Finding the origin of magnetic fields starting from unmagnetized plasmas constitutes a long-standing quest in plasma physics . This fundamental and intriguing problem has implications not only in astrophysics but also in laboratory plasmas. In fact, one of the main unanswered questions regards the genesis of the magnetic fields found throughout the universe. On the other hand, in a laboratory, long-lived magnetic fields can be created via the interaction of intense lasers with plasmas, in what is considered one of the most exciting emerging new branches of plasma physics. Laboratory experiments will indeed allow reproducing astrophysical mechanisms under controlled conditions favoring a physical insight that would be otherwise inaccessible.

The Biermann battery has often been invoked as a possible mechanism for magnetic field generation in unmagnetized plasmas . The Biermann battery acts in an unmagnetized plasma when its temperature and pressure gradients are perpendicular to each other. In such plasmas,…

Figure 1: Speiser and Dahu orbits: the trappings that heat you up!

High-energy particles are ubiquitous in the universe, from jets at the center of the galaxies to the Solar Wind. Understanding the different mechanisms responsible for this acceleration is one of the most challenging questions of contemporary plasma studies. The standard approach to address this question is using kinetic models, both numerical and theoretical, in order to deeply understand the dynamics of the particles involved.

Figure 1 shows the result of a numerical simulation of a collisionless plasma performed with the particle-in-cell code OSIRIS 4.0 in which is represented one mechanism for thermal acceleration (heating). As the title says, this mechanism is connected with the trapping of particles between two approaching current sheet profiles.

Figure 2: (Top) Radial current jx (black) and toroidal magnetic field By (blue) profile. The dashed gray lines delineate the regions of the different particle trajectories: the…

In astronomical phenomena such as neutron stars, the strength of the magnetic field can be extremely large, approaching the quantum Schwinger field (4.4 x 1013 Gauss). At these strengths, pair production can occur leading to a plasma dominated by electron-positron pairs. It is inevitable that magnetic reconnection occurs at some points in the magnetosphere. In the context of this reconnection, multiple flux-tubes referred to as magnetic islands can be found. Compression of the fields of a magnetic island leads to enhanced radiative cooling, and in turn further compression. The resulting runaway process leads to quantum electrodynamic (QED) effects such as pair production.

We have performed a 3D ab initio PIC simulation of magnetic reconnection, starting from a Harris equilibrium pair plasma, where most of the energy is in the form of magnetic fields, which takes into account hard photon emission and pair production at the rates predicted from QED. The…

The scales associated with this radiation are typically orders of magnitude smaller than the scales associated with the particle’s motion, thus, resolving such small scales would increase the number of operations beyond what is currently computationally possible. Consequently, the spatiotemporal behaviour of the radiation emitted by charged particles either in a plasma or a synchrotron environment remains fairly unexplored and to the best of our knowledge, no theoretical predictions have been made on the spatiotemporal signature of any kind of motion. However, the spectral properties of radiation are well documented for some common types of motion as typical synchrotron light sources generate radiation by making relativistic, ultra-fast particles undergo a sinusoidal or helical motion inside periodic magnetic structures known as wigglers or undulators. Such conditions can also be obtained in a plasma using relativistic plasma waves instead of magnetic structures or EM fields. The Radiation Diagnostic for OSIRIS (RaDiO)…

Weak dark matter (DM) self-interactions are a natural and well-motivated possibility, with possible observable consequences for large scale structure and halo dynamics. A minimal type of DM self-interaction is “dark electromagnetism”, which could cause DM to exhibit collisionless plasma-like collective behavior . This possibility raises the question of whether plasma instabilities may have a significant impact on the galaxy and cluster dynamics. Resolving this will determine whether or not such an interaction is consistent with current observations, and whether plasma instabilities may have a significant impact on galactic dynamics.

Two-dimensional PIC simulations were performed using two identical, collisionless e− e+ DM plasma clouds (shown in blue color). This picture depicts the interaction of two dark e− e+ plasma clouds mimicking the merger of two DM halos, such as in the Bullet Cluster. A transverse Weibel magnetic field (shown in red color) grows during the interactions which deflect the…

While conventional particle accelerators have been used in the past to answer fundamental physical questions, their technical limits have been reached. An intrinsically different approach is provided by plasma-based accelerators. Utilizing the properties of a plasma, acceleration gradients which are orders of magnitude higher compared to conventional accelerators can be achieved. Recent experiments have shown utilization of a long proton bunch provided by CERN’s Super Proton Synchrotron (SPS). In the experiment, a long proton bunch is co-propagated through a 10 m long gas vapor together with a laser pulse which ionizes the gas and creates an electron plasma column. With the help of the self‑modulation instability (SMI) the long proton bunch is sliced in micro‑bunches driving plasma wakes. Thus wakes allow the acceleration of electrons up to several gigaelectronvolts over a short distance.

