Magnetohydrodynamic instability modulated runaway electron losses in the ADITYA-U tokamak

Abstract In the ADITYA-U tokamak, modulation in the Hard x-ray (HXR) emission from the limiter synchronous with a m/n = 2/1 tearing mode is observed during the flat-top and pre-disruption phases of a discharge with a significant runaway electron (RE) population. The oscillation in the HXR signal is observed for a minimum magnitude of a poloidal magnetic field perturbation <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>δ</mml:mi> <mml:mrow> <mml:msub> <mml:mi>B</mml:mi> <mml:mi>θ</mml:mi> </mml:msub> </mml:mrow> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mrow> <mml:msub> <mml:mi>B</mml:mi> <mml:mi>ϕ</mml:mi> </mml:msub> </mml:mrow> <mml:mstyle scriptlevel="0"/> <mml:mo>∼</mml:mo> <mml:mstyle scriptlevel="0"/> <mml:mrow> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>4</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:mrow> </mml:math> , induced by a m/n = 2/1 magnetic island. Moreover, m/n = 2/1 tearing mode harmonics are pronouncedly visible in the HXR signal. The measured HXR energy spectrum in the presence of a 2/1 tearing mode exhibits high counting statistics of HXR photons in the energy range of 1–3 MeV. In contrast, in the absence of a 2/1 tearing mode, relatively low counting statistics of HXR photons are recorded in the same energy range. The modulation of the HXR signal in the presence of the m/n = 2/1 magnetic island is explained by a periodic RE loss due to resonance between the RE drift orbit and the m/n = 2/1 magnetic island radial magnetic field perturbation.