Understanding the dynamics of ultra-massive astrophysical objects requires detailed knowledge of the rich microphysics occurring in its vicinity. Complex phenomena, spanning the realms of particle physics, nuclear physics, and plasma physics result in macroscopic observables, which are monitored by ground and space telescopes. However, the colossal distances at which these events occur (sometimes even exceeding one billion light years) makes a detailed characterisation of the local physical properties of these objects virtually impossible. Most of our knowledge of these objects must then be inferred from numerical modelling and analytical work.
The recent development of high-power and high-energy lasers is now opening up a different route; reproducing small-scale versions of these phenomena in the laboratory where their dynamics can be studied in detail. The interaction of high-power lasers with matter can indeed generate extreme, and yet controllable and reproducible, environments, with temperatures exceeding the MeV (> 1 billion degrees) and pressures in the Mbar up to the Gbar regime.
Our group is actively engaged in this area of research, broadly defined as laboratory astrophysics, with particular attention to the plasma physics involved in the formation and dynamics of collision-less shock waves, neutral matter-antimatter beams, and photo-ionised plasmas in ultra-intense radiation fields.
The onset and propagation of shock waves (SW) in tenuous, collisionless plasmas is a phenomenon of central interest in astrophysics. SWs are common in the interstellar medium following the occurrence of energetic events such as supernova explosions, as well as in near-Earth plasmas such as the bow-shock region and the magnetosphere. Several means of producing collisionless shock waves in the laboratory exist, including through the expansion of a dense plasma, resulting from laser ablation of a solid target, through a rarefied background plasma. By employing this production technique, coupled to advanced diagnostics, we are able to characterize the generation and propagation of purely electrostatic as well magnetized shocks. In particularly, using a laser-driven, proton radiography technique pioneered at QUB allows us to obtain detailed maps of the electric and magnetic field across the shock front, as well as to investigate the shock stability and evolution.
Neutral electron-positron beams represent a unique state of matter, whereby an intrinsic and complete symmetry exists between the negatively charged (matter) and positively charged (anti-matter) particles. Electron positron beams are emitted, in the form of ultra-relativistic winds or collimated jets, by some of the most energetic or powerful objects in the Universe, such as black-holes, pulsars, and quasars. Only recently, our group has succeeded in generating the first neutral and high-density electron-positron beam in the laboratory and give the first characterisation of its dynamics. We are currently working on experimentally characterising plasma dynamics and subsequent field generation of an electron-positron beam propagating in a background plasma, in order to provide the first laboratory platform for the small-scale study of astrophysical jets and GRB emission, against which numerical models used to interpret astrophysical observations can be compared and refined.
Photoionized plasmas are thought to be regularly associated with accretion-powered astrophysical objects, where the radiation field is sufficiently intense that photoexcitation and ionization rates are high relative to electron collisional excitation and ionization rates. The distribution of ionization is characterised by the photoionization parameter ξ, which may reach ≃ 1000 erg cm s-1 or greater. There have been several attempts to create such photoionized plasmas in the laboratory, to allow plasma modelling codes to be benchmarked against well-diagnosed experimental data. However, as the electron density is generally much larger than the astrophysical counterpart, so too must be the X-ray flux to generate high values of ξ. We are attempting to achieve these conditions by the use of large-scale laser facilities, such as the VULCAN laser at the Central laser facility, to generate extremely intense, nanosecond duration X-ray sources.
KEY RECENT PUBLICATIONS
1. G. Sarri et al., Generation of neutral and high-density electron-positron pair plasmas in the laboratory, Nature Communications 6, 6747 (2015)
2. J. Warwick et al., Experimental observation of a current-driven instability in a neutral electron-positron beam, Physical Review Letters 119, 185002 (2017).
3. H. Ahmed et al., Time-resolved characterisation of the formation of a collision-less shock, Physical Review Letters 110, 205001 (2013).
4. H. Ahmed et al., Experimental observation of a thin-shell instability in a collision-less plasma, Astrophysical Journal Letters 834, L21 (2017)
5. S. White et al., Production of photoionized plasmas in the laboratory with x-ray line radiation, Physical Review E 97, 063203 (2018)
Laboratory studies of neutral and collimated electron-positron beams, EPSRC EP/N027175/1