2-Dimensional Measurements of Brehmsstrahlung X-rays Emitted by Trapped Electrons Impacting Tungsten pellets in an Microwave-Heated Magnetic Mirror in the LAPD

In December 2024 the Caltech 2D X-ray Camera was brought to the LAPD to diagnose the temperature of electrons trapped with a magnetic mirror by a 2.4GHz X-mode microwave heater. Electrons in this mirror form a confined electron ring. The temperature distribution of this confined, heated electron population is difficult to measure, but of great interest. It is extracted by injecting tungsten pellets into the interior region of the magnetic mirror during the afterglow of the plasma discharge. While the pellet is injected, electrons are heated by X-mode microwaves. As electrons impact the tungsten ball they emit brehmsstrahlung x-rays, which can be measured by the Caltech 2D x-ray camera. We assume that nearly all the energy of the confined electron ring is in perpendicular motion to the magnetic field, and thus the x-ray emissions are expected to correlate to electron energization and the perpendicular velocity distribution. The 2D x-ray camera recorded several hundred movies of tungsten balls dropping through the plasma. We present an examination of the overall intensity of emitted x-rays and an energy spectrum deduced from transmissions through multi-metal filters, as well as its correlation to the underlying electron energy spectrum. This work has been supported by NSF award no. 2403814, and was performed at the Basic Plasma Science Facility at UCLA, which is funded by the Office of Fusion Energy at the Department of Energy.