MAGNETIC WELL AND INSTABILITY THRESHOLDS IN THE TJ-II STELLARATOR

Experimental evidence for the controlled onset of fluctuations by varying the magnetic well in the plasma edge region of the TJ-II stellarator (major radius R=1.5 m, minor radius a<0.22 m, magnetic field B=1 T) has been obtained. The level of fluctuation, the degree of intermittency and the radial correlation of relevant quantities increase as the magnetic well is reduced. TJ-II possesses a high degree of flexibility, making it an ideal device for investigating the characteristics of transport close to instability thresholds.