Radiation sources from Langmuir waves has been a topic of interest for their relevance to experimental approaches in plasma laboratories as well as for estimating physical models to explain cosmic radio bursts. Since the mechanism for converting energy from electrostatic Langmuir waves to electromagnetic waves is complex, diverse scenarios of such energy conversion have been studied, e.g. mode conversion, antenna radiation, nonlinear scattering, etc. Previously, we introduced a novel perspective of plasma dipole oscillation (PDO) which generates strong radiation bursts at the plasma frequency and high harmonics. In this paper, we report our discovery of radiation that result from electron-laser beam driven Langmuir waves and their interactions. In 2-D PIC simulations, we have observed that obliquely colliding Langmuir waves or even a single Langmuir wave generate localized radiation sources at the plasma frequency and high harmonics. These mechanisms differ from conventional two-plasmon mergers, where only the second harmonic of the plasma frequency is dominant: a strong radiation is observed even at the fundamental harmonic. In addition, from 3-D PIC simulations of electron laser beam driven plasma oscillators in magnetized plasma, the radiation from a local plasma oscillator, i.e. PDO, is found to be robust with diverse spectral peaks at the X-mode and the upper-hybrid mode. Nonlinear theory demonstrates that the relative strength of the harmonics of the plasma frequency depends on the shape of the PDO. The studies imply that the PDO has a more complicated internal structure than the simple model of a solid charge. We discuss the potential of the PDO generated from electron-beam driven plasmas or laser-driven plasmas as a radiation source and its relevance to cosmic radio bursts.
Volume density gratings produced by degenerate, counterpropagating laser pulses in plasma have several useful optical properties. Here we report on one of these in an investigation into creation of a transient plasma density grating that functions as a waveplate.
The increasing demand for high laser powers is placing huge demands on current laser technology. This is now reaching a limit, and to realise new areas of research promised at high intensities, new cost-effective and technically feasible ways of scaling up the laser power and controlling its optical characteristics are required. It is likely that ultra-compact, time-dependent, plasma-based amplifiers and optical elements (such as mirrors, polarisers, waveplates, etc.) will be required to reach powers exceeding tens of petawatts and possibly exawatts, and to manipulate the laser beams. Plasma is a robust optical medium as it is broken-down and can sustain extremely high electric fields. Plasma-based optical elements will most likely require the production of transient plasma gratings (TPGs), which take advantage of the ponderomotive force of the beat of at least two laser pulses. To create an amplifier, the TPG is formed through the action of the beat wave of a pump and seed pulse, which has a phase velocity satisfying the conditions for energy and momentum conservation of the three waves, where the third wave is a Langmuir or ion acoustic wave. Other optical elements will normally require static TPGs produced by degenerate driver pulses. Here we present the results of an experimental campaign conducted at the Central Laser Facility, where we have studied chirped pulse Raman amplification at high intensities. We have used a relatively long duration, frequency chirped, pump pulse to limit the growth of noise amplification, while ensuring amplification of the short seed pulse. We show that by changing the sign of the frequency chirp of the pump, the measured back-scattered and amplified seed energies change significantly. A negative chirp leads to a strong reduction in scattering from thermal density fluctuations, but seed amplification saturates. In contrast, for a positive frequency chirp, scattered energy continues to increase with increasing plasma density, without showing any sign of saturation, for the range of densities studied. From simulations we attribute this observation to the production of a local, long-lived, static TPG that continues to scatter the pump pulse long after it has passed. We will discuss the specific conditions that should be satisfied to produce such a grating. The ability to produce and maintain robust TPGs may provide a breakthrough in technologies for manipulating, reflecting and compressing ultra-intense laser pulses.
The accelerating structure of the laser wakefield accelerator (LWFA) is dynamic and highly sensitive to the local laser and plasma properties. It can expand and contract as it responds to the evolution of the laser and plasma fields. As a result, the position of, and environment within, the LWFA bubble are usually time dependent, which is not ideal for stable acceleration. Variations can have a negative impact on electron bunch properties, and are deleterious for ion channel lasers and plasma wigglers. We demonstrate how a laser pre-pulse improves the stability of the LWFA, and controls the evolution of the laser group and bubble velocity, which are important for determining LWFA dephasing and ultimately the electron bunch energy.
