Overcritical Plasma Research Articles

Numerical study of breakdown pattern induced by an intense microwave under nitrogen and argon gases

Filamentary plasma induced by microwave beam irradiation was reproduced in nitrogen and argon by combining fluid or particle plasma models with electromagnetic wave propagation. Transport coefficients used in the fluid model are estimated from particle simulation to maintain consistency of the breakdown structure between the fluid and particle models. A discrete structure was obtained using the one-dimensional (1D) fluid model, because a standing wave is generated in front of the plasma when the incident microwave beam is reflected by the overcritical plasma, which agrees with the breakdown structure obtained using the 1D particle model. A 2D plasma filament was also reproduced using the fluid model in nitrogen and argon. Reflection of the incident microwave in argon becomes stronger than that in nitrogen because of the denser argon plasma. Change in filament shape is induced in argon because the electric field is deformed at the plasma tip owing to stronger wave reflection from the neighboring filament. The propagation speed of the plasma front becomes larger in argon breakdown because of the larger ionization frequency and the larger diffusion coefficient.

Control of transmission of right circularly polarized laser light in overdense plasma by applied magnetic field pulses.

The effect of a transient magnetic field on right-hand circularly polarized (RHCP) laser light propagation in overcritical-density plasma is investigated. When the electron gyrofrequency is larger than the wave frequency, RHCP light can propagate along the external magnetic field in an overcritical density plasma without resonance or cutoff. However, when the magnetic field falls to below the cyclotron resonance point, the propagating laser pulse will be truncated and the local plasma electrons resonantly heated. Particle-in-cell simulation shows that when applied to a thin slab, the process can produce intense two-cycle light pulses as well as long-lasting self-magnetic fields.

Radiation emission from ultra-relativistic plasma electrons in short-intense laser light interactions

Intense femtosecond laser light incident on overcritical density plasmas has shown to emit a prolific number of high-order harmonics of the driver frequency, with spectra characterised by power-law decays. When the laser pulse is p-polarised, plasma effects do modify the harmonic spectrum, weakening the so-called universal decay index p = 8/3 to 5/3. In this work appeal is made to a single particle radiation model in support of the predictions from particle-in-cell (PIC) simulations. Using these, we further show that the emission radiated by electrons -those that are relativistically accelerated inside the plasma, after being expelled into vacuum, the so-called Brunel electrons- is characterised not only by the plasma line but also by ultraviolet harmonic orders characterised by the 5/3 decay index.

Overcritical plasma ignition and diagnostics from oncoming interaction of two color low energy tightly focused femtosecond laser pulses inside fused silica

We report overcritical (3.3 × 1021 cm−3) microplasma produced by low energy colliding IR (infrared) (1.24 μm) and visible (0.62 μm) femtosecond pulses tightly focused (NA = 0.5) into the bulk of fused silica with on-line monitoring based on third harmonic generated by the IR beam. It was established that the absorbed energy density is the key parameter that determines the micromodification formation threshold and in our experimental conditions it is close to 4.5 kJ cm−3. Non-monotonic behavior of the third harmonic signal as a function of time delay between visible (0.62 μm) and IR (1.24 μm) femtosecond pulses demonstrates the qualitative differences about the two phenomena: one is the seed electrons generation by the visible pulse via multiphoton ionization and second is the avalanche ionization by the IR pulse. We predict that the tandem two-color excitation of wide-bandgap dielectric in comparison with single-color pulse interaction regime allows providing a much higher absorbed energy density and overcritical plasma.

Extending laser plasma accelerators into the mid-IR spectral domain with a next-generation ultra-fast CO2 laser

Expanding the scope of relativistic plasma research to wavelengths longer than the λ/≈ 0.8–1.1 μm range covered by conventional mode-locked solid-state lasers would offer attractive opportunities due to the quadratic scaling of the ponderomotive electron energy and critical plasma density with λ. Answering this quest, a next-generation mid-IR laser project is being advanced at the BNL ATF as a part of the user facility upgrade. We discuss the technical approach to this conceptually new 100 TW, 100 fs, λ = 9–11 μm CO2 laser BESTIA (Brookhaven Experimental Supra-Terawatt Infrared at ATF) that encompasses several innovations applied for the first time to molecular gas lasers.BESTIA will enable new regimes of laser plasma accelerators. One example is shock-wave ion acceleration (SWA) from gas jets. We review ongoing efforts to achieve stable, monoenergetic proton acceleration by dynamically shaping the plasma density profile from a hydrogen gas target with laser-produced blast waves. At its full power, 100 TW BESTIA promises to achieve proton beams at an energy exceeding 200 MeV.In addition to ion acceleration in over-critical plasma, the ultra-intense mid-IR BESTIA will open up new opportunities in driving wakefields in tenuous plasmas, expanding the landscape of laser wakefield accelerator (LWFA) studies into the unexplored long-wavelength spectral domain. Simple wavelength scaling suggests that a 100 TW CO2 laser beam will be capable of efficiently generating plasma ‘bubbles’ a thousand times greater in volume compared with a near-IR solid state laser of an equivalent power. Combined with a femtosecond electron linac available at the ATF, this wavelength scaling will facilitate the study of external seeding and staging of LWFAs.

