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Publication# Controlling the rotation of drift tearing modes by biased electrode in ADITYA-U tokamak

Résumé

The influence of background plasma poloidal rotation on the rotation frequency of the m/n = 2/1 drift tearing mode (DTM) has been studied in ADITYA-U tokamak. The poloidal rotation velocity of the background plasma in the ion diamagnetic direction is increased or decreased by inducing an outward or inward radial electric field, respectively, through a biased-electrode placed in the edge region of the plasma. The rotation frequency of the preexisting drift tearing mode, rotating in the electron diamagnetic direction, concomitantly decreased or increased with the application of bias depending on its polarity. The positive-bias increases the background plasma rotation in the ion-diamagnetic direction from its pre-bias value, hence decreasing the DTM rotation frequency, whereas the negative bias reduces the plasma rotation velocity in the ion-diamagnetic direction, hence increasing the mode rotation. In addition to that, a short gas puff introduced during the positive and negative bias pulse further reduces the mode frequency, however, with different amplitudes in different bias-polarities. These observations suggest that the background plasma rotation contributes significantly toward the rotation of DTMs, and the rotation frequency of the magnetohydrodynamic modes can be modified by varying the poloidal rotation of the background plasma and/or the diamagnetic drift frequency.

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Controlled thermonuclear fusion is the main goal of plasma physics. At the Swiss Plasma Center, the Tokamak `a Configuration Variable (TCV) constitutes the main experiment on fusion research, where high temperature plasmas are confined by means of magnetic fields. The confinement of plasma energy and particles is limited by transport arising from the gradients between the hot-dense plasma core and the cold-rarefied plasma edge. Due to the tokamak topology, plasma can rotate in the toroidal and poloidal directions. Plasma rotation has a strong influence on confinement and stability, which makes its understanding a priority. There are many discrepancies between the theoretical rotation description and experiments, which stimulated research in the field. In this context this work provided experimental results of unprecedented accuracy, where plasma impurity parameters are measured with the charge exchange recombination spectroscopy (CXRS) diagnostic. CXRS exploits the CX signal induced by a diagnostic neutral beam injector (DNBI), permitting localised measurements of impurity rotation, density and temperature. During this work, the CXRS diagnostic was extended with the development of a new high resolution system, termed CXRS-EDGE, devoted to the study of edge profiles. The accuracy improvements with respect to the legacy systems were obtained through an high throughput lens spectrometer and numerical aperture matching optics, resulting in rotation uncertainties

It has been observed experimentally that magnetically confined plasmas, characterised by the safety factor q with a small or slightly inverted magnetic shear, have good confinement properties. Such plasmas typically have no internal transport barrier, operate with q95 around 4 and are good candidates for long pulse operation at high fusion yield in the reactor ITER. These hybrid scenarios are an intermediate step between the reference standard H-mode (high confinement) scenario with monotonic q and inductive current, and advanced scenarios with strongly reversed magnetic shear in which the entire plasma current is ideally generated non-inductively. This thesis focuses on the study of the dynamics of hybrid plasmas, with weak or almost zero magnetic shear, in tokamak and Reversed Field Pinch (RFP) configurations, when q in the central region assumes values close to one (tokamaks) or to a rational number (tokamaks, RFPs), though the exact resonance is avoided. The first part of this thesis is focused on the study of tokamak and RFP equilibria with slightly reversed shear when an extremum in the safety factor is close to a low order rational. These equilibria are characterised by the possible presence of internal helical cores, although the plasma edge is symmetric in the toroidal direction. Such 3D structures can be understood as the result of the nonlinear saturation of ideal MHD modes. The amplitude of large scale m=1 helical displacements in tokamak and RFP plasmas is investigated using contrasting approaches, namely 3D equilibrium and non-linear stability codes. The non-linear amplitude of such saturated modes obtained with the stability code is compared both with the helical core structure resulting from equilibrium numerical calculations, and with analytic predictions which extend the nonlinear treatment of reversed q plasmas to arbitrary toroidal mode numbers. A preliminary study of the impact of a n=1 RMP on the equilibrium helical distorsion is also presented. The second part of the thesis is devoted to the analytical and numerical study of the stability of an initially axisymetric tokamak configuration when the safety factor is almost flat and very close to a rational value over a macroscopically extended region in the plasma centre. Such conditions typically occur either in hybrid scenarios or following reconnection of a global instability such as a sawtooth. This configuration is characterised by non-negligible coupling between a fundamental mode and its Fourier adjacent modes. A dispersion relation has been derived both for ideal and resistive modes, with additional non-MHD effects such as plasma diamagnetism, viscosity and equilibrium velocity flows. The analytical results show that the resistive sidebands coupled to a core kink-like mode exhibit extremely fast growth, though additional non-MHD effects tend to moderately reduce the extreme growth rate of the resistive modes. The existence of such modes has been confirmed numerically, where the sensitivity of the growth rate to changes in resistivity and two-fluid effects has been demonstrated, and thus in turn provides generally good agreement with the analytical theory developed. A family of modes are obtained, including modes with novel scaling against plasma resistivity, some of which rotate in the electron diamagnetic direction, and others in the ion diamagnetic direction, consistent with experimental observations in e.g. TCV during hybrid-like operation.

Basil Duval, Emiliano Fable, Giovanni Tardini

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