Ion Injection in a Helium Atmosphere

Almost immediately we noticed a great intensification of the circular discharges (Figure 5). As we raised the pressure of Helium from 60 mTorr to 200 mTorr, the circular lightning became more distinct, and with further pressure increase it reduced in frequency as well as diameter. The top panel of Figure 5 is 100 mTorr with electrodes at 900 and 1400 V (briefly); the bottom panel is identical but at 400 mTorr. Note the many discharges in the top panel, increasing in occurrence on the edge closer to the ion injector. The point discharges along the cylindrical sides of the magnet often occur in pairs, with at least one pair connected by a loop. The discharges from the top can be seen to follow larger magnetic loops. The trapped plasma produces a noticeable bright band at the equator. In the bottom panel, only two discharges are observed over the same 30 second exposure, though comparatively brighter. The larger of the two is closest to the ion injector.

The parameters show that lower pressures are more easily ionized and produce more discharges, while the higher pressures have fewer but brighter discharges. We therefore use 200 mTorr as a good intermediate pressure to show the effect of adjusting the other parameters. In particular, we want to know if spinning the magnet can intensify the process. Note that very little trapped plasma is visible in the higher pressure system, most probably due to the shorter scattering length preventing the ions from drifting completely around the magnet.

Both panels of Figure 6 are taken at 200 mTorr, with 900 and 1400 V on the electrodes. The panel at the bottom shows the effect of spinning the magnet at approximately 1000 rpm. Note that the discharges are very evenly distributed across the magnet. The torus of trapped plasma appears more intense as well. This suggests that even as highly collisional as this plasma is, the induced electric field enhances the separation of charges and produces a more intense trapped ion plasma. Close inspection of the top panel with the stationary magnet reveals an arcade of discharges near the equatorial region at the spot where the advancing cloud of ions $\nabla B$-drift away from the magnet.