[en] The surfaces of the first wall of the CTR will be bombarded by energetic helium ions with ion energy distributions in the approximate range 100 eV to 50 keV with an energetic tail up to 3.5 MeV and with flux rates ranging from 10¹³ to 10¹⁸ ions/cm². s. Although the mechanisms for bubble formation and blistering have been extensively studied at higher energies (>10 keV) comparatively little work has been done on the effects of low-energy (<3 keV) helium ion bombardment. This is partly due to the difficulty involved with the observation of the small damage volumes produced, since the depth of damage is <20 nm and so is less readily investigated using techniques such as electron microscopy. The field-ion microscope (FIM) is a useful technique for the study of the radiation damage caused by low-energy ions since not only does it possess atomic resolution, but it also allows the precise variation of the damage with depth to be investigated by removing successive atomic layers by field evaporation. In the present study an ultra-high vacuum FIM has been used to observe tungsten surfaces bombarded with helium ions in the energy range 0.2 to 3 keV with doses ranging from 10¹⁵ to 5 x 10¹⁷ ions/cm². The tungsten surfaces were initially characterised and then bombarded with a uniform ion beam incident along the [110] axis of the emitter. The specimens were bombarded at ground potential and at room temperature using a monoenergetic ion beam produced by a 3 keV Varian ion gun with ion current densities of 20 μA/cm². (Auth.)