To understand the effect of micro-instabilities on plasma confinement
has been a major physics issue in fusion plasma research. Experimentally,
detailed analyses of both temporal and spatial fluctuations of
plasma physics quantities (density and temperature fluctuations,
etc.) induced by micro-instabilities are essential for this purpose.
Density fluctuations in the plasma are measured by microwave reflectometry.
A microwave launched at the plasma is reflected at a certain layer
in the plasma which has a specific plasma electron density corresponding
to the frequency of the incident microwave (the cut-off layer).
Density fluctuations in the plasma cause displacements of the
cut-off layer, which lead to changes in the phase difference between
the incident and reflected waves. Thus, the local density fluctuation
in a plasma can be estimated from the measurement of the phase
difference (Fig.2-9).
The reflected waves from a plasma consist of waves of frequencies
different from the incident wave, scattered by the density fluctuations
near the cut-off layer, as well as the ordinary reflected wave
from the cut-off layer (Fig. 2-10). The effect of scattered waves
is negligible in a conventional scattering measurement, however
it becomes large in fusion plasmas when large density fluctuations
exists near the cut-off layer, and the conventional data analysis
is inaccurate or sometimes impossible. We have developed a new
analysis method based on the idea of separating the ordinary wave
and the scattered waves accurately using spectrum analysis and
a low-pass filter. By this method both the electron density at
the cut-off layer and the local density fluctuations can be measured
simultaneously with satisfactory accuracy. Figure 2-11 shows a
result obtained with the application of the new method to analyze
the experimental data of the JFT-2M tokamak, locating for the
first time where the reduction of density fluctuations occurs
in the plasma in a mode transition of confinement. |
