(EX/P3-4) Interaction of Runaway Electrons with Magnetic Field Ripple in the HT-7 Tokamak

Zhongyong Chen1), B.N. Wan, S.Y. Lin, Y.J. Shi, X.Z. Gong, Y. Jiang, J. Younis, L.Q. Hu, X. Gao, HT-7 Team
1) Institute of Plasma physics,Chinese Academy of Sciences, Hefei, China

Abstract.  The runaway electrons have been measured in combination with hard x-ray detectors and thermographic camera in the HT-7 tokamak. The dynamics of runaways in the core and edge regions is monitored simultaneously. The synchrotron radiation signal diagnoses the runaways in the interior of the plasma, and the HXR signal served as a supplement to monitor the lost runaways and the dynamics of runaways in the edge region. The magnetic field ripple can play an important role on the energy of runaways located in the edge region. The maximum energy of runaways in the edge region could be blocked by the resonance of gyromotion with the nth harmonic of the magnetic field ripple. This resonance interaction creates a barrier to a further increase in the runaway energy. The mechanism can quantitatively account for the observed energy limit of the runaways in HT-7. Runaway electrons in the core have energy of about 26 MeV, while the energy limit of runaways in the edge is only several MeVs. The resonance interaction of runaways with magnetic ripple is experimentally investigated in the HT-7 tokamak. There are abnormal energy gap in the HXR spectra. The energy presents a cutoff which is resulted from the resonance interaction between the runaway electron gyromotion and the nth harmonic of magnetic ripple. The energy limit of HXR spectra increases with increasing plasma current (loop voltage). The value of energy limit is consistent with the resonance energy with harmonic number n. The energy gap increases with decreasing resonance harmonic number. It is shown that, the strength of the resonance increases with decreasing harmonic number. The energy gap under low harmonic number is larger due to the strong interaction. Interaction of runaways with magnetic ripple act an additional barrier to limit the energy of runaways to a few MeVs in the edge is favorable for reducing the effect of runaways on first wall during disruptions. By exploiting this virtue, safer operation can be achieved.

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