Collisionless Shocks in Active Galaxies and in a Laboratory Plasma
Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
Final Report Abstract
Collisionless shocks are connected to many of the highest-energy phenomena in space, from supernova explosions to gamma-ray bursts and the jets of active galactic nuclei. This unusual type of shock forms wherever a beam of charged particles streams through a charged gas and is so dense and fast that the electromagnetic field of the beam compresses and accelerates the gas to velocities even higher than the speed of plasma waves. They are similar to the normal, collisional shocks that can form in air when an object is faster than the speed of sound, but the particles in space are too far apart to collide with each other very often, as they do in our atmosphere. Instead, they synchronise their motion such that their flow produces strong electromagnetic waves, and the collective fields of these waves then push other charged particles in the same direction as a direct collision would. Different types of synchronising mechanisms, or instabilities, can be excited by these beams, depending on what kinds of particles are involved and if there is a previous magnetic field present. Because charged particles follow magnetic fieldlines, we know that parallel ion-beam instabilities, where fast ion beams excite waves by streaming parallel to a preexisting magnetic field, can be found in many different environments in space. They drive the turbulence that exists where fast ions, which are accelerated on the Sun’s surface, hit the Earth’s magnetic field, and some of the fastest particles in our galaxy are energised when these instabilities occur around supernova remnants. In this project, I showed that two of these instabilities, called the right-hand instability and the nonresonant instability, are actually mirror images of each other. In the same way in which the beam particles excite the right-hand instability with their rotation around the magnetic fieldlines, the charged particles of the background gas drive the non-resonant instability. This means that, to drive a collisionless shock, the waves belonging to both instabilities must reach large amplitudes, because a shock can only form if one type of wave accelerates the beam particles in one direction and the other type accelerates the background gas in the opposite direction. I showed that, if the beam is very dense, the waves grow more slowly than we had previously thought, but that two new instabilities can appear in that case. And I explained how the waves, if they become strong enough, can accelerate the charged particles in the right direction and where this happens close to a parallel collisionless shock. The original idea of this project was to create the first man-made parallel collisionless shock at the Large Plasma Device (LAPD) at UCLA in Los Angeles. I collaborated with the team of Professor Niemann. In this experiment, a powerful laser hits a plastic target in a plasma tube that is 17 metres long and one metre thick. As the laser pulse heats the ions inside the target, some of them leave the target and stream along a magnetic field inside the LAPD, with a velocity that is high enough that they excite the right-hand instability. Because the target ions spread too quickly perpendicular and parallel to the magnetic field, the beam is not dense enough to excite the second instability, and the waves from the right-hand instability are weaker than we had hoped. So, our team could not actually form a collisionless shock. But we know that the waves that we measure are the first step towards shock formation, and this is the first time that we can observe waves of this type with the flexibility that only a laboratory experiment can provide. We were able to measure the magnetic fields of the right-hand instability at different locations by moving a probe in 5-millimetre increments during a series of laser shots. The result is a number of movies that show the pattern of magnetic fieldlines in the right-hand instability. This substantially helps us with understanding the waves that we can usually just observe at a couple of points in the Earth’s magnetosphere.
Publications
- Towards a parallel collisionless shock in LAPD. Journal of Physics: Conf. Ser. 900 (2017)
Martin S. Weidl, Peter Heuer, Derek Schae er, Robert Dorst, Dan Winske, Carmen Constantin, Christoph Niemann
(See online at https://doi.org/10.1088/1742-6596/900/1/012020) - Observations of a Field-Aligned Ion/Ion-Beam Instability in a Magnetized Laboratory Plasma. Physics of Plasmas 25 (2018)
Peter V. Heuer, Martin S. Weidl, Robert S. Dorst, Derek B. Schae er, Anton S. Bondarenko, Shreekrishna K. P. Tripathi, Bart Van Campernolle, Stephen Vincena, Carmen G. Constantin, Christoph Niemann, Dan Winske
(See online at https://doi.org/10.1063/1.5017637) - On the Background-gyroresonant Character of Bell’s Instability in the Largecurrent Regime. Astrophysical Journal 872 (2019)
Martin S. Weidl , Dan Winske, Christoph Niemann
(See online at https://doi.org/10.3847/1538-4357/aafad0) - Three Regimes and Four Modes for the Resonant Saturation of Parallel Ion-beam Instabilities. Astrophysical Journal 873 (2019)
Martin S. Weidl, Dan Winske, Christoph Niemann
(See online at https://doi.org/10.3847/1538-4357/ab0462)
