Preformed Plasma Research Articles

Wakefield generation as the mechanism behind spectral shift of a short laser pulse

An equation describing the dynamics of plasma wave generation by a short intense laser pulse is analyzed to find a relation between the difference in mean-square pulse frequency before and after laser-matter interaction and the electric field amplitude in the wakefield plasma wave generated by the laser pulse. This relation can be effectively used in systems for wakefield diagnostics. The relation is applied to several geometries of interaction between a pulse and an ionizing gas or preformed plasma.

The Effect of Pre-plasma and Self-focusing on Characteristics of Laser Produced Ions

Generation of various ion groups was studied using lasers of different pulse length and intensities. Up to thirteen ion subgroups have been distinguished when using the PALS iodine laser system in experiments that involve a laser interaction with a preformed plasma. In addition to generally accepted thermal, fast, and slow ion groups, the existence of the second fast (superfast) group was clearly proven. The threshold laser intensity IL necessary fo appearance of nonlinear effects was confirmed to be ≈ × 1014W/cm2 at our experimental conditions. Above this value, there exists an interval of laser focus positions in front of the target in which ions with significantly higher charge states and energies are produced, compared to experiments without pre-plasma interactions, due to nonlinear processes in preformed plasma.

Space-charge neutralization of heavy ion beams via electron injection

One key issue in heavy ion beam fusion (HIBF) is how to effectively focus high-current and high-perveance ion beams onto a small target area. Space charge neutralization is needed to prevent defocusing as the ion density compresses during focusing. The present study is concerned with the physics of space-charge neutralization by use of various concepts for injection of electrons into the ion beam in addition to plasma neutralization. To date the most widely studied approaches to neutralization use a preformed plasma or ionize a background gas in the beam line. However, these schemes alone face difficulties due to local non-uniformities in charge neutralization due to beam dynamic/focusing effects. Thus, two supplemental techniques are under study and will be discussed here. One involves axially injection of electrons created in a thin foil in the beam path. The other uses a magnetic field to guide the electrons into the direction of the beam path. Both techniques would be used in combination with conventional background plasma, providing added control of neutralization.

Relativistic self-focusing of fs-laser pulses and their heating effect on the preformed plasma

Experiments in the context of fast ignition were carried out by focusing a 6-TW laser beam into a preformed plasma. An electron population with a quasi-temperature of 2 MeV was generated. The 2/spl omega/ self-emission image reveals a self-focused plasma channel formation, whereas X-ray pinhole photography shows a postheating of the preformed plasma.

Comparison of experimental data and three-dimensional simulations of ion beam neutralization from the Neutralized Transport Experiment

The Neutralized Transport Experiment at Lawrence Berkeley National Laboratory has been designed to study the final focus and neutralization of high perveance ion beams [E. Henestroza, S. Eylon, P. Roy, S. Yu, A. Anders, F. Bieniosek, W. Greenway, B. Logan, R. MacGill, D. Shuman et al., Phys. Rev. ST-Accel. Beams 7, 083501 (2004)]. Preformed plasmas in the last meter before the target of the scaled experiment provide a source of electrons which neutralize the ion current and prevent the space-charge-induced spreading of the beam spot. Neutralized Transport Experiment physics issues are discussed and experimental data are analyzed and compared with three-dimensional (3D) particle-in-cell simulations. Along with detailed target images, 4D phase-space data at the entrance of the neutralization region have been acquired. These data are used to provide a more accurate beam distribution with which to initialize the simulation. Previous treatments have used various idealized beam distributions which lack the deta...

Effective coupling of ultraintense laser pulse to funnel-mouthed plasma waveguides

The injection of laser pulses into hydrodynamically preformed plasma channels can be hindered by the conditions at the entrance of the channel. In particular, neutral gas and narrowing of the channel prevent efficient coupling of laser pulse entering into the channel. To solve this problem, a funnel-shaped plasma lens can be grafted onto the channel using an auxiliary formation pulse. Simulations of channel formation show that such a funnel can be made in the density ramp of a gas jet. Simulations of laser pulse propagation show that such a funnel efficiently couples pulse energy into the channel. For a backfill target with a funnel, the coupling efficiency is lower and required funnel parameters are more restrictive than for the gas jet case.

