Magnetic field threshold for runaway generation in tokamak disruptions
Journal article, 2009
Experimental observations show that there is a magnetic field threshold for runaway electron
generation in tokamak disruptions. In this work, two possible reasons for this threshold are studied.
The first possible explanation for these observations is that the runaway beam excites whistler waves
that scatter the electrons in velocity space prevents the beam from growing. The growth rates of the
most unstable whistler waves are inversely proportional to the magnetic field strength. Taking into
account the collisional and convective damping of the waves it is possible to derive a magnetic field
threshold below which no runaways are expected. The second possible explanation is the magnetic
field dependence of the criterion for substantial runaway production obtained by calculating how
many runaway electrons can be produced before the induced toroidal electric field diffuses out of the
plasma. It is shown, that even in rapidly cooling plasmas, where hot-tail generation is expected to
give rise to substantial runaway population, the whistler waves can stop the runaway formation
below a certain magnetic field unless the postdisruption temperature is very low.