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Magnetic Confinement Fusion ScienceStatus and Challenges
S. Prager
University of Wisconsin
February, 2005
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Two approaches to fusion
Inertial confinementextremely dense, short-lived
Magnetic confinementrelatively dilute, long-lived
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The tokamak
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Fusion energy requires
Heating the plasmato 100 million degrees
Confining the plasmaenergy replacement time ~ 1 second
for density ~ 1015 cm-3
Extracting energy from the plasmaco-existence of hot plasma and material surface
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progress in confinement is measured by
fusion triple product =
(density)(temperature)(energy replacement time)
(1021m-3) (108K) (1 second)
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Progress of fusion triple product
triples every 1.8 years
ITER
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Huge advance in plasma parameters
year
fusion power
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Progress accomplished through research infundamental plasma physics and technology
Challenges and opportunities remain
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Scientific issues for fusion(not exhaustive)
Maximize the plasma pressure
Control plasma turbulence and energy transport
Control plasma disruptions
Develop new magnetic configurations
Control the plasma-wall interaction
Develop new materials
Produce a burning plasma
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Pressure-driven instability
Rayleigh-Taylor instability in fluid,driven bygravity
heavyfluid
lightfluid
gravity
plasma instability,
driven bycentrifugal forceof
particles moving alongcurvedmagnetic field
plasma
magneticfield
Centrifugal force
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Centrifugal force in a torus
centrifugalforce
magneticfield
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Stability theory is highly developed
Stability depends upon
magnetic curvature, twist, shear
plasma rotation
location of conducting boundaries
All plasmas disassemble above a pressure limit
Feedback techniques have been developed to attainhigh pressure
(plasma pressure ~ 10% of magnetic pressure)
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Maximizing pressure through feedback
plasmapressure
with feedback
without feedback
plasmainstability(magnetic fielddisturbance)
1.0 1.5 2.0 2.5
time (seconds)
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Turbulence and transport
free energy sourcese.g., nonuniform Te, Ti, n, j.
turbulencefluctuating electric and magnetic fields
forces on charged particlesloss of particles and energy
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fusion power gain depends strongly on confinement
Q = fusion powerheating power
energy confinement time
0 1
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Plasma turbulence no longer considered
unavoidable force of nature
Fundamental understanding evolving
Control techniques evolving
Recent insight: sheared plasma flow canreduce turbulence
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Most Dangerous Eddies:Transport long distances
+Sheared Flows
Sheared Eddies
Less effective Eventually break up
=
Sheared Flows can Reduce or Suppress Turbulence
EB
vEB
~
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Computation of plasma turbulence
Without sheared flow
With sheared flow
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Ion transport barrier forms
Sheared flow and ion transport barrier formsspontaneously
Next frontiers: electron turbulence, magnetic
turbulence
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Disruptions
localized heat flux (tens of GW/m2 for 1 ms in ITER)
Induced currents in structures (hundreds of tons)
runaway electrons (tens of MA )
plasma current (MegAmps)
plasma temperature (K)
0 0.5 1.0time (seconds)
107
2
disruption
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Disruptions arise from sudden rearrangement ofmagnetic field
physics similarities tosolar flares
magnetic reconnection
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A practical method for disruption control
Permit disruption to occur,
Control its behavior by rapid injection of jet ofneutral gas
Causes energy to be radiated isotropically,
No localized heat deposition
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temperature(107K)
radiatedpower(GW)
Time after gas injection (ms)
radiates nearly 100% of power in 200 microseconds,
power radiated isotropically - no local damage
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Plasma confinement is an optimization problem withmany physics and engineering variables
Physics variables Magnetic field curvature, twist, shear, symmetry
Plasma flow
Spatial structure of electron temperature, iontemperature, current density..
Optimizing the magnetic configuration requires
fundamental physics and invention
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Configuration optimization is an essentialpartner to tokamak research
For fundamental plasma physics andfusion energy science
To evolve an improved fusion energyconcept
To contribute to scientific problems yetconfronting the tokamak
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The spectrum of magnetic configurations
magnetic field strong weak
self-
organized
externally
controlled
emerging highlydeveloped
examples:
compact reversed field spherical stellarator
torus pinch tokamak
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Tokamak stellarator
simple coils
symmetric around axis
current carrying
Complex coils
Helical symmetry withinplasma
No need for plasma current
Steady-state, no disruptions
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Reduce aspect ratio of tokamak
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tokamak spherical tokamak
Instability from centrifugalforce yields mediumpressure
Centrifugal forceMagnetic fieldline
Effect of centrifugalforce weakened, yieldinghigher pressure
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tokamak reversed field pinch
strong toroidal magnetic field weak toroidal field
simplifies engineering
but weakens confinement
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Spherical tokamak compact torus
Small hole in center No hole in center of torus
Very compact
Stability under study
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How to interface a 100 million degreeplasma to a room temperature wall?
Need to control the edge plasma
Need new wall materials
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A temperature pedestal forms at the plasma edge
radius
107
radius
Temperature (k)
~3 cm
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The temperature at the height
of the pedestal is important
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Structures developed to withstand aheat flux of 25 MW/m2
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Liquid walls for fusion
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Scientific issues for fusion
Maximize the plasma pressure
Control plasma turbulence and energy transort
Control plasma disruptions
Develop new magnetic configurations
Control the plasma-wall interaction
Develop new materials
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Conclusions
Many scientific challenges remain
Fusion energy science is highly advanced
We are ready to build a burning plasmaexperiment - a new frontier is fusion energy andplasma physics
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