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

VINETA
Head: Prof. Dr. Olaf Grulke
Device and Operational Parameters
VINETA is a long, cylindrical helicon plasma device, specifically
tailored to study plasma waves and instabilities. The nonresonant
rf helicon wave heating provides high plasma densities
at relatively low electron temperatures (n=1019 m-3 , T ≈3eV e
for a rf frequency f =13.56 MHz, rf power of P =5 kW, and
rf rf
magnetic field B=0.1 T). The main diagnostic tools are electrostatic
and magnetic probes. In addition to standard Langmuir
probe diagnostics advanced active induction probes have
been developed to achieve reasonable signal to noise ratios
at the low frequencies of drift wave and Alfvén wave existence
(f=50 kHz). In the high frequency range induction
probes with high capacitive pickup rejection are used for the
measurements of the magnetic wave field of whistler waves.
Nonlinear Interaction of Drift Waves and Driven Parallel Currents
A key feature of the dynamics of drift wave modes and turbulence
is the parallel electron response associated with the
drift wave instability. Using highly sensitive magnetic probes
the parallel current structure of single saturated drift wave
modes has been measured. Figure 1 shows plasma density
fluctuation pattern and the associated perpendicular magnetic
field fluctuations of a m=3 drift wave mode for two phases of
the drift mode in half of the azimuthal cross section. The magnetic
field fluctuations show the signature of parallel current
filaments, which are correlated with the drift mode pressure
perturbations. To directly influence of the parallel drift wave
currents two approaches are persued: First, a shear Alfvén
wave is excited with a frequency close to the drift wave frequency.
The parallel currents of the drift wave and the Alfvén
wave interact nonlinearly, which is observed as frequency
pulling of the drift mode by the Alfvén wave. Second, a more
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Laboratory Plasma Devices WEGA and VINETA
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Figure 1: Pressure fluctuations (color coded) and associated perpendicular
magnetic field fluctuations (arrows) for a single m=3 drift mode in the
azimuthal plane perpendicular to the ambient magnetic field.
density fluctuations [a.u.]
62
advanced scheme is to drive mode selective parallel currents
either inductively by a set of eight azimuthally arranged magnetic
saddle coils or with electric contactors in the plasma.
With this mode selective current drive we achieved a full
suppression of the drift wave turbulence. Figure 2 shows an
example of a typical power spectral density for drift wave
turbulence as observed in VINETA and the result of the mode
selective current drive with mode number m=2. In the turbulent
state without current drive the spectrum is broad and displays
a power law decrease for frequencies f≥10 kHz. The
spatiotemporal measurement shows incoherent fluctuations
without any distinct mode number. When parallel currents are
driven, drift wave turbulence is fully suppressed and the fluctuation
energy is transferred into a coherent m=2 drift mode.
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Whistler Waves
In previous measurements we observed a systematic deviation
of the whistler wave behaviour from the linear dispersion
relation at a frequency of roughly half the electron cyclotron
frequency. At this frequency the whistler wave phase velocity
equals its group velocity and it has been suggested that whistler
wave solitons, so-called oscillitons, might be responsible for the
observed deviations. Dedicated measurements of those deviations
and the associated damping of whistler waves in VINETA
revealed first evidence for the existence of whistler solitons. The
unambiguous identification of oscillitons by wave package dispersion
measurements will be the subject of further investigations.
Scientific Staff
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C. Brandt, O. Grulke, T. Klinger, J. Pfannmöller, K. Rahbarnia,
A. Stark, N. Sydorenko, S. Ullrich, T. Windisch.
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Figure 2: Power spectral density of plasma density fluctuations and associated
spatiotemporal plot of density fluctuation time series along an
azimuthal circumference for the case of drift wave turbulence (top row) and
the controlled situation when a m=2 current pattern is driven
Part of the VINETA program is carried out under the auspices of the Transregional
Special Collaborative Research Center SFB-TR24 “Fundamentals
of Complex Plasmas”.
δ n [a.u.]
δ n [a.u.]