arXiv subjects

V. A. Izzo

Publications and source records attributed to V. A. Izzo.

3 recordsLinked to original sources

arXiv2023

On the minimum transport required to passively suppress runaway electrons in SPARC disruptions

In [V.A. Izzo et al 2022 Nucl. Fusion 62 096029], state-of-the-art modeling of thermal and current quench (CQ) MHD coupled with a self-consistent evolution of runaway electron (RE) generation and transport showed that a non-axisymmetric (n = 1) in-vessel coil could passively prevent RE beam formation during disruptions in SPARC, a compact high-field tokamak projected to achieve a fusion gain Q > 2 in DT plasmas. However, such suppression requires finite transport of REs within magnetic islands and re-healed flux surfaces; conservatively assuming zero transport in these regions leads to an upper bound of RE current ~1 MA compared to ~8.7 MA of pre-disruption plasma current. Further investigation finds that core-localized electrons, within r/a < 0.3 and with kinetic energies 0.2-15 MeV, contribute most to the RE plateau formation. Yet only a relatively small amount of transport, i.e. a diffusion coefficient ~18 $\mathrm{m^2/s}$, is needed in the core to fully mitigate these REs. Properly accounting for (i) the CQ electric field's effect on RE transport in islands and (ii) the contribution of significant RE currents to disruption MHD may help achieve this.

physics.plasm-ph↗

arXiv2022

Runaway electron deconfinement in SPARC and DIII-D by a passive 3D coil

The operation of a 3D coil--passively driven by the current quench loop voltage--for the deconfinement of runaway electrons is modeled for disruption scenarios in the SPARC and DIII-D tokamaks. Nonlinear MHD modeling is carried out with the NIMROD code including time-dependent magnetic field boundary conditions to simulate the effect of the coil. Further modeling in some cases uses the ASCOT5 code to calculate advection and diffusion coefficients for runaway electrons based on the NIMROD-calculated fields, and the DREAM code to compute the runaway evolution in the presence of these transport coefficients. Compared with similar modeling in Tinguely, et al [2021 Nucl. Fusion 61 124003], considerably more conservative assumptions are made with the ASCOT5 results, zeroing low levels of transport, particularly in regions in which closed flux surfaces have reformed. Of three coil geometries considered in SPARC, only the $n=1$ coil is found to have sufficient resonant components to suppress the runaway current growth. Without the new conservative transport assumptions, full suppression of the RE current is maintained when the TQ MHD is included in the simulation or when the RE current is limited to 250kA, but when transport in closed flux regions is fully suppressed, these scenarios allow RE beams on the order of 1-2MA to appear. Additional modeling is performed to consider the effects of the close ideal wall. In DIII-D, the current quench is modeled for both limited and diverted equilibrium shapes. In the limited shape, the onset of stochasticity is found to be insensitive to the coil current amplitude and governed largely by the evolution of the safety-factor profile. In both devices, prediction of the q-profile evolution is seen to be critical to predicting the later time effects of the coil.

physics.plasm-ph↗

arXiv2021

Cold bubble formation from 2/1 tearing mode during massive gas injection in a tokamak

Massive gas injection (MGI) experiments have been carried out in many tokamaks to study disruption dynamics and mitigation schemes. Two events often observed in those experiments are the excitation of the $m=2, n=1$ magnetohydrodynamic (MHD) mode, and the formation of cold bubble structure in the temperature distribution before the thermal quench (TQ). Here $m$ is the poloidal mode number, $n$ the toroidal mode number. The physics mechanisms underlying those phenomena, however, have not been entirely clear. In this work, our recent NIMROD simulations of the MGI process in a tokamak have reproduced the main features of both events, which has allowed us to examine and establish the causal relation between them. In these simulations, the $3/1$ and $2/1$ islands are found to form successively after the arrival of impurity ion cold front at the corresponding $q = 3$ and $q = 2$ rational surfaces. At the interface between impurity and plasma, a local thin current sheet forms due to an enhanced local pressure gradient and moves inward following the gas cold front, this may contribute to the formation of a dominant $2/1$ mode. Following the growth of the $2/1$ tearing mode, the impurity penetration into the core region inside the $q=2$ surface gives rise to the formation of the cold bubble temperature structure and initiates the final TQ. A subdominant $1/1$ mode developed earlier near the $q=1$ surface alone does not cause such a cold bubble formation, however, the exact manner of the preceding impurity penetration depends on the nature of the $1/1$ mode: kink-tearing or quasi-interchange.

physics.plasm-ph↗