Behaviour of tokamak plasma in the dynamic limiter experiment

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Abstract
A description of an experiment with a dynamic limiter – a pneumatically driven limiter of the JAERI tokamak, JFT-2 – is presented. Plasma behaviour is studied for the case of the dynamic limiter being quickly retracted during the discharge; this is done to clarify the role a material limiter plays in equilibrium and stability of a tokamak plasma. It has been seen from the response of a plasma ring to the high-frequency ( 1 kHz) electric field superimposed on the main electric field that the edge of the hot-plasma region, with effective radius a and a conductivity electron temperature of 20-100 eV, expands together with the dynamic limiter; this fact has been confirmed by the time behaviour of the plasma boundary as derived from the time variation of the light intensity. The ion temperature (Ti), which is obtained by the charge-exchanged neutral-particle energy analyser, obviously decreases after retraction of the limiter. Moreover, it is shown that, in this phase, a2/3Ti is almost constant. This behaviour (a2/3Ti const.) is shown to be due to the fact that, by neoclassical transport theory, is almost independent of time, where λ is the proportionality coefficient of Artsimovich's ion temperature scaling law and Ne is the total number of electrons.