Experimental and Theoretical Studies of Unstable Dynamics of Caltech’s Plasma Jet: X-Rays, Ultraviolet, and Visible Light

Citation

Zhou, Yi (2023) Experimental and Theoretical Studies of Unstable Dynamics of Caltech’s Plasma Jet: X-Rays, Ultraviolet, and Visible Light. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/yfha-cs19. https://resolver.caltech.edu/CaltechTHESIS:05262023-165151068

Abstract

The Caltech plasma jet experiment launches a laboratory plasma jet that is analogous to an astrophysical jet. Even though the temperature of the plasma jet is around 2 eV, 6 keV X-rays and 20--60 eV extreme ultraviolet (EUV) radiation were detected when the plasma jet was perturbed by magnetohydrodynamic instabilities. How charged particles in a plasma are accelerated to suprathermal energy has been a key question in plasma physics, solar physics, and astrophysics. Studying these surprisingly energetic radiations from Caltech’s plasma jet can help answer this question. Toward this goal, this thesis contains an experimental study of the X-rays and a theoretical study of the EUV radiation.

In the experimental study, a PIN-diode-based 1D X-ray camera has been developed to spatially, temporally, and spectrally resolve the transient, low-intensity, and suprathermal X-rays detected to be simultaneous with magnetohydrodynamic instabilities that disrupt the plasma jet. This X-ray camera has high detection efficiency over the 5–10 keV X-ray band, an over 20-degree field of view (FOV), and the capability to produce more than 50 time-resolved frames with a submicrosecond time resolution. The X-ray images are formed by a pinhole or by a coded aperture placed outside the vacuum chamber in which the plasma jet is launched. The 1D imaging shows that the location of the X-ray source is either a few centimeters away from an inner disk electrode or near a spatially translatable metal frame that is 30–40 cm away from the electrode.

In the theoretical study, we propose a collisional two-fluid model which involves a novel two-stream instability that is indifferent to collisions, even though collisions have been traditionally presumed to damp the two-stream instability. This model is used to explain previously observed localized dimming of visible light and a simultaneous, localized burst of EUV radiation from a plasma jet the cross section of which is constricted by a kink-instigated Rayleigh-Taylor instability. On being triggered by the constriction of the plasma cross section, the proposed two-stream instability produces a region of low density where an electric double layer leads to localized electron heating. The low-density region is consistent with and so likely explains the visible light dimming, and the localized electron heating is consistent with and likely explains the EUV radiation. The numerical solution of the collisional two-fluid model demonstrates good agreement with the apparent electron velocity and density profiles in the plasma jet.

Thesis Files