Equilibrium and axisymmetric stability of dee-shaped plasmas in Doublet III

Abstract

Stable plasmas with surface elongations of up to 1.8 (aspect ratio = 3.4) have been produced in the upper lobe of Doublet III with the use of both passive and active controls. The growth rate of the vertical instability has been measured at various values of elongation by disabling the feedback circuit of the vertical position control power supply. A rigid-shift analysis of growth rates indicates that the passive stabilization effect of the field-shaping coils plays a key role in achieving a high elongation of 1.8. Experiments have demonstrated that the maximum stable elongation is determined by the strength of the passive stabilization effect even with active feedback control. The dee-shape is found to be preferable to an elliptical shape because the triangularity reduces the absolute value of the decay index required to produce a given elongation.

Stability limit of feedback control of vertical plasma position in the JFT-2M tokamak

“…If there is no conductive material around the plasma, the growth rate of this instability is larger than 10 s s" 1 , which is too large for the feedback control to suppress the instability by practically feasible power supply. Growth rate reduction by the passive feedback effect of the conductive material has been confirmed experimentally in some tokamak devices [1][2][3]. This effect is necessary for vertical instability stabilization by practical power supply.…”

“…The maximum growth rate in Doublet-Ill reported in Ref. [2] is also about 500 s" 1 , with the same type In Fig. 11, the decay rates of the exponentially changing response are adopted as data because JD for the oscillating response is not a good representation of the response characteristics.…”

Stability conditions of feedback control system of vertical plasma position in tokamaks with a resistive shell

“…If the plasma duration time is shorter than the skin time of the shell, the positional instability is stabilized by the shell or the passive conductors [7][8][9][10][11][12][13][14][15][16]. For long-pulse operation, however, active feedback control is necessary to maintain the noncircular tokamak plasma in the centre [17][18][19].…”

The fusion power research programme at GA Technologies Inc

“…Prime considerations were theoretical work [43] and reactor studies indicating that reactor-relevant values of (3 could be attained in dees. At the same time, the JAERI team [44] demonstrated that comparable performance could be obtained in such plasmas with fewer control problems than experienced with doublets. A powerful incentive was the ability to use the auxiliary heating power coming on line in a configuration of higher power density, by depositing heat in a single lobe rather than spreading it in the heating of doublets.…”