Wake Of Pulse Research Articles

The dynamic coupling between the pulse wake mixing strategy and floating wind turbines

Abstract. In recent years, control techniques such as dynamic induction control (often referred to as “the pulse”) have shown great potential in increasing wake mixing, with the goal of minimising turbine-to-turbine interaction within a wind farm. Dynamic induction control disturbs the wake by varying the thrust of the turbine over time, which results in a time-varying induction zone. If applied to a floating wind turbine, this time-varying thrust force will, besides changing the wake, change the motion of the platform. In light of the expected movement, this work investigates if applying the pulse to a floating wind turbine yields similar results to that of the pulse applied to bottom-fixed turbines. This is done by considering first the magnitude of motions of the floating wind turbine due to the application of a time-varying thrust force and secondly the effect of these motions on the wake mixing. A frequency response experiment shows that the movement of the floating turbine is heavily frequency dependent, as is the thrust force. Time domain simulations, using a free-wake vortex method with uniform inflow, show that the expected gain in average wind speed at a distance of 5 rotor diameters downstream is more sensitive to the excitation frequency compared to a bottom-fixed turbine with the same pulse applied. This is due to the fact that, at certain frequencies, platform motion decreases the thrust force variation and thus reduces the onset of wake mixing.

Delayed ionization and excitation dynamics in a filament wake channel in a dense-gas medium

A unified theoretical description is developed for the formation of an ionized filament channel in a dense-gas medium and the evolution of electronic degrees of freedom in this channel in the laser pulse wake, as illustrated on an example of high-pressure argon. During the laser pulse, the emerging free electrons gain energy via inverse Bremsstrahlung on neutral atoms, enabling impact ionization and extensive collisional excitation of the atoms. A kinetic model of these processes produces the radial density distributions in the immediate wake of the laser pulse. After the pulse, the thermalized electron gas drives the system evolution via impact ionization (from the ground and excited states) and collisional excitation of the residual neutral atoms, while the excited atoms are engaged in Penning ionization. The interplay of these three processes determines the electron gas cooling dynamics. The local imbalance of the free-electron and ion densities induces a transient radial electric field, which depends critically on the electron temperature. The evolving radial profiles of the electron, ion, and excited-atom densities, as well as the profiles of electron temperature and induced electric field are obtained by solving the system of diffusion-reaction equations numerically. All these characteristics evolve with two characteristic timescales, and allow for measuring the electronic stage of the wake channel evolution via linear and nonlinear light-scattering experiments.

Charge coupling in multi-stage laser wakefield acceleration

The multi-stage technique in the laser driven acceleration of electrons has become a critical part for full-optical jitter-free accelerators. Several independent laser drivers and shorter plasma targets allow the stable and reproducible acceleration of electron bunches (or beam) at the GeV energies with narrower energy spreads. Moreover, the charge coupling, necessary for efficient acceleration in the consecutive acceleration stages, depends collectively on the parameters of the injected electron beam, the booster stage, and the nonlinear transverse dynamics of the electron beam in the laser pulse wake. An unmatched electron beam injected in the booster stage and its nonlinear transverse evolution may result in perturbations and even a reduction of the field strength in the acceleration phase of the wakefield. Thus, the analysis and characterization of charge coupling in multi-stage laser wakefield acceleration (LWFA) are important. Here, we investigate acceleration of an externally injected electron beam in the laser wakefield, emulating two-stage LWFA, using fully relativistic multi-dimensional particle-in-cell simulations. We also study the underlying critical parameters that affect the efficient coupling and acceleration of the injected electron beam in the booster stage.

Characterization of ionization injection in gas mixtures irradiated by subpetawatt class laser pulses

Effects of ionization injection in low and high Z gas mixtures for the laser wake field acceleration of electrons are analyzed with the use of balance equations and particle-in-cell simulations via test probe particle trajectories in realistic plasma fields and direct simulations of charge loading during the ionization process. It is shown that electrons appearing at the maximum of laser pulse field after optical ionization are trapped in the first bucket of the laser pulse wake. Electrons, which are produced by optical field ionization at the front of laser pulse, propagate backwards; some of them are trapped in the second bucket, third bucket and so on. The efficiency of ionization injection is not high, several pC/mm/bucket. This injection becomes competitive with wave breaking injection at lower plasma density and over a rather narrow range of laser pulse intensity.

