IPP Annual Report 2007 - Max-Planck-Institut für Plasmaphysik ...

an admixture of 10 % D 2 ) were performed. The start-up
procedure was slower than usual, because of the technical
constraints (due to the missing fly-wheel generator EZ4)
and a less reliable plasma breakdown and current ramp-up
phase compared to boronized conditions. However, after
about 20 plasma discharges the current flat-top could be
reached and it took only about 5 more discharges to obtain
the first H-modes.
He Conc. (Div) (%)
H98
6
4
2
0
1.2
0.8
0.4
0.0
21700 21800
shot
21900
Figure 2: Temporal evolution of the divertor He/D influx ratio (top) and the
ITER H factor H98 (bottom) in I =0.8 MA, P =7.5 MW discharges. The
p NBI
vertical dashed line denotes the discharge from which on He-glow was no
longer performed.
Figure 2 shows the temporal evolution of the divertor He concentration,
and the ITER H factor H98 during I p =0.8 MA,
P NBI =7.5 MW discharges, starting with # 21700 (the re-commissioning
started around # 21600, but mostly technical trials
were performed and only about 100 s of plasma operation
were accomplished in this initial phase). Although the H
concentration was already quite low (~10 %) and the total
radiation was in the range of 50 %, the confinement remained
at H factors between 0.6-0.8. In parallel, an increasing amount
of He was observed in the plasma discharges, obviously
due to the He overnight glows and the inter-shot He glow
discharges (5 min. duration). Since it is known from experiments
and code calculations that He is de-enriched in the
divertor by a factor of 0.25-0.35, the He concentrations in
the main plasma could have reached values up to 20 %,
consistent with exploratory CXRS measurements. The storage
in and the strong release of He from W surfaces was already
ASDEX Upgrade
6
known from the previous campaign and from accompanying
laboratory experiments, but no strong influence on the confinement
was expected. However, after omitting He glow
completely and performing 3 minutes of D 2 glow only after
disruptions, the He concentrations decreased rather quickly
and, concomitantly, the confinement increased (figure 2,
bottom). Edge density and temperature measurements suggested
that a weak edge transport barrier was at least partly
responsible for the lower confinement with high He content.
Additionally, after the initial conditioning phase it was
found, that inter-shot glow discharges are much less important
for plasma ramp-up and density control, than they have
been with graphite PFCs.
Astonishingly, similar levels of oxygen as in previous campaigns
have been achieved after the initial conditioning
phase with the full W wall without boronization. Post
mortem analysis of the PFCs revealed very low C deposition
at remote areas in the divertor (see MF section), which
reflects the strongly reduced primary C sources. However,
neither C influx measurements at the central column nor
CXR spectroscopy show this strong reduction yet. Typical
values are several 10 20 /s for the gross C-influx and 0.3 % for
the edge C concentration. The reason for the persisting (low)
C influx and content is not yet completely understood, but in
principle it can be explained by the remaining small C
sources and the observed strong C recycling. Therefore, all
PFC has been thoroughly cleaned to start the 2008 campaign
with pure W surfaces.
2.3 Tungsten Influxes
Special emphasis is given to the determination of the tungsten
influx in order to elucidate the details of the erosion processes
and to assign them to specific plasma scenarios. Figure 3
shows the temporal evolution of the W influx at the low
field side limiters, the central column and in the divertor in
an I p =0.8 MA, n e ~7.5⋅10 19 /m³ discharge with continuous
NBI and ECRH as well as alternately switched ICRH.
Simultaneously, the outer radius of the plasma was varied.
The W fluxes from the divertor and the central column (HS)
presented in the bottom part of the figure were calculated
assuming toroidal symmetries. The W influx at the low field
side (LFS) ICRH limiter (W L34 ) increases immediately by
almost one order of magnitude as soon as the corresponding
antenna is powered and it increases generally for larger plasma
radii leading to a smaller gap between separatrix and LFS
limiters. From the temporal behaviour of c W it can be concluded
that the divertor W source, although it is the largest
one compared to the sources from the other areas has only a
minor impact on the W contamination in the plasma. This is
in line with earlier investigations comparing discharges with
the upper W divertor with ones using the lower graphite
divertor and dedicated experiments with W injections at the
mid-plane and in the divertor.