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

the reference profile database. The neoclassical transport
simulations for W7-X are based on the DKES database of
mono-energetic transport coefficients for the low-mirror, the
standard, and the high-mirror configurations with ι(a)=1.
Here, the standard configuration has the lowest neoclassical
transport which is further reduced with increasing β (whereas
the bootstrap current increases with β). The high-mirror
configuration fulfils the optimisation criterion of minimized
bootstrap current as long as “electron-root” scenarios are
avoided, although the bootstrap current is also expected to
increase with β (due to reduction of the toroidal mirror term).
The ray-tracing code TRAVIS has been included as a module
in the transport code to self-consistently simulate modifications
of electron cyclotron resonance heating (ECRH), noninductive
current drive, and plasma profiles. The upgraded
codes can now handle multi-beam heating schemes with reflection
from mirrors and the vacuum vessel which is especially
important for O2 (second harmonic of the ordinary mode)
and X3 (third harmonic of the extraordinary mode) scenarios
with incomplete first-pass absorption of the ECRH power.
Transport Simulations of High-Power O2 Heating in W7-X
ECRH at densities as high as 2×10 20 m -3 is envisaged for
W7-X at the second harmonic of the ordinary mode (O2).
First-pass absorption of O2 ECRH is relatively poor for the
projected plasma parameters, enforcing the adoption of a
multi-pass absorption scheme with a fixed launching geometry
to conform with the placement of mirrors and reflecting
surfaces within the vacuum vessel. Using the transport/raytracing
package, this scheme is predicted to deliver threepass
absorption in excess of 95 % and produce plasmas with
〈β〉≈4 % for 10 MW of ECRH power. For such simulations,
the combination of Shafranov shift and diamagnetic effect
moves the ECRH deposition zone significantly inwards
which is most easily counteracted by increasing the initial
strength of B by several percent.
ITER Benchmark of the Ray-Tracing Code TRAVIS
A successful benchmark with the reference scenario 2 for
ITER has been performed with the recently developed raytracing
code TRAVIS. In particular, the electron cyclotron current
drive (ECCD) was calculated for both the upper and equatorial
launcher. It has been found that for the range of angles
expected to be optimal for ECCD, accounting for momentum
conservation is mandatory. Additionally, the scenario with
reduced magnetic field has been considered, where it was
found that the ECCD efficiency (as well as the deposition profile)
may be significantly altered for the equatorial launcher
due to unwanted absorption at the higher (parasitic) harmonics.
Divertor Transport Studies for Helical Devices
Transport essentials of the helical divertor in LHD were
investigated and compared to those already intensively studied
Theoretical Plasma Physics
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for the island divertor of W7-AS, aiming at identifying common
physics issues of two helical devices with completely
different divertor concepts and geometries. For W7-X, a
systematic numerical study of the island screening effect on
recycling neutrals and intrinsic impurities has been started
with the standard island divertor configuration; preliminary
results are being analyzed. The EMC3-EIRENE code is
being implemented for NCSX.
Configuration Studies for W7-X
The configuration space of Wendelstein 7-X has been explored
at large toroidal mirror ratios. These configurations
show worsened interchange stability properties as both
shear and depth of the vacuum magnetic well are reduced so
that most configurations become interchange unstable at
medium to high β values. Another interesting quality is a
further reduction of the parallel current density which completely
suppresses the dipole component of the Pfirsch-
Schlüter currents for mirror fields around 30 %. Configurations
with negligible Shafranov shift at 〈β〉≈10 % can
be found.
Recovery of External Coil Currents for W7-X Vacuum
Configurations
The fast equilibrium reconstruction technique, called function
parametrisation (FP), was applied, indirectly, to recover
the external coil currents of W7-X in an inverse transformation.
The “inversion” is in the mapping of (known or
pre-determined) physical parameters of a configuration
onto the coil currents. The importance of this lies in the
fact that it serves to answer queries on the feasibility of
desired configurations with respect to allowed values of
external currents, something very crucial during the operation
of stellarators. The method employs an iterative minimisation
of target functions using the FP with the coil currents
as independent parameters since direct inversion is
not possible.
Scientific Staff
C. D. Beidler, M. Borchardt, S. Braun, M. Drevlak, D. Eremin,
Y. Feng, J. Geiger; P. Helander, K. Kauffmann, R. Kleiber,
A. Könies, H. Maaßberg, N. Marushchenko, A. Mishchenko,
C. Nührenberg, J. Nührenberg, J. Riemann, A. Runov, F. Sardei,
A. Sengupta, Yu. Turkin, P. Xanthopoulos, X. Zha.
Guests
J. Canic (ORNL), T. Fülöp (TU Chalmers Göteborg),
L. Guazzotto (MIT), M. Kobayashi (NIFS), L. Koziol (USC),
R. Maingi (ORNL), M. Mikhailov (Kurchatov Institute),
D. Mikkelsen (PPPL), A. Simakov (LANL), Z. Unterberg
(DIII-D).