Filamentation in the RTP tokamak plasma

Experimental data are presented showing filamentation of the Rijnhuizen tokamak project (RTP) plasma. These filaments are only resolved by the high-resolution double pulse Thomson scattering diagnostic, and appear as multiple peaks in the T-e profile with a typical width of 5-10 mm and an amplitude as high as 1 keV in a 2 keV ambient plasma. This paper shows the occurrence of filaments under various plasma conditions. It shows that filaments are statistically significant plasma physical phenomena. A parameter study shows a weak dependence of their amplitude on q(a), whereas a strong inverse dependence on plasma density has been found. It takes filaments several milliseconds to develop after the switch-on of electron cyclotron heating; they are wiped out by a sawtooth crash and take only a few hundred microseconds to reappear after such a crash. They mainly occur in the centre of additionally heated plasmas by means of electron cyclotron heating, but have also been observed in transiently heated plasmas and off-axis in non-centrally heated plasmas. Finally, two interpretations of filament topology are tested by means of three experiments. It turns out that the interpretation of filaments as independent closed tube-like structures seems to best fit the RTP data.