Abstract
Operation of fusion confinement experiments in full steady state is a major challenge for the development towards fusion energy. Critical to achieving this goal is the availability of actively cooled plasma facing components and auxiliary systems withstanding the very harsh plasma environment. Equally challenging are physics issues related to achieving plasma conditions and current drive efficiency required by reactor plasmas. RF heating and current drive systems have been key instruments for obtaining the progress made until today towards steady state. They hold all the records of long pulse plasma operation both in tokamaks and in stellarators. Nevertheless much progress remains to be made in particular for integrating all the requirements necessary for maintaining in steady state the density and plasma pressure conditions of a reactor. This is an important stated aim of ITER and of devices equipped with superconducting magnets. After considering the present state of the art, this review will address the key issues which remain to be solved both in physics and technology for reaching this goal. They constitute very active subjects of research which will require much dedicated experimentation in the new generation of superconducting devices which are now in operation or becoming close to it.
Highlights
In the past decades, emphasis of fusion research has been placed on demonstrating, albeit during short pulses, plasma parameters required, at least in dimensionless form, by future fusion reactors
We review the progress made by the actuators based on waves in the electron cyclotron (EC), the ion cyclotron (IC) and the lower hybrid (LH) range of frequencies both in their technical realization and for their properties for driving plasma current
These records have been achieved in devices equipped with super-conducting magnets potentially allowing of CW plasma operation but, interestingly, in these cases the duration was not limited by the capability of some components of the machine but by unexpected events such as a radiation collapse triggered by detached flakes (LHD), very low frequency oscillations (TRIAM) or MHD events (Tore-Supra)
Summary
Emphasis of fusion research has been placed on demonstrating, albeit during short pulses, plasma parameters required, at least in dimensionless form, by future fusion reactors. The world tokamak community has adopted a “wind tunnel” approach which has led to dimensioning the ITER tokamak for achieving, with high confidence, a fusion gain Q = 10 lasting in excess of 300 seconds This approach left the goal of running in true steady state or at least during very long pulses as an objective to be achieved later in a second phase. A comprehensive physics basis is provided in a recent review [2] It puts the emphasis on the profound coupling between MHD stability, bootstrap current, transport and edge plasma conditions and shows the narrow way forward towards a steady state tokamak. We outline the major issues which need to be resolved in line with the ITER programme
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