Our work presented in the video depicts a simulation for the generation of micro‑bunches by the…

In astronomical phenomena such as neutron stars, the strength of the magnetic field can be extremely large, approaching the quantum Schwinger field (4.4 x 1013 Gauss). At these strengths, pair production can occur leading to a plasma dominated by electron-positron pairs. It is inevitable that magnetic reconnection occurs at some points in the magnetosphere. In the context of this reconnection, multiple flux-tubes referred to as magnetic islands can be found. Compression of the fields of a magnetic island leads to enhanced radiative cooling, and in turn further compression. The resulting runaway process leads to QED effects such as pair production.

We have performed a 3D ab initio PIC simulation of magnetic reconnection, starting from a Harris equilibrium pair plasma, where most of the energy is in the form of magnetic fields, which takes into account hard photon emission and pair production at the rates predicted from QED. The image…

Intense and energetic gamma ray sources with tabletop earmark may be available soon. Such photon beams can be generated through beamstrahlung radiation emitted when two dense and relativistic charged beams collide.

Density and energy of the colliding beams is crucial to approach the quantum regime, where the efficiency of the emission process allows to convert a relevant fraction of the kinetic energy of the primary beams into electromagnetic radiation. The quantum regime occurs when the electromagnetic field strength approaches the Schwinger limit.

Such extreme condition can be reached when dense and relativistic charged beams collide as the strong collective field of one beam is boosted in the relativistic frame of the counter propagating beam thus approaches the Schwinger limit in one particle bunch rest frame. Beamstrahlung radiation is connected with other two phenomena: disruption effect and pair creation.

Disruption rises when the particles of one beam are strongly bent by…

Recent experiments have unravel the potential to efficiently accelerate electrons of plasma wakefield accelerators .
In this context we have been investigating the formation of a plasma scheme suitable for accelerating positrons to high energies. The setup consists of a tightly focused energetic positron bunch that travels through an uniform plasma of electrons and ions. It’s space charge is capable of not only bringing electrons towards its axis of propagation as it repels ions strongly enough, and for long enough, to have them form…

Relativistic high-density neutral electron-positron (fireball) beams have been recently generated in the laboratory providing a platform to explore processes directly to address unsolved problems in astrophysics. Ab initio PIC simulations of astrophysical plasmas have provided new directions to identify the role of plasma instabilities in the formation of collisionless shock, the particle acceleration processes, and the generation of magnetic field structures compatible with the intense radiation bursts of synchrotron radiation. Recently, it has been proposed that the available 20 GeV electron and positron bunches at the Stanford Linear Accelerator Center could be used to investigate a purely transverse filamentation instability using a short fireball (e-, e+) bunch (i.e. shorter than the plasma wavelength) .

In this work, we performed 2D cylindrical PIC simulation considering the interaction of a long fireball beam (longer than the plasma wavelength) with static neutral plasmas. The development of oblique instability has been observed due…

Collisionless shocks and turbulent plasma regions are often found in space and astrophysical magnetized plasma-obstacle interactions. In the moderate magnetic fields of objects such as comets or small lunar magnetic anomalies, the plasma electrons are tied to the field lines, whereas the ions may stream across them. In these conditions, plasma waves in the lower-hybrid range can be excited. Lower-hybrid waves can be resonant with both electrons propagating along the background magnetic field and ions moving perpendicularly to it, efficiently mediating an energy transfer between the two species. This property of lower-hybrid waves has been extensively explored to drive current in magnetic fusion devices .

This figure depicts the acceleration process of electrons (spheres) in the magnetic field (lines in orange-red) of a dipolar object through the waves generated on the shock front (wavy line pattern). The results shown here were obtained in a three-dimensional particle-in-cell simulation performed to reproduce the conditions…

In extreme astrophysical environments, where plasmas are composed of electron positron pairs, the magnetic field energy is often so strong that the process of magnetic reconnection can generate flows that approach the speed of light. The significant energy contained in the magnetic fields is directed to acceleration of particles, heating, and the generation of bulk flows of charged particles via relativistic magnetic reconnection. We have performed a 3D ab initio PIC simulation of relativistic magnetic reconnection, starting from a Harris equilibrium, where most of the energy is in the form of magnetic fields. The movie shows a snapshot of oppositely directed magnetic fields and the associated current sheet after magnetic reconnection has developed. The magnetic field lines are colored by magnitude from weak (purple) to strongest (green), and are separated by a current sheet shown in red. Three key phenomena that occur during this process are highlighted: 1) The…

High quality ion beam can potentially be used for wide-ranging fields. The advent of ultra-hight intensity laser provides access to laboratory-sized compact ion accelerator via the intense accelerating fields in the laser-plasma interactions. Magnetic vortex acceleration (MVA) is proposed to produce collimated energetic ion beams in near-critical/underdense gas plasmas. Furthermore, the gas plasma allows the laser ion acceleration to be operated with a high repetition rate. The picture shows the generation of a collimated ion beam (moving upward along the vertical axis of the picture) when a relativistic laser is incident into an underdense plasma. However, for the conventional MVA, the number of accelerated ions are rather low and the corresponding spectrum is exponentially decreasing. We have proposed in our research an advanced target design in which the underdense plasma is contained within an overdense tube. In such integrated targets, the electrons in the tube can be replenished to…

The upgrade of the electron positron collider at SLAC (FACET II) promises to deliver dense, energetic and high-quality beams with the aim to study High Field QED (see FACET-II Science Workshop 2017).

