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

the future, a new safety interlock was introduced which
switches off the RF system in case of an unrequested power
output. Whenever the limited time between shots allowed,
conditioning of source no. 2 was continued. This source had
been installed close to the end of the previous campaign as a
replacement for a source which had suffered a water leak.
At the beginning of the 2007 campaign, the source’s extraction
grid system was in a completely unconditioned state.
After about 1300 conditioning shots at the end of the campaign
the source could be operated more or less reliably at
40 kV, where 60 kV is the standard operation voltage of this
injector. Conditioning is expected to be finished during the
2008 campaign.
In the following two months shutdown phase maintenance
of both injectors could be finished in due time for the start of
the next campaign despite an extensive list of tasks. Among
them, all eight titanium evaporator pumps were overhauled.
Traces of melting were discovered on bellows of the cooling
connections to the ion dumps of the second injector. The
bellows were replaced and protection shields installed.
3.5 Ion Cyclotron heating
The ICRF system was ready when ASDEX Upgrade restarted
operation. Now all antennas are equipped with W-coated
limiters. One antenna has an optically closed Faraday
screen, to investigate the influence of shielding the antenna
from plasma, possibly expelled by ELMs into the antenna,
on its voltage standoff capability. One antenna has also been
fitted with the option to locally inject gas and with diagnostics
to measure currents drawn by some limiters. Whereas
experimentation with ICRF has become more difficult, a way
was found to still use ICRF efficiently to heat high density
discharges centrally: a large plasma-antenna distance is beneficial,
together with a high gas puff rate. In terms of plasma
and impurity behaviour, there is no difference between
standard gas puff and local gas puff near the antenna. The
latter however has the advantage that it increases the antenna
coupling resistance, which is low at a large antennaplasma
distance.
One of the power tetrodes failed during operation, increasing
the need to find a solution for the unavailability of replacement
tetrodes. While this tetrode is being refurbished, the
ICRF systems have to operate without any spare tetrode.
The failure of another tetrode would result in a reduction of
the available power by half. The option to modify the generators
to be able to use commercially available tetrodes is
being investigated.
During the AUG vessel opening at the end of the year, diagonal
plates covering the uncompensated top and bottom
parts of the antenna straps have been installed. Calculations
show that the dominant contributions to the electric fields are
due to currents in the antenna box. Consequently, this installation
should not lead to a substantial change in impurity
ASDEX Upgrade
10
production, as long as the fields due to the antenna box
remain the major contribution. Our modelling also shows
that those dominant fields can be reduced, by placing the
antenna in a wall and by having 4 straps with the proper
phasing, rather than 2. A new vacuum chamber, where such
a 4-strap antenna can be tested, was ordered for the new test
facility located in the L7 hall. The contract for the automatic
matching was cancelled as it became clear that the company
that had been working on it since 2000 could not fulfil its
obligations. Alternative options are being investigated. A
complete overhaul of the compressor system providing pressurized
air for the transmission lines is ongoing. It will allow
for faster refilling of those lines and easier maintenance.
3.6 Electron Cyclotron Resonance Heating
In 2007 both ECRH systems were routinely requested for a
large number of ASDEX Upgrade pulses. The main drive
for the use of ECRH was the prevention of central accumulation
of tungsten (see section 2). The old system was back
to full power (i.e. 1.6 MW, 2 s or 0.8 MW, 4 s) after a gyrotron
broken in 2004 was replaced in the last shutdown by a
similar tube, formerly used at W7-AS.
In February 2007 the new 2-frequency gyrotron Odissey-2
was commissioned. The achieved maximum output power
was 910 kW at 140 GHz and 650 kW at 105 GHz, both for
10 s. Using the new gyrotron, the new stainless-steel highpower
long-pulse load from GYCOM has been conditioned
rapidly and has operated very reliably since then. With
140 GHz the gyrotron was used routinely for heating (700 kW,
limited by arcing in torus-side mirror box, for 5 s, limited by
plasma duration). With 105 GHz, first experiments for collective
Thomson Scattering were performed with the new
RisØ-System which receives its signal through the transmission
line of unit 2. The extension of the new ECRH-2 system
with three 4-frequency gyrotrons 105-140 GHz/4×1 MW/10 sec
is ongoing. First short-pulse tests of the modified gyrotron
Odissey-1, which has been equipped with a BN-Brewster
window, were performed at GYCOM up to 1 MW/0.8 MW
at 140 GHz/other frequencies. No arcing on the air-side was
observed, proving the viability of the concept. Actually the
BN-window has been replaced by a diamond window. Final
factory tests are expected soon. For details on the future
planning for the new system see section 4.
4 AUG enhancements
Continuous progress in tokamak research needs hardware
upgrades that enhance the machine’s capabilities into new
directions, according to recent theoretical and experimental
findings. In line with its mission to prepare for ITER,
ASDEX Upgrade is presently being equipped with new
tools that will enable active control of MHD modes both in
the plasma core (sawteeth, NTMs) as well as in the edge