The larger ion lasers use a cylindrical electromagnet around the bore that "squeezes" the plasma along the length of the bore, decreasing its volume, which increases the energy density (and hence output), and greatly assists in keeping the plasma away from the bore walls, reducing erosive effects.
Also as mentioned above, some helium-neon lasers, especially those with longer tubes, use magnets placed at intervals along the tube; these are not used for the same purpose as in the ion laser, but rather to enhance the gain of the visible laser transitions over the naturally higher gain of infrared laser transitions by decreasing the gain of those transitions, which originate from the same levels as the visible lines; because of their higher gain the infrared lines tend to "take over" in longer tubes and rob energy that would otherwise show up in the visible lines.
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