Acceleration Regime Research Articles

Laser Technology for Advanced Acceleration: Accelerating Beyond TeV

The implementation of the suggestion of thin film compression (TFC) allows the newest class of high power, ultrafast laser pulses (typically 20[Formula: see text]fs at near-infrared wavelengths) to be compressed to the limit of a single-cycle laser pulse (2[Formula: see text]fs). Its simplicity and high efficiency, as well as its accessibility to a single-cycle laser pulse, introduce a new regime of laser–plasma interaction that enhances laser acceleration. Single-cycle laser acceleration of ions is a far more efficient and coherent process than the known laser-ion acceleration mechanisms. The TFC-derived single-cycle optical pulse is capable of inducing a single-cycle X-ray laser pulse (with a far shorter pulse length and thus an extremely high intensity) through relativistic compression. The application of such an X-ray pulse leads to the novel regime of laser wakefield acceleration of electrons in the X-ray regime, yielding a prospect of “TeV on a chip.” This possibility of single-cycle X-ray pulses heralds zeptosecond and EW lasers (and zeptoscience). The additional invention of the coherent amplification network (CAN) fiber laser pushes the frontier of high repetition, high efficiency lasers, which are the hallmark of needed applications such as laser-driven LWFA colliders and other, societal applications. CAN addresses the crucial aspect of intense lasers that have traditionally lacked the above properties.

Collimated proton beams by ultra-short, ultra-intense laser pulse interaction with a foil–ramparts target

AbstractA foil–ramparts target interaction with an ultra-short, ultra-intense laser pulse (pulse duration between 10−12 and 10−15 s, intensity above 1018 W cm−2) to produce proton beams with controlled divergence and concentrated energy density in target normal sheath acceleration regime is studied. Two-dimension-in-space and three-dimension-in-velocity particle-in-cell simulations show that the foil–ramparts target helps to reshape the sheath electric field and generate a transverse quasi-static electric field of ~6.7 TV m−1 along the inner wall of the ramparts. The transverse electric field suppresses the transverse expansion of the proton beam effectively, as it tends to force the produced protons to focus inwards to the central axis, resulting in controlled divergence and concentrated energy density compared with that of a single plain target. The dependence of proton beam divergence on length of the rampart h is investigated in detail. A rough estimation of h ranges depending on dimensionless parameter a0 of the incident laser is also given.

High field plasmonics and laser-plasma acceleration in solid targets

The interaction of low intensity laser pulses with metal nano-structures is at the basis of plasmonics and the excitation of surface plasmon polaritons (SP) is one of its building blocks. Some of the configurations adopted in classical plasmonics can be explored considering high intensity lasers interacting with properly structured targets. SP excitation at intensities such that the electrons quiver at relativistic velocities, poses new questions and might open new frontiers for manipulation and amplification of high power laser pulses. Here we discuss two configurations which show evidence of the resonant coupling between relativistically intense laser pulses with the SPs on plasma targets with surface modulations. Evidences of SP excitation were observed in a recent experiment when a high contrast (1012), high intensity laser pulse ( W cm−2) was focussed on a grating target (engraved surface at sub-micron scale); a strong emission of multi-MeV electron bunches accelerated by SPs was observed only in conditions for the resonant SP excitation. Theoretical and numerical analysis of the Light-Sail (LS) Radiation Pressure Acceleration (RPA) regime show how the plasmonic resonant coupling of the laser light with the target rippling, affects the growth of Rayleigh Taylor Instability (RTI) driven by the radiation pressure.

