TRIAC Progress Report - KEK

with different plasma volumes [2-6], a plasma chamber around 1 l appears to be enough
for our purpose. Furthermore, the volume determined in this way can accommodate a
hot ECR zone with an axial length of 100 mm and a radial diameter of 50 mm. It would
be sufficiently large so that the ECR plasma could efficiently capture injected ions for
charge breeding.
The deceleration system in Fig. 2-8 is a simplified version of that used in the pilot
breeder [2-7]. Instead of the multi-step deceleration, two-step deceleration is adopted by
using two concentric cylindrical electrodes. The outer cylinder, 40 mm in diameter, is
operated as a final stage of deceleration, whereas the inner cylinder, 20 mm in diameter,
stays on the ground potential. Therefore, the main deceleration happens between two
electrodes and further smooth potential drop exists between the outer cylinder and the
plasma chamber. Both of the electrodes are altogether movable axially around the
position of the maximum axial magnetic field for the optimization of the injection under
the influence of the field. The outer cylinder is now removed because of the aging
problem of the insulator on which the electrode was attached. Instead, a similar shape of
electrode is directly attached to the inner tube (plasma chamber). The aging effect was
found due to the irradiation effect of RF wave unintentionally leaking from the ECR
chamber. In the present system, only the ground electrode (i.e. inner cylinder) is
movable and the outer electrode is always on the same potential as the plasma chamber.
Fig. 2-8. Cross-sectional view and specifications of the 18-GHz KEKCB
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