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

use of generator power consumption applying lower PF coil
voltages reducing inductive loads and new shape and scenario
evolution accounting better for PF coil interactions. Plasma
currents up to 1 MA at limited triangular plasma shapes,
heating powers up to 7.5 MW and flattop times of more than
5 s were achieved.
In spring 2007 the transition of the AUG vessel to fully
tungsten coated plasma facing surfaces (PFCs) was completed
with the 200 μm W-cladding of the lower outer divertor target
tiles. Careful cleaning of all other tiles was performed to remove
old carbon and boron layers. Accordingly the main emphasis
of the 2007 experimental campaign was on the operation in
an all-metal machine. The proposals were prioritized into W
compatibility of ITER scenarios, extension of the working
space with a metal wall and other ITER related physics
investigations. Plasma operation was accomplished from
April to July and September to October. We conducted 54 long
shifts at high availability with a total of 1150 pulses (technical
tests, diagnostic calibrations and plasma discharges).
1.2 ITER relevant results in 2007
In April 2007 the restart of AUG was done without prior
boronization as the first divertor tokamak operating with a
full tungsten wall. The reduced generator capacity caused
limitations of transformer flux swing, plasma current, plasma
shaping and heating power. In spite of these tough boundary
conditions, a “milestone” pulse was already achieved six
weeks after the first plasma attempt with a full performance
H-mode discharge at β N =2, confinement factor H 98 =1 and a
density of 70 % of the Greenwald density as in previous H-mode
discharges with boronization. No boronizations were performed
throughout the whole campaign. The W concentration
remained at an acceptable level by using central ECRF
at higher heating powers enhancing the turbulent impurity
transport. Obviously the divertor W source has only minor
influence on the plasma W content, which is mainly determined
by the local influx from the low field side limiters
and the central column. The carbon content of a few per
mille did not decrease further, but the net C deposition in the
inner divertor was strongly reduced. This is important for
the tritium co-deposition problem in ITER, as is the whole
W programme for DEMO where the erosion of low-Z material
and the destruction of graphite under neutron bombardment
will be unacceptable (section 2).
The use of ICRF heating is restricted with tungsten walls
by W sputtering caused by accelerated light impurities in
the ICRF antenna near-fields. Therefore the beneficial effect
of ICRH for suppression of W accumulation is offset under
un-boronized conditions. Simulations show an important
influence of the antenna geometry and the resulting box
currents on these electric fields, which will be checked in
2008 after the installation of covering metal corner plates
on one of the antennas.
ASDEX Upgrade
4
During the last years on ASDEX Upgrade the physics base
for ITER operation was significantly extended in both the
foreseen standard H-mode scenario as well as the stationary
improved H-mode (ITER Hybrid) scenario. The progress was
slowed down in 2007 in favour of the tungsten experiments.
Concerning anomalous transport, zonal flows, geodesic acoustic
modes and turbulence k-spectra were investigated by
Doppler reflectometry (section 6.1-6.2). New insights were
gained on the interaction of energetic particles (produced by
ICRH) with TAEs and other modes driven by the fast ions
based on new diagnostics (fast ion loss detectors, SXR, fast
ECE, hopping millimetre-wave reflectometer (from IST),
CO 2 laser) and theoretical tools (section 6.3-6.6). The structures
of ELM produced filaments, pellet ELM triggering,
wall heat loads from ELMs and disruptions, fuel retention in
W surrounding, and disruption mitigation by a fast gas puff
valve close to the plasma have been looked at (section 7).
1.3 Technical Enhancements and Programme in 2008
The restart of AUG is planned for February 2008, again
without boronization and with the main emphasis on reactor
relevant scenarios in an all-metal machine. Assessment of
plasma performance, divertor properties, C retention, erosion,
hydrogen and noble gas balance and operation without intrinsic
C radiation are the main themes. Improved H-modes
at low ITER-like collisionality (may need the revival of
boronizations) and at high densities will be possible with the
increased power from EZ3 extending our operational window.
At the start of the new campaign the EZ3 generator current
will be enhanced providing an increase from 144 MVA to
175 MVA. With the EZ4 flywheel back in operation in
October 2008 and with the experience gained last year the
AUG operation will be boosted into shaped plasma operation
with up to 1.6 MA and more than 20 MW heating power
at pulse lengths lasting for 10 s. This is supported by reliable
tokamak operation including NTM stabilization, ELM and
disruption mitigation. For the 2008 AUG campaign 171 experimental
proposals have been submitted by 84 scientists,
with more than 50 % from outside IPP and including 44 ITPA
joint experiments. A prioritized programme was approved by
the AUG PC and will be conducted by new TF leaders including
one from the UKAEA.
In order to achieve AUG´s programmatic goals and to maintain
a leading position parallel to the ITER construction, it is
necessary to continuously upgrade the AUG diagnostic and
technical systems (section 4). The successful H-mode operation
with an all-tungsten wall has set a milestone. The preparations
of the next major hardware extensions are well
underway with support from other EU Associations. Fast ion
loss detectors for the poloidal distribution of fast ions, CTS
(from RisØ) and beam fluctuation measurements (from HAS)
are the next new diagnostics. The tube geometry of the
blower gun extending the pellet ELM-triggering capability