Classical electrodynamics basically assumes that a charged particle is a point charge, whose size is infinitesimal. This assumption caused a lot of problems with the radiation reaction (also called the self-force), by which a radiating particle feels friction. For example, Lorentz-Abraham-Dirac (LAD) model, which is the earliest theory about radiation reaction based on a point charge, leads to unphysical solutions such as runaway solutions and preacceleration. It has been tried to modify LAD model, and the most successful result was Landau-Lifshitz (LL) model, which also interpret well the recent experimental data of radiation reaction from laser-plasma interactions. However, even LL model has some problems, and particularly it cannot explain the radiation reaction of a uniformly accelerated charge. In this talk, we assume that the size of a particle is finite but the particle itself is completely indistinguishable from a point charge. By this assumption, we calculate the self-force of this particle undergoing uniform acceleration and uniform circular motion. The calculations show that the self-force of uniform acceleration can increase the effective mass of the particle, and it explain why a uniformly accelerated particle needs more energy to radiate. The calculations also indicate that the self-force of uniform circular motion only depends on the acceleration at the retarded time. In addition, we propose that there is a classical limit of the acceleration, and by combining it with the Schwinger limit we suggest that the classical radius of a particle is somehow related to the reduced Compton wavelength.
There is significant international effort focussed on developing ultra-high-power systems for next-generation laser facilities, such as the Extreme Light Infrastructure (ELI). Existing amplification methods are based on chirped-pulse amplification (CPA). However, the low damage threshold of conventional solid-state optics results in very large amplifiers and compressors. To overcome this challenge, we use stimulated Raman backscattering of a long pump laser in plasma to provide amplification for a low intensity seed pulse. Plasma has the advantage that it is already a broken down medium and therefore field intensities are not constrained as they are in conventional laser amplifiers. This offers the potential to reduce the size and cost of these devices significantly, while providing a possible route to reach exawatt powers, which will enable investigation of extremely high field physics.
Despite its advantages, efficient Raman amplification has not yet been demonstrated experimentally. Efficiencies are limited to only a few percent for seed energies of a few mJ, in contrast with theoretical predictions. Several phenomena lead to saturation or inhibit the amplification process – such as detuning, wavebreaking and particle trapping – depending on the amplification regime. Amplification is therefore highly sensitive to the conditions and parameters used. Raman amplification experiments are challenging, and careful planning is required to ensure that controlled and sustained amplification can take place. Numerical simulation is an essential ingredient to this preparation yet, like the experiments themselves, this is not a trivial task. The amplification process takes place over several millimetres, while structures on the short beat wavelength of the lasers need to be adequately resolved. Since particle kinetic effects are also important, a large number of particles are required. Simulation of the entire domain therefore requires significant computing resources, and therefore many investigations are only performed in 1-dimension. Moreover, the long propagation times involved allow numerical artefacts from processes such as grid heating or numerical dispersion to become significant. These can become pathological and artificially seed or disturb the amplification process.
Using state-of-the-art numerical techniques, we investigate the amplification of low- and high-intensity seed pulses in plasma, and compare their amplification growth rates and efficiencies with experimental results obtained by our group. The use of a chirped pump laser pulses is discussed and compared.
The increasing demand for high laser powers is placing huge demands on current laser technology. This is now reaching a limit, and to realise the existing new areas of research promised at high intensities, new cost-effective and technically feasible ways of scaling up the laser power will be required. Plasma-based laser amplifiers may represent the required breakthrough to reach powers of tens of petawatts to exawatts, because of the fundamental advantage that amplification and compression can be realised simultaneously in a plasma medium, which is also robust and resistant to damage, unlike conventional amplifying media. Raman amplification is a promising method, where a long pump pulse transfers energy to a lower frequency, short duration counter-propagating seed pulse through resonant excitation of a plasma wave that creates a transient plasma echelon, which backscatters the pump into the probe. While very efficient, this comes at the cost of noise amplification (from plasma density fluctuations) that needs to be controlled. Here we present the results of an experimental campaign where we have demonstrated chirped pulse Raman amplification (CPRA) at high intensities. We have used a frequency chirped pump pulse to limit the growth of noise amplification, while trying to maintain the amplification of the seed. In non-optimised conditions we show that indeed noise amplification can be controlled but reducing noise scattering also limits the seed amplification factor. Finally, we show that the gross efficiency is a few percent, consistent with previous measurements of CPRA obtained in capillaries with pump pulses of duration of a few hundred picoseconds.
