Chirping instabilities produced by a runaway electron beam at a spherical tokamak

• Published in: Plasma science & technology Vol. 25; no. 7; pp. 75102 - 75107
• Main Authors: BALACHENKOV, I M, BAKHAREV, N N, GUSEV, V K, ILIASOVA, M V, KHILKEVICH, E M, KORENEV, P S, KONKOV, A E, MINAEV, V B, MITRIZHKIN, Yu V, PATROV, M I, PETROV, Yu V, SAKHAROV, N V, SHEVELEV, A E, SKREKEL, O M
• Format: Journal Article
• Language: English
• Published: IOP Publishing 01.07.2023
• Subjects: Alfvén waves; chirping instability; runaway electrons; spherical tokamak
• ISSN: 1009-0630; 2058-6272
• DOI: 10.1088/2058-6272/acb875

Abstract Two different types of MHD instabilities with rapidly chirping frequency were found to arise in the Globus-M2 spherical tokamak in substantially different frequency ranges. The first type arises at frequencies of an order of 1 MHz in ohmic plasmas at relatively low density 〈 n e 〉 < 2 × 10 19 m − 3 in a wide range of toroidal magnetic fields and plasma currents. This type of instability was identified as compressional Alfvén waves, driven by electrons, accelerated during a sawtooth crush. It was found that the mode frequency is sweeping in time, according to the Berk–Breizman hole–clump nonlinear chirping model. The second type of wave arises in a specific single-swing regime of the central solenoid current with a very narrow plasma column, when the plasma tends to decay at extremely low density 〈 n e 〉 < 2 × 10 18 m − 3 and, in fact, is an instability of the runaway electron beam. The exited modes cover the whole observed frequency range and are divided into several (two or three) frequency regions: approximately 0–30 MHz, 60–120 MHz and sometimes 30–60 MHz. Reconnection of the branches was also observed. Single chirps are more rapid than for 1 MHz Alfvén instability and follow an exponential law. This paper, to our knowledge, is the first report of frequency chirping instabilities excited by accelerated electrons at a spherical tokamak.