Beta-limiting in the IR-T1 tokamak

Summary form only given, as follows. An experimental investigation has been carried out on the /spl beta/-limiting phenomena in the IR-T1 plasma during ohmic heating discharges. The IR-T1 is a small air-core transformer tokamak without conducting shell and divertor: (R=0.45 m; a=0.125 m; B/spl phi/<1 T; I/sub p/=35 kA; Te(O)=230 ev; Z/sub eff/<2.2; q(a)>2). One goal was the determination of the maximum achievable value of /spl beta//sub m/ (Troyon limit) as a function of q(a), in steady state operation. Another goal of this paper is a detailed study of the MHD phenomena giving rise to the observed /spl beta//sub m/-limit. In particular, we studied the hard disruptive instability occurring in the IR-T1 plasma under a range of reproducible conditions, for 3>q(a)>2. We use soft X-ray, ECE, visible spectrometer and Mirnov diagnostic systems to infer the structure of the MHD modes at the /spl beta//sub m/=1.8%. It is found that the maximum /spl beta//sub m/ is limited by the occurrence of coupled (m/n=3/2) and (m/n=2/1) modes.