultaneous evolution of spot size and length of short laser pulses in a plasma channel

Nonlinear, nonparaxial propagation of an intense, short, Gaussian laser pulse in a preformed plasma channel having a parabolic radial density profile is analyzed. The variational technique is used to obtain equations describing the evolution of laser spot size and pulse length in the channel. Relativistic nonlinearity and group velocity dispersion effects have been taken into account and numerical methods are used to study the simultaneous evolution of the laser spot and pulse length.

Simulations of grazing-incidence pumped Ni-like Sn X-ray laser at 11.9 nm

Grazing-incidence pumped Ni-like Sn X-ray laser media at 11.9 nm (4d–4p, J = 0–1) is modelled using 1 1 2 d code EHYBRID and a post-processor code. The required atomic data are obtained using the Cowan code. In this study the pre-formed plasma is pumped on longitudinal direction with a grazing angle. Detailed simulations were performed to optimize the driving laser configurations. Relatively high gain is produced for the Ni-like Sn X-ray laser at 11.9 nm with long pre-pulse and short main pulse drive energy of only 100 mJ on 4 mm slab targets. Using low intensity pre-pulse prior to long pulse decreases the electron density gradient. X-ray resonance lines between 13 and 25 Å emitted from tin plasma have been simulated using post-processor coupled with EHYBRID. The ratio of these resonance lines can be used to measure electron temperature of the laser produced Sn plasma.

Optical-field-ionization collisional-excitation x-ray lasers with an optically preformed plasma waveguide

Optical-field-ionization x-ray lasers in an optically preformed plasma waveguide for pure xenon, krypton, and argon gases, respectively, are achieved. In addition to the 46.9 nm main lasing line for Ne-like argon, the 45.1 and 46.5 nm lasing lines are also observed, indicative of the strong enhancement effect and the large gas density in the plasma waveguide. With this technique multispecies parallel x-ray lasing is also demonstrated in a Kr-Ar mixed-gas waveguide. Extensive experimental results including the pump-energy dependence, the density dependence, and the effects of parameters that control the waveguide fabrication are reported and discussed.

Propagation of sinusoidal pulse laser beam in a plasma channel

The propagation of an intense sinusoidal pulse laser beam, in a preformed plasma channel having a parabolic density profile, is analyzed. Considering a nonparaxial, nonlinear wave equation and using a variational technique, simultaneous equations describing the evolution of laser spot and pulse length are obtained. Numerical methods are used to study the effect of pulse length variation on betatron oscillations of the laser spot size as it propagates in the plasma channel. Approximate conditions for propagation of the laser pulse with a constant spot size and pulse length (matched beam propagation) are obtained.

Studies of craters’ dimension for long-pulse laser ablation of metal targets at various experimental conditions

Long pulse laser shots of the PALS iodine laser in Prague have been used to obtain metal target ablation at various experimental conditions. Attention is paid mainly to the dependencies of the crater diameter on the position of minimum laser-focus spot with regard to the target surface, by using different laser wavelengths and laser energies. Not only a single one, but two minima, independently of the wavelength, of the target irradiation angle and of the target material, were recorded. Significant asymmetries, ascribed to the non-linear effects of intense laser beam with pre-formed plasma, were found, too. Estimations of ejected mass per laser pulse are reported and used to calculate the efficiency of laser-driven loading. Results on metal target ablation and crater formation at high intensities (from 2 × 10 13 to 3 × 10 16 W/cm 2) are presented and compared. Crater depth, crater diameter and etching yield are reported versus the laser energy, in order to evaluate the ablation threshold fluence.

The influence of an intense laser beam interaction with preformed plasma on the characteristics of emitted ion streams

AbstractIntense laser-beam interactions with preformed plasma, preceding the laser-target interactions, significantly influence both the ion and X-ray generation. It is due to the laser pulse (its total length, the shape of the front edge, its background, the contrast, the radial homogeneity) as well as plasma (density, temperature) properties. Generation of the super fast (FF) ion groups is connected with a presence of non-linear processes. Saturated maximum of the charge states (independently on the laser intensity) is ascribed to the constant limit radius of the self-focused laser beam. Its longitudinal structure is considered as a possible explanation for the course of some experimental dependencies obtained.

Dramatic Enhancement of Optical-Field-Ionization Collisional-Excitation X-Ray Lasing by an Optically Preformed Plasma Waveguide

Dramatic enhancement of optical-field-ionization collisional-excitation x-ray lasing is achieved by using an optically preformed plasma waveguide. With a 9-mm-long pure krypton plasma waveguide prepared by using the axicon-ignitor-heater scheme, lasing at 32.8 nm is enhanced by 400 folds relative to the case without the plasma waveguide. An output level of 8 x 10(10) photon/shot is reached at an energy conversion efficiency of 2 x 10(-6). The same method is used to achieve x-ray lasing in a gas jet for the high-threshold low-gain transition at 46.9 nm in neonlike argon.

