Callen: NTM Physics Discussion (Nov 14, 2000)
Electrons: bootstrap current (w<ne)
Ions: poloidal ion flow damping (¶/¶t < ni )
Neo-enhanced polarization flow
(from Ń·J ~ Ń·[B´Ń·P_parallel/B2] )
Longer-term … kinetic-based closures (¶/¶t > ni , kparallel vT> w)
Temperature cparallel >> cperp , diffusive to “free-streaming”
Pressure, density (sound waves) evolution
? need Landau-type closure to limit mode interactions to
Dx < (Ls/rp)ri ~10ri
need gyroviscosity to obtain correct nonlinear behavior
Possible physics tests of NTM simulations:
o Seed island threshold…due to Chi_parallel, classical and neoclassical ion polarization effects
o Flow effects…diamagnetic flows, toroidal flow, island rotation
o Stabilization by localized current (ECCD), heating (ECH)
o Error field, helical perturbation effects ?
· Explore NTM Nonlinear Physics:
o NTM excitation via mode coupling … sawtooth crash, ELMs, …(need flow differences on different rational surfaces)
o Why does only one NTM mode occur at a time?
o Why does 2/1 NTM mode in DIII-D evolve to a disruption?
o NTM interactions with resistive wall modes (RWMs)
o What happens when NTM islands get large ?
…toroidal mode coupling, island hits “limiter”?
Hegna (Presented at Oct APS) Issues for NTM modeling
· Neoclassical flow damping – damping of flows within magnetic surfaces (bootstrap current/poloidal ion flow damping)
Current implementation of the flow damping forms encapsulates this physics – produces bootstrap current drive for NTM’s – damping of flows along perturbed surfaces. Analytic forms used are based on long-timescale asymptotics (t >> 1/nc) and quasi-equilibrium structure of helically distorted magnetic surfaces -> time and space issues for theorists to ponder.
· Anisotropic heat and particle flux-transport along field lines >> transport across field lines.
Local diffusion coefficients with disparate scales present in NIMROD. Wave-like physics to equilibrate along field lines in M3D – mocks up free streaming physics – long mean free part regime equilibration along B is nonlocal (Held’99) – implementation ?
· Ion polarization threshold requires 2-fluid drift effects (w*) – differential flow between ion and electron fluids.
In two-fluid formulation, what is the natural frequency of a nonlinear island? Depends parametrically on viscosity ? ion flow damping ? neoclassical elements? Other dissipation processes ? Island width?
· Seed island formation requires another MHD event (sawtooth crash, ELM). Differential rotation between geometrically coupled magnetic surfaces.
Self-consistent edquilibrium with sheared flow. Accurate description of sawteeth? ELM’s?
· Ultimate nonlinear evolution – isolated small island dynamics explained by Rutherford-like theory- what happens when islands gets big ? – little theoretical insight.
Disruption phenomenology ? Coupled island dynamics ? Nonlinear NTM sawtooth interaction ? NTM coupled to free boundary modes (RWM)?