1 - Nuclear Sciences and Applications - IAEA

498 BOLTON et al.
*
3 -
5 T
3 -
•
la)
(b)
)
0.5
t (s)
Profile
» Peak
J Average
$ Calculated from Vi
0.1 0.2
r (ml
FIG. 3. Xeg versus time (a) and minor radius (b), from visible bremsstrahlung; single point from
resistivity.
VACUUM VESSEL CONDITIONING
Prior to discharge cleaning, the vacuum vessel wall has been outgassed by ultra-violet
radiation from 4 mercury arc lamps (7 watts each). This outgassing method, novel for
tokamaks, has been found to be effective in reducing the level of all monitored gases (mass
2,16,18,28,44), resulting in the removal of about IS equivalent monolayers in 4 hours and
a pressure reduction by a factor of 5 to a final pressure of 8-10" 8 mbar.
Discharge cleaning has been carried out in hydrogen both by DC glow at 0.2 mbar
and RF-assisted glow at 10 mbar. Auger electron depth profiles are shown in Fig. 1 before
and after the two types of glow discharge. It is concluded that the DC glow discharge leads
to an erosion of the surface contamination (C and O) at the rate of 0.6 nm/h (Fig. lb) while
the low-pressure RF-assisted glow discharge leads to efficient removal of the surface oxygen
but also deposition of carbon (Fig. ic) probably originating from the graphite limiters. The
form of the C KLL peak (insert) and the ratio Fe/C in the latter case indicate carbide
u
1.0
-