A Machine-Learning (ML) based detection scheme that automatically detects Alfvén Eigenmodes (AE) in a labelled DIII-D database is presented here. Controlling AEs is important for the success of planned burning plasma devices such as ITER, since resonant fast ions can drive AEs unstable and degrade the performance of the plasma or damage the first walls of the machine vessel. Artificial Intelligence could be useful for real-time detection and control of AEs in steady-state plasma scenarios by implementing ML-based models into control algorithms that drive actuators for mitigation of AE impacts. Thus, the objective is to compare differences in performance between using two different recurrent neural network systems (Reservoir Computing Network and Long Short Term Memory Network) and two different representations of the CO2 phase data (simple and crosspower spectrograms). All CO2 interferometer chords are used to train both models, but only one is processed during each training step. The results from the model and data comparison show higher performance for the RCN model (True Positive Rate = 90% and False Positive Rate = 14%), and that using simple magnitude spectrograms is sufficient to detect AEs. Also, the vertical CO2 interferometer chord passing near the center is better for ML-based detection of AEs.

Comparison of machine learning systems trained to detect Alfvén eigenmodes using the CO2 interferometer on DIII-D

The formation of electron thermal internal transport barriers (eITBs) is investigated by critical gradient threshold analysis for recent EAST experiments with dominant RF heating and low torque injection. The ratio of electron temperature to ion temperature, τ = T e/T i, is identified to be the key parameter that triggers eITB in the hot electron mode through electron turbulence suppression by the effect of ion shielding. The critical gradient of electron-temperature-gradient turbulence can be greatly increased with τ due to strong electron heating and weak electron–ion energy coupling. The formation and evolution of eITB is found to be determined by the critical temperature gradient of electron turbulence.

Electron thermal internal transport barriers triggered by the effect of ion shielding

A broadband (BB) mode is observed by collective Thomson scattering diagnostics in repeatable shots of EAST and analyzed for the first time. This BB mode usually grows during L–H transitions, featuring a BB quasi-coherent mode with increasing frequency. During H-mode operations, it is characterized by steady-state BB in the high-frequency range (f ∼ 200–2000 kHz), at the electron scale (k θ ρ s = 1‒2), mainly driven by the density gradient, and is sensitive to the value of η e in the region of interest (ρ = 0.4‒0.8), where ηe=R/LTe/R/Lne is the ratio of the normalized electron temperature gradient and density gradient, and the region ρ = 0.4‒0.8 usually has a relatively low collisionality (v eff < 5). The frequency of BB is found to be dependent on the electron temperature and density gradient, which is a typical feature of electron-driven turbulence. A negative correlation between the energy confinement and the intensity of the BB turbulence during H-mode has been found, which indicates a strong electron thermal transport induced by the BB turbulence. The BB significantly decreases the electron temperature and causes flatter electron temperature profiles in the region of interest (ρ = 0.4‒0.8), thus making η e decrease and the BB destabilize further. These characteristics of BB are related to the theoretical density gradient-driven trapped electron mode. It should be noted that this mode is not observed by other diagnostics in EAST, and shows very different features to the coherent modes in the edge.

Experimental study of density gradient-driven micro-instabilities and the confinement degradation during H-mode in EAST


 A Broad-Band(BB) mode is observed by the Collective Thomson Scattering (CTS) diagnostics in repeatable shots of EAST. This BB mode usually grows during L-H transitions, featuring a Broad-Band Quasi-Coherent (BB-QC) mode with increasing frequency. During H-mode operations, it is characterized by steady-state BB in high frequency range(f~200-2000kHz), electron-scale (k_θ ρ_s=1~2), mainly driven by density gradient and is sensitive to the value of η_e in the region of interest (ρ=0.4~0.8), where η_e=(R/L_Te)/(R/L_ne) is the ratio of normalized electron temperature gradient and density gradient, and the region ρ=0.4~0.8 usually has relatively low collisionality (ν_eff<5). The frequency of BB is found to be dependent on the electron temperature and density gradient, which is a typical feature of electron-driven turbulence. A negative correlation between energy confinement and the intensity of the BB turbulence during H-mode has been found, which indicates a strong electron thermal transport induced by the BB turbulence. The BB significantly decreases the electron temperature and causes flatter electron temperature profiles in the interested region (ρ=0.4~0.8), thus making η_e decrease and BB destabilize further. These characteristics of BB are related to the theoretical density gradient driven trapped electron mode (DGTEM). It should be noted that this mode is not observed by other diagnostics in EAST, and shows very different features with the coherent modes in the edge.