The laser wake-field accelerator (LWFA) traditionally produces high brightness, quasi-monoenergetic electron beams with Gaussian-like spatial and angular distributions. In the present work we investigate the generation of ultra-relativistic beams with ring-like structures in the blowout regime of the LWFA using a dual stage accelerator. A density down-ramp triggers injection after the first stage and is used to produce ring-like electron spectra in the 300 - 600 MeV energy range. These well defined, annular beams are observed simultaneously with the on-axis, high energy electron beams, with a divergence of a few milliradians. The rings have quasi-monoenergetic energy spectra with an RMS spread estimated to be less than 5%. Particle-in-cell simulations confirm that off-axis injection provides the electrons with the initial transverse momentum necessary to undertake distinct betatron oscillations within the plasma bubble during their acceleration process.
We present an investigation into counter-streaming electron beams converging towards, and diverging from, a single point in two dimensions, leading to two-stream and current filamentation instabilities, which have radial and azimuthal density modulations, respectively. Using a semi-analytical approach and numerical simulations, we find no evidence for the two-stream instability in this geometry, but show that the system is unstable to the development of current filamentation.
High energy attosecond electron bunches from the laser-plasma wakefield accelerator (LWFA) are potentially useful sources of ultra-short duration X-rays pulses, which can be used for ultrafast imaging of electron motion in biological and physical systems. Electron injection in the LWFA depends on the plasma density and gradient, and the laser intensity. Recent research has shown that injection of attosecond electron bunches is possible using a short plasma density ramp. For controlled injection it is necessary to keep both the laser intensity and background plasma density constant, but set to just below the threshold for injection. This ensures that injection is only triggered by an imposed density perturbation; the peak density should also not exceed the threshold for injection. A density gradient that only persists over a short range can lead to the injection of femtosecond duration bunches, which are then Lorentz contracted to attoseconds on injection. We consider an example of a sin2 shaped modulation where the gradient varies until the downward slope exceeds the threshold for injection and then reduces subsequently to prevent any further injection. The persistence above the threshold determines the injected bunch length, which can be varied. We consider several designs of plasma media including density perturbations formed by shaped Laval nozzles and present an experimental and theoretical study of the modulated media suitable for producing attosecond-duration electron bunches.
Stimulated electron self-injection in the laser wakefield accelerator (LWFA) using density downramps is well known and regularly used to produce high energy electron bunches. The use of density gradients not only to stimulate injection but also control the properties of the injected electron bunch was recently presented by Tooley et al. [Phys. Rev. Lett. 119, 044801 (2017)], in which the authors put forward a model for controlling the velocity of the back of the bubble and compared to 2D and 3D particle-in-cell (PIC) data. This model is discussed and used to identify suitable LWFA parameters for ultra-short injection and repeated injection of multiple bunches. Quasi-3D PIC data is used to demonstrate injection of multiple bunches well separated in energy.
Optically pumped CO2 lasers can operate with high efficiency, high repetition rate and large bandwidths, suitable for producing ultra-short pulses at terawatts to petawatts, in contrast to conventional discharge-pumped CO2 lasers, which are restricted by the requirements of discharge dynamics in high-pressure gas. We show how an optically pumped CO2 laser can be realised and we consider its application in laser-driven acceleration. There is potential to replace conventional transversely excited atmospheric CO2 lasers with diode-pumped solid-state lasers as a pump laser for a high-pressure CO2 gain medium, making it suitable for amplifying ultra-short pulses. We show that by driving a laser plasma wakefield accelerator with an ultra-short pulse CO2 laser, a very high charge, high average current, high energy accelerator can be constructed. This could have a major impact on the application of these novel accelerators and radiation sources based on them.
Here we explore ways of transforming laser radiation into incoherent and coherent electromagnetic radiation using laserdriven plasma waves. We present several examples based on the laser wakefield accelerator (LWFA) and show that the electron beam and radiation from the LWFA has several unique characteristics compared with conventional devices. We show that the energy spread can be much smaller than 1% at 130-150 MeV. This makes LWFAs useful tools for scientists undertaking time resolved probing of matter subject to stimuli. They also make excellent imaging tools. We present experimental evidence that ultra-short XUV pulses, as short as 30 fs, are produced directly from an undulator driven by a LWFA, due to the electron bunches having a duration of a few femtoseconds. By extending the electron energy to 1 GeV, and for 1-2 fs duration pulses of 2 nm radiation peak powers of several MW per pC can be produced. The increased charge at higher electron energies will increase the peak power to GW levels, making the LWFA driven synchrotron an extremely useful source with a spectral range extending into the water window. With the reduction in size afforded by using LWFA driven radiation sources, and with the predicted advances in laser stability and repletion rate, ultra-short pulse radiation sources should become more affordable and widely used, which could change the way science is done.
