To shed some light on the development of the fast m/n = 1/1 precursor to
the sawtooth crash and its influence on plasma transport properties in
the vicinity of the q = 1 surface, series of dedicated experiments have
been conducted on the Tore Supra and TEXTOR tokamaks. It has been
concluded that, before a crash, the hot core gets displaced with respect
to the magnetic axis, drifts outwards by as much as 8-10 cm and may
change its shape. Observation of the magnetic reconnection process has
been made by means of electron cyclotron emission diagnostics. The heat
pulse is seen far outside the inversion radius. The colder plasma
develops a magnetic island on the former magnetic axis, after the hot
core expulsion. Different kinds of behaviour of the m = 1 precursor
before the crash, with respect to the displacement of the hot core and
the duration of the oscillating phase, have been observed. An ideal kink
model alone cannot be used for explanation; therefore, resistive effects
play an important role in the mode development. Possible mechanisms that
lead an m = 1 mode to such behaviour, and their links to the change in
the central q-profile, are discussed. Results have been discussed in the
light of various theoretical models of the sawtooth.
This is an extended version of the paper presented at the 31st EPS Conf.
on Plasma Physics (London, 28 June-2 July 2004).