Magnet Aspect Ratio

In Figure we show the glow discharge around each type of magnet.

In table we observe that the more prolate the aspect ratio of the magnet, or the more negative the voltage bias put on the magnet, the smaller the annulus of the trapped plasma. Eventually, the trapped plasma is making direct collisions with the nickel-plating and the plasma glow becomes quite white with obvious feedback from surface ablation. This type of discharge heats the magnet, and if sustained, raises the temperature of the outer portion of the ceramic Nd-B magnets above their Curie point. Since this weakens the field, the gyroradii of the trapped plasma enlarge and even more plasma contacts the magnet, resulting in runaway failure of the trap (and a dead magnet). Since we would rather avoid this mode, we select the magnet with the most stable plasma, which is the oblate or pancake shaped magnet.