Solitary electrostatic structures involving density depletions have been observed in the upper ionosphere by the Freja satellite [Dovner et al., 1994]. If these are interpreted as ion sound solitons, the difficulty arises that the standard Korteweg‐de Vries description predicts structures with enhanced rather than depleted density. Here we show that the presence of non‐thermal electrons may change the nature of ion sound solitary structures and allow the existence of structures very like those observed.

We propose in this White Paper a concept for a space experiment using cold atoms to search for ultra-light dark matter, and to detect gravitational waves in the frequency range between the most sensitive ranges of LISA and the terrestrial LIGO/Virgo/KAGRA/INDIGO experiments. This interdisciplinary experiment, called Atomic Experiment for Dark Matter and Gravity Exploration (AEDGE), will also complement other planned searches for dark matter, and exploit synergies with other gravitational wave detectors. We give examples of the extended range of sensitivity to ultra-light dark matter offered by AEDGE, and how its gravitational-wave measurements could explore the assembly of super-massive black holes, first-order phase transitions in the early universe and cosmic strings. AEDGE will be based upon technologies now being developed for terrestrial experiments using cold atoms, and will benefit from the space experience obtained with, e.g., LISA and cold atom experiments in microgravity.KCL-PH-TH/2019-65, CERN-TH-2019-126

Magnetic fields are ubiquitous in the Universe. The energy density of these fields is typically comparable to the energy density of the fluid motions of the plasma in which they are embedded, making magnetic fields essential players in the dynamics of the luminous matter. The standard theoretical model for the origin of these strong magnetic fields is through the amplification of tiny seed fields via turbulent dynamo to the level consistent with current observations. However, experimental demonstration of the turbulent dynamo mechanism has remained elusive, since it requires plasma conditions that are extremely hard to re-create in terrestrial laboratories. Here we demonstrate, using laser-produced colliding plasma flows, that turbulence is indeed capable of rapidly amplifying seed fields to near equipartition with the turbulent fluid motions. These results support the notion that turbulent dynamo is a viable mechanism responsible for the observed present-day magnetization.

Contemporary high-power laser systems make use of solid-state laser technology to reach petawatt pulse powers. The breakdown threshold for optical components in these systems, however, demands metre-scale beams. Raman amplification of laser beams promises a breakthrough by the use of much smaller amplifying media, that is, millimetre-diameter plasmas, but so far only 60 GW peak powers have been obtained in the laboratory, far short of the desired multipetawatt regime. Here we show, through the first large-scale multidimensional particle-in-cell simulations of this process, that multipetawatt peak powers can be reached, but only in a narrow parameter window dictated by the growth of plasma instabilities. Raman amplification promises reduced cost and complexity of intense lasers, enabling much greater access to higher-intensity regimes for scientific and industrial applications. Furthermore, we show that this process scales to short wavelengths, enabling compression of X-ray free-electron laser pulses to attosecond duration

