ReNeW Theme 5 Workshop

1
Status of the Stellarator Panel

• Members
• David Anderson
• Jeff Freidberg
• Jeff Harris
• Chris Hegna
• Steve Knowlton
• Mike Mauel
• Pete Politzer
• Allen Reiman
• Andrew Ware
• Harold Weitzner
• (and a host of contributors Thank you!)

Contact any of the above for input!
2
Process

• The stellarator panel has had numerous conference
  calls to discuss the TAP issues gt Panel feels
  the issues/conclusions reflect the needed
  activities
• Working on requirements and thrusts draw upon
  input from this workshop
• The Panel has significant representation (and
  input) in other Themes (especially II)
• Need to build community-wide support for
  contributions alternates can make to
  understanding toroidal science and our own
  concepts
• Begin definition of staged program elements and
  rough timeline needed to accomplish this research
  with enough detail to permit external costing
• Draft thrusts which reflect the goals of the
  issues put forth
• Develop cross-connects within Theme 5 and between
  the other Themes on our thrusts which permit
  distillation into the major program thrust
  outcomes of the overall ReNeW process

Goals for This Workshop
3
Stellarators and 3D Plasma Physics

• Stellarator confinement requires non-symmetric
  fields to generate rotational transform
• inherently 3D devices, requiring 3D analysis
• Advantages
• Inherently steady-state
• Empirically disruption-free
• Benign limiting behavior at high b
• High density
• Long connection length for divertor
• Disadvantages
• 3D geometry more complex to build
• Enhanced low-collisionality transport in
  unoptimized stellarators

Focus items of US program

• Well-developed understanding of 3D stellarator
  plasma geometry scientifically beneficial to
  other toroidal systems, including tokamaks
• e.g. weak 3D fields (lt10-3 B0) for ELM control

4
Steady progress in international program supports
steady-state, robustly stable stellarator concept

• Sustained ltbgt 4.5 benign limiting behavior
• ne0 1021 m-3 at B 2.7 T
• 3-5 X Greenwald limit
• Discharge duration 1 hr with P 0.6 MW,
  limited only by PWI (divertors important!)
• Ti 6.8 keV without impurity accumulation
• tE gt 0.2s
• Confinement scaling similar to tokamak

?E similar to ELMy H-mode
5
Stellarators contribute to PGO themes

• Predictable, high-performance steady-state
  plasmas
• Equilibrium from external fields ? minimal
  off-normal events
• Quiescent high-beta plasmas with confinement
  similar to tokamaks
• Good alpha particle confinement in optimized QS
  configurations
• No(minimal) need for auxiliary current drive,
  rotation drive, or profile control systems in
  reactor.
• Very high density operation reduces fast-ion
  instability drive
• Close coupling to theory gt predictability
• Taming the plasma material interface
• High density operation leads to radiative
  divertor
• No disruptions, avoids ELMs
• Talk by Boozer later today on 3-D Benefits

6
Quasi-symmetry is at core of US stellarator
program

• QS in B benefits stellarators by
• Tokamak-like neoclassical confinement
• reduced prompt a-losses
• Reduced flow damping
• Flexibility for lower aspect ratio
• QS restores 2D collisional transport physics to
  3D configuration
• Bridge to tokamak physics
• Benefits of QS are generic
• extend to quasi-axisymmetry and quasi-poloidal
  symmetry
• QS is US leadership activity. Key focus of ReNeW

HSX - quasi-helical symmetry
along field line
HSX Te profile, with and w/o QS
Where does the US stellarator program go and how
can it best contribute to toroidal fusion science?
7
From the TAP Report
Mission To achieve sufficient scientific
understanding and plasma conditions to justify
designing a fusion reactor based on a fully
steady-state, passively stable stellarator ITER-er
a goal Develop and validate the scientific
understanding necessary to assess the feasibility
of a burning plasma experiment based on the
quasisymmetric stellarator TAP concludes there
is little doubt that a stellarator configuration
can confine plasma at the parameters necessary
for fusion burn. Scientific and technical
questions to achieve this goal Tier 1 Simpler
coil systems Integrated high-performance of
quasisymmetric optimized stellarators Predictive
Capability Divertors
8
Simpler coil systems

• Stellarators clearly can be, and have been, built
  successfully to specification
• Simplifying stellarator design, construction,
  assembly and maintenance while preserving
  requisite physics properties is highly desirable
• Do fundamentally simpler coil solutions exist
  that perform confinement missions demanded of
  them?
• Refine and validate physics input on b-limits,
  confinement
• Incorporate engineering metrics in coil design
  process
• Incorporate engineering experience from recent
  devices into design basis
• Model use of magnetic materials (ferritics, S/C
  diamagnets)
• Fabrication and assembly tolerances are cost
  drivers.
• Can coil fabrication and assembly/alignment
  tolerances be relaxed through use of trim coils?