Trapping of intense light in hollow shell

A small hollow shell for trapping laser light is proposed. Two-dimensional particle-in-cell simulation shows that under appropriate laser and plasma conditions a part of the radiation fields of an intense short laser pulse can enter the cavity of a small shell through an over-critical density plasma in an adjacent guide channel and become trapped. The trapped light evolves into a circulating radial wave pattern until its energy is dissipated.

Modeling relativistic soliton interactions in overdense plasmas: a perturbed nonlinear Schrödinger equation framework.

We investigate the dynamics of localized solutions of the relativistic cold-fluid plasma model in the small but finite amplitude limit, for slightly overcritical plasma density. Adopting a multiple scale analysis, we derive a perturbed nonlinear Schrödinger equation that describes the evolution of the envelope of circularly polarized electromagnetic field. Retaining terms up to fifth order in the small perturbation parameter, we derive a self-consistent framework for the description of the plasma response in the presence of localized electromagnetic field. The formalism is applied to standing electromagnetic soliton interactions and the results are validated by simulations of the full cold-fluid model. To lowest order, a cubic nonlinear Schrödinger equation with a focusing nonlinearity is recovered. Classical quasiparticle theory is used to obtain analytical estimates for the collision time and minimum distance of approach between solitons. For larger soliton amplitudes the inclusion of the fifth-order terms is essential for a qualitatively correct description of soliton interactions. The defocusing quintic nonlinearity leads to inelastic soliton collisions, while bound states of solitons do not persist under perturbations in the initial phase or amplitude.

Dynamics of femtosecond laser absorption of fused silica in the ablation regime

We investigate the ultrafast absorption dynamics of fused silica irradiated by a single 500 fs laser pulse in the context of micromachining applications. A 60-fs-resolution pump-probe experiment that measures the reflectivity and transmissivity of the target under excitation is developed to reveal the evolution of plasma absorption. Above the ablation threshold, an overcritical plasma with highly non-equilibrium conditions is evidenced in a thin layer at the surface. The maximum electron density is reached at a delay of 0.5 ps after the peak of the pump pulse, which is a strong indication of the occurrence of electronic avalanche. The results are further analyzed to determine the actual feedback of the evolution of the optical properties of the material on the pump pulse. We introduce an important new quantity, namely, the duration of absorption of the laser by the created plasma, corresponding to the actual timespan of laser absorption by inverse Bremsstrahlung. Our results indicate an increasing contribution of plasma absorption to the total material absorption upon raising the excitation fluence above the ablation threshold. The role of transient optical properties during the energy deposition stage is characterized and our results emphasize the necessity to take it into account for better understanding and control of femtosecond laser-dielectrics interaction.

Measuring the strong electrostatic and magnetic fields with proton radiography for ultra-high intensity laser channeling on fast ignition.

In order to investigate the intense laser propagation and channel formation in dense plasma, we conducted an experiment with proton deflectometry on the OMEGA EP Laser facility. The proton image was analyzed by tracing the trajectory of mono-energetic protons, which provides understanding the electric and magnetic fields that were generated around the channel. The estimated field strengths (E ∼ 10(11) V/m and B ∼ 10(8) G) agree with the predictions from 2D-Particle-in-cell (PIC) simulations, indicating the feasibility of the proton deflectometry technique for over-critical density plasma.

Resonant Conversion of the Point-Source Field to Multipole Radiation Using Ring Layer of the Overcritical Plasma

Resonant Conversion of the Point-Source Field to Multipole Radiation Using Ring Layer of the Overcritical Plasma

Effect of electron heating on self-induced transparency in relativistic-intensity laser-plasma interactions

The effective increase of the critical density associated with the interaction of relativistically intense laser pulses with overcritical plasmas, known as self-induced transparency, is revisited for the case of circular polarization. A comparison of particle-in-cell simulations to the predictions of a relativistic cold-fluid model for the transparency threshold demonstrates that kinetic effects, such as electron heating, can lead to a substantial increase of the effective critical density compared to cold-fluid theory. These results are interpreted by a study of separatrices in the single-electron phase space corresponding to dynamics in the stationary fields predicted by the cold-fluid model. It is shown that perturbations due to electron heating exceeding a certain finite threshold can force electrons to escape into the vacuum, leading to laser pulse propagation. The modification of the transparency threshold is linked to the temporal pulse profile, through its effect on electron heating.

