Identifying Trajectories of Energetic Particles in the Caltech Plasma Jet Using a PIN-Diode-Based 1D Coded Aperture/Pinhole X-ray camera.*

The Caltech plasma jet experiment produces a magnetohydrodynamically-driven, collisional plasma jet with a length increasing to roughly 40 cm in ~20 μs, an electron-ion collision mean free path of a few micrometers, and a nominal electron temperature of 2 eV. Even though this plasma jet is cold and highly collisional, transient (~1 µs duration), low-intensity (a few photons per mm² at 1 m), and highly energetic (> 6 keV) X-rays with a broad energy spectrum (ΔE ≈ 4 keV) have been observed simultaneously when the plasma jet is perturbed by magnetohydrodynamic instabilities. In order to understand how charged particles inside this collisional plasma jet are accelerated from ~ 2 eV to ~ 6 keV, a PIN-diode-based 1D X-ray camera has been developed to spatially, temporally, and spectrally resolve the X-rays. This X-ray camera has high detection efficiency over the 5–10 keV X-ray band, an over 20-degree field of view, and the capability to produce more than 50 time-resolved frames with a submicrosecond time resolution. 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 X-ray images suggest that charged particles in the plasma jet are accelerated in two opposite directions and collide with metal objects inside the vacuum chamber to produce bremsstrahlung X-rays.