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Article overview
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Statistical analysis of $m/n$ = 2/1 locked and quasi-stationary modes with rotating precursors at DIII-D | | R. Sweeney
; W. Choi
; R.J. La Haye
; S. Mao
; K.E.J. Olofsson
; F.A. Volpe
; DIII-D Team
; | | Date: |
14 Jun 2016 | | Abstract: | A database has been developed to study the evolution, the nonlinear effects
on equilibria, and the disruptivity of locked modes with poloidal and toroidal
mode numbers $m=2$ and $n=1$ at DIII-D. The analysis of 22,500 discharges shows
that more than 18% of disruptions are due to locked modes with rotating
precursors. A parameter formulated by the plasma internal inductance $l_i$
divided by the safety factor at 95% of the toroidal flux, $q_{95}$, exhibits
predictive capability over whether a locked mode will cause a disruption, and
does so up to hundreds of milliseconds before the disruption. Within 20 ms of
the disruption, the shortest distance between the island separatrix and the
unperturbed last closed flux surface, referred to as $d_{edge}$, performs
comparably to $l_i/q_{95}$ in its ability to discriminate disruptive locked
modes. Out of all parameters considered, $d_{edge}$ also correlates best with
the duration of the locked mode. Disruptivity following a 2/1 locked mode as a
function of normalized beta, $eta_N$, peaks at an intermediate value, and
decreases for high values. The decrease is attributed to the correlation
between $eta_N$ and $q_{95}$ in the DIII-D operational space. Within 50 ms of
a locked mode disruption, average behavior includes exponential growth of the
$n=1$ perturbed field, which might be due to the 2/1 locked mode. Even assuming
the aforementioned 2/1 growth, disruptivity following a locked mode shows
little dependence on island width up to 20 ms before the disruption. Greater
deceleration of the rotating precursor is observed when the wall torque is
large, and at locking, modes often align at a particular phase. Timescales
associated with the mode evolution, the effects of poloidal beta on the
saturated width, the evolution of $eta_N$, and the reduction in Shafranov
shift during locking are also presented. | | Source: | arXiv, 1606.4183 | | Services: | Forum | Review | PDF | Favorites | |
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