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Michael Ulrickson - One of the best experts on this subject based on the ideXlab platform.

J.d. Kotulski - One of the best experts on this subject based on the ideXlab platform.

Hyunseok Kim - One of the best experts on this subject based on the ideXlab platform.

  • observation of the loss of pre disruptive runaway electrons in kstar ohmic Plasma Disruptions
    Nuclear Fusion, 2016
    Co-Authors: Munseong Cheon, Junghee Kim, D C Seo, Hyunseok Kim
    Abstract:

    A newly-developed fast neutron detector revealed a close relationship between the loss of pre-disruptive runaway electrons and the Plasma Disruption in KSTAR ohmic Plasmas. It is observed that a burst of photoneutrons is generated exactly before the start of thermal quenches, indicating a bunch of runaway electrons which had existed before the Disruption impacts the first wall at the time. The loss of runaway electrons could be identified also as a decrease in the measured electron temperature, forming a typical two-stage thermal quench trace. From the MHD pattern in the neutron signal during a low-q Disruption, it could be identified that pre-disruptive runaway electrons are localized in the Plasma, especially on the q = 2 drift surface. These new findings suggest the pre-disruptive runaway electrons might play an important role in the Plasma Disruption mechanism.

Rebecca S. Coats - One of the best experts on this subject based on the ideXlab platform.

L. K. Zeng - One of the best experts on this subject based on the ideXlab platform.

  • Mechanisms of Plasma Disruption and runaway electron losses in the TEXTOR tokamak
    Journal of Plasma Physics, 2015
    Co-Authors: Sadrilla Abdullaev, K.h. Finken, K. Wongrach, M. Tokar, H. R. Koslowski, Oswald Willi, L. K. Zeng
    Abstract:

    Based on the analysis of data from the numerous dedicated experiments on Plasma Disruptions in the TEXTOR tokamak the mechanisms of the formation of runaway electron (RE) beams and their losses are proposed. The Plasma Disruption is caused by a strong stochastic magnetic field formed due to nonlinearly excited low-mode-number magneto-hydro-dynamics (MHD) modes. It is hypothesized that the RE beam is formed in the central Plasma region confined by an intact magnetic surface due to the acceleration of electrons by the inductive toroidal electric field. In the case of Plasmas with the safety factor $q(0)1~(q=5/4,q=4/3,\dots )$ . The thermal quench (TQ) time caused by the fast electron transport in a stochastic magnetic field is calculated using the collisional transport model. The current quench (CQ) stage is due to the particle transport in a stochastic magnetic field. The RE beam current is modelled as a sum of a toroidally symmetric part and a small-amplitude helical current with a predominant $m/n=1/1$ component. The REs are lost due to two effects: (i) by outward drift of electrons in a toroidal electric field until they touch the wall and (ii) by the formation of a stochastic layer of REs at the beam edge. Such a stochastic layer for high-energy REs is formed in the presence of the $m/n=1/1$ MHD mode. It has a mixed topological structure with a stochastic region open to the wall. The effect of external resonant magnetic perturbations on RE loss is discussed. A possible cause of the sudden MHD signals accompanied by RE bursts is explained by the redistribution of runaway current during the resonant interaction of high-energetic electron orbits with the $m/n=1/1$ MHD mode.

  • Mechanisms of Plasma Disruption and runaway electron losses in tokamaks
    arXiv: Plasma Physics, 2015
    Co-Authors: Sadrilla Abdullaev, K.h. Finken, K. Wongrach, M. Tokar, H. R. Koslowski, Oswald Willi, L. K. Zeng
    Abstract:

    Based on the analysis of data from the numerous dedicated experiments on Plasma Disruptions in the TEXTOR tokamak the mechanisms of the formation of runaway electron beams and their losses are proposed. The Plasma Disruption is caused by strong stochastic magnetic field formed due to nonlinearly excited low-mode number magnetohydrodynamic (MHD) modes. It is hypothesized that the runaway electron beam is formed in the central Plasma region confined by an intact magnetic surface due to the acceleration of electrons by the inductive toroidal electric field. In the case of Plasmas with the safety factor $q(0) 1$ ($q=5/4$, $q=4/3$, ...). The thermal quench time the current quench time are estimated. The runaway electron beam current is modeled as a sum of toroidally symmetric part and a small amplitude helical current with a predominant $m/n=1/1$ component. The runaway electrons are lost due to two effects: ($i$) by outward drift of electrons in a toroidal electric field until they touch wall and ($ii$) by the formation of stochastic layer of runaway electrons at the beam edge. Such a stochastic layer for high--energy runaway electrons is formed in the presence of the $m/n=1/1$ MHD mode. It has a mixed topological structure with a stochastic region open to wall. The effect of external resonant magnetic perturbations on runaway electron loss is discussed. A possible cause of the sudden MHD signals accompanied by runaway electron bursts is explained by the redistribution of runaway current during the resonant interaction of high--energetic electron orbits with the $m/n=1/1$ MHD mode.