Speaker
Description
Parametric decay instabilities might play a significant role in various plasma physics phenomena and have garnered considerable interest in recent years [1]. In this study, we compare a model of parametric decay instabilities against the data observed during experiments conducted in the AUG (ASDEX Upgrade) fusion device while performing Electron Cyclotron Wall Conditioning (ECWC)[2].
During these experiments, a remarkable observation was made—clear spectral signals appeared at approximately half the main gyrotron frequency. Tomator-1D simulations [3] of the electron density and temperature evolution in the plasma demonstrated excellent
agreement with measurements obtained from AUG diagnostics.
The observed signals are generally associated to a parametric decay instability [4], which arises from a non-linear three-wave interaction where energy is transferred from a pump wave to daughter waves at shifted frequencies. The daughters, such as slow X-mode and Electron Bernstein waves (EBW), are naturally present in the plasma as thermal background. When these waves become trapped within a density bump formed between two reflective layers [5], an absolute decay is established, leading to exponential growth in the amplitude of the daughter waves. Subsequently, further decays into lower frequency waves, including EBW, Lower Hybrid (LH), or Ion Bernstein waves (IBW), might occur as the primary decay saturates.
In this work, we present a model that describes the necessary conditions for a primary decay to occur in conditions found during ECWC. By comparing the density profiles that create trapping conditions against Tomator-1D simulations and experimental profiles, we investigate the mechanisms that account for the observed features, such as half-frequency lines and localized light emission. This study contributes to the understanding of parametric decay instabilities in the context of electron cyclotron wall conditioning and its implications for the
future devices like ITER.
This work has been carried out within the framework of the EUROfusion Consortium, funded by the European Union via the Euratom Research and Training Programme (Grant Agreement No 101052200 — EUROfusion). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Commission. Neither the European Union nor the European Commission can be held responsible for them.
References
[1] S.K. Hansen, et al. Nucl. Fusion 60 106008 (2020)
[2] T. Wauters, et al. Nucl.Fusion 63 066018 (2023)
[3] T. Wauters, et al. Plasma Phys. Control. Fusion 62 3 (2020)
[4] A. Popov, et al. Plasma Phys. Control. Fusion 57 025022 (2015)
[5] M. G. Senstius, et al. Phys. Plasmas 27 062102 (2020)