Coupling Effects in Multistage Laser Wake-field Acceleration of Electrons

Staging laser wake-field acceleration is considered to be a necessary technique for developing full-optical jitter-free high energy electron accelerators. Splitting of the acceleration length into several technical parts and with independent laser drivers allows not only the generation of stable, reproducible acceleration fields but also overcoming the dephasing length while maintaining an overall high acceleration gradient and a compact footprint. Temporal and spatial coupling of pre-accelerated electron bunches for their injection in the acceleration phase of a successive laser pulse wake field is the key part of the staging laser-driven acceleration. Here, characterization of the coupling is performed with a dense, stable, narrow energy band of <3% and energy-selectable electron beams with a charge of ~1.6 pC and energy of ~10 MeV generated from a laser plasma cathode. Cumulative focusing of electron bunches in a low-density preplasma, exhibiting the Budker–Bennett effect, is shown to result in the efficient injection of electrons, even with a long distance between the injector and the booster in the laser pulse wake. The measured characteristics of electron beams modified by the booster wake field agree well with those obtained by multidimensional particle-in-cell simulations.

Intrapulse impact processes in dense-gas femtosecond laser filamentation

The processes of energy gain and redistribution in a dense gas subject to an intense ultrashort laser pulse are investigated theoretically for the case of high-pressure argon. The electrons released via strong-field ionization and driven by an oscillating laser field collide with neutral neighbor atoms, thus effecting the energy gain in the emerging electron gas via a short-range inverse bremsstrahlung interaction. These collisions also cause excitation and impact ionization of the atoms, thus reducing the electron-gas energy. A kinetic model of these competing processes is developed which predicts the prevalence of excited atoms over ionized atoms by the end of the laser pulse. The creation of a significant number of excited atoms during the pulse in high-pressure gases is consistent with the delayed ionization dynamics in the pulse wake, recently reported by Gao et al. [X. H. Gao et al., Phys. Rev. A 95, 013412 (2017)]. This energy redistribution mechanism offers an approach to manage effectively the excitation vs ionization patterns in dense gases interacting with intense laser pulses and thus opens avenues for diagnostics and control in these settings.

Effect of halo on high power laser pulse wake in underdense plasma

Strong disturbance in the wake of the laser pulses propagating in underdense plasma and consequent unstable electron acceleration by the wakefield can be provoked by pulse's halo, which always exists as a result of an imperfect optical focusing. When the power in the halo part exceeds a critical level for the self-focusing, it evolves in the plasma as an independent mode, which later gets coupled with the propagation of the central Gaussian spot of the pulse resulting in a novel instability. Here, this instability is investigated numerically via fully relativistic 3D particle-in-cell simulations and is shown to be partially suppressed by using plasma channels for pulse guiding.

Breaking symmetry in propagation of radially and azimuthally polarized high power laser pulses in underdense plasma

Propagation of relativistically intense azimuthally or radially polarized laser pulses (RPP) is demonstrated, via 3D particle-in-cell simulations, to be unstable in uniform underdense plasma. Strong breaking of the pulse symmetry occurs for RPP with power exceeding the critical one for self-focusing in transversely uniform plasma with an increment, Γ, close to the well-known Rayleigh-Taylor-like instability depending on the acceleration, α, and the modulated density gradient length, L, as Γ≈(α/L)1/2. In deeper plasma channels, the instability vanishes. Electron self-injection in the pulse wake and resulting acceleration is explored.