A model experiment to understand the oral phase of swallowing of Newtonian liquids

A model experiment to understand the oral phase of swallowing is presented and used to explain some of the mechanisms controlling the swallowing of Newtonian liquids. The extent to which the flow is slowed down by increasing the viscosity of the liquid or the volume is quantitatively studied. The effect of the force used to swallow and of the gap between the palate and the roller used to represent the contracted tongue are also quantified. The residual mass of liquid left after the model swallow rises strongly when increasing the gap and is independent of bolus volume and applied force. An excessively high viscosity results in higher residues, besides succeeding in slowing down the bolus flow. A realistic theory is developed and used to interpret the experimental observations, highlighting the existence of an initial transient regime, at constant acceleration, that can be followed by a steady viscous regime, at constant velocity. The effect of the liquid viscosity on the total oral transit time is lower when the constant acceleration regime dominates bolus flow. Our theory suggests also that tongue inertia is the cause of the higher pressure observed at the back of the tongue in previous studies.The approach presented in this study paves the way toward a mechanical model of human swallowing that would facilitate the design of novel, physically sound, dysphagia treatments and their preliminary screening before in vivo evaluations and clinical trials.

Shock ion acceleration by an ultrashort circularly polarized laser pulse via relativistic transparency in an exploded target.

We investigated ion acceleration by an electrostatic shock in an exploded target irradiated by an ultrashort, circularly polarized laser pulse by means of one- and three-dimensional particle-in-cell simulations. We discovered that the laser field penetrating via relativistic transparency (RT) rapidly heated the upstream electron plasma to enable the formation of a high-speed electrostatic shock. Owing to the RT-based rapid heating and the fast compression of the initial density spike by a circularly polarized pulse, a new regime of the shock ion acceleration driven by an ultrashort (20-40 fs), moderately intense (1-1.4 PW) laser pulse is envisaged. This regime enables more efficient shock ion acceleration under a limited total pulse energy than a linearly polarized pulse with crystal laser systems of λ∼1μm.

Nanostructured target fabrication with metal and semiconductor nanoparticles

The development of ultra-intense high-energy (≫1 J) short (<1 ps) laser pulses in the last decade has enabled the acceleration of high-energy short-pulse proton beams. A key parameter for enhancing the acceleration regime is the laser-to-target absorption, which heavily depends on the target structure and material. In this work, we present the realization of a nanostructured target with a sub-laser wavelength nano-layer in the front surface as a possible candidate for improving the absorption. The nanostructured film was realized by a simpler and cheaper method than using conventional lithographic techniques: A colloidal solution of metallic or semiconductor nanoparticles (NPs) was produced by laser ablation and, after a heating and sonication process, was spray-dried on the front surface of an aluminum target. The obtained nanostructured film with a thickness of 1 μm appears, at morphological and chemical analysis, uniformly nanostructured and distributed on the target surface without the presence of oxides or external contaminants. Finally, the size of the NPs can be tuned from tens to hundreds of nanometers simply by varying the growth parameters (i.e., irradiation time, fluence, and laser beam energy).

Escape forces and trajectories in optical tweezers and their effect on calibration

Whether or not an external force can make a trapped particle escape from optical tweezers can be used to measure optical forces. Combined with the linear dependence of optical forces on trapping power, a quantitative measurement of the force can be obtained. For this measurement, the particle is at the edge of the trap, away from the region near the equilbrium position where the trap can be described as a linear spring. This method provides the ability to measure higher forces for the same beam power, compared with using the linear region of the trap, with lower risk of optical damage to trapped specimens. Calibration is typically performed by using an increasing fluid flow to exert an increasing force on a trapped particle until it escapes. In this calibration technique, the particle is usually assumed to escape along a straight line in the direction of fluid-flow. Here, we show that the particle instead follows a curved trajectory, which depends on the rate of application of the force (i.e., the acceleration of the fluid flow). In the limit of very low acceleration, the particle follows the surface of zero axial optical force during the escape. The force required to produce escape depends on the trajectory, and hence the acceleration. This can result in variations in the escape force of a factor of two. This can have a major impact on calibration to determine the escape force efficiency. Even when calibration measurements are all performed in the low acceleration regime, variations in the escape force efficiency of 20% or more can still occur. We present computational simulations using generalized Lorenz-Mie theory and experimental measurements to show how the escape force efficiency depends on rate of increase of force and trapping power, and discuss the impact on calibration.

