Speaker
Description
Abstract:
JET deuterium experiments in an advanced tokamak scenario with an internal transport barrier (ITB) exhibit unstable electromagnetic (EM) perturbations in the sub-TAE frequency range. In JET pulse number (JPN) 92054, a high-beta plasma ($\beta_N = \beta_T B_T a / I_P \sim 4.38 [\%Tm/MA]$) with high power neutral beam injection (NBI), $P_{NBI} = 25.1 MW$, contained EM perturbations identified as beta-induced ion temperature gradient (BTG) eigenmodes and not beta-induced Alfvén eigenmodes (BAE) nor beta-induced Alfvén acoustic eigenmodes (BAAE) which are often destabilised in similar plasma conditions. The EM perturbations are localised near the $q=2$ magnetic surface related to the ITB, and their frequency correlates well with the BTG characteristic frequency (ion diamagnetic frequency, $\omega_i^*$) and the thermal ion temperature gradient ($\nabla T_i$). BTG modes are the most unstable modes due to the high thermal ion temperature gradient in the ITB, high thermal ion temperature compared to thermal electron temperature ($T_i / T_e > 1$), and a high ion beta. Three well-defined conditions for BTG modes to exist, defined by BTG analytical theory [1], are fulfilled in JPN 92054: (1) a positive relative ion temperature gradient, (2) ion beta higher than a critical value, and (3) a low magnetic shear. BTG theory also predicts a mode location in the vicinity of a rational magnetic surface, a frequency scaling with $\omega_i^*$, and a coupling between Alfvén and drift waves. We have performed linear gyrokinetic simulations with validated plasma profiles and equilibrium, and find a mode with features resembling those of the experimental and theoretical BTG modes; specifically the mode is kinetically driven by thermal ions, is localised near the $q=2$ magnetic surface, has a dominant Alfvénic polarisation, and its frequency scales with $\omega_i^*$ dependent on the toroidal mode number ($n$). Parts of this work have been reported in [2].
BTG modes are also observed in more recent JET plasmas during energetic particle scenario experiments aimed at studying alpha-particle driven AEs, performed in JET 2019/2020 deuterium campaigns. Reflectometer diagnostic data confirm that the mode location is around the $q=2$ magnetic surface. We also present evidence for a systematic correlation between the BTG mode stability and the neutron rate roll-over (i.e. $d(R_{NT})/dt$ transiting from positive to negative).
References:
[1] A. B. Mikhailovskii and S. E. Sharapov. Beta-induced Temperature-gradient Eigenmodes in Tokamaks. Kinetic Theory. JET Joint Undertaking Reports, JET–P(98)12:1–16, 1998.
[2] N. Fil, et al. Interpretation of electromagnetic modes in the sub-TAE frequency range in JET plasmas with elevated monotonic q-profiles. Physics of Plasmas, Accepted, 2021.
Acknowledgments:
This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 and 2019-2020 under grant agreement No 633053 and from the RCUK [grant number EP/T012250/1]. The views and opinions expressed herein do not necessarily reflect those of the European Commission.
This work was supported by U.S. Department of Energy (DOE) through DEFG02-99ER54563, DE-AC05-00OR22725 and DE-AC02-05CH11231).
| Speaker's Affiliation | UKAEA, CCFE, Abingdon. |
|---|---|
| Member State or IGO | United Kingdom |
