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Published by Oak Ridge National Laboratory
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Editor: James A. Rome Issue 155 December 2016
E-Mail: jamesrome@gmail.com Phone (865) 482-5643
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Power loads to the limiters
in the initial W7-X campaign
Wendelstein 7-X (W7-X) is an optimized superconducting
stellarator with a helical magnetic axis. Its main objective
is to demonstrate steady-state plasma operation at fusionrelevant
plasma parameters and thereby verify that the
stellarator is a viable fusion power-plant concept. The
design of W7-X is based on an elaborate optimization procedure
to overcome the shortcomings of traditional stellarators.
As the machine is rather complex, during the design,
development, and assembly important know-how in engineering
and technology was acquired. The main construction
phase of W7-X was completed in 2014. This was
followed by the commissioning of the superconducting
coils, which was successfully concluded with an assessment
of a series of careful measurements of the magnetic
field, not only confirming the basic magnetic field topology,
but also demonstrating that potential error fields are
within the correction capabilities of the W7-X trim coils
[1]. After the operating permit was granted, first plasma
operation started in December 2015 and lasted until March
2016. During integral commissioning of plasma start-up
and operation, the plasma was heated using electron cyclotron
resonance heating (ECRH), and an extensive set of
completed plasma diagnostics allowed initial physics studies
during the first operational campaign.
In the first operational phase, most of the graphite armor
components and the island divertor were not installed. To
protect metal parts (in particular the divertor frame structure)
and to guarantee reasonable performance, a limiter
configuration was used. Five graphite limiters, matching
the five-fold symmetry of the plasma, were installed in
symmetry planes at the inboard side of the vacuum vessel
(see Fig. 1). The magnetic vacuum configuration of W7-X
was chosen such that it has a smooth scrape-off layer
(SOL), with no stochastic region and no large magnetic
islands, such that the limiters efficiently intercept ~99% of
the convective plasma heat load in the SOL. The typical
connection length of magnetic field lines in the scrape-off
layer was on the order of a few tens of meters, and observation
revealed three separate helical magnetic flux bundles
of different connection lengths. Therefore, separate
heat flux channels were expected featuring localized peaks
in the limiter power deposition patterns.
Fig. 1. (a) Magnetic flux surface calculated for the limiter
configuration of W7-X during its initial campaign.
(b) Graphite limiter on the inboard side of W7-X in module
5. Tips of Langmuir probes are visible above and below the
midplane. The dashed rectangles indicate the camera
views.
In this issue . . .
Power loads to the limiters in the initial W7-X
campaign
In the initial campaign of Wendelstein 7-X, the
machine was operated in a limiter configuration using
up to 4 MW of ECRH power. Power loads to the limiters
due to the 3-dimensional topology of the scrapeoff
layer were measured and calculated. ................ 1
Stellarator News -2- December 2016
Both in helium and hydrogen, plasma breakdown was easily
achieved. However, the very first plasmas were small
in diameter and highly radiating, and consequently the
limiters initially received little deposited energy (only a
few ºC surface temperature increase). Then plasmas
improved as we conditioned the walls with helium glow
discharge cleaning (during periods without magnetic
field), and with repetitive helium plasma discharges
between main pulses. A typical discharge with an input
energy of 3 MW and plasma duration of about 0.5 s is presented
in Fig. 2. This led to a central electron temperature
of Te 9 keV. As experience was gained with plasma vessel
conditioning, discharge lengths could be extended
gradually. Eventually, discharges lasted up to 6 s, reaching
an injected energy of 4 MJ, which is twice the limit originally
agreed upon for the limiter configuration [2]. At
higher powers of 4 MW, central electron densities reached
3 × 1019 m3, central electron temperatures reached
12 keV, and ion temperatures reached just above 2 keV
[3]. Interestingly, as shown in bottom time trace of Fig. 2,
the power loads to the limiter showed also transient
events, which we attribute to anomalous transport in the
SOL. Those transient events had mode-like structure with
helical stripes on the limiter and a poloidal mode number
around 15. The transient power loads could exceed the
steady-state values by more than a factor of 2. To our
knowledge this is the first indication in a helical device
that turbulent transport in the SOL plays a major role.
In the initial campaign important physics studies including
the assessment of power balance and the heat load distribution
over the inboard limiters could be studied. The surface
temperature on the limiters was investigated using a
set of infrared (IR) diagnostics: immersion tubes with near
IR cameras with observation wavelength range between
0.8 μm and 1.0 μm. Those cameras had rather poor spatial
resolution and their dynamic range was also very limited
(8 bit). Additionally we used high-resolution IR cameras:
a microbolometric camera from DIAS (8–14 μm, spatial
resolution of order 5 mm) and a mid-IR camera from FLIR
(3–5 μm, spatial resolution of order 1 mm) [4]. The
microbolometric camera observes the left side of the limiter
in module 5 from the top (as indicated in Fig. 1), and
the high-resolution IR camera observes five of the nine
tiles above the midplane of limiter 3. With a frame rate of
up to 50 Hz, the microbolometric camera can resolve a
half limiter with a time resolution of 20 ms, while the
high-resolution camera can run with a frame rate up to 400
Hz (for a cropped image) to measure fast temperature
changes during transient events.