Estimate of the inertial torque in rotating shafts - a metrological approach to signal processing

This article studies the setting of the method to model the inertial torque in the acceleration regimes of a rotating axis system. The angular speed measurement to obtain an estimated acceleration and the moment of inertia axis of the variables are used to calculate the inertial torque. The evaluated dynamic regimes are acceleration ramps between levels (intervals) of angular speed, which are implemented using variable speed drives that are coupled to AC motors. The article focuses on assessing the main parameters in signal processing, such as the derivation of the speed signal, application of digital filters and data- smoothing methods, and their effects on the main features of interest in the generated torque curves, such as the peak torque amplitude, ascending and downward slopes, stability and repeatability. A qualitative approach was performed to identify the measurement uncertainty contributions. Thus, the validation methodology of the processing methods and the proposed physical principle are feasible.

Bus Following Model: A Case Study in Edinburgh

A number of car following models have been developed and calibrated for the purpose of understanding and simulating the behaviour of driving under various traffic and transportation management systems. Such studies are useful for safety impact studies, network analysis and capacity analysis. However bus following is a special behaviour, which impacts on other traffic especially in the absence of dedicated bus lanes. This paper reports on a study in the City of Edinburgh which was undertaken to investigate and understand the behaviour of buses in the city. A commuter bus equipped with a data logging device was followed by an instrumented vehicle to collect data to study bus following behaviour. A model of distance difference as a function of the acceleration/deceleration, speed and speed difference has been developed for bus following behaviour. The GHR model was used to investigate and calibrate the parameters of the model of the car following a bus. The study findings show that the speed and acceleration of the following vehicle are influenced by the leading commuter bus in both acceleration and deceleration regimes. The results also show that a car following a bus behaves differently to a car following a car. This is because of the limited visibility, huge delays for buses on the corridor and the different driving characteristics of the bus with respect to the car.

The effect of ego-motion on environmental monitoring

Air pollution has a proven impact on public health. Currently, pollutant levels are obtained by high-priced, sizeable, stationary Air Quality Monitoring (AQM) stations. Recent developments in sensory and communication technologies have made relatively low-cost, micro-sensing units (MSUs) feasible. Their lower power consumption and small size enable mobile sensing, deploying single or multiple units simultaneously. Recent studies have reported on measurements acquired by mobile MSUs, mounted on cars, bicycles and pedestrians. While these modes of transportation inherently present different velocity and acceleration regimes, the effect of the sensors' varying movement characteristics have not been previously accounted for.This research assesses the impact of sensor's motion on its functionality through laboratory measurements and a field campaign. The laboratory setup consists of a wind tunnel to assess the effect of air flow on the measurements of nitrogen dioxide and ozone at different velocities in a controlled environment, while the field campaign is based on three cars mounted with MSUs, measuring pollutants and environmental variables at different traveling speeds. In both experimental designs we can regard the MSUs as a moving object in the environment, i.e. having a distinct ego-motion.The results show that MSU's behavior is highly affected by variation in speed and sensor placement with respect to direction of movement, mainly due to the physical properties of installed sensors. This strongly suggests that any future design of MSU must account for the speed effect from the design stage all the way through deployment and results analysis. This is the first report examining the influence of airflow variations on MSU's ability to accurately measure pollutant levels.

Control of the dynamic interaction of a “magnetized” sphere with a hypersonic flow of rarefied plasma

Parametric analysis has been performed, and dependences of the “magnetic” components coefficients of the drag force and lifting force of a “magnetized” sphere in a hypersonic flow of rarefied plasma for the angle between the flow-velocity vector and the induction vector of the intrinsic magnetic field of the body, as well as on the ratio of the magnetic pressure to the gas-dynamic pressure of the plasma flow have been obtained. It is shown that the change in the mutual orientation of the vectors of the magnetic field of the body and the incoming flow velocity is an efficient means of controlling the dynamic interaction in the plasma—sphere system and makes it possible to implement the interaction mode with nonzero aerodynamic quality and the regimes of deceleration and acceleration of the “magnetized” sphere in a hypersonic flow of rarefied plasma.

