Effect of periodic gas-puffs on drift-tearing modes in ADITYA/ADITYA-U tokamak discharges

The effect of a periodic train of short gas-puff pulses on the rotation frequency and amplitude of drift-tearing modes has been studied in ADITYA/ADITYA-U tokamak. The short gas puffs, injecting approximately similar to 10(17)-10(18) molecules of fuel gas (hydrogen) at one toroidal location, are found to concomitantly decrease the drift-tearing mode rotation frequency and the mode amplitude during the period of injection and then recover back to its initial values when the gas pulse is over. This leads to a periodic modulation of the rotation frequency and amplitude of the drift-tearing modes that is correlated with the periodicity of the gas pulse injection. The underlying mechanism for this change in the mode characteristic appears to be related to gas puff induced change in the radial profile of the plasma pressure in the edge region that brings about a reduction in the diamagnetic drift frequency. Detailed experimental measurements and BOUT++ code simulations support such a reduction in diamagnetic drift frequency. Our results reveal a close interaction between the edge dynamics and core MHD phenomena in a tokamak that could help us better understand the rotation dynamics and amplitude pulsations of magnetic islands.

Towards integrated control of tokamak plasmas: physics-based control of neoclassical tearing modes in the TCV tokamak

Mengdi Kong

Neoclassical tearing modes (NTMs), magnetic islands located at rational

q

surfaces, are an important class of resistive magnetohydrodynamics (MHD) instabilities in tokamak plasmas, with

q

the safety factor. NTMs are one of the main constraints of the achievable plasma pressure by increasing the local radial transport and NTMs can lead to plasma disruptions. It is therefore crucial to understand the physics of NTMs and ensure their reliable control. This thesis explores the physics and control of NTMs, by means of dedicated experiments in the TCV tokamak and interpretative simulations with the modified Rutherford equation (MRE), a model widely used in interpreting island width evolutions. Triggerless NTMs originating from unstable tearing modes (TMs, stability index

\Delta'>0

) and saturating under the effects of the perturbed bootstrap current are the main focus of this thesis. In TCV, triggerless NTMs are reproducibly observed in low-density discharges with strong near-axis electron cyclotron current drive (ECCD), providing an excellent opportunity of studying these modes. Instead of direct computations of

\Delta'

, a model for

\Delta'

is developed based on extensive experiments and interpretative simulations. This model facilitates the clarification of the complete evolution of triggerless NTMs, from onset as TMs to saturation as NTMs. Our

\Delta'

model also explains an unexpected density dependence of the onset of NTMs, where NTMs only occur with a certain range of density that broadens with increasing near-axis ECCD power and with lower plasma current. The density range is found to result from the density and plasma current dependence of the stability of ohmic plasmas and the density dependence of ECCD efficiency. Given its high localization and flexibility, off-axis ECH/ECCD will be used for NTM control in future tokamaks. Comprehensive experimental and numerical studies of the dynamics of NTMs are carried out in this thesis, concerning both the stabilization of existing NTMs and the prevention of NTMs by means of preemptive off-axis ECCD. It is shown and predicted that the prevention of NTMs is much more efficient than NTM stabilization in terms of EC power. Interpretative simulations of the complex set of experiments constrain well the coefficients in the MRE and quantify NTM evolutions. The prevention effects from off-axis ECCD are found to result from local ECH/ECCD instead of a change of

\Delta'

. A key element of a reliable real-time (RT) control of NTMs is the alignment of EC beams with the target mode location. A small sinusoidal sweeping of the deposition location of EC beams around the target location proves to be effective for both NTM stabilization and prevention, making it a promising technique. Integrated control of NTMs, plasma pressure and model-estimated

q

profiles is demonstrated on TCV, including advanced plasma state reconstruction, monitoring, supervision and actuator management. A RT-capable MRE module, based on our validated MRE, is developed for the first time and tested by extensive offline simulations for TCV and AUG. It provides a more intelligent physics-based NTM controller, aware of the EC power it requires to stabilize or prevent a given NTM. The information from the RT-MRE is also valuable for RT actuator allocations and decision-making in view of the overall integrated control, in particular for future long-pulse tokamaks like ITER and DEMO.