Short pulse laser train for laser plasma interaction experiments

A multiframe, high-time resolution pump-probe diagnostic consisting of a consecutive train of ultrashort laser pulses (approximately ps) has been developed for use with a chirped pulse amplification (CPA) system. A system of high quality windows is used to create a series of 1054 nm picosecond-laser pulses which are injected into the CPA system before the pulse stretcher and amplifiers. By adding or removing windows in the pulse train forming optics, the number of pulses can be varied. By varying the distance and thickness of the respective optical elements, the time in between the pulses, i.e., the time in between frames, can be set. In our example application, the CPA pulse train is converted to 527 nm using a KDP crystal and focused into a preformed plasma and the reflected laser light due to stimulated Raman scattering is measured. Each pulse samples different plasma conditions as the plasma evolves in time, producing more data on each laser shot than with a single short pulse probe. This novel technique could potentially be implemented to obtain multiple high-time resolution measurements of the dynamics of physical processes over hundreds of picoseconds or even nanoseconds with picosecond resolution on a single shot.

Characterization of an Intense Laser-Produced Preformed Plasma for Proton Generation

High-energy protons are generated with a 3-μm-thick tantalum target irradiated with a highintensity laser under the preformed plasma condition. We observe the electron density distributions of preformed plasmas with two-color probe beams as interferometers. The preformed plasma at the front side of the target is generated by pre-pulses of a high-intensity Ti:sapphire laser. In this preformed plasma condition, the maximum proton enegy is ∼900 keV. The half angle of the generated proton beam (>500 keV) is about 10◦.

An Acoustically Driven Magnetized Target Fusion Reactor

We propose a new acoustic compression scheme for a MTF power plant. A strong acoustic wave is produced by piston impacts. The wave focuses in liquid PbLi to compress a pre-formed FRC plasma. Simulations indicate the possibility of building an economical 60 MWe power plant. A proof-of-principle experiment produces a small D-D fusion yield of 2000 neutrons per shot.

GeV electron beams from a centimeter-scale channel guided laser wakefield accelerator

Laser wakefield accelerators can produce electric fields of order 10–100GV∕m, suitable for acceleration of electrons to relativistic energies. The wakefields are excited by a relativistically intense laser pulse propagating through a plasma and have a phase velocity determined by the group velocity of the light pulse. Two important effects that can limit the acceleration distance and hence the net energy gain obtained by an electron are diffraction of the drive laser pulse and particle-wake dephasing. Diffraction of a focused ultrashort laser pulse can be overcome by using preformed plasma channels. The dephasing limit can be increased by operating at a lower plasma density, since this results in an increase in the laser group velocity. Here we present detailed results on the generation of GeV-class electron beams using an intense femtosecond laser beam and a 3.3cm long preformed discharge-based plasma channel [W. P. Leemans et al., Nature Physics 2, 696 (2006)]. The use of a discharge-based waveguide permitted operation at an order of magnitude lower density and 15 times longer distance than in previous experiments that relied on laser preformed plasma channels. Laser pulses with peak power ranging from 10–40TW were guided over more than 20 Rayleigh ranges and high quality electron beams with energy up to 1GeV were obtained by channeling a 40TW peak power laser pulse. The dependence of the electron beam characteristics on capillary properties, plasma density, and laser parameters are discussed.

Tuning of the high-order harmonics generated from laser plasma plumes and solid surfaces by varying the laser spectrum, chirp, and focal position

Tuning of the harmonic wavelength by varying the driving radiation wavelength, chirp, and intensity during the interaction of Ti:sapphire laser radiation with preformed laser plasma and solid surfaces was studied. High-order harmonics (HHG) were studied at different spectral, phase, and intensity characteristics of femtosecond radiation. Analysis of the variation of the high-order harmonic distribution in the case of different plumes is presented. We show that the tuning of harmonic wavelength allows achievement of resonance enhancement of single harmonics in the plateau distribution of HHG.

