Wakefield Acceleration Research Articles

Optical ionization effects in kHz laser wakefield acceleration with few-cycle pulses

We present significant advances in laser wakefield acceleration (LWFA) operating at a 1 kHz repetition rate, employing a sub-TW, few-femtosecond laser and a continuously flowing hydrogen gas target. We conducted a comprehensive study assessing how the nature of the gas within the target influences accelerator performance. This work confirms and elucidates the superior performance of hydrogen in LWFA driven by few-cycle, low-energy laser pulses. Our system generates quasimonoenergetic electron bunches with energies up to 10 MeV, bunch charges of 2 pC, and angular divergences of 15 mrad. Notably, our scheme relying on differential pumping enables continuous operation at kHz repetition rates, contrasting with previous systems that operated in burst mode to achieve similar beam properties. Particle-in-cell simulations explain hydrogen's superior performances: the ionization effects in nitrogen and helium distort the laser pulse, negatively impacting the accelerator performance. These effects are strongly mitigated in hydrogen plasma, thereby enhancing beam quality. This analysis represents a significant step forward in optimizing and understanding kHz LWFA. It underscores the critical role of hydrogen and the imperative need to develop hydrogen-compatible target systems capable of managing high repetition rates, as exemplified by our differential pumping system. These advances lay the groundwork for further developments in high-repetition-rate laser plasma accelerator technology. Published by the American Physical Society 2024

Energy depletion and re-acceleration of driver electrons in a plasma-wakefield accelerator

For plasma-wakefield accelerators to fulfill their potential for cost effectiveness, it is essential that their energy-transfer efficiency be maximized. A key aspect of this efficiency is the near-complete transfer of energy, or depletion, from the driver electrons to the plasma wake. Achieving full depletion is limited by the process of re-acceleration, which occurs when the driver electrons decelerate to nonrelativistic energies, slipping backward into the accelerating phase of the wakefield and being subsequently re-accelerated. Such re-acceleration is unambiguously observed here for the first time. At this re-acceleration limit, we measure a beam driver depositing (57 ± 3)% of its energy into a 195-mm-long plasma. This suggests that the energy-transfer efficiency of plasma accelerators could approach that of conventional accelerators. Published by the American Physical Society 2024

Combined plasma lens and rephasing stage for a laser wakefield accelerator

Electrons from a laser wakefield accelerator have a limited energy gain due to dephasing and are prone to emittance growth, causing a large divergence. In this paper, we experimentally show that adjusting the plasma density profile can address both issues. Shock-assisted ionisation injection is used to produce 100 MeV quasi-monoenergetic electron bunches in the primary part of the accelerator. Downstream from the accelerator, a second, independently tuneable density region is added, which can be used to either boost the energy of the electron bunches or as a plasma lens for significant divergence reduction. An additional energy gain of 25% and a 40% divergence reduction are obtained. Theoretical models validate the effects.

GALADRIEL: A facility for advancing engineering science relevant to rep-rated high energy density physics and inertial fusion energy experiments.

Many current and upcoming laser facilities used to study high-energy-density (HED) physics and inertial fusion energy (IFE) support operating at high rep-rates (HRRs) of ∼0.1-10Hz, yet many diagnostics, target-fielding strategies, and data storage methods cannot support this pace of operation. Therefore, established experimental paradigms must change for the community to progress toward rep-rated operation. To this end, we introduce the General Atomics LAboratory for Developing Rep-rated Instrumentation and Experiments with Lasers, or GALADRIEL, to serve as a test bed for developing and benchmarking the engineering science advancements required for HRR experiments. GALADRIEL was constructed from the ground up around a commercial 1TW (∼25 mJ in ∼25fs at 800nm) laser with diverse experimental applications in mind. Assembly of the basic framework of GALADRIEL concluded with commissioning shots generating ∼1-4MeV electrons via laser-wakefield acceleration (LWFA) using a nitrogen gas jet. Subsequent LWFA experiments operated at 1Hz, utilized instrument feedback for optimization, and stored all data in a custom-built NoSQL database system. From this database called MORIA, or the MOngodb Repository for Information Archiving, data are retrievable via individual files or en masse by query requests defined by the user. GALADRIEL focuses on outstanding questions in engineering science, including targetry, diagnostics, data handling, environmental and materials studies, analysis and machine learning algorithm development, and feedback control systems. GALADRIEL fills a niche presently missing in the US-based user-facility community by providing a flexible experimental platform to address problems in engineering science relevant to rep-rated HED and IFE experiments.

