JET Snake Magnetohydrodynamic Equilibria

A long-lived density perturbation labelled a ``snake'' has been observed in the JET tokamak with pellet injection for toroidal field $B_t=3.1T$ and toroidal current $I_t=3MA$.\footnote{R.~D.~Gill {\em et al.}, Nucl.~Fusion \textbf{32} (1992) 723.} Spontaneous snakes triggered by core impurity accumulation have also been reported at $B_t=2.8T$ and $I_t=4.2MA$.\footnote{ibid.} We compute model magnetohydrodynamic equilibria with the 3D ANIMEC code\footnote{W.~A.~Cooper {\em et al.}, Comput.~Phys.~Commun.~\textbf{180} (2009) 1524.} that can recover snake-like conditions by prescribing peaked pressure and hollow toroidal current profiles which are consistent with those in the experimental discharges.\footnote{M.~Hugon {\em et al.}, Nucl.~Fusion \textbf{32} (1992) 33.} The internal helical distortions that look like snake structures have been obtained with the following parameters: $B_t\simeq 2.65T$, $I_t=3.75MA$, $q_0\simeq 1.7$, $q_{min}\simeq 1$, $q_{edge}\simeq 7.5$, $\ell_i\simeq 1.2$, $\left<\beta\right>\simeq 2.4\%$, $\beta_N\simeq 2.3$.