Kinetic and fluid Langmuir wave nonlinearities driven by stimulated Raman scattering in a diffraction limited single-hot-spot

A diffraction limited focal spot is used to create a single speckle in a preformed plasma. The single speckle, or hot spot, is the simplest irreducible element for studying laser plasma interactions. Using the single hot spot configuration, the nonlinear behavior of SRS driven Langmuir waves has been investigated in both a fluid dominated and a kinetic regime. A transition between the two regimes has been observed as a function of kλD where k is the Langmuir wave-number and λD is the Debye length.

Charge-state and energy enhancement of laser-produced ions due to nonlinear processes in preformed plasma

At laser intensities above IL∼1×1014W∕cm2(ILλ2∼1×1014Wμm2∕cm2), nonlinear processes in preformed plasma, such as self-focusing, influence ion generation significantly and ions with higher charge states and energies can be produced than without interaction with preformed plasma. The step (spread) in plots of experimental data of ion energy per nucleon versus ILλ2 reported by other researchers most likely reflects high-intensity laser interactions with and without preformed plasma.

Two-stream stability assessment of intense heavy ion beams propagating in a plasma immersed in an axial magnetic field

The stability of intense heavy ion beams propagating in preformed plasmas is being assessed for heavy-ion fusion reactor chamber parameters. A dispersion analysis is used to determine unstable two-stream modes for charge- and current-neutralized heavy ion beams propagating in a cold background plasma. Growth rates are compared directly with 2D particle-in-cell simulations. These simulations are also examined to determine saturation amplitudes of the electron thermal energy as well as any deleterious effects on the ion beam.

Kαyields from Ti foils irradiated with ultrashort laser pulses

We have studied the emission of Kalpha radiation from Ti foils irradiated with ultrashort (45 fs) laser pulses. We utilized the fundamental (800 nm) light from a Ti:sapphire laser on bare foils and foils coated with a thin layer of parylene E (CH). The focusing was varied widely to give a range of intensities from approximately 10(15) - 10(19) W cm(-2). Our results show a conversion efficiency of laser to Kalpha energy of approximately 10(-4) at tight focus for both types of targets. In addition, the coated targets exhibited strong secondary peaks of conversion at large defocus, which we believe are due to modification of the extent of preformed plasma due to the dielectric nature of the plastic layer. This in turn affects the level of resonance absorption. A simple model of Kalpha production predicts a much higher conversion than seen experimentally and possible reasons for this are discussed.

Laser production of highly charged ions

A summary of ion generation experiments at both low (10 9 W=cm 2 ) and high (10 16 W=cm 2 ) laser intensities, using Nd:YAG and iodine lasers, as well as the properties of the ions produced, are presented. Different medium- and high-Z elements were tested as a target: Ag, Al, Au, Co, Cu, Nb, Ni, Pb, Pt, Sn, Ta, W. A maximum charge state 55+ and maximum energy 34 MeV were recorded for Ta ions at high laser intensities, while the charge states up to 10+ and energies lower than 10 keV are characteristic for the region of low limit intensities. At laser intensities above 2£10 14 W=cm 2 conditions for the presence of nonlinear processes are fulfilled and ion acceleration due to ponderomotive force appears, in addition to the thermal and hot electron guided ambipolar acceleration. The existence of two production mechanisms for highly charged ions with charge states above z = 50+ has been demonstrated by changing the minimum focus position with regard to the target surface (by changing the interaction length with pre-formed plasma for relativistic self-focusing). Various potential applications of LIS are mentioned

The bubble regime of laser–plasma acceleration: monoenergetic electrons and the scalability

The bubble regime of electron acceleration in ultra-relativistic laser plasma is considered. It has been shown that the bubble can produce ultra-short dense bunches of electrons with quasi-monoenergetic energy spectra. The first experiment in this regime done at LOA has confirmed the peaked electron spectrum (Faure J et al 2004 Nature at press). The generated electron bunch may have density an order of magnitude higher than that of the background plasma. The bubble is able to guide the laser pulse over many Rayleigh lengths, thus no preformed plasma channel is needed for high-energy particle acceleration in the bubble regime. In this work we discuss a simple analytical model for the bubble fields as well as the scaling laws.