However, at the interaction point (IP) of the collider, there are effects (disruption, kink instability) that must be accounted for and coupled with QED processes in the design of fruitful experiments. With the PIC code OSIRIS, we simulated the self-consistent beam-beam interaction dynamics including the effect of a transverse misalignment of the two colliding beams and incorporating as well the QED processes of photon emission (non-linear Compton scattering) and pair production (Breit-Wheeler).

The video shows the evolution of the positron beam (red) and the electron beam (green) density. The two beams propagate horizontally. During the beam-beam interaction time, disruption effect, QED pair production and kink unstable modes come into play all together. Each beam pinches under the…

In our ongoing collaboration modelling and simulating recent experiments done at SLAC, we have a similar setup as the one from the energy doubling experiment . An initially neutral Lithium gas has longitudinal profile as shown in Fig. 1.

Figure 1: Neutral Lithium longitudinal profile. The red dots represent experminetal data and the blue line is the fit used in our simulations.

A 20 GeV electron beam, with typical SLAC parameters, is used to both ionize the gas and to drive wakefield. In the first centimetres of propagation the gas density is so low that there is not significant ionization happening and the beam shape is unchanged.
As the beam propagates up the ramp of the density profile, ionization starts to become significant. A key feature is that the timescale to field-ionize the gas is shorter than the bunch duration. Therefore, the tail of the beam starts to interact with the wakefield….

The possibility of using a plasma density down-ramp injection scheme for laser wakefield acceleration has been known for almost two decades . While electrons with small impact parameters are deflected by the wake and the ones with large are hardly perturbed, somewhere in between there is a narrow sheath of impact parameters whose electrons eventually create a high-density region in the rear of the wake. These electrons are also very energetic, but they will only be useful for trapping and acceleration if their velocity is bigger than the phase velocity of the wake.

At this point, we call attention for the relevance of the down-ramp injection scheme. The phase velocity of the wake is slowed while it passes through…

The figure shows electron plasma density and the energy of trapped electrons (for clarity, we show only a random sample of particles with energy over 1 GeV). The channel wall density is about 30 – 40 % of the critical plasma density (the density opaque for laser light), while in the inside of the channel the plasma density is below 5 % of that value. This structure guides light such that most of the energy travels through the low-density region.

Due to the high laser intensity (a0 ~ 600, I ~ 5 x 1023 W/cm2 ), the interaction between the light and plasma electrons is highly nonlinear. The electrons quiver in the laser field, while at the same time being affected by the collective plasma electromagnetic fields formed within the channel. In addition to the inherent nonlinearity of the interaction, relativistic electrons also emit high-frequency radiation. Sometimes, they convert a…

The ability to shape the topology of plasma waves is a remarkable feature, which remains largely unexplored, and that may have deep ramifications into basic plasma physics and relativistic nonlinear optics. This feature is particularly interesting in the context of particle acceleration as it allows to shape the structure of the plasma in unique and novel ways, which are currently inaccessible to more conventional approaches.

Magnetorotational instability (MRI) is a crucial mechanism for the amplification of magnetic field in astrophysical accretion disks, characterised by a state of differential rotation around a massive central object, such as neutron stars or black holes. The video shows a particle-in-cell (PIC) simulation of the evolution of the toroidal magnetic field in the meridian plane of a portion of a pair plasma (electron-positron) accretion disk, where the dimensions of the simulation box are small compared to the distance of the simulation from the centre of rotation. During the early time of the simulation, the MRI amplifies the magnetic field on the proper wavelength of the instability. Due to the collisionless nature of PIC simulations, the growth of the magnetic field activates a pressure anisotropy in the plasma. This anisotropy triggers another instability called mirror instability, which modifies the structure of the MRI magnetic field with oblique filaments of the…

Our knowledge of matter and energy is shaped by extreme scenarios explored only in current colliders. Thus, unveiling the physical mysteries beyond the energy frontier requires future, more compact and efficient, schemes where relativistic colliding particles reach even higher energies and unknown phenomena take place. For light elementary particles, such as leptons, our community resorts to linear configurations, without synchrotron radiation energy losses, typically RF accelerators. However, in standard RF technologies the accelerating gradients are limited to about 100MeV/m in order to prevent its constituent material breakdown. Budker proposed the use of the plasma media collective fields as means for the energy transfer. Their pioneer concept consisted of sending a short laser pulse trough a plasma gas cell where it developed a wake of plasma density oscillations with associated fields capable of accelerating particles with gradients in the order of the GeV/m. Since then…