ELECTRON SOURCE BASED ON EMERGENCE OF SELF-INJECTED ELECTRON BUNCH AT PLASMA WAKEFIELD EXCITATION BY A TW LASER PULSE

Wakefield acceleration methods are known due to some their advantages. The main of them is the high accelerating gradient up to several teravolts per meter. In the paper another important advantage is concluded to the possibility of using a wakefield accelerator as a source of electrons by means of obtaining self-injected bunches and their acceleration. The result is the simulation of the process of plasma wakefield excitation by a laser pulse with an energy of tens of mJ and a power of 1…2 TW for obtaining the promising electron source. Homogeneous and Gaussian plasma profiles were investigated and compared to increase the energy of the self-injected bunches. The laser parameters were taken that corresponded to the parameters of the laser setup in the Institute of Plasma Electronics and New Methods of Acceleration of the National Scientific Center “Kharkiv Institute of Physics and Technology”. Based on the results of the simulation, the possibility of obtaining relativistic self-injected bunches that can be used for further laser acceleration experiments, including dielectric laser acceleration, was demonstrated.

Coupling of Fluid and Particle-in-Cell Simulations of Ambipolar Plasma Thrusters

Ambipolar plasma thrusters are an appealing technology due to multiple system-related advantages, including propellant flexibility and the absence of electrodes or neutralizer. Understanding the plasma generation and acceleration mechanisms is key to improving the performance and capabilities of these thrusters. However, the source and plume regions inside are often simulated separately, and no self-consistent strategy exists which can couple these different simulations together. This paper introduces the MUlti-regime Plasma Equilibrium Transport Solver (MUPETS), a self-consistent coupled model integrating a fluid solver for the plasma dynamics in the source, which are collision-driven, with a kinetic Particle-In-Cell (PIC) code for the plasma dynamics in the magnetic nozzle, which involve expansion across a diverging magnetic field. The methodology begins by solving the plasma source with the classical Bohm condition at the thruster’s throat. The resulting plasma profiles (density, temperature, speed) are input into the PIC code for the magnetic nozzle. The PIC code calculates the plasma plume expansion and determines the electric field at the thruster’s throat. This electric field is then used as a boundary condition in the fluid code, where it replaces the Bohm assumption, and the fluid simulation is repeated. This iterative process continues until convergence. In comparing the MUPETS results with those for an experimental thruster, the plasma densities at the thruster’s throat differed by less than 2–5% between the fluid and PIC regions. The thrust predictions agreed with the experimental trend, and were kept well within the measurement’s uncertainty band. These results validate the effectiveness of the coupling strategy for enhancing plasma thruster simulation accuracy.

Effects of discharge parameters on plasma acceleration and transmission characteristics of a coaxial gun operated in gas-prefilled mode

The effects of discharge parameters on the discharge process and plasma transport characteristics of a coaxial gun in the gas-prefilled mode are studied. The plasma optical intensity and ejection velocity are measured by photodiodes, the optical emission spectrum is taken by a spectroscopic system, and the plasma evolution in the transport process is captured by a high-speed camera. The plasma acceleration characteristics under different discharge parameters show that the velocity and electron density of the ejection plasma are mainly determined by the pre-filled pressure and discharge current, which is consistent with the snowplow model. The kinetic energy of ejection plasma can be significantly increased by reducing the outer loop inductance, which is conducive to increasing the energy utilization efficiency. The time-varying images of plasma radiation and the plasma density at different transport locations illuminate the transmission characteristics of coaxial gun discharge plasma. The results show that the snowplow effect continues to play a role in the plasma transport process, and the plasma accumulation is induced by the combination of shock wave compression. The current-driven magneto hydrodynamics instability occurs during the transport process, and the luminous signal of the plasma current sheet oscillates periodically. In addition, the plasma impact effect is obvious and the gas retarding effect is enhanced with the increase in the gas pressure. These results give us a more comprehensive view of the coaxial gun discharge process and plasma transport and provide a certain reference for optimizing the parameters selection and physical design of coaxial gun discharge plasma characteristics.

