Kansas Annual NSF EPSCoR Statewide ConferenceWichita, KS January 12-13, 2012
Simulation of pellet ablation in DIII-D
Tianshi Lu
Patrick RinkerDepartment of Mathematics
Wichita State University
In collaboration with
Roman Samulyak, Stony Brook University
Paul Parks, General Atomics
Model for pellet ablation in tokamak
• MHD system at low ReM
• Explicit discretization• EOS for partially ionized gas• Free surface flow• System size ~ cm, grid size ~ 0.1 mm
Courtesy of Ravi Samtaney, PPPL
Tokamak (ITER) Fueling
• Fuel pellet ablation• Striation instabilities• Killer pellet / gas ball for
plasma disruption mitigation
Schematic of pellet ablation in a magnetic field
Schematic of processes in the ablation cloud
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1.Spherical model• Excellent agreement with NGS model
2.Axisymmetric pure hydro model• Geometric effect found to be minor (Reduction by 18% rather than 50%)
3.Plasma shielding without rotation• Subsonic ablation flow everywhere in the channel• Ablation rate depending on the ramp-up time
4.Cloud charging and rotation• Supersonic rotation causes wider channel and faster ablation• Ablation rate independent of the ramp-up time
Simulation results of pellet ablation
Spherical model Axis. hydro model Plasma shielding
Plasma shielding without rotation
Mach number distribution
Double transonic flow evolves to subsonic flow
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Formation of the ablation channel and ablation rate strongly depends on plasma pedestal properties and pellet velocity.
Plasma shielding without rotation
Supersonic rotation of the ablation channel
Cloud charging and rotation
Isosurfaces of the rotational Mach number in the pellet ablation flow
Density redistribution in the ablation channel
Steady-state pressure distribution in the widened ablation channel
2TB
• Gsteady of a rotating cloud is independent of tramp
• G(tramp) < Gsteady
• G(tramp) increases with tramp
• Fast pellet
• Short ramp-up distance
Fixed pellet: effect of ramp up time
Shrinking pellet: tumbling pellet model
“Pancake” pellet
• Due to anisotropic heating, the pellet would evolve to a pancake shape.
• In reality, the pellet is tumbling as it enters the tokamak, so its shape remains approximately spherical.
• In the simulation, the pellet shrinking velocity is averaged over the surface to maintain the spherical shape.
Tumbling spherical pellet
Shrinking pellet: DIII-D temperature profile
DIII-D Temperature and Density Profile G from simulation agrees with 0.8 GNGS
Conclusions and future work
Conclusions
• Supersonic rotation causes wider channel and faster ablation• Good agreement with NGS model for DIII-D profile • Smaller Ablation rate during fast ramp-up
Future work
• Inclusion of grad-B drift in the simulation• Non-transient radial current for smaller B field – finite spin up• Mechanism of striation