The increasing demand for high laser powers is placing huge demands on current laser technology. This is now reaching a limit, and to realise the existing new areas of research promised at high intensities, new cost-effective and technically feasible ways of scaling up the laser power will be required. Plasma-based laser amplifiers may represent the required breakthrough to reach powers of tens of petawatt to exawatt, because of the fundamental advantage that amplification and compression can be realised simultaneously in a plasma medium, which is also robust and resistant to damage, unlike conventional amplifying media. Raman amplification is a promising method, where a long pump pulse transfers energy to a lower frequency, short duration counter-propagating seed pulse through resonant excitation of a plasma wave that creates a transient plasma echelon that backscatters the pump into the probe. Here we present the results of an experimental campaign conducted at the Central Laser Facility. Pump pulses with energies up to 100 J have been used to amplify sub-nanojoule seed pulses to near-joule level. An unprecedented gain of eight orders of magnitude, with a gain coefficient of 180 cm−1 has been measured, which exceeds high-power solid-state amplifying media by orders of magnitude. High gain leads to strong competing amplification from noise, which reaches similar levels to the amplified seed. The observation of 640 Jsr−1 directly backscattered from noise, implies potential overall efficiencies greater than 10%.
The ponderomotive force is an important concept in plasma physics and, in particular, plays an important role in many aspects of the theory of laser plasma interactions including current concerns like wakefield acceleration and Raman amplification. The most familiar form of this gives a force on a charged particle that is proportional to the slowly varying gradient of the intensity of a high frequency electromagnetic field and directed down the intensity gradiant. For a field amplitude simply oscillating in time there is a simple derivation of this formula, but in the more general case of a travelling wave the problem is more difficult. Over the years there has been much work on this using Hamiltonian or Lagrangian averaging techniques, but little or no investigation of how well these theories work. Here we look at the very basic problem of a particle entering a region with a monotonically increasing electrostatic field amplitude and being reflected. We show that the equation of motion derived from a widely quoted ponderomotive potential only agrees with the numerically computed orbit within a restricted parameter range and that outside this range it shows features which are inconsistent with any ponderomotive potential quadratic in the field amplitude. Since the ponderomotive force plays a fundamental role in a variety of problems in plasma physics we think that it is important to point out that even in the simplest of configurations standard theories may not be accurate.
In the coming years, a new generation of high-power laser facilities (such as the Extreme Light Infrastructure) will become operational, for which it is important to understand how the interaction with intense laser pulses affects the bulk properties of relativistic electron bunches. At such high field intensities, we expect both radiation reaction and quantum effects to have a dominant role to play in determining the dynamics. The reduction in relative energy spread (beam cooling) at the expense of mean beam energy predicted by classical theories of radiation reaction has been shown to occur equally in the longitudinal and transverse directions, whereas this symmetry is broken when the theory is extended to approximate certain quantum effects. The reduction in longitudinal cooling suggests that the effects of radiation reaction could be better observed in measurements of the transverse distribution, which for real-world laser pulses motivates the investigation of the angular dependence of the interaction. Using a stochastic single-photon emission model with a (Gaussian beam) focussed pulse, we find strong angular dependence of the stochastic heating.
KEYWORDS: Electron beams, Particles, Electroluminescent displays, Pulsed laser operation, Atmospheric particles, Physics, High power lasers, Photons, Quantum physics, Laser energy
The next few years will see next-generation high-power laser facilities (such as the Extreme Light Infrastructure) become operational, for which it is important to understand how interaction with intense laser pulses affects the bulk properties of a relativistic electron beam. At such high field intensities, we expect both radiation reaction and quantum effects to play a significant role in the beam dynamics. The resulting reduction in relative energy spread (beam cooling) at the expense of mean beam energy predicted by classical theories of radiation reaction depends only on the energy of the laser pulse. Quantum effects suppress this cooling, with the dynamics additionally sensitive to the distribution of energy within the pulse. Since chirps occur in both the production of high-intensity pulses (CPA) and the propagation of pulses in media, the effect of using chirps to modify the pulse shape has been investigated using a semi-classical extension to the Landau-Lifshitz theory. Results indicate that even large chirps introduce a significantly smaller change to final state predictions than going from a classical to quantum model for radiation reaction, the nature of which can be intuitively understood.
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