Temporal and spectral evolution of runaway electron bursts in TEXTOR disruptions

Abstract

Novel observations of the burst-like runaway electron losses in tokamak disruptions are reported. The runaway bursts are temporally resolved and first-time measurements of the corresponding runaway energy spectra are presented. A characteristic shape and burst to burst changes of the spectra are found. The runaway energy content of the disruptions and the conversion of the predisruptive magnetic energy are estimated. The radial decay of the runaways can be approximated by an exponential distribution. Deriving from the measurements, resistive tearing modes or kink modes are suggested to trigger the formation of the bursts.

Structure of the runaway electron loss during induced disruptions in TEXTOR

“…This is consistent with the earlier publication, 8 in which the energy spectrum of the lost RE was analyzed by the different channel of the scintillator probe. The spectrum of the lost runaways was described by an exponential decay function with an exponent of n r 0 $ 10 MeV.…”

“…In addition to the continuous decay during the plateau phase, a small stepwise decays of the plasma current and loss spikes accompanied by MHD activity are observed. 8 The main emphasis of this article is laid on an imaging of the plasma cross section by the synchrotron radiation measurement in order to provide a better understanding of the mechanism behind the loss. We also try to show similarities and differences between the observations during the runaway plateau phase of two different groups of disruptions.…”

Measurements of the runaway electron energy during disruptions in the tokamak TEXTOR

“…Additional results are used as an independent check for the calorimetry data: A scintillator probe measured RE energy spectra at the plasma edge during TEXTOR disruptions. 6 These disruptions were induced in the same way as described in Sec. III.…”

“…Eventually, the RE current plateau is depleted and the REs leave the plasma in sharp bursts which were first observed via X-ray diagnostics. 2,5 The bursts are around 0.1 ms long 6 and they strike the PFCs on areas of a few tens of square centimeters. The energetic REs can propagate through several centimeters of low atomic number material like graphite or beryllium.…”

Mechanism of runaway electron beam formation during plasma disruptions in tokamaks

“…However, no regular strategy to solve this problem has been developed because up to now the physical mechanisms of the formation of REs during plasma disruptions are not well understood. In spite of the numerous dedicated experiments to study the problem of runaway current generation during plasma disruptions in different tokamaks (see, e.g., [7][8][9][10][11][12][13][14][15]) no clear dependence of RE formation on plasma parameters has been established. These numerous experiments show the complex nature of plasma disruption process especially the formation of RE beams.…”