Study of runaway electron behaviour during electron cyclotron resonance heating in the HL-2A Tokamak

During the current flat-top phase of electron cyclotronresonance heating discharges in the HL-2A Tokamak, the behaviour ofrunaway electrons has been studied by means of hard x-ray detectorsand neutron diagnostics. During electron cyclotron resonanceheating, it can be found that both hard x-ray radiation intensityand neutron emission flux fall rapidly to a very low level, whichsuggests that runaway electrons have been suppressed by electroncyclotron resonance heating. From the set of discharges studied inthe present experiments, it has also been observed that theefficiency of runaway suppression by electron cyclotron resonanceheating was apparently affected by two factors: electron cyclotronresonance heating power and duration. These results have beenanalysed by using a test particle model. The decrease of thetoroidal electric field due to electron cyclotron resonance heatingresults in a rapid fall in the runaway electron energy that may leadto a suppression of runaway electrons. During electron cyclotronresonance heating with different powers and durations, the runawayelectrons will experience different slowing down processes. Thesedifferent decay processes are the major cause for influencing theefficiency of runaway suppression. This result is related to thesafe operation of the Tokamak and may bring an effective control ofrunaway electrons.