Coherent high-harmonic generation with laser-plasma beams

Active energy compression scheme is presently being investigated for future laser-plasma accelerators. This method enables generating laser-plasma accelerator electron beams with a small, ∼10−5, relative slice energy spread. When modulated by a laser pulse, such beams can produce coherent radiation at very high, ∼100th harmonics of the modulation laser wavelength, which are hard to access by conventional techniques. The scheme has a potential of providing additional capabilities for future plasma-based facilities by generating stable, tunable, narrow-band radiation. Published by the American Physical Society 2024

Guiding properties of Bessel–Gaussian and super-Gaussian pulses in inhomogeneous parabolic plasma channels

The guidance and stable propagation of laser pulses over many Rayleigh lengths are crucial for the plasma electron acceleration in the laser wakefield accelerators. Using plasma channels with specific characteristics can lead to the proper guidance of the laser pulse. Here, quasi-three-dimensional particle-in-cell (PIC) simulations are performed to investigate the guidance of Bessel–Gaussian pulse (BGP) of zeroth order and super-Gaussian pulse (SGP) of 3rd and 4th orders in an axially and radially inhomogeneous plasma channel. The effects of the channel radius and depth, the laser wavelength and initial spot size, and the plasma channel inhomogeneity on the guidance of the laser pulse are also examined. The results indicate that the guidance of a laser pulse in the plasma channel depends on the pulse profile, and under certain conditions, the pulses can be guided with the least variation of spot size in the inhomogeneous plasma channel. It is shown that the channel depth and the initial laser spot size are very effective in pulse guiding, as the values of these parameters increase, the pulse guidance is done better. In addition, the results show that the guidance of laser pulse is dependent on the type of plasma inhomogeneity represented by three different kinds of initial conditions, as considering the nonlinear-axial inhomogeneity in the parabolic plasma channel can lead to more convergence than the axially homogeneous and linear-axially plasma density profiles.

MEASUREMENT OF THE VELOCITY OF PULSED PLASMA FLOW AT THE PW-7 INSTALLATION

The paper considers two independent methods for measuring the velocity of the plasma flow generated in the PV-7 pulsed plasma accelerator: a method based on observation and evaluation of the Doppler shift of spectral lines, and a method of high-speed visualization of plasma motion. To record the plasma flow radiation spectrum, a monochromator M833 was used. High-speed video recording was carried out at 640,000 fps using a Phantom VEO710S CMOS camera. The results of measurements of the average flow velocity obtained at a working gas pressure of 2⋅10-2 Torr, capacitance and voltage of the capacitor bank of 400 μF and 4 kV are presented. The results obtained by two independent methods were compared with each other. Argon was used as the working gas in the experiments. It is shown that the value of the plasma flow velocity estimated by the first method is 12.5 m/s, and the value of the plasma flow velocity estimated by the second method is 16.7 m/s. From these data the measured flow velocity values have a small discrepancy. Thus, it has been established that high-speed video recording and Doppler shift methods make it possible to obtain comparable estimates of flow velocity within the measurement errors. Determining the magnitude of the plasma flow velocity is of great practical importance.

Enhanced pointing and charge properties of electron beams from few-TW laser wakefield acceleration with an asymmetric density profile in a sub-millimeter nitrogen gas jet