Laser–plasma interaction experiments in the context of inertial fusion

In laser fusion, the coupling and the propagation of the laser beams in the plasma surrounding the pellet must be well controlled for to succeed in producing a high energy level. To achieve thermonuclear ignition and high gain, the coupling efficiency must be as high as possible, the uniformity of the energy deposition must be very good and the fast electron generation must be minimized. This implies a deep understanding of the laser–plasma interaction mechanisms to keep the nonlinear processes at a low level. Important advances in laser–plasma interaction physics have been achieved thanks to the converging efforts of the experimental and theoretical approaches.Among the different studies of the last few years, we will report results on three themes which are important for future fusion experiments. The first concerns the ability of plasmas to induce temporal and spatial incoherence to the laser beams during their propagation. Beam smoothing, beam spraying and increased incoherence may in turn reduce the level of backscattering instabilities.In laser fusion, multiple beams are used to irradiate the target. The effect of the overlap of the laser beams on parametric instabilities may complicate the problem. Not only is there the interplay between instabilities driven by one beam, but also the interplay between instabilities driven by different beams. In the Laboratoire pour l'Utilisation des Lasers Intenses (LULI) experiment, although the overall stimulated Brillouin scattering (SBS) reflectivity was reduced, a well-defined resonance of the amplitude of ion acoustic waves (IAWs) associated with SBS has been observed for waves propagating along the bisecting direction between two laser beams. Energy transfer between two identical laser beams has been observed and correlated with plasma induced incoherence.The nonlinear saturation of stimulated scattering instabilities is a fundamental ingredient of the understanding of the observed and future reflectivity levels. Using Thomson scattering, the decay of the primary IAWs associated with SBS in secondary IAWs has been observed and correlated with the saturation of SBS.The experiments were performed with the six-beam laser facility of the LULI at Ecole Polytechnique, using one or two 1.053 µm interaction beams in the nanosecond regime and a well-characterized preformed plasma. Multiple diagnostics, including Thomson scattering of a probe beam with spatial, temporal and spectral resolution, were used. The comparison between the experimental results and numerical simulations is used to improve the physics included in the codes.

Experiments and Simulations of Short-Pulse Laser-Pumped Extreme Ultraviolet Lasers

Recent experimental work on the development of extreme ultraviolet lasers undertaken using as the pumping source the VULCAN laser at the Rutherford Appleton Laboratory is compared to detailed simulations. It is shown that short duration (/spl sim/picosecond) pumping can produce X-ray laser pulses of a few picosecond duration and that measurement of the emission from the plasma can give an estimate of the duration of the gain coefficient. The Ehybrid fluid and atomic physics code developed at the University of York is used to simulate X-ray laser gain and plasma emission. Two postprocessors to the Ehybrid code are utilized: 1) to raytrace the X-ray laser beam amplification and refraction and 2) to calculate the radiation emission in the kiloelectronvolt photon energy range. The raytracing and spectral simulations are compared, respectively, to measured X-ray laser output and the output of two diagnostics recording transverse X-ray emission. The pumping laser energy absorbed in the plasma is examined by comparing the simulations to experimental results. It is shown that at high pumping irradiance (>10/sup 15/ Wcm/sup -2/), fast electrons are produced by parametric processes in the preformed long scalelength plasmas. These fast electrons do not pump the population inversion and so pumping efficiency is reduced at high irradiance.

High-quality electron beams from a laser wakefield accelerator using plasma-channel guiding.

