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JT-60U Experimental Report No. 53 (August 11, 1999)

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Quasi Steady Sustainment of High Performance Reversed Shear Plasma

While a very high equivalent fusion power gain of QDTeq =1.25 was achieved in a JT-60U reversed shear discharge in 1998, its high performance phase was transient and the duration of QDTeq =1.25 was short as ~0.01 s. Therefore, the extension of the high performance duration has been an issue towards steady state tokamak operation.

Recently, a quasi steady state for high performance reversed shear plasmas was successfully created as shown below, where the high fusion performance of QDTeq ~ 0.5 was sustained for ~ 0.8 s, which is comparable to the energy confinement time. A new technique of stored energy feedback control by using neutral beam injection power was applied for maintaining the stored energy constant slightly below beta limits. Occurrence of a beta collapse limited the duration of the high performance when the magnetic pitch minimum qmin in the current profile decreased down to ~ 2.

Further extension of the high performance duration could be obtained with current profile control for avoidance of the beta collapse.

 

Figure
Figure Sustaining the equivalent fusion power gain of 0.5 for 0.8 s

Table Major plasma parameters
Table Major plasma parameters