ELEMENTARY PARTICLE PHYSICS

Plasmas can sustain electric fields that can be orders of magnitude higher than material breakdown thresholds. Thus, even in the presence of extremely high electric fields, a plasma in an equilibrium will simply readjust in order to reach a new equilibrium state, without loosing any of its properties. This unique feature has been exploited towards the development of novel, more compact, plasma-based accelerators. Although there is still much room for improvement, plasma accelerators have the potential to either improve or to assist conventional particle acceleration techniques towards the construction of increasingly powerful accelerator devices and associated light sources.

There are many university scale laboratories where plasma based acceleration experiments are routinely performed, and several experiments are being planned in national and international facilities around the world. One of the most exciting of such projects is the proton driven plasma wakefield accelerator currently under preparation at CERN. In this project, a long proton beam from the Super-Proton Synchrotron will excite large amplitude plasma waves that will be used to accelerate electrons to high energies. A large collaboration has been set up, called the AWAKE collaboration (AWAKE).

One of the challenges for AWAKE is to stabilise proton beam propagation over very long distances. With the unique computational power of high performance computing (HPC) system SuperMUC, researchers of the Group for Lasers and Plasmas (GoLP) at the Institute for Plasmas and Nuclear Fusion of Instituto Superior Técnico in Lisboa (Portugal), of the Max Plank Institute for Physics in Munich (Germany), and of the University of California in Los Angeles (USA) have performed three-dimensional simulations of AWAKE relevant scenarios. Their numerical findings, obtained using OSIRIS, a massively parallel fully relativistic particle-in-cell code, provided a set of conditions for which the long proton bunches could propagate over arbitrarily long distances [1]. The work also establishes a set of conditions for which the beam propagation becomes unstable. As a result, the physicists defined sets of parameters that can be used to stabilise the propagation, to study particle acceleration, or explore new physics associated with unstable beam modes (Figure 1).

In addition to particle acceleration, similar setups could also be exploited towards understanding fascinating astrophysical mysteries. The scientists have then performed three-dimensional simulations that can be related with the origin and amplification of galactic or extra galactic magnetic fields. In collaboration with an experimental team at Laboratoire d'Optique Appliquée (Paris, France), they have then found a new mechanism for the generation of large-scale, persistent magnetic fields that could occur in astrophysical scenarios [2] (Figure 2). The three-dimensional simulations used a laser pulse driver, but similar findings could also be found by using proton beam drivers. In addition, it was also shown that electron-positron particle bunches from laser-driven plasma accelerators could greatly amplify ambient magnetic fields (Figure 3). Similar physics could also occur during the propagation of a long proton bunch in conditions close to the AWAKE experiment [3].