Density Mapping of Negative Hydrogen Ions in Cold Electron Plasma

In a room temperature electron plasma, a small fraction of negative hydrogen ions (H⁻ ions) gradually accumulates due to dissociative attachment in collisions of cold electrons with excited hydrogen molecules, and it is continuously carried away radially by faster ion transport. This fraction of H⁻ ions changes many properties of the plasma column; including a straightforward decrease in the frequency of plasma waves, and it makes a large contribution to the waves (collisional) damping rates. In our work, the radial density distribution of H⁻ ions is mapped in an electron plasma column confined in a Penning-Malmberg trap. The trap is equipped with an electron-sensitive Phosphor Screen/CCD (PS/CCD) only, so a direct imaging of H⁻ ions is not possible. Rather, we find n_H(r) by comparing images of n_e(r) with and without H⁻ ions stripped of their extra electrons. Here, the H⁻ ion stripping can be done by intense (laser) light (hν ≥1eV) or by electron detachment in collisions of accelerated H⁻ ions with heavy (background) molecules. Fortunately, the electron detachment cross section in collisions of 10÷20eV H⁻ ions with heavy molecules is quite big (∼20Ų). At a residual pressure of 10^-9 Torr, the H⁻ stripping rate is about 0.5/sec. At a selected moment of plasma evolution, the H⁻ ions are quickly (<0.1sec) energized in axial velocity by bounce-resonant sloshing of the plasma column. Then, during a few seconds of confinement, the accelerated H⁻ ions lose their extra electrons, which now contribute to the electron density n_e(r) at subsequent dump to PS/CCD. By subtracting a similar image taken without the H⁻ acceleration/stripping cycle, we obtain the distribution n_H(r).