Publications

@article{pfefferle-m3dc1,

title = {Modelling of NSTX hot vertical displacement events using M3D-C1},

author = {D Pfefferl\'{e} and N Ferraro and S C Jardin and I Krebs and A Bhattacharjee},

doi = {10.1063/1.5016348},

year  = {2018},

date = {2018-04-03},

journal = {Physics of Plasmas},

volume = {25},

number = {5},

pages = {056106},

abstract = {The main results of an intense vertical displacement event (VDE) modelling activity using the implicit 3D extended MHD code M3D-C1 are presented. A pair of nonlinear 3D simulations are performed using realistic transport coefficients based on the reconstruction of a so-called NSTX frozen VDE where the feedback control was purposely switched off to trigger a vertical instability. The vertical drift phase is solved assuming axisymmetry until the plasma contacts the first wall, at which point the intricate evolution of the plasma, decaying to large extent in force-balance with induced halo/wall currents, is carefully resolved via 3D nonlinear simulations. The faster 2D nonlinear runs allow to assess the sensitivity of the simulations to parameter changes. In the limit of perfectly conducting wall, the expected linear relation between vertical growth rate and wall resistivity is recovered. For intermediate wall resistivities, the halo region contributes to slowing the plasma down, and the characteristic VDE time depends on the choice of halo temperature. The evolution of the current quench and the onset of 3D halo/eddy currents are diagnosed in detail. The 3D simulations highlight a rich structure of toroidal modes, penetrating inwards from edge to core and cascading from high-n to low-n mode numbers. The break-up of flux-surfaces results in a progressive stochastisation of field-lines precipitating the thermalisation of the plasma with the wall. The plasma current then decays rapidly, inducing large currents in the halo region and the wall. Analysis of normal currents flowing in and out of the divertor plate reveals rich time-varying patterns.},

keywords = {disruption, M3D-C1, resistive MHD, vertical displacement event},

pubstate = {published},

tppubtype = {article}

}

@article{pfefferle-vdemagneto,

title = {Algebraic motion of vertically displacing plasmas},

author = {D Pfefferl\'{e} and A Bhattacharjee},

doi = {10.1063/1.5011176},

year  = {2018},

date = {2018-01-27},

journal = {Physics of Plasmas},

volume = {25},

number = {2},

pages = {022516},

abstract = {The vertical motion of a tokamak plasma is analytically modelled during its non-linear phase by a free-moving current-carrying rod inductively coupled to a set of fixed conducting wires or a cylindrical conducting shell. The solutions capture the leading term in a Taylor expansion of the Green's function for the interaction between the plasma column and the surrounding vacuum vessel. The plasma shape and profiles are assumed not to vary during the vertical drifting phase such that the plasma column behaves as a rigid body. In the limit of perfectly conducting structures, the plasma is prevented to come in contact with the wall due to steep effective potential barriers created by the induced Eddy currents. Resistivity in the wall allows the equilibrium point to drift towards the vessel on the slow timescale of flux penetration. The initial exponential motion of the plasma, understood as a resistive vertical instability, is succeeded by a non-linear “sinking” behaviour shown to be algebraic and decelerating. The acceleration of the plasma column often observed in experiments is thus concluded to originate from an early sharing of toroidal current between the core, the halo plasma, and the wall or from the thermal quench dynamics precipitating loss of plasma current.},

keywords = {eddy current, inductance, vertical displacement event},

pubstate = {published},

tppubtype = {article}

}