Disruptions, Disruptivity, and Safer Operating Windows in the High-β Spherical Torus NSTX

Detection of Disruptions in the High-β Spherical Torus NSTX

“…Note that both of these trends are consistent with the disruptivity analysis in Ref. [116]; see that paper for additional discussion of the underlying physics.…”

“…As in Ref. [116], the core is defined as the average of CHERS channels 4 through 7 spanning R=0.99 to 1.09 m, while the midradius is defined as the average of the CHERS channels 16-20 spanning R=1.27-1.34m; these midradius chords are in the vicinity of the q=2 surface for typical NSTX NBI heated discharges. In this case, there is a rapid drop in the mid-radius rotation following time t 1 , with the core rotation dropping to zero shortly afterward.…”

“…For most examples in this paper, the disruption time will be defined as the time of the current quench, t quench , determined using an automated method as described in [14]. However, an additional relevant time is that when I P first deviates meaningfully from its requested value (t IpDev ) [116]; this time is typically, but not always, before t quench , due to the oftensignificant pre-disruption energy and current losses. Hence, in Section 4e), the definition t dis =min(t quench ,t IpDev ) will be used.…”

“…These have been observed both during the flat-top phase of high-elongation discharges, and during the phase after a locked mode or resistive wall mode drives the plasma out of H-mode [116]. Hence, observing vertical motion can be a useful indicator of proximity to disruption.…”

“…Hence, it is important to apply error field correction techniques that maintain the rotation [47,143], while monitoring the rotation for drops that may render the plasma more susceptible to these modes. Secondly, the core rotating n=1 modes that often occur in NSTX H-mode discharges [116,122,126,[144][145][146] have a tendency to first reduce the core rotation compared to the edge value, locking the plasma into a rigidly rotating system, followed by a rapid damping of the total plasma rotation. Hence, these modes can be detected by their impact on the rotation.…”