Abstract
Experiments in the TCV tokamak show that when high power central electron cyclotron heating (ECH) and current drive (ECCD) are deposited in the central plasma region, tearing-like instabilities can develop. In the present work we address, both analytically and numerically, the problem of tearing mode (TM) generation analysing extensively the behaviour of L-mode and low plasma current TCV discharges when high power heating is deposited in the plasma core region. Three possible mechanisms are discussed. The first is the modification of the equilibrium current density profile due to the power deposition leading to variation of the sign of the classical tearing stability index Δ', where finite β effects could also be taken into account. The second mechanism still the modification of the tearing stability parameter Δ', but this time due to the presence of an equilibrium sheared toroidal plasma rotation. Finally, the third mechanism considered is the role of infernal-type instabilities in driving tearing-like modes when the safety factor becomes sufficiently close to a rational in the core region.
Highlights
One of major issues in present day tokamak theory, is to understand the mechanisms which cause the onset of dangerous tearing-like instabilities
Experimental observations in L-mode and low plasma current TCV discharges, showed the presence suddenly growing neoclassical tearing modes (NTMs) when high power heating is deposited in the plasma core region
This work we address the problem of NTM generation analysing a realistic TCV configuration when central power deposition and electron current drive (ECCD) are applied
Summary
One of major issues in present day tokamak theory, is to understand the mechanisms which cause the onset of dangerous tearing-like instabilities. Experimental observations in L-mode and low plasma current TCV discharges, showed the presence suddenly growing NTMs when high power heating is deposited in the plasma core region. This occurred without usual triggers such as sawteeth or ELMs [2] a small 1/1 MHD activity was detected. We can think of three possible mechanisms for the triggering of these sudden tearing modes:
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