EPFL2020

Observation of short time-scale spectral emissions at millimeter wavelengths with the new CTS diagnostic on the FTU tokamak

Cristian Galperti, Gabriele Grosso

On the FTU tokamak, the collective Thomson scattering (CTS) diagnostic was renewed for investigating the possible excitation of parametric decay instabilities (PDI) by electron cyclotron (EC) or CTS probe beams in presence of magnetic islands and measure their effects on the EC power absorption. The experiments were performed launching a gyroton probe beam (140 GHz, 400 kW) and observing the scattered radiation in symmetric and asymmetric directions (with respect to the equatorial plane) in different conditions of plasma density and magnetic field (with or without the EC resonance in the plasma), and with magnetic islands generated by Neon injection. The acquisition with a fast digitizer allowed observing spectral features with very high time and frequency resolution. Shots were performed at 7.2 T, with the fundamental EC resonance out of the plasma region, at 4.7 T, with the resonance on the high field side of the plasma column, and at 3.6 T, in this last case with the plasma between the first and the second EC harmonics both lying outside the plasma volume. Several types of spectral features characterized by their frequency and their fast time evolution were identified in the observed signal after a proper treatment. The paper reports the observations in the different experimental cases and the correlation of the features with the existence of MHD modes as witnessed by magnetic probes signals and with macroscopic plasma parameters.

Iop Publishing Ltd2017

Non-linear modelling of saturated internal and external MHD instabilities in tokamaks

Andreas Kleiner

Macroscopic instabilities and their unfavourable effects on plasma confinement pose a central challenge for the development of reactor relevant tokamak scenarios. Some promising operation scenarios feature extended regions of low magnetic shear. These are in the core region for hybrid scenarios, and in the pedestal-edge region for e.g. the ELM-free quiescent H-mode (QH-mode). This thesis presents a non-linear study of macroscopic magnetohydrodynamic (MHD) instabilities with a focus on plasmas with regions of low magnetic shear. In the first part, we investigate the triggering of fast growing resistive modes in hybrid tokamak plasmas, where infernal modes can couple to neoclassical tearing modes (NTMs). Numerical simulations with the non-linear resistive initial value code XTOR-2F with and without inclusion of bootstrap current effects allow us to determine the evolution of MHD modes from the linear to the late non-linear phase. An analytical model is developed to describe the vanishing of mode coupling in the early non-linear regime. In this context, we extend the tearing stability parameter

\Delta'

from the linear to the non-linear phase and calculate the individual contributions to the growth of the resistive mode. This allows for an identification of the triggering mechanism in the initial phase and the dominant terms in the non-linear phase. A comparison with the numerical results shows that infernal mode coupling can destabilise otherwise stable NTMs and thus seed

2/1

magnetic islands. The helically perturbed bootstrap current is found to further destabilise the magnetic islands in the non-linear phase. In the second part of the thesis, 3D free-boundary equilibrium computations are employed to describe saturated external kink-type modes. The approach is first demonstrated to capture the salient features of non-linearly saturated external kink modes in standard baseline tokamak scenarios and is then applied to QH-mode plasmas. A method to conveniently extract the saturated displacement amplitude from edge-corrugated VMEC equilibria in terms of straight field line Fourier modes is presented. For standard current-driven modes the amplitude is compared with a non-linear analytical model, for which a numerical solver is implemented. In QH-mode plasmas, dangerous edge localised modes (ELMs) are replaced by benign edge harmonic oscillations (EHOs). The latter are commonly assumed to be connected to saturated external kink states. In our study of QH-mode plasmas, we consider two different driving mechanisms for external kink type-modes separately. We find that standard current-driven external kinks are linearly unstable, and non-linearly stable in a wide parameter range, especially where

q_{\text{edge}} \lesssim m/n

. But, where standard current-driven kinks are linearly stable we find that coupling of pressure-driven infernal modes can cause instability, and their upper sideband drives edge corrugations that appear to have external kink features. Both types of modes are identified with the VMEC equilibrium code, and the spectra are compared favourably with those of linear numerical approaches and analytic methods. EHOs could be connected to both type of modes.