diii-d shot #87009 observes a plasma disruption during neutral beam heating at high plasma beta...

DIII-D SHOT #87009 Observes a Plasma Disruption During Neutral Beam Heating At High Plasma Beta

Callen et.al, Phys. Plasmas 6, 2963 (1999)

• Rapid loss of thermal energy ~100 microsecond time scale

• Energy deposited on material wall and divertor. If ablative limit reached, material wall is damaged.

• For ITER, stored energy 100x greater than DIII-D

Control and mitigation of disruptions important for advancement of tokamak concept

Free-Boundary Simulations Based on Equilibrium Reconstruction

• Pressure raised 8.7% above most accurate equilibrium reconstruction (EFIT code)

• Pressure raised ideal MHD marginal stability limit to simplify physics

• Simulation includes:– n = 0, 1, 2– Anisotropic heat conduction

(with no T dependence)parperp

• Ideal modes grow with finite resistivity (S = 105)

First Macroscopic Feature is 2/1 Helical Temperature Perturbation Due to Magnetic Island

• Island result of 1/1 and 3/1 ideal perturbation causing forced reconnection

Magnetic Field Rapidly Goes Stochastic with Field Lines Filling Large Volume of Plasma

• Region near divertor goes stochastic first

• Islands interact and cause stochasticity

• Rapid loss of thermal energy results. Heat flux on divertor rises

Maximum Heat Flux in Calculation Shows Poloidal And Toroidal Localization

• Heat localized to divertor regions and outboard midplane

• Toroidal localization presents engineering challenges - divertors typically designed for steady-state symmetric heat fluxes

• Qualitatively agrees with many observed disruptions on DIII-D

Investigate Topology At Time of Maximum Heat Flux

• Regions of hottest heat flux are connected topologically

• Single field line passes through region of large perpendicular heat flux. Rapid equilibration carries it to divertor

• Complete topology complicated due to differences of open field lines and closed field lines

Initial Simulations Above Ideal Marginal Stability Point Look Promising

• Qualitative agreement with experiment: ~200 microsecond time scale, heat lost preferentially at divertor.

• Plasma current increases due to rapid reconnection events changing internal inductance

• Wall interactions are not a dominant force in obtaining qualitative agreement for these types of disruptions.

Future Directions

• Direct comparison of code against experimental diagnostics

• Increased accuracy of MHD model– Temperature-dependent thermal diffusivities– More aggressive parameters– Resistive wall B.C. and external circuit modeling

• Extension of fluid models– Two-fluid modeling– Electron heat flux using integral closures– Energetic particles

• Simulations of different devices to understand how magnetic configuration affects the wall power loading