Theory: Addressing RFP Physics Issues and Broader Issues in Magnetic Confinement

1
Theory Addressing RFP Physics Issuesand Broader
Issues in Magnetic Confinement

• P.W. Terry, A. Boozer, J.S. Sarff, R. White, C.
  Sovinec

2
General considerations

• Toroidal Alternates Panel (TAP) identified key
  issues (prioritized in tiers) for major toroidal
  alternates, including RFP
• Theory (Analytic, numerical) will be
  indispensable for progress
• White paper organized by TAP report topics
• More broadly, RFP complements tokamak in
  confinement issues
• Impacts key confinement questions, e.g.,
  electron and ion thermal transport, momentum
  transport, density peaking, etc.
• Theory is key player in making connections
  across device types

3
Tier 1Identify transport mechanisms and
establish confinement scaling

• What is cause of weaker decrease of transport
  with S in experiment relative to theory?
• Need better understanding of stochastic
  regions, including
• -Better theoretical description of finite
  amplitude islands
• -Transition from global tearing to small-scale
  drift Alfvén, including role of two-fluid and
  kinetic effects
• Need better understanding of transport in
  fluctuating magnetic field
• -Wave particle resonances in overlapping,
  nonoverlapping island regimes
• -Ion and electron transport, including potential
  -gt ExB rotation
• -Assess diffusive paradigm sub diffusive? super
  diffusive?
• -Transport in transient events like sawtooth
• Consider axisymmetric shaping (make m gt 1?)

4
Tier 1Transport mechanisms continued

• What is the nature of the single helicity state
  and its transport scalings?
• What is the role of dissipation (Hartman
  number)?
• Mechanism for decoupling single helicity
  perturbation from other helicities
• Temporal behavior of switching between single,
  multiple helicity states
• Current threshold condition
• Does it operate as an H-mode-like transport
  barrier?
• Does pressure gradient from barrier drive flow
  shear?
• Simulation with macroscopic codes understand,
  optimize, NC transport

5
Tier 1Transport mechanisms continued

• What is residual transport and scaling after
  reducing stochastic transport?
• Role of small scale, electrostatic
  instabilities like ITG
• Bad poloidal field curavature everywhere,
  shorter connection length and role on k,
  negative magnetic shear, q lt 1, finite beta
• What instabilities dominate?
• What is their role on transport?
• Can residual transport be controlled with
  transport barriers?

6
Tier 1Transport mechanisms continued

• What is the physics of rotation and momentum
  transport in RFP?
• What are relative roles of
• Magnetic vs. electrostatic fluctuations?
• Global vs. local fluctuations?
• Core processes vs. edge processes?
• What is connection between momentum transport
  and other transport channels?
• Is there a connection between current profile
  and rotation?
• Study through calculations of
• Constrained relaxation
• Diamagnetic effects in tearing modes
• Calculations of Reynolds, Maxwell stresses

7
Tier 1Transport mechanisms continued

• What governs the magnetic fluctuation spectrum?
• Questions on saturation of global tearing
  modes
• What are relative roles of current profile
  flattening and wavenumber cascade?
• How does this change through sawtooth cycle?
• What is width of saturated islands?
• What is appropriate description for small
  scale fluctuations?
• What is their effect on large scales?
• How is energy dissipated?
• What is connection to ion heating?
• What is the mechanism for anomalous ion heating?
• Will it extend to reactor conditions (alpha
  channeling)?
• To validate theoretical mechanisms, calculate
  observable consequences ofeach theory for
  experimental comparison

8
Tier 1Understanding the physics of current
sustainment in the RFP

• Can oscillating field current drive (OFCD) supply
  100 of plasma current?
• To limit AC modulation of field, high
  Lundquist number needed in experiment
• Explore two-fluid effects in simulation
• Is it possible to drive 100 of current?
• Effects on transport?
• Are there other means for current sustainment?
• RF inefficient for current drive, but could be
  used for profile control
• How much non inductive current needed for
  desired confinement improvements?
• Can helical shaping broaden current profile
  sufficiently to improve tearing stability?
• Could study with VMEC and codes that do not
  require toroidal symmetry

9
Tier 1Integration of current sustainment and
improved confinement

• Can good confinement be maintained during 100
  current drive?
• Issues OFCD deposition, inward current
  relaxation, effect on flux surfaces
• Theory and simulation can answer to what
  extent confinement is degraded
• Do non MHD fluctuations play a role in
  relaxation, confinement?
• Does outward relaxation pose a danger to wall?
• Is a hybrid scenario of OFCD and current
  ramp-down attractive and possible?
• Use computation to see if non steady scenario
  offers advantages
• Can current profile be controlled efficiently in
  steady state?
• What are overall effects on confinement of
  current profile control techniques?

10
Tier 2Plasma Boundary Interactions

• To what extent is RFP boundary physics like that
  of the tokamak?
• What mechanisms dominate transport in the
  boundary region?
• -Is transport bursty?
• -Is it related to core events, or is it
  indigenous to edge?
• -What is the role of the edge shear flow layer?
• What is combined effect of island overlap,
  small magnetic fluctuation level in edge?
• -Relationship to RMP, particularly with respect
  to transport
• Distance from plasma to wall and active
  feedback coils
• Does RFP boundary physics allow for a divertor?
• Does the short connection length affect
  divertor detachment?
• What are proximity effects of divertor x-point
  and reversal radius?

11
Tier 2Energetic Particle Effects

• What aspects of energetic particle effects are
  relevant to the RFP?
• What is the spectrum of toroidal Alfvén
  eigenmodes in RFP?
• Is it driven by global tearing modes?
• How is TAE stability affected by NBI fast
  particles?
• What is the role of high RFP beta?
• Examine issues relating to energetic particle
  beta scaling and need for high T to limit Ohmic
  dissipation
• Can high density operation limit energetic
  particle population growth?
• What aspects of RFP energetic particle effects
  differ from the tokamak?
• Approximate calculations suggest effect on
  tearing mode stability
• -Self consistently treat coupling of energetic
  particles and bulk collective excitation

12
Tier 2Determine and Understand the Beta Limiting
Mechanism

• What mechanisms limit beta how do they depend on
  geometry and profiles?
• 26 beta values with pellet injection appear
  to exceed Suydam limit
• -Need to study pressure gradient effects, drifts
  for variety of potential magnetic modes
• -Does helical equilibrium with axisymmetric
  shaping have benefits by increasing local shear?
• How do beta limiting mechanism affect the plasma?
• Is beta limit hard, leading to disruption, or
  soft, leading to transport increase?
• -Evolution of instability at finite amplitude
• -Role of pressure gradient

13
Tier 3Optimization of RWM control for fusion
environmentSelf-consistent reactor scenarios

• Quantify requirements on wall conductivity,
  proximity issues for RWM control
• Are controls for plasma/wall interaction and RWM
  compatible with good confinement and efficient
  current sustainment?
• Integrates confinement, sustainment
  understanding with effect of external control
  perturbations long term issue