The chamber is filled with deuterium gas at low pressure (0.7 pascals). As the crystal is heated, the potential builds across the crystal. Deuterium ions (deuterons) are generated at the tungsten tip, and accelerated towards the target; the electrons fall back to the crystal electrode. The ions strike the deuterium target (ErD3), and some generate 2.5-MeV neutrons. Electrons knocked from the target surface are repelled by the suppression grid and fall back on to the target rather than being accelerated back to the crystal.