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

oscillations) keeps the profiles in a stable regime. Pellet
injection into ohmic plasmas showed a directly pellet driven
MHD perturbation exceeding by far that one present at the
visible ELM onset thought to be in the order required for
ELM triggering. No correlation was found between the
MHD perturbations with pellet particle ablation or deposition
rate, indicating an already saturated driving mechanism.
Lower local plasma energies result however in a reduction
of the MHD excitation attainable by the pellet perturbation.
The correlation between pellet MHD drive and plasma
parameters are now under investigation.
7.4 Parallel plasma flow and radial E-field in the SOL
The fast reciprocating probe located 30 cm above the torus
midplane was used to investigate the SOL from the limiters
up to the separatrix. It was equipped with swept Langmuir
probes facing in co- and counter-current direction. This
allowed the floating potential (V fl ), the electron density (n e )
and temperature (T e ) and the Mach number (M) of the parallel
plasma flow to be measured all at once. Also the plasma
potential V pl =V fl +3.1T e and the radial electric field E r =-∇ r V pl
could be deduced. In a series of ohmic discharges the density
was varied covering a Greenwald density fraction of
f GW =0.21-0.49 with I p =1 MA and B t =-2 T. The outer divertor
was fully attached at f GW =0.21 and fully detached at
f GW =0.49. On both sides of the probe head the same T e was
found, but compared to values from Thomson scattering
(TS) T e was considerably lower for ΔR=R-R sep 1 cm). When the outer
divertor was (partially) detached M≤0.6 was found at the
separatrix decreasing to M≈0.2 at ΔR≈0.5 cm and rising
again further outward (M≤0.4). Close to the separatrix the
Pfirsch-Schüter and “return flow” can cause the observed
flows but for ΔR>0.5 cm there has to be another contribution
like e.g. a transport related flow. Flow and divertor
detachment are possibly related.
7.5 Impurity flux in SOL
Ion fluxes in the SOL were measured by exposure of collector
probes with the midplane manipulator system and subsequent
quantification of deposited deuterium and impurity
elements by ex-situ ion beam analysis. Discharge-resolved
and even time-resolved measurements within one discharge
can be carried out employing rotating cylindrical samples
inside a shield with a 6 mm wide slit aperture extending
88 mm in a radial direction. Increased impurity fluxes are
observed during the low density current ramp-up phase as well
as in plasma configurations with a small separatrix-sample
distance and increased ion cyclotron resonance heating
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(ICRH) power. The increase of the impurity flux in the first
two cases is attributed to an increased wall flux and correspondingly
higher sputtering flux. In the latter case the interaction
of the ICRH antenna with the plasma is expected to
create a local impurity source increasing with heating power.
In recent experimental campaigns, apart from the dominant
first wall element tungsten, the main impurities were calcium
(from ceramic isolation) and steel constituents (Fe, Ni) from
the vacuum vessel. The derived concentrations of c Fe ≈10 -3
and c W ≈10 -5 at the plasma edge are in good agreement with
spectroscopic measurements. In order to quantify the edge
carbon flux with a full tungsten wall configuration a full
metallic probe head (TZM shield, Al cylinder) has been constructed.
A typical residual carbon deposition rate of a few
10 21 m -2 / discharge was detected and no significant temporal
evolution of the carbon flux throughout the 2007 campaign
could be observed similar to spectroscopic investigations
(see Chapter 2).
7.6 Analysis of divertor profiles
Dedicated discharges with strike point sweeps allowed power
load profiles derived from Langmuir probes to be compared,
assuming P LP =(7 k B T e +E rec )Γ i, and IR thermography.
Figure 20 shows profiles of electron temperature, density
and power fluxes along the outer target for the inter-ELM
phase of a medium and a high density H-mode discharge.
Coherent averaging over about 100 ELM cylces was used to
reduce the statistical uncertainties. While the T e profiles
along the target are flat, the density profiles are peaked close
to the separatrix. For the high density H-mode the profiles
indicate partial detachment around the strike point. The target
profiles (T e , n e , j sat , P IR ) are very similar comparing discharges
with carbon (divertor IIb) and tungsten (divertor
IIc) target plates. The electron power width obtained from
the Langmuir probes using a sheath transmission factor of 8 is
a factor 2 shorter than the one obtained from thermography.
Both widths increase with density, compatible to the trend
seen for the electron temperature decay length as obtained
from Thomson scattering measurements. One exponential
decay length is sufficient to fit the power decay for these
medium/high density conditions. While the power widths
obtained from the probes are a factor 2-3 larger than the
expectation 2/7·λ Te midplane , the thermographic widths are
clearly out of range, suggesting dominant effects of fast ion
impact and/or radiation.
During ELMs, similar power loads are observed at the outer
target from Langmuir probe and thermography analysis for
both divertors IIb and IIc. The large deposited ELM energies
observed with thermography at the inner target have
shifted upward, away from the strike zone from divertor
IIb to IIc. A corresponding redistribution of the target currents
indicates that in fact the impact region of fast ions
has shifted upward.