Transitions in confinement with characteristics of H-mode are observed in JET
inner wall limited plasmas in experiments performed at a magnetic field of
0.8 T and at a current of 0.9 MA, using up to 7MWof auxiliary heating power.
These transitions in confinement are short lived, with durations of up to 20 ms,
but the overall stored energy increases by up to 5% and edge density by up to
20%. The termination of the period of good confinement correlates with the
observation of a burst of magnetic fluctuations, similar to those associated with
edge localized modes (ELMs).
These transitions in confinement also correlate with a significant decrease
in the density fluctuations measured by the microwave reflectometer and a
small decrease in the magnetic fluctuations measured using ‘Mirnov’ probes,
accompanied by a reduction of the D(alpha) emission. The comparison of the
fluctuations in the limiter H-mode in JET with the divertor H-modes shows
a similar behaviour, in particular the reduction of the fluctuation levels at
high frequencies (f > 10 kHz) and an increase in the fluctuation levels at
lower frequencies (f < 10 kHz). The strong electrostatic nature of the plasma
edge turbulence is confirmed, where a reduction of up to ∼50% in the density
fluctuation levels correlates with an increase of up to 50% in the confinement
time during the transition to H-mode. However, the importance of instabilities
with a dominant magnetic component (such as ELMs) is also highlighted,
which cause significant increase in transport, including the loss of the good
confinement phase in limiter H-modes. |