Testing models of ion orbit loss (IOL) · Particle drifts lead to trapping and ion orbit loss...
Transcript of Testing models of ion orbit loss (IOL) · Particle drifts lead to trapping and ion orbit loss...
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J. R. KingWith contributions from
E. Howell, S. Kruger & A. Pankin (Tech-X); B. Grierson, S. Haskey (PPPL); R. Groebner (General Atomics);
J. Callen (U. Wisc); U. Schumlak & S. Taheri (U. Washington);
Work supported by the US Department of Energy, DE-SC0018311, DE-SC0018313 and DE-FC02-04ER54698Fusion Energy SciencesComputational support from NERSC
Testing models of ion orbit loss (IOL)
APS-DPP 2019
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Our understanding of the interaction between flow and extended-MHD dynamics is limited
● For fast MHD dynamics (e.g. disruption simulation) and transport time scales separated by multiple orders of magnitude
● Long time-scale MHD simulations run on the flow evolution time– NTM, QH, RMP simulations routinely run 1 to 100 of milliseconds
– NTM, QH, RMP dynamics critically depend on flow
● Predictive modeling of MHD dynamics requires models for flow in next generation devices
● Anticipate many relevant physics effects: – e.g. Ion-orbit loss (IOL), neoclassical poloidal flow, fast parallel, neutral
particle, and neutral beam torques
We work to incorporate IOL and other flow drives into extended-MHD simulations with the NIMROD code
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Focus on validation with DIII-D shot 164988
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Multispecies (D+C6 here) model required for correct collisionality parameters (T
i and Z
*)
Single species (deuterium)
Multi-species (D + C6)
Inclusion of Carbon permits profiles consistent with transport analysis in reconstructed state
All subsequent slides use multispecies
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Particle drifts lead to trapping and ion orbit loss
● Particle orbits depend on combination of drifts
● Dominant drift from ExB is from ErBpol and within a flux surface
● The loop voltage (Eφ) provides a small confining cross-field drift
● Cross field drifts (GradB and curvature) combine with mirror force to cause trapped ‘banana’ orbits
● Near the LCFS particles drift to open field lines and cause prompt losses
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Consider single particle motion to establish conceptual picture
Particle motionequations:(neglect ExBdrift for now)
Start near LCFS:
Track 4 particles with speed =
Passing Trapped in
Trapped out
Lost
-1.8 -0.77 0.62 1.8
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Mapping out phase space establishes a “loss cone”
Trajectories at s=2.63 vTi
(blue line in lower left fig)
Lost
Trapped in
Passing
Trapped out
If particle hits wall: lostOtherwise do 3 outboard midplane crossingsIf u
|| changes sign: trapped
Otherwise: passing
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Orbit losses vanish away from the separatrix
Most Ions with s<2.6 vTi NOT lost
Most thermal ions in loss cone lost
Passing
Trapped
Lost
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A culinary representation of IOL
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Shaing develops closure for IOL
FSA particleloss rate
Without collisions no pitch-angle scattering into loss cone
Losses are limited by collisions and distance from LCFS
S is ‘orbit-squeezing’ factor that decreases IOL outside minimum of E
r well
IOL particle flux acts like a current
IOL leads to viscous forces &co-current torque
Shaing et al., PFB 2 June (1990); Shaing PFB 2 Jan (1992); Shaing PFB 2 Oct (1992)
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Collisionality limits IOL in DIII-D shot 164988
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IOL rate is slightly limited by orbit squeezing
Orange – no orbit squeezing
IOL torque is not “turned off” by orbit squeezing from the generation of E
r
Need balancing torques
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Torque balance establishes flow – need to consider neoclassical poloidal flow damping
Callen UW-CPTC 09-06R
Residual stress leadsto neoclassical offset torquetowards offset velocity
Fast parallel viscous damping
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Poloidal flow damping alone does not set the poloidal flow
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Neutrals also provide edge torque
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Neutral closures
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1D neutral test with frozen plasma establishes neutral profiles
wall (neutral source)
Ballistic expansion
Inwardparticle flux
Initial state is local EQ:
Ionization in SOL
Te(wall)=10 eV
(sets wallneutral density) N
n ~ 1016 m-3
core
Time dependence:Red (initial)
toBlue (final)
Freeze plasma profiles similar to 164988 and establish neutral steady state
x (m)x (m)
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Local ODE equations become stiff at low T
Time-centered neutral implementation numerically unstable at low Te / low neutral population for this test case
New time-split algorithm extendsNIMROD’s implicit leapfrog:
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Computation with low neutral temperatures now tractable
Ballistic expansion
Inwardparticle flux
Initial state is local EQ:
Ionization in SOL & pedestal
Te(wall)=1.25 eV
(sets wallneutral density)N
n ~ 1019 m-3
core
wall (neutral source)
Time dependence:Red (initial)
toBlue (final)
Freeze plasma profiles similar to 164988 and establish neutral steady state
x (m)x (m)
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Next steps
● Add neutral beam torque
● Compute 2D neutral state
● Run time-dependent simulation for the steady-state flow in for 164988 with IOL, neoclassical poloidal flow, fast parallel viscous, neutral particle and neutral beam torques
● Compare to flows measured by CER (right)
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Summary
● Flow critical to MHD edge-mode dynamics on millisecond time scales
● Moving toward predictive modeling requires a model for the underlying flow
● Ion-orbit loss (IOL) produces a significant edge co-current torque
● This balances neoclassical poloidal flow, fast parallel, neutral particle, and neutral beam torques
● Work underway to include these effects in NIMROD simulations
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Extra slides (not included in poster)
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Trapping fraction calculations
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FSA magnetic field calculation
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Nustar and collisionality regimes
Plateau Pfirsch-Schluter(collisional)