Photoionised plasmas in the laboratory

Abstract

The experiments in this thesis advance previous research into photoionised plasmas in the laboratory and the identification of spectral lines relevant to Neutron Star Mergers (NSMs), both of which relate to laboratory astrophysics.

Photoionised plasmas were created in the laboratory using double-sided irradiated gas cell targets filled with argon at various pressures with Ag foils at each side. The Ag foils are the source of the x-rays. The photoionisation parameter (ξ = 4πF/n_e), where F is the x-ray flux and n_e is the electron density, is an important astrophysical plasma parameter and was calculated for each experimental shot. The photoionisation parameter is usually a steady state parameter but is often used in laboratory astrophysics experiments that are not steady state as an indicator of relevance. By utilising laser-plasma x-ray sources, ξ values of > 100 erg cm s^−1 were produced in the range relevant to a number of astrophysical scenarios. In particular, a keV spectral temperature was generated by using a keV line source instead of the typical quasi-blackbody radiation fields typically used in other experiments. This is also a key element for comparing the results of experiments to the predictions made by various codes used to model astrophysical sources. Results were compared with an in-house plasma modelling code and a good level of agreement was discovered for the temporal evolution of various parameters such as the electron temperature and average ionisation.

A laser plasma x-ray source was created for use in the photoionisation research described above at the VULCAN laser facility. Thin Ag or Sn foil targets coated on a parylene substrate were exposed to a nanosecond laser pulse with an intensity of order 10^15 Wcm^−2, and the L-shell emission in the region of 3.3-4.4keV was observed on both the laser-irradiated and non-irradiated sides of the targets. Ag produced higher laser to x-ray conversion yields than Sn, according to both experimental and simulated results, with the simulations predicting yields that were roughly two times higher than those reported in the experiments. The simulations reveal that the emission is almost isotropic on the laser-irradiated side of the target but exhibits close to a cosine variation on the non-irradiated side of the target as seen experimentally in earlier work.

The experiment related to NSMs aimed to obtain accurate experimental line wavelengths for Au, in the 300-1000nm wavelength range, that are not recorded in the National Institute of Standards and Technology (NIST) database. By applying laser intensities of up to 10^9 Wcm^−2 onto a solid high purity Au target under vacuum (10^−5 mBar), a laser-produced Au plasma was created. A section of the optical radiation was taken out of the vacuum chamber and directed onto a 50μm slit of a high-resolution spectrometer. The results show that there are 54 unknown Au lines seen experimentally that are not found on the NIST database and are not from any large impurities of Au (Cu and Ag). Of these 54 lines, 21 match strong lines in the produced atomic calculations for Au I and Au II using the calculated plasma conditions (n_e ∼ 6×10^17cm^−3 and T_e ∼ 0.85eV). The intensity distribution of the 21 lines from the atomic calculations are in good agreement with the experimental lines and are virtually identical in behaviour. These 21 lines are, most likely, newly identified Au I and Au II lines (8 new Au I lines and 13 new Au II lines).

Date of Award	Jul 2024
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Northern Ireland Department for the Economy
Supervisor	Gianluca Sarri (Supervisor) & David Riley (Supervisor)