This work demonstrates the feasibility of creating a sub-millimeter, subsonic nitrogen gas jet using a 178-μm diameter orifice to conduct laser wakefield acceleration (LWFA) with 1-TW, 40-fs laser pulses. More importantly, our findings reveal that using a blade to impede part of the gas flow and create an asymmetric density profile with a shortened down-ramp leads to a notable reduction in pointing fluctuations and an increase in the total charge of the output electron beams. As evidenced by the corresponding particle-in-cell simulation, the laser intensity is more effectively sustained toward the downstream end of the shaped gas jet, allowing for effective excitation of low-amplitude plasma waves that help preserve the accelerated electrons over the target rear side. In contrast, the pulse intensity drops significantly within the rear side of the unshaped gas jet, resulting in continuously diminishing plasma waves and decreased beam charge. The steeper gradient of the density down-ramp in the shaped gas jet also leads to a more rapid increase in the plasma wavelength over a reduced propagation distance, which helps mitigate the dephasing of accelerated electrons and increase the charge at the high-energy side of the spectrum. Our study paves the way for the future development of few-TW LWFA using a subsonic gas jet with sharp edges to further enhance the properties of output electron beams.

Deep learning based x-ray spectrometer for high repetition rate characterization of betatron radiation

Betatron radiation produced from a laser-wakefield accelerator is a broadband, hard x-ray (>1 keV) source that has been used in a variety of applications in medicine, engineering, and fundamental science. Further development and optimization of stable, high repetition rate (HRR) (>1 Hz) betatron sources will provide a means to extend their application base to include single-shot dynamical measurements of ultrafast processes or dense materials. Recent advances in laser technology used in such experiments have enabled increases in shot-rate and system stability, providing improved statistical analysis and detailed parameter scans. However, unique challenges exist at high repetition rate, where data throughput and source optimization are now limited by diagnostic acquisition rates and analysis. Here, we present the development of a machine-learning algorithm for the real-time analysis of betatron radiation. We report on the fielding of this deep learning algorithm for online source characterization at the Institut National de la Recherche Scientifique's Advanced Laser Light Source. By fine-tuning an algorithm originally trained on a fully synthetic dataset using a subset of experimental data, the algorithm can predict the betatron critical energy with a percent error of 7.2 % with a reconstruction time of 1.5 ms, providing a valuable tool for real-time, multi-objective optimization at HRR.

On estimation of betatron radiation spectrum characteristics of DLA electrons in NCD plasma

Action of a relativistically intense subpicosecond laser pulse on the near critical density (NCD) plasma can give rise to the formation of ion channel inside the plasma and effective acceleration of background electrons. Thus, one can produce high current (electron charges from tens nCl to several mkCl), high energy (from several MeV to several hundreds of MeV) electron bunches, demanded in different practical applications. Synchrotron (betatron) radiation of these electrons can serve as an important tool both for practical applications and also for diagnostic of the process in laser plasma, which is important for better understanding of these processes and for optimization of experimental conditions. For the last goals, an approximate model is proposed for calculating the spatial and energy characteristics of a bunch of DLA (direct laser accelerated) electrons in the ion channel formed in the NCD plasma and the characteristics describing the spectrum of their synchrotron radiation. For the considered example of a powerful laser pulse action on NCD plasma, the predictions of the proposed model are in good agreement with the results of particles in cell simulations and with the experimental measurements of the synchrotron radiation specter. It is shown that with the assumption of the rotation of the initial plane of motion of DLA electrons, the experimental data on the measurements of synchrotron radiation specters can be explained on the basis of the concept of betatron radiation of electrons accelerated in NCD plasma by DLA mechanism.

On the effect of the maximal proper acceleration in the inertia

The effect of a hypothetical maximal proper acceleration on the mass of a charged particle is investigated in the context of particle accelerators. In particular, it is shown that maximal proper acceleration implies an increase in the kinetic energy of the particle being accelerated with respect to the relativistic energy. Such an increase in kinetic energy leads to a reduction of the luminosity of the bunches with respect to the expected luminosity in the relativistic models of the bunches. This relative loss in luminosity is of the order 10 −3 to 10 −5 for the Large Hadron Collider (LHC) bunches and can be of order up to 10 −3 for certain laser-plasma accelerator facilities. Although the effect is small, it increases with the square of the bunch population.