Experimental study of density gradient driven micro-instabilities and the confinement degradation during H-mode in EAST

Abstract A new Hard X-ray and soft gamma-ray spectrometer imaging System (HXS) has been built for the 2-Dimensional measurements of plasma emitted photons on the Experimental Advanced Superconducting Tokamak (EAST). The system uses a 2-D Cadmium Zinc Telluride (CZT) detector and an integrated electronics, which is as a whole shielded by a tungsten box with a pinhole and is tangential to the toroidal field. Three classes of typical energy spectra have been summarized in differently experimental scenarios over the past campaigns. After performing tomography calculations, the local emissivity contours have been obtained in different energy ranges which obviously show the asymmetry on the plasma cross section. The spatial perturbation structure is similar to the magnetohydrodynamics (MHD) modes with low mode numbers. In particular, the runaway island found by an infrared camera [R. Jaspers et al, Phys. Rev. Lett. 72, 4093 (1994)] is also measured by the HXS. There exists a reversal population on the energy spectra in both slide-away and strong neutral beam injection (NBI) shots. It is consistently observed that the count rate is increased in the low energy range before the plasma disruptions. Calculations in phase space indicate that the accelerated momentum flux can be deflected back to the low energy region by the large pitch-angle scattering. In the post-disruption phase the plasma current is not replaced by runaway electrons due to tearing modes or transiently bursting instabilities. This paper constructs the basics for the proper use of HXS for Hard X-ray and Soft Gamma-ray measurements in future investigations of plasma disruptions. 

First results of a newly-built hard-X-ray/soft-Gamma spectrometer imaging system: on the aspect of plasma disruptions

Dynamics between toroidal Alfvén eigenmode evolution and turbulence suppression under resonant magnetic perturbations on EAST

Avoiding the plasma instability, especially the disruption instability, is an important problem for the stable operation of tokamak. Large-scale instabilities driven by free energy evolve nonlinearly and lead to the disruption. The microscale turbulence is highly sensitive to the change of free energy. The paper show the electron-scale turbulence evolution in the pre-precursor phase of TMs included disruption with the CO2 laser coherent scattering system in EAST. In the pre-precursor phase of disruption, it is observed that the characteristics of turbulence (e.g. intensity, spatial correlation) have obviously changed for more than 30 ms. In addition, before TM ( n=1 ) included major disruption, the spatial-correlation of turbulence in two regions ( ρ = 0–0.4 and ρ = 0.4–0.8) increase obviously, while opposite turbulence spatial-correlation evolution was observed before TM ( n=1 ) included minor disruption. The warning time for disruption with microscale turbulence is competitive while 30 ms for ITER. According to the experimental results in EAST, it may provide a new experimental evidence for the method improvement of predicting disruption.

Evolution of turbulence characteristics in the pre-precursor phase of tearing mode included disruption in the core of EAST

Spontaneous evolution of density peaking factor in TEM turbulence-dominated H-mode plasma on the EAST Tokamak

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Dynamics between toroidal Alfvén eigenmode evolution and turbulence suppression under resonant magnetic perturbations on EAST

Evolution of turbulence characteristics in the pre-precursor phase of tearing mode included disruption in the core of EAST

Spontaneous evolution of density peaking factor in TEM turbulence-dominated H-mode plasma on the EAST Tokamak