9
Trim coils reduce errors with RMPs
10
Integrated High-Performance of Quasisymmetric
Optimized Stellarators
There is a need to demonstrate the ability of
quasisymmetric configurations to confine high-b,
low collisionality plasmas with comparable ion
and electron temperature Can this be accomplished
without current drive, active stabilization, and
without danger of disruption? Does the density
limit degrade in the presence of large
currents? How do impurities behave in a
high-performance quasisymmetric plasma? To
satisfy the ITER-era goal we need to understand
the scaling of confinement and performance in
such a device How do we define a staged program
to accomplish this goal with acceptable risk?
11
High-? equilibrium limits rather than stability?
11

• Transient instabilities at intermediate b do not
  impede access to high b
• No disruptions observed

• Equilibrium reconstruction analysis indicates
  loss of 35 of minor radius surface break-up as ?
  increases. Trim coils can improve flux surfaces.

??? 2.7
12
Predictive Capability
Development of a validated predictive capability
is necessary for extrapolation to a fusion energy
system for any concept. Significant efforts are
being devoted to tokamak simulation codes many
are inherently three dimensional Modest efforts
could connect these codes to 3D equilibrium
configurations with benefit to all of toroidal
confinement (e.g. 3D RMP or rotation driven by
NTV) Most models assume simply-connected
well-formed flux surfaces. Magnetic surface
break-up may be the b-limiting process in some
configurations Equilibrium determination and
finite-b surfaces in 3D is an overlapping issue
for all toroidal systems Quasisymmetry permits
plasma flow (and flow shear) How can this be used
to improve configurations?
13
Divertors

• Divertor operation has been essential in
  achieving present stellarator results
• Island divertor in W7-AS (W7-X) and LHD
  (helical upgrade on LHD)
• Structure is complicated by 3-D nature
• Power loads far below that needed for the
  ITER-era goal
• Key questions
• Can a divertor configuration be developed for a
  stellarator which reduces power flux to
  acceptable levels to target plates?
• Can the interaction region be greatly expanded?
• Can this be resolved within the constraints
  imposed by the other physics design requirements
  of the device?
• There is a strong need to continue development
  and validation of edge/divertor models and for
  investigation into options for future designs

14
TAP Tier 2 Issues

• Operational limits
• Density limits well understood (radiation)
  b-limits appear benign
• Allowable parallel (bootstrap) current an open
  issue
• Impurity and ash accumulation
• High confinement modes without impurity
  accumulation exist
• Not well understood at present Wagner
  Impurity confinement may be the most critical
  physics aspect along the road to steady-state
  operation
• Anomalous transport reduction
• Flows and flow shear can result in reductions of
  anomalous transport
• Not critical to goal, but could result in design
  improvements
• 3-D non-linear gyro-kinetics/turbulence
  simulations needed

15
TAP Tier 3 Issues

• Four issues were relegated to Tier 3 based on the
  existence of known solutions or non-critical
  elements of the ITER-era goal (but merit
  investigation)
• Energetic particle instabilities high density
  reduces EPM drive
• Disruptions can be designed for low/moderate
  currents
• ELM-free high performance modes of operation
  observed
• High-temperature superconducting coils great
  potential for application to stellarators, but
  not essential to the goal

TAP report states It is clear that to achieve
the ITER-era goal a quasi-symmetric experiment of
sufficient scale needs to be undertaken within
this timeframe to demonstrate, in an integrated
fashion, that the benefits of quasi-symmetry seen
at the CE level can be extended to high
performance, high beta plasmas
16
Goals for the Workshop

• To incorporate comments from the committee as a
  whole on research requirements to advance the TAP
  stellarator goal
• Refine (and shortly complete) technical
  requirements on the required research
• Begin definition of staged program elements and
  rough timeline needed to accomplish this research
  with enough detail to permit external costing
• Draft thrusts which reflect the goals of the
  issues put forth
• Develop cross-connects within Theme 5 and between
  the other Themes on our thrusts which permit
  distillation into the major program thrust
  outcomes of the overall ReNeW process

17
Possible (Strawman) Thrusts

• Employ 3D stellarator tools to contribute to
  understanding and improvement of toroidal
  confinement
• Use of steady-state systems without current drive
  to address issues of toroidal confinement
• Develop broad understanding of QS stellarator as
  basis of credible approach to fusion energy

18
Possible (Strawman) Thrusts (2)

• Understand plasma confinement in disruption-free
  quasi-symmetric 3D configurations at high beta
  and low collisionality
• Develop 3D QS configurations using simpler coils
  and assembly strategies
• Assess the suitability of 3D divertor designs for
  high power exhaust fluxes.