High-speed flows in the plasma accelerator channel for the regime of electron current-transport on coaxial electrodes

High-speed flows in the plasma accelerator channel for the regime of electron current-transport on coaxial electrodes

High brightness betatron x-ray source driven by chirped laser pulses

We demonstrate high brightness betatron x-ray generation from a chirped laser pulses driven plasma accelerator. It is shown that positively chirped laser pulse leads to the initiation and enhancement of collective oscillations of electrons inside plasma bubbles, due to associated pulse front tilt (PFT). The PFT causes transverse drift of the bubbles with respect to the laser axis, which results in high brightness x-ray generation. At an optimum chirp, enhanced x-ray emission of >108 photons/pulse/sr in 0.1% BW with a critical energy of ∼18 keV was observed by a factor >2 in comparison to the case of no chirp. The role of collective oscillation in enhancing x-ray emission is also validated in the Geant4 simulations.

Operating Characteristics of a Wave-Driven Plasma Thruster for Cutting-Edge Low Earth Orbit Constellations

This paper outlines the development phases of a wave-driven Helicon Plasma Thruster for cutting-edge Low Earth Orbit (LEO) constellations. The two-stage ambipolar electric propulsion (EP) system combines the efficient ionization of an ultra-compact helicon reactor with plasma acceleration based on an ambipolar electric field provided by a magnetic nozzle. This paper reveals maturation challenges associated with an emerging EP system in the hundreds-watt class, followed by outlook strategies. A 3 cm diameter helicon reactor was operated using argon gas under a time-modulated RF power envelope ranging from 250 W to 500 W with a fixed magnetic field strength of 400 G. Magnetically enhanced inductively coupled plasma reactor characteristics based on half-wavelength right helical and Nagoya Type III antennas under capacitive (E-mode), inductive (W-mode), and wave coupling (W-mode) were systematically investigated based on Optical Emission Spectroscopy. The operation characteristics of a wave-heated reactor based on helicon configuration were investigated as a function of different operating parameters. This work demonstrates the ability of two-stage HPT using a compact helicon reactor and a cusped magnetic field to outperform today’s LEO spacecraft propulsion.

Overview and Recent Developments of the Frascati Laser for Acceleration and Multidisciplinary Experiments Laser Facility at SPARC_LAB

An overview of the 200 TW Frascati Laser for Acceleration and Multidisciplinary Experiments (FLAME) at the SPARC_LAB Test Facility at the National Laboratories of Frascati (LNF-INFN) is presented. The FLAME laser is employed to investigate different laser–matter interaction schemes, i.e., electron acceleration and secondary radiation sources through Laser Wakefield Acceleration (LWFA) or ion and proton generation through Target Normal Sheath Acceleration (TNSA), for a wide range of scientific areas including the biomedical applications. Finally, recently performed experimental campaigns within the EuAPS and EuPRAXIA frameworks are reported.

Compact laser wakefield acceleration toward high energy with micro-plasma parabola

Laser wakefield acceleration (LWFA) promises compact accelerators toward the high-energy frontier. However, the approach to the 100 GeV milestone faces the obstacle of the long focal length required for optimal acceleration with high-power lasers, which reaches hundreds of meters for 10–100 PW lasers. The long focal length originates from optimal laser intensity required to avoid nonlinear effects and hence large spot size and Rayleigh length. We propose a “telescope” geometry in which a micro-plasma parabola (MPP) is coupled with a short-focal-length off-axis parabola, minimizing the focal length to the meter range for LWFA under optimized conditions driven by lasers beyond 1 PW. Full-dimensional kinetic simulations demonstrate the generation of a 9 GeV electron bunch within only 1 m optical length—only one-tenth of that required with the conventional approach with the same performance. The proposed MPP provides a basis for the construction of compact LWFAs toward single-stage 100 GeV acceleration with 100 PW class lasers.

Electron Dynamics in a Relativistic Central Field Approach Associated with Laser Acceleration of Electrons

We perform 2D PIC simulations and observe different scenarios of electron trajectories trapped inside the bubble formed during the laser wakefield acceleration process (LWFA). The trajectories can close on the inside or on the outside of the bubble. The evolution of maximum energy of the injected electrons in both cases is evaluated. We propose an analytic approach based on a relativistic central field model for describing the attraction of electrons near the bubble edge and compare it with the simulations.