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

Most of the power transfer takes place at larger scales. For
experimental conditions (C=1), the direction of power transfer
of the density fluctuations has changed to the opposite
direction and one can identify a local direct cascade at
smaller scales, which is also expected for the enstrophy
transfer. This cascade is also present in the potential fluctuations,
but due to the weighting of the vorticity with k 2 it is
much more pronounced in the enstrophy cascade. The simulation
at the realistic value of C=1 agrees with the experimental
result.
Investigations of microwave material properties in a
three-mirror resonator configuration
For the construction of future fusion experiments like ITER
and W7-X, where long-pulse, high-power ECR applications
are planned, the definition of realistic cooling requirements
of mm-wave components is crucial. To enlarge the data base
on ohmic losses of reflectors due to finite conductivity,
imperfections of the material, and especially surface modifications
by long-term plasma exposition, various mirror samples
have been investigated at 140 and 170 GHz using a
three-mirror resonator. The losses depend on the position on
the mirror, confirming strong influence of plasma on the invessel
components. The absorption coefficients for 170 GHz,
E-plane, 45° angle of incidence for a ASDEX Upgrade mirror
are 0.36-0.44 % and for a W7-AS mirror 0.39-0.43 %.
Additional measurements of the W7-AS mirror surface with
a roughness detection instrument and spectroscopic analysis
confirmed the strong influence of different surface deposits
on mirrors, depending on the placement inside the torus.
Measurements on non-exposed W7-X TZM mirrors yield
comparable absorption with previous results for copper and
aluminium surfaces, all significantly lower (0.21 %) than for
the exposed samples.
Optimization of smooth-wall HE 11 Horn antennas
The existing waveguide codes were extended by a scattering
matrix module, which allows the calculation of mode conversions
due to diameter changes in cylindrical waveguides.
In contrast to the faster coupled mode equation algorithm,
the scattering matrix method does not neglect reflected
waves and is not limited to slight diameter variations. Based
on this, a program was developed, which optimizes the
function r(z) (r: waveguide radius, z: axial coordinate) for
given input- and output radii. Since mode conversion is
highly dependent on the phase differences of coupled modes,
the total length can also be changed by the optimizer. This
code can be used to optimize any kind of mode converter
with varying diameter. For a horn antenna, the wanted aperture
field is expanded into a spectrum of complex mode
amplitudes, which are then used as the optimization goal.
University of Stuttgart
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Figure 2 shows the measured far field of an optimized horn
at 140 GHz in co- and cross polarization. The side-lobe level
is very low the level of cross-polarization, however, is -17 dB,
which is too high for some applications. The improvement
of the codes for better results is an ongoing task.
Figure 2: Far field of the horn antenna in co-polarization (left) and crosspolarization
Materials with negative index of refraction
The propagation of electromagnetic waves in waveguide
structures with negative index of refraction has been investigated
with a commercial full-wave solver (CST-Microwave
Studio). Calculations and experiments on waveguides with
different filling like thin wire structures, which exhibit behaviour
similar to plasma (negative permittivity), and/or
resonant structures like split rings (negative permeability)
have been performed. Especially, experiments on TM 11 waveguides
with wire structures confirmed the backward wave
propagation at resonant frequency.
For high-power applications in experimental plasma physics,
feasibility studies on application of the principle of a negative
refractive index, e.g. for backward-forward scanning
leaky-wave antennas and other microwave components have
been started. A design based on coaxial balanced CL-LC
transmission lines is proposed. The optimization of the
structures that allow high current-density handling, and
numerical simulations using CST are underway. The excitation
of backward waves based on coupled cavity chains was
shown. The back-fire to end-fire scanning capability of this
new metamaterial-based leaky-wave antenna for high power
applications was numerically confirmed.
Scientific Staff
P. Brand, G. Birkenmeier, E. Holzhauer, H. Höhnle, A. Jooß,
W. Kasparek, A. Köhn, H. Kumric, C. Lechte, N. Mahdizadeh,
P. Manz, B. Nold, B. Plaum, K. Rahbarnia, M. Ramisch,
L. Stollenwerk, U. Stroth.