Laser-driven accelerators, in which particles are accelerated by the electric field of a plasma wave (the wakefield) driven by an intense laser, have demonstrated accelerating electric fields of hundreds of GV m(-1) (refs 1-3). These fields are thousands of times greater than those achievable in conventional radio-frequency accelerators, spurring interest in laser accelerators as compact next-generation sources of energetic electrons and radiation. To date, however, acceleration distances have been severely limited by the lack of a controllable method for extending the propagation distance of the focused laser pulse. The ensuing short acceleration distance results in low-energy beams with 100 per cent electron energy spread, which limits potential applications. Here we demonstrate a laser accelerator that produces electron beams with an energy spread of a few per cent, low emittance and increased energy (more than 10(9) electrons above 80 MeV). Our technique involves the use of a preformed plasma density channel to guide a relativistically intense laser, resulting in a longer propagation distance. The results open the way for compact and tunable high-brightness sources of electrons and radiation.

Femtosecond pump-probe study of preformed plasma channels

We report femtosecond pump–probe experiments in He plasma waveguides using 800-nm, 80-fs pump pulses of 0.2×1018 W/cm2 peak guided intensity and single orthogonally polarized 800-nm probe pulses ~0.1% as intense as the pump. Single-shot spectra and spatial profiles of the probe pulses exiting the channels are measured through a crossed-polarization analyzer at various pump–probe time delays Δt. At |Δt|≳100 fs, we observe frequency-domain interference between the probe and a weak component of the exiting pump created by hybridization of its polarization state through interaction with the channel. Frequency-domain interference measurements show this “depolarized” component differs substantially in mode structure from the injected pump pulse. This component is nearly undetectable by direct measurements of pump leakage without the probe. We analyze possible causes of depolarization within the channel and near its entrance and exit regions. At Δt≲0, i.e., the probe pulse propagates in the leading edge of the pump pulse, we observe spectral blueshifts in the transmitted-probe spectrum that are not evident in the transmitted pump. The evidence indicates that pump depolarization and probe blueshifts both originate primarily near the channel entrance.

Quantitative simulations of short pulse x-ray laser experiments

EHYBRID fluid and atomic physics code simulations are compared with experimentally measured Ne-like nickel x-ray laser output at 23.1 nm and emission from nickel and germanium plasmas produced by line focus irradiation into a pre-formed plasma with 1.2 ps pulses of 1.06 µm wavelength and peak irradiance 7 × 1015 W cm−2. A particle-in-cell (PIC) code is used to simulate the laser energy absorption and resulting electron energy distribution in the experiment. It is shown that some absorbed laser energy is distributed to fast electrons that do not produce collisional excitation of the population inversion.

Self-focusing and channel-coupling effects on short laser pulses propagating in a plasma channel

Nonparaxial, nonlinear propagation of a relativistic laser pulse in a preformed plasma channel having a parabolic density profile is analyzed. Nonlinearity arises due to relativistic motion of plasma electrons and their coupling with plasma channel. The effect of nonlinearities on betatron oscillations is analyzed and the nonlinear critical channel depth required for propagation of a matched laser pulse is evaluated. Mismatch due to nonlinear effects and additional condition for matched pulse propagation are obtained. Amplitude of the matched, as well as mismatched, beam is derived and finite pulse effects are studied.

Integrated implosion/heating studies for advanced fast ignition

Integrated experiments to investigate the ultrafast heating of implosions using cone/shell geometries have been performed at the Rutherford Appleton Laboratory. The experiments used the 1054 nm, nanosecond, 0.9 kJ output of the VULCAN Nd:glass laser to drive 486 μm diameter, 6 μm wall thickness Cu-doped deuterated plastic (CD) shells in 6-beam cubic symmetry. Measurements of the opacity of the compressed plasma using two-dimensional spatially resolved Ti-Kα x-ray radiography suggest that densities of 4 g cm−3 and areal densities of 40 mg cm−2 were achieved at stagnation. Upper limits on the heating with both 1 ps and 10 ps pulses were deduced from the fluorescent yield from the Cu dopant. The data suggest that control of the preformed plasma scale-length inside the cone is necessary for efficient coupling to the compressed plasma.

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.