1 - Nuclear Sciences and Applications - IAEA

400 MANICKAM et al.
5. TURBULENCE, ABM INSTABILITY AND LIMITER H-MODES
Turbulent magnetic fluctuations have long been considered a possible cause
of anomalous electron thermal transport. In TFTR, magnetic fluctuations in
the scrape off region of the plasma have been measured with stationary poloidal
field coils (Mirnov loops) and with a movable probe having poloidal and toroidal
coils. Both coherent modes and a turbulent background level of fluctuations
have been found [8]. The background turbulence is characterized by a spectrum
that has an approximately 1/f dependence in the frequency range of 1 kHz to 1
MHz. For some high power NB heated plasmas very large correlation lengths
have been observed; essentially all coils located on the outer half of the machine
are correlated at the 80-90% level. The correlation extends over the 30-50 kHz
band. The relative phases are consistent with an m = 1, n = 0 standing wave
structure [9].
Over a wide range of parameters the background turbulence has been
measured and correlated with power balance estimates of the global
confinement time. In general, very good correlation between the fluctuation
level and the inverse of the global confinement time has been found for L-mode
type discharges. Supershots are an exception to this correlation, where it is found
that the fluctuation levels increase with beam power but the global confinement
time does not change. Thus the cause and effect connection between transport
and these measured magnetic fluctuations is not clear.
A new type of instability is observed on TFTR when the neutral beams are
first injected. This Axisymmetric Beam Mode (ABM) has a standing mode
structure of m/n = 2/0, with an anti-node in BQ at 45 to the poloidal mid-plane.
At high intensities the mode is coherent with a frequency in the range 30-60 kHz.
This frequency is close to the fast ion poloidal transit time at the plasma edge,
and the duration of the mode is comparable to the fast ion thermalization time.
The amplitude of the mode correlates with the beam power (especially counter
power) and with low target density. These features point to a fast ion resonant
coupling, but the specific background plasma mode to which these ions couple
has not yet been identified. In its present form this instability does not appear to
have any detrimental effects on the plasma.
Limiter H-mode discharges [10] have been obtained on TFTR, with Ip in the
range of 0.9 to 1.25 MA and injected beam power of 14 to 24 MW. Figure 3 shows
the characteristics of the TFTR limiter H-mode, the Ha signal drops and the
edge density increases. Simultaneous with the ne increase is a rise in edge Te.
In this example the H-mode is triggered by a sawtooth at 4.04 s, although this is
not always the case. Gas puffing during neutral beam injection enhances the
Ha spikes [or ERP/ELM's (edge localized modes)] during the H-mode phase. The
Ha spikes are clearly observed on the Mirnov coils and soft x-ray signals. The
spikes are dominated by an m/n = 1/0 mode at =• 0.5 kHz with a higher frequency
mode at 5 - 20 kHz that also has an n = 0 component. The H-modes observed on