Simple scalings for various regimes of electron acceleration in surface plasma waves

Different electron acceleration regimes in the evanescent field of a surface plasma wave are studied by considering the interaction of a test electron with the high-frequency electromagnetic field of a surface wave. The non-relativistic and relativistic limits are investigated. Simple scalings are found demonstrating the possibility to achieve an efficient conversion of the surface wave field energy into electron kinetic energy. This mechanism of electron acceleration can provide a high-frequency pulsed source of relativistic electrons with a well defined energy. In the relativistic limit, the most energetic electrons are obtained in the so-called electromagnetic regime for surface waves. In this regime, the particles are accelerated to velocities larger than the wave phase velocity, mainly in the direction parallel to the plasma-vacuum interface.

Enhancement of proton acceleration by frequency-chirped laser pulse in radiation pressure mechanism

The transition from hole-boring to light-sail regime of radiation pressure acceleration by frequency-chirped laser pulses is studied using particle-in-cell simulation. The penetration depth of laser into the plasma with ramped density profile increases when a negatively chirped laser pulse is applied. Because of this induced transparency, the laser reflection layer moves deeper into the target and the hole-boring stage would smoothly transit into the light-sail stage. An optimum chirp parameter which satisfies the laser transparency condition, a0≈πnel/ncλ, is obtained for each ramp scale length. Moreover, the efficiency of conversion of laser energy into the kinetic energy of particles is maximized at the obtained optimum condition. A relatively narrow proton energy spectrum with peak enhancement by a factor of 2 is achieved using a negatively chirped pulse compared with the un-chirped pulse.

Ion acceleration from intense laser-generated plasma: methods, diagnostics and possible applications

Abstract Many parameters of non-equilibrium plasma generated by high intensity and fast lasers depend on the pulse intensity and the laser wavelength. In conditions favourable for the target normal sheath acceleration (TNSA) regime the ion acceleration from the rear side of the target can be enhanced by increasing the thin foil absorbance through the use of nanoparticles and nanostructures promoting the surface plasmon resonance effect. In conditions favourable for the backward plasma acceleration (BPA) regime, when thick targets are used, a special role is played by the laser focal position with respect to the target surface, a proper choice of which may result in induced self-focusing effects and non-linear acceleration enhancement. SiC detectors employed in the time-of-flight (TOF) configuration and a Thomson parabola spectrometer permit on-line diagnostics of the ion streams emitted at high kinetic energies. The target composition and geometry, apart from the laser parameters and to the irradiation conditions, allow further control of the plasma characteristics and can be varied by using advanced targets to reach the maximum ion acceleration. Measurements using advanced targets with enhanced the laser absorption effect in thin films are presented. Applications of accelerated ions in the field of ion source, hadrontherapy and nuclear physics are discussed.

On the feasibility of increasing the energy of laser-accelerated protons by using low-density targets

Optimal regimes of proton acceleration in the interaction of short high-power laser pulses with thin foils and low-density targets are determined by means of 3D numerical simulation. It is demonstrated that the maximum proton energy can be increased by using low-density targets in which ions from the front surface of the target are accelerated most efficiently. It is shown using a particular example that, for the same laser pulse, the energy of protons accelerated from a low-density target can be increased by one-third as compared to a solid-state target.

Laser–plasma acceleration of electrons for radiobiology and radiation sources

Laser-driven acceleration in mm-sized plasmas using multi-TW laser systems is now established for the generation of high energy electron bunches. Depending on the acceleration regime, electrons can be used directly for radiobiology applications or for secondary radiation sources. Scattering of these electrons with intense laser pulses is also being considered for the generation of X-rays or γ-rays and for the investigation of fundamental electrodynamic processes. We report on laser-plasma acceleration in the 10TW regime in two different experimental configurations aimed at generating either high charge bunches with properties suitable for radiobiology studies or high collimation bunches for secondary radiation sources with high quality and good shot-to-shot stability. We discuss the basic mechanisms and describe the latest experimental results on injection threshold and bunch properties.

