Abstract
The evolution of runaway electrons in disruptive plasmas in TEXTOR is determined by observing the synchrotron radiation (hard component E > 25 MeV) and by measuring the runway electrons with an energy of a few MeV using a scintillator probe. Disruptions are initiated by a massive argon gas injection performed by a fast valve. The observed runaway beam of the high energy component (synchrotron radiation) fills about half of the diameter of the original plasma. The beam is smooth and shows no indication of filamentation. The initial conditions are in all cases very similar. The temporal development of the runaway electrons, however, is different: one observes cases with and without subsequent mode excitation and other cases in which the hard runaway component survives the apparent end of the runaway plateau. Several methods are applied to remove the runaway electrons including massive gas injection from two additional valves of different sizes as well as external and internal ergodization by inducing a tearing mode. The mitigation is only marginally successful and it is clearly found that the runaways in disruptions are substantially more robust than runaways created in stationary, low density discharges.
Highlights
During a disruption, a sudden loss of magnetic confinement, the energy stored in the plasma is rapidly lost to the plasma facing components (PFCs) [1]
We present initially some examples of the evolution of runaways without additional mitigation methods and a systematic study of different runaway mitigation methods used in TEXTOR including a gas puff and mitigation by the dynamic ergodic divertor (DED)
In order to study the development of runaway electrons in disruptive discharges, a massive gas injection of argon is applied by valve 1
Summary
A sudden loss of magnetic confinement, the energy stored in the plasma is rapidly lost to the plasma facing components (PFCs) [1]. The injection of a large amount of gas may affect the vacuum system It has been shown in [19] that a fast gas puff with a moderate amount of helium leads to the loss of the existing REs and the plasma current decay time is shortened. Magnetic perturbations are another method applied to mitigate the REs. The suppression of the runaway avalanche during TEXTOR disruptions is observed when a perturbation field with n = 1 and n = 2 is applied [20]. An overview of the different observations detected by these diagnostics is given
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