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An international journal of news from the stellarator community
Editor: James A. Rome Issue 164 February 2019
E-Mail: James.Rome@stelnews.info Phone: +1 (865) 482-5643
On the Web at https://stelnews.info
Second divertor operation
campaign: High-density, longpulse
operation
The most recent operation campaign, which was finished
in fall 2018, marked the completion of the initial divertor
campaign of Wendelstein 7-X (W7-X) at Max Planck
Institut für Plasma Physics, Greifswald, Germany. During
the 14 weeks of operation, W7-X operated for 35 days,
and a total of 1500 scientific plasma discharges were
accomplished. As in the previous campaign in 2017, operation
was done in full island divertor geometry with 10
inertially cooled divertor targets. Thus, heat loads to the
plasma-facing components needed to be monitored closely
and led to an initial limit on the allowed heating energy
input of 80 MJ, which could be increased to 200 MJ later
in the campaign. In addition to enhanced diagnostics capabilities,
two major new components were implemented:
improved wall conditioning via boronization, and neutral
beam injection heating. Primary goals of the campaign
were to develop stationary high-density hydrogen discharges
and to operate the divertor in a safe and controlled
way. More than 300 proposals were selected by the topical
task forces and the proposals were executed to address
these goals.
Boronization has a tremendous effect on the plasma performance.
The associated reduction in the impurity content
leads to much lower plasma radiation losses and
allows W7-X to operate at considerably higher plasma
densities in hydrogen.
Figure 1 displays a comparison of stationary hydrogen
plasma densities achieved before and after boronization. It
is evident that the impurity radiation fraction is strongly
reduced after boronization and stationary line-averaged
plasma densities exceeding 1·1020 m3 could be routinely
achieved. The maximum density was only limited by the
available ECRH power of PECRH6 MW. At these high
plasma densities, a transition in the divertor heat load is
observed. This is shown in Fig 2. In the right-hand plot,
the longest discharge so far achieved at W7-X with a duration
of 100 s is shown. At an ECRH power of 2 MW and
relatively small plasma density, the divertor surface temperature
tdiv steadily increases. The length of this stationary
discharge is limited by the maximum allowed divertor
temperature of only 1800° C.
Fig. 1. Comparison of achieved mean plasma densities
before and after boronization with an estimate of the
expected density for a certain concentration of impurities
fimp.
Second divertor operation campaign:
High-density, long-pulse operation
In the second divertor campaign, Wendelstein 7-X
operated for the first time with high-density hydrogen
discharges exceeding 1·1020 m3. At these high densities,
divertor detachment is achieved; this allowed for
stationary discharges of up to 30 s with 5 MW of heating
power. The key for these operational achievements
was to apply boronization, which reduced the
plasma impurity content considerably. Neutral beam
injection heating was operated for the first time at
Wendelstein 7-X and could sustain the plasma for 5 s.
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Stellarator News -2- February 2019
If, however, the plasma density is increased (left-hand side
of Fig. 2), the divertor temperature drops to well below
500°C, which is an indication of divertor detachment.
Without the definition of a general heating energy limit,
this scenario can be extended to discharge times much longer
than 30 s, even without water-cooled divertor elements.
In this context, it was important to achieve control
over the position of the heat load onto the divertor target,
particularly if, e.g., the bootstrap current is evolving in
time. Additionally, 2 separate target elements (the socalled
scraper elements) were qualified. These elements
are designed to protect the divertor edges from large heat
loads. Neutral beam injection heating was applied at W7-
X for the first time. Up to 3.5 MW of heating power could
be coupled for 5 s into the plasma. The heating efficiency
was best demonstrated by the fact that the plasma discharge
could be sustained by neutral beam injection heating
only, without any additional ECRH. Due to beambeam
fueling, the highest plasma density of up to
2·1020m3 with centrally peaked density profiles was
achieved in this case.
In summary, the most recent campaign was very successful,
and the major physics goal were accomplished. We
achieved a lot of experimental results and the detailed data
analysis and physics exploitation is ongoing. Over the next
few years, W7-X will be completed by installing the highheat-
flux divertor and water cooling. This technical prerequisite
and the experience obtained in the last two divertor
campaigns will form the basis for the development of
long-pulse, high-performance operation in the next campaigns.
Olaf Grulke for the W7-X Team
E-mail: grulke@ipp.mpg.de
Max-Planck Institute for Plasma Physics
Wendelsteinstr.1
17491 Greifswald, Germany
Fig. 2. Discharge overview of two stationary hydrogen discharges displaying, from top to bottom; the time evolution of the
ECRH power, the central plasma density, the electron temperature, the diamagnetic plasma energy, and the divertor surface
temperature.