Coupling the nongravitational forces and modified Newton dynamics for cometary orbits

In recent work (Milgrom 2009, Blanchet & Novak 2011), the authors showed that MOdified Newton Dynamics (MOND) have a non-negligible secular perturbation effect on planets with large semi-major axes (gaseous planets) in the Solar System. Some comets also have a very eccentric orbit with a large semi-major axis (Halley family comets) going far away from the Sun (more than 15 AU) in a low acceleration regime where they would be subject to MOND perturbation. They also approach the Sun very closely (less than 3 AU) and are affected by the sublimation of ices from their nucleus, triggering so-called non-gravitational forces. The main goal of this paper is to investigate the effect of MOND perturbation on three comets with various orbital elements (2P/Encke, 1P/Halley and 153P/Ikeya-Zhang) and then compare it to the non-gravitational perturbations. It is motivated by the fact that when fitting an outgassing model for a comet, we have to take into account all of the small perturbing effects to avoid absorbing these effects into the non-gravitational parameters. Otherwise, we could derive a completely wrong estimation of the outgassing. For this work, we use six different forms of MOND functions and compute the secular variations of the orbital elements due to MOND and non-gravitational perturbations. We show that, for comets with large semi-major axis, the MONDian effects are not negligible compared to the non-gravitational perturbations.

Advanced targets, diagnostics and applications of laser-generated plasmas

High-intensity sub-nanosecond-pulsed lasers irradiating thin targets in vacuum permit generation of electrons and ion acceleration and high photon yield emission in non-equilibrium plasmas. At intensities higher than 1015 W/cm2 thin foils can be irradiated in the target-normal sheath acceleration regime driving ion acceleration in the forward direction above 1 MeV per charge state. The distributions of emitted ions in terms of energy, charge state and angular emission are controlled by laser parameters, irradiation conditions, target geometry and composition. Advanced targets can be employed to increase the laser absorption in thin foils and to enhance the energy and the yield of the ion acceleration process. Semiconductor detectors, Thomson parabola spectrometer and streak camera can be employed as online plasma diagnostics to monitor the plasma parameters, shot by shot. Some applications in the field of the multiple ion implantation, hadrontherapy and nuclear physics are reported.

Influence of target curvature on ion acceleration in radiation pressure acceleration regime

AbstractIon acceleration from submicron thick foil target irradiated by a circularly polarized laser is studied using multidimensional particle-in-cell simulations. Convex, flat, and concave target shapes are considered. Radius of curvature of curved target is of the order of laser width in transverse direction. Accelerated ion beam of highest peak energy and least energy spread is obtained from concave target, whereas total accelerated charge is highest in convex target. It is attributed to the change in the growth of transverse instabilities and geometrical effects due to target curvature in initial stages of acceleration process. The variation in the radius of curvature of the foil depends on the ratio of initial spot size to the radius of curvature. Faster reduction in curvature is achieved for tightly focused Gaussian pulses as conjectured by the model.

Enhancement of maximum attainable ion energy in the radiation pressure acceleration regime using a guiding structure.

Radiation pressure acceleration is a highly efficient mechanism of laser-driven ion acceleration, with the laser energy almost totally transferrable to the ions in the relativistic regime. There is a fundamental limit on the maximum attainable ion energy, which is determined by the group velocity of the laser. In the case of tightly focused laser pulses, which are utilized to get the highest intensity, another factor limiting the maximum ion energy comes into play, the transverse expansion of the target. Transverse expansion makes the target transparent for radiation, thus reducing the effectiveness of acceleration. Utilization of an external guiding structure for the accelerating laser pulse may provide a way of compensating for the group velocity and transverse expansion effects.