Description
Relativistic outflows enriched with electron-positron pair plasma can be found in various highly energetic astrophysical environments, such as those around active galactic nuclei, black holes, or in the jets of gamma ray bursts. Understanding the mechanisms involved in the generation of particular types of plasma instabilities during the propagation of such jets is important in order to explain the radiative signature of their outflows observed on Earth. Until now, the understanding of electron-positron pair plasmas has been based on numerical studies limited in the quasi-linear regime, while experimental realizations of high charge density, quasi-neutral beams have been challenging. In our experiments, inaugurating and improving a newly developed experimental platform for such studies at the HiRadMat facility of CERN, high intensity, high density, ultra-relativistic, quasi-neutral electron-positron pair beam production was achieved, opening up the possibility to study the microphysics of such pair plasmas via experimental means.
In our experiments, we confirmed that the growth of plasma instabilities as the pair beam propagates through ambient plasma is greatly affected by the divergence and momentum spread of the pair beams. We present a study on the implementation and the performance limitations of a beam focusing setup used in one of our experiments. We designed a quadrupole triplet, based on custom-made permanent magnet quadrupoles, aiming to improve the optical and momentum properties of the electron-positron pair beam towards an increased e-/e+ density. A comparison between the experimental results and the expected performance given by different computational tools, along with the challenges of such a setup that should be considered for future implementations, is presented in this work.