On the feasibility of electron cyclotron heating of overcritical plasma in a magnetic mirror trap

The feasibility of matching electromagnetic radiation in the electron cyclotron frequency range to a dense plasma in an open magnetic trap by producing an inverted (with a minimum on the axis) plasma density profile is discussed. The use of such a profile shows promise for the implementation of efficient cyclotron heating at plasma densities above the critical density, at which the Langmuir frequency is equal to the heating radiation frequency. Examples of the magnetic field and plasma density distributions in a mirror trap are presented for which analysis of the beam trajectories shows the feasibility of efficient electron cyclotron absorption of microwave beams in overcritical plasma.

Adiabatic nonlinear waves with trapped particles. III. Wave dynamics

The evolution of adiabatic waves with autoresonant trapped particles is described within the Lagrangian model developed in Paper I, under the assumption that the action distribution of these particles is conserved, and, in particular, that their number within each wavelength is a fixed independent parameter of the problem. One-dimensional nonlinear Langmuir waves with deeply trapped electrons are addressed as a paradigmatic example. For a stationary wave, tunneling into overcritical plasma is explained from the standpoint of the action conservation theorem. For a nonstationary wave, qualitatively different regimes are realized depending on the initial parameter S, which is the ratio of the energy flux carried by trapped particles to that carried by passing particles. At S < 1/2, a wave is stable and exhibits group velocity splitting. At S > 1/2, the trapped-particle modulational instability (TPMI) develops, in contrast with the existing theories of the TPMI yet in agreement with the general sideband instability theory. Remarkably, these effects are not captured by the nonlinear Schrödinger equation, which is traditionally considered as a universal model of wave self-action but misses the trapped-particle oscillation-center inertia.

Laser Shaping of a Relativistic Intense, Short Gaussian Pulse by a Plasma Lens

By 3D particle-in-cell simulation and analysis, we propose a plasma lens to make high intensity, high contrast laser pulses with a steep front. When an intense, short Gaussian laser pulse of circular polarization propagates in near-critical plasma, it drives strong currents of relativistic electrons which magnetize the plasma. Three pulse shaping effects are synchronously observed when the laser passes through the plasma lens. The laser intensity is increased by more than 1 order of magnitude while the initial Gaussian profile undergoes self-modulation longitudinally and develops a steep front. Meanwhile, a nonrelativistic prepulse can be absorbed by the overcritical plasma lens, which can improve the laser contrast without affecting laser shaping of the main pulse. If the plasma skin length is properly chosen and kept fixed, the plasma lens can be used for varied laser intensity above 10(19) W/cm(2).

Transmission of electromagnetic waves through a warm over-dense plasma layer with a dissipative factor

The high transparency condition of an overcritical warm plasma layer due to the excitation of electromagnetic surface modes was studied. This procedure requires evanescent incident waves on plasma layers, which is prepared here by placing two dielectric layers on both sides of a plasma film. The consequences for a cold plasma are also discussed and compared. Additionally, the transmission losses due to thermal effects and collisions in the considered structure are studied.

Laser-induced cavities and solitons in overcritical hydrogen plasma

A picosecond CO2 laser was used successfully in a number of experiments exploring advanced methods of particle acceleration [1]. Proton acceleration from gas-jet plasma exemplifies another advantage of employing the increase in laser wavelength from the optical to the mid-IR region. Recent theoretical- and experimental-studies of ion acceleration from laser-generated plasma point to better ways to control the ion beam’s energy when plasma approaches the critical density. Studying this regime with solid-state lasers is problematic due to the dearth of plasma sources at the critical electron density ∼1021 cm−3, corresponding to laser wavelength λ = 1 μm. CO2 laser offers a solution. The CO2 laser’s 10 μm wavelength shifts the critical plasma density to 1019 cm−3, a value attainable with gas jets. Capitalizing on this approach, we focused a circular polarized 1-TW CO2 laser beam onto a hydrogen gas jet and observed a monoenergetic proton beam in the 1–2 MeV range. Simultaneously, we optically probed the laser/plasma interaction region with visible light, revealing holes bored by radiation pressure, as well as quasi-stationary soliton-like plasma formations. Our findings from 2D PIC simulations agree with experimental results and aid in their interpretation.