Transient magnetized plasma as an optical element for high power laser pulses

Underdense plasma produced in gas jets by low intensity laser prepulses in the presence of a static magnetic field, $B\ensuremath{\sim}0.3\text{ }\text{ }\mathrm{T}$, is shown experimentally to become an optical element allowing steering of tightly focused high power femtosecond laser pulses within several degrees along with essential enhancement of pulse's focusability. Strong laser prepulses form a density ramp perpendicularly to magnetic field direction and, owing to the light refraction, main laser pulses propagate along the magnetic field even if it is tilted from the laser axis. Electrons generated in the laser pulse wake are well collimated and follow in the direction of the magnetic field; their characteristics are measured to be not sensitive to the tilt of magnetic field up to angles $\ifmmode\pm\else\textpm\fi{}5\ifmmode^\circ\else\textdegree\fi{}$.

Generation of hard x rays by femtosecond laser pulse interaction with solid targets in atmosphere

X ray radiation as high as 50 keV, including K(α) of Ba and Mo, have been observed from a solid target during the interaction of low energy ~0.65 mJ, 1 kHz 40 femtosecond laser pulses focused in air at atmospheric pressure. Energetic electrons generating such x rays are possibly produced when the field strength in laser pulse wake exceeds the runaway threshold in air. Two dimensional particle-in-cell simulations that include optical field ionization of air and elastic collisions support this mechanism.

Electron self-injection during interaction of tightly focused few-cycle laser pulses with underdense plasma

We study the interaction of short laser pulses tightly focused in a tiny volume proportional to the cube of the pulse wavelength (lambda3) with underdense plasma by means of real-geometry particle-in-cell simulations. Underdense plasma irradiated by relatively low-energy lambda3 (and lambda2) laser pulses is shown to be an efficient source of multi-MeV electrons, approximately 50 nC/J , and coherent hard x rays, despite a strong pulse diffraction. Transverse wave breaking in the vicinity of the laser focus is found to give rise to an immense electron charge loading to the acceleration phase of a laser wake field. A strong blowout regime provoked by the injected electrons resulting in the distribution of accelerated electrons is found for lambda3 pulses (further electron acceleration driving by lambda2 pulses runs in the usual way). With an increase of pulse energy, wiggling and electron-hose instabilities in the lambda3 pulse wake are recognized in the blowout regime. For higher-energy lambda3 pulses, the injected beams are well modulated and may serve as a good source of coherent x rays.

Giant electromagnetic vortex and MeV monoenergetic electrons generated by short laser pulses in underdense plasma near quarter critical density region

Very efficient generation of monoenergetic, about 1MeV , electrons from underdense plasma with its electron density close to the critical, when irradiated by an intense femtosecond laser pulse, is found via two dimensional particle-in-cell simulation. The stimulated Raman scattering of a laser pulse with frequency omega< or =2omega(pl max) gives rise to a giant electromagnetic vortex. In contrast to electron acceleration by the well-known laser pulse wake, injected plasma electrons are accelerated up to vortex ponderomotive potential forming a quite monoenergetic distribution. A relatively high charge of such an electron source makes very efficient generation of soft gamma rays with homega>300 keV .

Effects of density gradient on short-bunch injection by wave breaking in the laser wake field acceleration

Effects of density gradient on the self-injection of plasma electrons in the phase of laser pulse wake for further acceleration, is studied for moderate laser intensities, a0⩽3. It is shown that transverse wave breaking can shorten the length of accelerated electrons, whereas effective longitudinal wave breaking requiring steep plasma density interface increases their total charge. For the considered range of laser intensities, the total charge of electrons injected by wave breaking rises exponentially with a0.

Plasma Ion Evolution in the Wake of a High-Intensity Ultrashort Laser Pulse

Experimental investigations of the late-time ion structures formed in the wake of an ultrashort, intense laser pulse propagating in a tenuous plasma have been performed using the proton imaging technique. The pattern found in the wake of the laser pulse shows unexpectedly regular modulations inside a long, finite width channel. On the basis of extensive particle in cell simulations of the plasma evolution in the wake of the pulse, we interpret this pattern as due to ion modulations developed during a two-stream instability excited by the return electric current generated by the wakefield.