Integrated simulation of an ion-driven warm dense matter experiment

Longitudinal compression factors in excess of 50 of a 300-keV, 20-mA K + ion beam have been demonstrated in the Neutralized Drift Compression Experiment (NDCX) in agreement with LSP particle-in-cell simulations using the experimental tilt voltage waveform. Here, pre-formed plasma provides beam neutralization for a 1–2-m drift length. To achieve simultaneous transverse and longitudinal compression, we must understand and account for the impact of the applied velocity tilt on the transverse phase space of the beam. Of equal importance to achieving warm dense matter and heavy ion fusion conditions, is quantifying the effect of beam plasma interactions, including stability and neutralization, on the beam transport throughout the drift section up to the target. Critical new issues relate to transverse focusing of the axially compressing ion beam in a high-field (3–15 T) solenoid that is filled with plasma. Integrated LSP simulations that include modeling of the diode, magnetic transport, induction bunching module, plasma neutralized transport, solenoidal focusing and beam target interaction, are assisting in the design of a near-term warm dense matter experiment. We discuss the simulation algorithms and present calculations of designs for such an experiment that will heat an aluminum target up to roughly 1-eV temperature.

Scale-length optimizing of short pulse CuKα laser-plasma sources

The authors present experimental results showing how the use of a high contrast femtosecond laser system allows better optimization of Kα emission from a Cu target. The shorter scale-length preformed plasma is better optimized for resonance absorption of the laser light when the laser is moved away from best focus. The experimental data show a central peak of Kα emission at tight focus with strong secondary peaks at large offset. The use of these secondary peaks results in a much reduced hard x-ray background and should lead to shorter Kα pulses than at tight focus.

SHEET CROSSING AND WAVE BREAKING IN THE LASER WAKEFIELD ACCELERATOR

The seminal work of J. M. Dawson on sheet crossing and wave breaking is extended to laser wakefield scenarios. We derive a quantitative estimate for the sheet crossing time. We find that the time for the onset of sheet crossing is reduced for lower densities, higher laser intensities and smaller spot sizes. Furthermore, we show that a preformed parabolic plasma channel can decrease the time required for sheet crossing.

Enhancement of Relativistic Harmonic Generation by an Optically Preformed Periodic Plasma Waveguide

Enhancement of relativistic third-harmonic generation by using an optically preformed periodic plasma waveguide was achieved. Resonant dependence of harmonic intensity on plasma density and density modulation parameters was observed, which is a distinct characteristic of quasi-phase-matching. The results demonstrate the potential of a modulated plasma waveguide in high-field applications.

Strong enhancement and extinction of single harmonic intensity in the mid- and end-plateau regions of the high harmonics generated in weakly excited laser plasmas

Strong intensity enhancement or extinction of some single harmonics is observed in high-harmonic generation from 48 fs Ti:sapphire laser pulses propagating through preformed low-excited laser-produced plasmas of various materials (GaAs, Cr, InSb, stainless steel). The intensities of some of the harmonics in the mid- and end-plateau regions vary from ~23-fold enhancement to near disappearance compared with those of the neighboring ones. It is also shown that the observed intensity enhancement (or extinction) can be varied by controlling the chirp of the driving laser radiation.

Characterization of preformed plasmas using a multi-dimensional hydrodynamic simulation code in the study of high-intensity laser–plasma interactions

We observed a preformed plasma of an aluminum slab target produced by a high-intensity Ti:sapphire laser. The expansion length of the preformed plasma at the electron density of 3 × 1018 cm−3, which was the detection limit, was around 100 μm measured with a laser interferometer. In order to characterize quantitatively and to control the preformed plasmas, we perform a two-dimensional hydrodynamic simulation. The expansion length of the preformed plasma was almost the same as the experimental result, if we assumed that the amplified spontaneous emission lasted 3.5 ns before the main pulse arrived.

High Harmonic Generation from Ions in a Capillary Discharge Plasma Waveguide

I recent years, high harmonic generation (HHG) has proven to be a useful light source for investigating surface dynamics, holographic imaging and, more recently, probing molecular structure and dynamics. One important goal in the further development of HHG as a light source is the production of higher photon flux at shorter soft X-ray wavelengths. Since HHG is the result of ionization of the nonlinear medium followed by rapid acceleration of the electron in the light field, the highest energy photons are generated from target atoms that ionize at high laser intensities—in other words, those with high ionization potentials, such as helium. However, the highest flux has been observed when high-Z atoms are used for HHG. Although these requirements conflict, there is one attractive method for improving conversion to very high photon energy: to use ions rather than neutral atoms. Gibson et al. showed that it is possible to generate higher energy harmonics from argon ions by using a hollow waveguide to mitigate ionization-induced defocusing of the laser beam—which limits the laser intensity and thus the highest photon energy obtained. However, this geometry does not eliminate the laser loss. Alternatively, using a pre-formed plasma waveguide could overcome defocusing, as well as mitigate the ionization-induced energy loss of the driving laser.2,3 A capillary discharge creates an ideal medium for HHG from ions by providing a preformed plasma waveguide with tunable density and ionization. In this work, we present the first observation of HHG from ions in a plasma discharge. monic generation shows great promise for increasing harmonic flux and extending efficient harmonic generation to shorter wavelengths. t