A multi-machine scaling of halo current rotation

Abstract

Halo currents generated during unmitigated tokamak disruptions are known to develop rotating asymmetric features that are of great concern to ITER because they can dynamically amplify the mechanical stresses on the machine. This paper presents a multi-machine analysis of these phenomena. More specifically, data from C-Mod, NSTX, ASDEX Upgrade, DIII-D, and JET are used to develop empirical scalings of three key quantities: (1) the machine-specific minimum current quench time, τ CQ ; (2) the halo current rotation duration, trot; and (3) the average halo current rotation frequency, f h. These data reveal that the normalized rotation duration, trot/τ CQ , and the average rotation velocity, v h , are surprisingly consistent from machine to machine. Furthermore, comparisons between carbon and metal wall machines show that metal walls have minimal impact on the behavior of rotating halo currents. Finally, upon projecting to ITER, the empirical scalings indicate that substantial halo current rotation above f h = 20 Hz is to be expected. More importantly, depending on the projected value of τ CQ in ITER, substantial rotation could also occur in the resonant frequency range of 6-20 Hz. As such, the possibility of damaging halo current rotation during unmitigated disruptions in ITER cannot be ruled out.

MHD stability and disruptions in the SPARC tokamak

“…Following the analysis in Myers et al. (2018), the expected number of full toroidal rotations of a halo current asymmetry can be calculated using multi-machine scalings of the rotation frequency and the halo current rotation duration. The rotation scaling from Myers et al.…”

“…The radial field produced by the poloidal field coil system during the plasma flattop is calculated and the maximum vertical excursion of a full current plasma with a flat q = 1 profile is estimated to be |ΔZ| ≈ 0.5 m. The radial field averaged over the displaced plasma is B R ≈ 0.5 T, giving FIGURE 9. Projected non-axisymmetric halo current behaviour in terms of rotation duration versus rotation count, as scaled from the tokamak database in Myers et al (2018). The plot is presented in a way that facilitates direct comparison with the ITER projection in that reference, where the shaded parallelogram and its unshaded extension represent the projected rotation ranges for the lower and upper bounds of the minimum CQ time, respectively.…”

“…The location of the asymmetry often rotates toroidally during the disruption, which can lead to dynamically amplified forces if the halo currents complete 2-3 full rotations at a frequency that resonates with critical machine components (usually of the order of 10 Hz). Following the analysis in Myers et al (2018), the expected number of full toroidal rotations of a halo current asymmetry can be calculated using multi-machine scalings of the rotation frequency and the halo current rotation duration. The rotation scaling from Myers et al (2018) is used to predict the intrinsic rotation using the SPARC V2 design variables.…”

“…Following the analysis in Myers et al (2018), the expected number of full toroidal rotations of a halo current asymmetry can be calculated using multi-machine scalings of the rotation frequency and the halo current rotation duration. The rotation scaling from Myers et al (2018) is used to predict the intrinsic rotation using the SPARC V2 design variables. Figure 9, which is analogous to figure 14 of Myers et al (2018), shows that halo current rotation may be damaging if SPARC has any critical machine or system resonances above 60 Hz.…”

“…The rotation scaling from Myers et al (2018) is used to predict the intrinsic rotation using the SPARC V2 design variables. Figure 9, which is analogous to figure 14 of Myers et al (2018), shows that halo current rotation may be damaging if SPARC has any critical machine or system resonances above 60 Hz. Analysis similar to those for JET (Riccardo, Walker & Noll 2000) or ITER (Schioler et al 2011) will be conducted to verify that all machine resonances are below 60 Hz.…”