The properties of four-wave interaction via the nonlinear quantum vacuum is investigated. The effect of the quantum vacuum is to generate photons with new frequencies and wave vectors, due to elastic photon-photon scattering. An expression for the number of generated photons is derived and using state-of-the-art laser data it is found that the number of photons can reach detectable levels. In particular, the prospect of using the high repetition Astra Gemini system at the Rutherford Appleton Laboratory is discussed. The problem of noise sources is reviewed, and it is found that the noise level can be reduced well below the signal level. Thus, detection of elastic photon-photon scattering may for the first time be achieved. PACS numbers: 12.20.Fv, 42.50.Xa Classically, elastic photon-photon scattering does not take place in vacuum. However, according to quantum electrodynamics (QED), such a process may occur owing to the interaction with virtual electron-positron pairs. Several suggestions to detect photon-photon scattering have been made, for example using harmonic generation in an inhomogeneous magnetic field [1], using resonant interaction between eigenmodes in microwave cavities [2,3], using ultra-intense fields occurring in plasma channels [4], as well as many others, see e.g. Refs. [5,6,7]. Related to, but physically different from, elastic photon-photon scattering is photon splitting [8] (see also [9]) and Delbrück scattering [10], the latter being detectable using high-Z atomic targets [10]. However, no suggestions have yet led to detection of elastic scattering among real photons.In the present Letter we will investigate the possibility to detect photon-photon scattering in vacuum using four-wave mixing, where three colliding laser pulses stimulate emission in a fourth direction with a new frequency. Four-wave mixing has the advantage of not being limited by the low cross section for photon-photon scattering [11]. A similar scheme was first studied by Ref. [12], and further theoretical [13,14,15] and experimental [16] studies of QED four-wave mixing have been performed since then. However, so far the available laser technology has not been sufficiently powerful to allow for successful detection of scattering events. Moreover, even with high laser power, the laser setup and geometry in such an experiment is important. In particular, we argue that a two-dimensional (2D) setup preferred in earlier literature is unlikely to produce a detectable signal. This is in contrast to the 3D setup presented in this Letter. We have calculated the coupling coefficient for four-wave mixing as a function of incident angles and laser polarization. For given data of the laser beams, together with the coupling coefficients, the number of scattered photons can be calculated. The analysis is then used * Currently at: Department of Physics, Stockholm University, SE-106 91, Sweden to suggest a novel concrete experiment, where the parameters are chosen to fit the Astra Gemini (AG) system (operational 2007) located at the Centr...

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Studies of charged-particle acceleration processes remain one of the most important areas of research in laboratory, space and astrophysical plasmas. In this paper, we present the underlying physics and the present status of high gradient and high energy plasma accelerators. We will focus on the acceleration of charged particles to relativistic energies by plasma waves that are created by intense laser and particle beams. The generation of relativistic plasma waves by intense lasers or electron beams in plasmas is important in the quest for producing ultra-high acceleration gradients for accelerators. With the development of compact short pulse high brightness lasers and electron positron beams, new areas of studies for laser/particle beam-matter interactions is opening up. A number of methods are being pursued vigorously to achieve ultrahigh acceleration gradients. These include the plasma beat wave accelerator mechanism, which uses conventional long pulse (∼100 ps) modest intensity lasers (I ∼ 10 14 -10 16 W cm −2 ), the laser wakefield accelerator (LWFA), which uses the new breed of compact high brightness lasers (<1 ps) and intensities >10 18 W cm −2 , the self-modulated LWFA concept, which combines elements of stimulated Raman forward scattering, and electron acceleration by nonlinear plasma waves excited by relativistic electron and positron bunches. In the ultra-high intensity regime, laser/particle beam-plasma interactions are highly nonlinear and relativistic, leading to new phenomena such as the plasma wakefield excitation for particle acceleration, relativistic self-focusing and guiding of laser beams, high-harmonic generation, acceleration of electrons, positrons, protons and photons. Fields greater than 1 GV cm −1 have been generated with particles being accelerated to 200 MeV over a distance of millimetre. Plasma wakefields driven by positron beams at the Stanford Linear Accelerator Center facility have accelerated the tail of the positron beam. In the near future, laser plasma accelerators will be producing GeV particles.

Twisted Laguerre–Gaussian lasers, with orbital angular momentum and characterized by doughnut-shaped intensity profiles, provide a transformative set of tools and research directions in a growing range of fields and applications, from super-resolution microcopy and ultra-fast optical communications to quantum computing and astrophysics. The impact of twisted light is widening as recent numerical calculations provided solutions to long-standing challenges in plasma-based acceleration by allowing for high-gradient positron acceleration. The production of ultra-high-intensity twisted laser pulses could then also have a broad influence on relativistic laser–matter interactions. Here we show theoretically and with ab initio three-dimensional particle-in-cell simulations that stimulated Raman backscattering can generate and amplify twisted lasers to petawatt intensities in plasmas. This work may open new research directions in nonlinear optics and high–energy-density science, compact plasma-based accelerators and light sources.