Electron Injection in Nitrogen Plasma

When a negative potential of $\sim$ 1 kV is applied to the electrode, a weak glow (in $N_2$) indicates that electrons are ionizing the background gas and electrons are injected into the plasma (Figure 3). Electrons that have small pitch angles are attracted to the grounded magnet, producing copious X-rays. Faint ``horns'' can be seen on the field lines that connect the electrode to the magnet. Electrons that have pitch angles near 90$^\circ$ gradient drift around behind the magnet and can be seen to the left of the magnet as a faint glow outlining the dipole field lines. The X-ray detector brightens as the voltage on the electrode is raised. Electrons that impact on the top of the magnet form a circular aurora made more visible by a light coating of phosphor dust.

The 16-bit resolution camera was able to better capture the density of electrons as they drifted around the magnet. We show the bottom two panels of Figure 3 in false color, the left at 2.5kV potential, the right at 4kV. One artifact of the digitization is the ``bleeding'' upward of the pixels as they are read from the camera. In both panels the electrode was bright enough to saturate the pixels in a small region. Note the auroral zone expands outward as the electron energy is raised. Other than slight density changes, the plasmas show no qualitative differences. Nor did spinning the magnet make much qualitative difference, though it did affect the X-ray shadow.