Characterization of disruption halo currents in the National Spherical Torus Experiment

Abstract

This paper describes the general characteristics of disruptions halo currents in the National Spherical Torus Experiment [M. Ono, et al. Nuclear Fusion 40, 557 (2000)].The commonly observed types of vertical motion and resulting halo current patterns are described, and it is shown that plasma discharges developing between components can facilitate halo current flow. The halo current fractions and toroidal peaking factors at various locations in the device are presented. The maximum product of these two metrics for localized halo current measurements is always significantly less than the worst-case expectations from conventional aspect ratio tokamaks (which are typically written in terms of the total halo current). The halo current fraction and impulse is often largest in cases with the fastest plasma current quenches and highest quench rates. The effective duration of the halo current pulse is comparable to or shorter than the plasma current quench time. The largest halo currents have tended to occur in lower β and lower elongation plasmas. The sign of the poloidal halo current is reversed when the toroidal field direction is reversed. PACS: 52.55. Ez, 52.55.Tn

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

“…Secondly, moving more current closer to the edge increases the coupling to the wall and improves the global n=1 stability in the wall stabilized regime. Third, equilibrium and current drive calculations show that increased l i is often associated with increases in the pressure peaking [34], which is independently associated with increases in disruptivity.…”

“…Halo currents in NSTX have been discussed in Ref. [34], and in the spherical torus MAST in Ref. [31].…”

“…The current quench can also lead to the generation of runaway electrons [6,7,[16][17][18][19][20][21], which can result in catastrophic vessel and PFC damage [33]. Finally, if the control of the plasma vertical position [23][24][25][26][27] is lost, the plasma can drift up or down and come in contact with the vessel and PFCs; the "halo currents" [28][29][30][31][32][33][34] that are shared between those components and the plasma can lead to destructive forces [15]. Clearly, it is necessary to avoid these events.…”

Detection of Disruptions in the High-β Spherical Torus NSTX

“…Finally, the plasma column often moves upward or downward during or proceeding the disruption, and makes contact with the first wall or divertor. When this happens, "halo" currents [3,4,[28][29][30][31][32][33][34][35][36] flowing between the plasma and the wall or divertor structures can lead to large forces on those items. For all these reasons, disruptions must be avoided in these large next-step devices.…”

Dynamics of the Disruption Halo Current Toroidal Asymmetry in NSTX

“…Currents have been observed to flow from the plasma into the PFCs, through the various in-vessel structures, and then out of a different set of PFCs back into the plasma. These currents, which have historically been known as "halo currents", have been observed in the conventional aspect ratio tokamaks DIII-D [3,[25][26][27][28], JET [29][30][31][32][33][34], ASDEX-Upgrade [35,36], COMPASS-D [37], JT-60 [38] and ALCATOR C-MOD [39], and in the spherical torii MAST [40] and NSTX [41]. When the current path in these in-vessel components crosses the strong tokamak magnetic field, the resulting JxB forces can result in severe damage.…”