On the Possibility of ECR-Discharge with Overcritical Plasma Density in Axisymmetrical Magnetic Trap

New approach to realizing high plasma parameters in electron cyclotron resonance discharge supported by microwave radiation in axisymmetric magnetic trap has been proposed. This approach is based on using the inverted radial distribution of plasma density (with minimum at the trap axis). Such plasma density distributions allow efficient cyclotron heating and sustaining the steady-state discharge with high plasma density including densities above the critical value (corresponding to the density for which plasma frequency equals to the frequency of heating radiation). Simulations of power deposition for EC heating of plasma with overcritical density in axisymmetric mirror magnetic trap are presented, in which the effect of increasing the heating efficiency for inverted radial plasma density profiles has been demonstrated.

Particle-in-cell modeling of relativistic laser–plasma interaction with the adjustable-damping, direct implicit method

Implicit particle-in-cell codes offer advantages over their explicit counterparts in that they suffer weaker stability constraints on the need to resolve the higher frequency modes of the system. This feature may prove particularly valuable for modeling the interaction of high-intensity laser pulses with overcritical plasmas, in the case where the electrostatic modes in the denser regions are of negligible influence on the physical processes under study. To this goal, we have developed the new two-dimensional electromagnetic code ELIXIRS (standing for ELectromagnetic Implicit X-dimensional Iterative Relativistic Solver) based on the relativistic extension of the so-called Direct Implicit Method [D. Hewett, A.B. Langdon, Electromagnetic direct implicit plasma simulation, J. Comput. Phys. 72 (1987) 121–155]. Dissipation-free propagation of light waves into vacuum is achieved by an adjustable-damping electromagnetic solver. In the high-density case where the Debye length is not resolved, satisfactory energy conservation is ensured by the use of high-order weight factors. In this paper, we first derive the electromagnetic direct implicit method as a simplified Newton scheme. Its linear properties are then investigated through numerically solving the relation dispersions obtained for both light and plasma waves, accounting for finite space and time steps. Finally, our code is successfully benchmarked against explicit particle-in-cell simulations for two kinds of physical problems: plasma expansion into vacuum and relativistic laser–plasma interaction. In both cases, we will demonstrate the robustness of the implicit solver for crude discretizations, as well as the gains in efficiency which can be realized over standard explicit simulations.

Electrode microwave discharge: Areas of application and recent results of discharge physics

The first paper on the electrode microwave discharge (EMD) appeared in 1996. Presently many problems of EMD physics and applications have already been solved. Several examples of EMD application are discussed: diamond growth, deposition of CNx films and nanotubes, deposition of metal films (Cu, Al), deposition of TiN and TiO2 films, generation of O2(a1Δ), and EMD as a plasma cathode. Results of EMD experiments and modeling give rise to the assumption that an EMD consists of a self-sustained domain (near-electrode plasma region with overcritical plasma density) which is surrounded by a region of a non-self-sustained discharge (ball shaped region with undercritical plasma density). We assumed that the layer of charge separation and of induced electrostatic field originated at the outer EMD boundary was one of the reasons for the abrupt decrease of the plasma density which leads to the formation of a compact plasma structure. Recent modeling results of the strongly nonuniform electrode microwave plasma based on a quasi static, 1D spherically symmetric model showed that such a layer can be generated at the point where a sudden increase of the total ionization rate takes place.

High harmonics from solid surfaces as a source of ultra-bright XUV radiation for experiments

The coherent high-order harmonic generation from the interaction of ultra-intense femtosecond laser pulses with solid density plasmas holds promise for tabletop sources of extreme ultraviolet (XUV) and soft x-ray radiation with attosecond duration and unprecedented intensities. Together with the generation of mono-energetic electron beams from gas jets and capillaries and the generation of mono-energetic ions from thin foils, this offers a unique tool box of tabletop-laser-generated radiation sources for a wide range of applications previously only accessible with large-scale accelerator and synchrotron-radiation facilities. Especially, the generation of high harmonics from laser plasmas has the potential of being applied to a wide range of experiments from plasma physics to molecular dynamics. So far the studies addressing the generation of high harmonics from laser-generated overcritical plasma surfaces have concentrated mainly on the characterization of the harmonic beams themselves not considering how, in a next step, these beams could be applied to experiments. In this paper we discuss the generation of surface harmonics with the ATLAS (800 mJ, 40 fs) laser system with the emphasis on the transport, spectral shaping refocusing of the harmonic beams, all of these being absolute prerequisites for multi-shot experiments. We also present considerations for future improvements and possible future experiments exploiting the full potential of high harmonic radiation from solid targets.