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

WEGA
Head: Dr. Matthias Otte
Laboratory Plasma Devices WEGA and VINETA
Electron Cyclotron Wave Physics
The plasma at WEGA is generated
for low magnetic field operation
of B 0 =60 mT by magnetrons
at a frequency of 2.45 GHz.
The over-dense plasma operation
(>10 times cut-off density for
2.45 GHz) is achieved with mode
conversion Bernstein wave heating.
Raytracing calculations by Preinhalter and Urban, (IPP
Prague/Czech Republic) confirmed the propagation of the
Bernstein waves in the over-dense plasma region. Although the
WEGA double slot antenna has a symmetric emission profile in
toroidal co- and counter direction, these calculations also showed
a beam propagation in one toroidal direction only, which should
generate Bernstein wave driven toroidal currents. The predicted
current was detected by an external out-of-vessel Rogowski
coil and by the primary coil current at the WEGA transformer.
Power modulation experiments (6 kW on-off at a modulation
frequency of 30 Hz) showed a maximum toroidal current amplitude
of I t =50 A. This current strongly depends on the magnetic
configuration and could even be reversed in case of magnetic
shear reversal. Furthermore, the current density profile inside the
plasma could be measured with small movable Rogowski coils.
This makes WEGA an unique experiment to investigate
Bernstein wave driven currents. The 28 GHz ECRH system was
successfully built and commissioned and is now fully operational
at the second harmonic resonance at 0.5 T. A microwave power
of up to 10 kW is typically used for 15 s plasma operation. The
waveguide system generates an HE11 beam, which is focussed
into the plasma center by an optimized double mirror system inside
the plasma vessel. The resonant character of the absorption
could be verified by on- and off-axis heating experiments.
In argon plasmas a density of n e =4.9×10 18 m -3 and a temperature
of T e =25 eV in the central region could be achieved. Furthermore,
the microwave stray radiation measurement indicates a
strongly enhanced resonant absorption by multiple wall reflections
as predicted by TRAVIS ray-tracing code calculations.
However, in hydrogen discharges an electron temperature
T e >20 eV is achieved already at the plasma edge. For such plasmas
the central region is not longer accessible with simple
Langmuir probes because they start to emit electrons distorting
the characteristics. Therefore, contact-less diagnostics, like Heavy
Ion Beam Probe (HIBP), Electron Cyclotron Emission (ECE),
a bolometer and a reflectometer are being implemented now.
Results from Plasma Experiments
Turbulence studies have been continued in order to identify the
driving instability mechanism and to give a three-dimensional
characterization of the structure of the turbulence. A small
On WEGA experiments were focused on electron
cyclotron wave physics, on fluctuation,
biasing studies and preparations for 0.5 T plasma
operation. Furthermore, the prototype installation
of the W7-X control system made significant
progress. In VINETA it has been achieved to
control drift wave turbulence by influencing nonlinearly
the intrinsic parallel drift wave currents.
The investigations of non-linear wave phenomena
was extended to whistler wave solitons.
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cross-phase between density and
potential fluctuations is a strong
indicator for drift waves. Additional
hints for drift wave turbulence
could be found, namely
a propagation of turbulent structures
in electron diamagnetic
drift direction after subtraction of
E×B convection and a finite
average parallel wavenumber in
the order of k || ≈10 -2 k θ . The precise
knowledge of the magnetic
field topology allows insight into the parallel dynamics of turbulence.
Turbulent structures in WEGA have a three-dimensional
character and arise preferably on the low field side of the
torus. Furthermore, the influence of magnetic islands on turbulence
was studied. WEGA offers the unique possibility to control
islands and to study turbulence in this region with high
spatial and temporal resolution. Initial experiments showed a
locally enhanced fluctuation amplitude and turbulent transport
in the presence of magnetic islands. Biasing experiments
using carbon probes fitting the shape of the last close flux surface
(LCFS) demonstrated in argon plasmas that a positive
biasing just inside the LCFS increases the poloidal rotation by
a factor of 2 in the maximum. With probe voltages exceeding
+70 V a high temperature regime has been reached with temperatures
of up to T e =25 eV in the vicinity of the LCFS.
Diagnostic Development
The HIBP is a first contactless diagnostic designed for 28 GHz
ECR heated plasmas operation. The ion source was modified
to increase the beam current stability. With an optimised ion
source first profiles of the plasma potential and the total current
were measured. The profiles are in good agreement with
Langmuir probe data. In collaboration with the HIBP group at
TJ-II (Madrid/Spain) and HIBP in KIPT (Kharkov/Ukraine)
modifications of data acquisition and control systems have
been performed in order to decrease the noise level of the
obtained signals. Furthermore, the design of a multi-channel
ECE-system started which will provide information on the
temperature profile of the electrons for 0.5 T operation.
Prototype of W7-X Control System
The setup of a prototype installation of the W7-X control
system is in progress. The central control system, power
supplies, cooling system could be successfully tested in
autonomous operation. An integrated test of all components
is anticipated for the beginning of 2008.
Scientific Staff
D. Andruczyk, O. Lischtschenko, S. Marsen, Y. Podoba,
M. Schubert, T. Stange, G. B. Warr, F. Wagner.