The power loads were calculated with the help of the wellknown
THEODOR (THermal Energy Onto DivertOR)
code developed at IPP Garching. Starting from the time
evolution of the surface temperature data, the code calculated
the heat flux density onto the surface by solving a
linear heat diffusion equation for the bulk of tile (2D). The
code takes a 1D profile of the surface temperature as input.
The results for helium plasmas at limiter 3 are presented in
Fig. 3(b). At each side of the limiter we can observe two
strike lines on the left and the right sides of the watershed.
The plateau insert and a large part of the linear extensions
at the edges are not loaded. The maximal loads are located
in the areas where the angle between the surface and the
field is about 20°.
Fig. 2. Time traces for a typical hydrogen discharge during
the initial campaign of W7-X. From top to bottom: electron
temperature and density, total input power, diamagnetic
energy, particle fluxes to the limiter, and at the bottom maximum
heat flux density onto the 3 tiles indicated in Fig.
3(a).
Fig. 3. (a) Infrared image of the limiter in module 3. Color
scale denotes temperature. (b) Heat flux density profiles
evaluated from the surface temperature at the locations
indicated by colored lines in (a).
Stellarator News -3- December 2016
On each limiter, three areas with different connection
lengths are present. This is clearly visible in Fig. 4.
Field lines in the first area with connection length Lc about
39 m meet the same limiter after a single toroidal turn; in
the second area with Lc about 43 m, they meet the next
adjacent limiter (after 6/5 of a full turn); the third bundle
with Lc about 80 m has field lines that make two full toroidal
turns and an extra 1/5 of a turn, hitting the adjacent
limiter. The heterogeneous distribution of the connection
lengths in the scrape-off layer resulted in nonuniform heat
flux densities. Figure 4(b) shows the heat flux densities
simulated with Monte Carlo field line diffusion for the
design parameters and ECRH power of 4 MW. In these
numerical calculations, field lines from inside the last
closed flux surface are traced with an additional perpendicular
diffusion.
Two stellarator-symmetric strike lines are present on each
limiter. The strike lines are not uniform vertically. Each
strike line has three areas with the aforementioned different
connection lengths and, as a consequence, with different
incident power densities. The highest power density on
the limiter is observed for the area with Lc about 80 m.
The overall structure of the strike lines is confirmed experimentally
with the infrared camera measurements of the
limiter surface temperature as shown in Fig. 4(c). Also ,the
measurement shows the vertical variation of the temperature
in the strike line. The highest temperatures, and therefore
heat flux densities, appear in the region of long
connection lengths with Lc about 80 m. These areas are
mainly at tiles 3 and 4 on the left and tiles 6 and 7 on the
right. In agreement with the stellarator symmetry, the left
and right strike lines are flip symmetric: the right one is
the left one rotated by 180 degrees.
Using a rather limited set of tools we were able to characterize
the heat fluxes on the poloidal limiters in W7-X
during the first helium and hydrogen plasmas during
OP1.1. A nonuniform distribution of power loads with two
heating stripes on each limiter was observed. The measured
patterns are consistent with modeling and can be
understood in terms of longer flux bundles carrying more
plasma heat flux. IR thermography and calorimetry using
multiple IR camera systems, combined with slow thermocouples
to account for toroidal asymmetries, allowed us to
estimate that the limiters intercepted up to 60% of the total
energy put into the vessel by the ECRH system for lowpower
(0.6 MW), long-pulse (6 s) shots, and a smaller
fraction (~35%) for high-power (4 MW) short duration
(1 s) discharges. More details may be found in upcoming
papers [5, 6].
References
[1] T. S. Pedersen et al., Nature Commununications 7
(2016) 13493.
[2] T. S. Pedersen et al., Nuclear Fusion, 55, 12 (2015)
126001.
[3] T. Klinger et al., Plasma Physics and Controlled Fusion
59, 1, (2017) 014018.
[4] R. König et al., Review of Scientific Instruments 85, 11
(2014) 11D818.
[5] S. Bozhenkov et al., Nuclear Fusion, submitted.
[6] G. Wurden et al., Nuclear Fusion, submitted.
M. Jakubowski, S. Bozhenkov, H. Niemann, T.S. Pedersen
IPP Greifswald
G. Wurden
Los Alamos National Laboratory
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Fig. 4. (a) Simulated connection lengths and (b) heat flux
density for the limiter during the initial campaign. (c) Temperature
distribution on the limiter measured with the
microbolometric IR camera in module 5 during a typical discharge.
Only half of the limiter is visible.