Transcript plasma

Tokamak physics and thermonuclear
perspectives
Alexei Dnestrovskij
Kurchatov Institute, Moscow Russia
This lecture was prepared during the visit to the
Culham Science Centre, UK
Outline
•Requirements for fusion energy release, Lawson criterion
•Tokamak device
•Tokamak basic physics
•Examples of Tokamak experiment
•The next step – ITER
It occurs when two light nuclei are forced together, producing a
larger nucleus
The combined mass of the two
small nuclei is greater than the
mass of the nucleus they produce
The extra mass is changed into
energy
We can calculate the energy
released using Einstein’s famous
equation:
D+T=He + n
3.5MeV 14.1MeV
E = mc2
To overcome strong repulsive forces,
fusion nuclei require very high
energies - matter becomes a …
pressure and confinement
(p=n*T and
 E)
<v>
Fusion requires high plasma
Cross section:
High temperatures (Te and Ti) are required
D-T
D-D
D-He3
1
1
10
10
100
100
Ion Temperature /keV
for fusion events < v >
High density (n ) is required for reaction rate
E
Fusion power  n2 < v >
characterises energy loss time = stored energy
loss rate
Requirement for ignition (Lawson criterion):
nE > 1.5x1020m-3s
At T~30keV ~300MoC
Fusion factor:
Q= Fusion power
External power
… on some history
• Anomalous Bohm confinement
1 cTe
DB 
16 eB
During 50s- late 60s it correlated with wide variety of data on radial
diffusion in different devices.
• Tokamak experiments in 1965
Artsimovich and colleagues reported an excess of Bohm confinement
by a factor approximately 3
• So it was the beginning of TOKAMAK ERA in
fusion
What is the TOKAMAK ?
• Tokamak, from the Russian words:
toroidalnaya kamera and magnitnaya katushka
meaning “toroidal chamber” and “magnetic coil”
A tokamak is a toroidal plasma
confinement device with:
– Toroidal Field coils to provide
a
toroidal magnetic field
– Transformer with a
primary winding to produce a
toroidal current in the plasma
– The current generates a poloidal
magnetic field and therefore twisted
field lines which creates a perfect “trap”
– Other coils shape the plasma
Major Progress Towards Fusion Power
Fusion factor:
Q= Fusion power
External power
Q=0.65 achieved in JET
Q~1 in Neutron Source
Q~10 in ITER
Q>50 in Power Plant
Magnetic fusion activities
Future steps
• ITER (International Thermonuclear Experimental
Reactor), project is ready
• 3 medium size tokamaks under construction: KSTAR
(S.Korea), HT-7U (China), SST-1 (India). Main aims:
steady-state long pulse operations
• CTF (Component Test Facility), preliminary studies
In operation
• 3 large fusion devices operational (JET, JT-60U, LHD),
a big stellarator under construction (W7-X)
• 11 medium size tokamaks are operational
• … plus ~ 50 small size devices
Progress on JET
(Joint European Torus)
JET parameters:
Major radius 3m
Minor radius 1m
Plasma height 3.5m
Plasma current 3MA
Toroidal B = 2.7T
Tokamak Basics
Drifts in nonuniform plasma
Example:
electrons
B drift
B
• Magnitude of B varies with position
• Larmor radius varies as 1/B
• What happens in
tokamak plasma due
to the drift?
•Charge separation
•Vertical electric field
•ExB plasma losses
B
ions
•OutwardsExB drift
------B
B
B
E
ExB
+++++
Tokamak Basics
Toroidal current in tokamak produces poloidal
magnetic field
Plasma current
Poloidal magnetic field
Toroidal
magnetic field
Important parameter
for stability analysis
safety factor q
toroidal turns
q= poloidal turns
Edge q >2 for stability
• Field lines become helical
• Particles can move to short out any E field produced
by gradient/curvature drifts
Tokamak Basics
ASDEX-Upgrade
in Germany
Plasma forms magnetic surfaces
Quasineutrality of plasma
Σnjej=0
provides to use fluid MHD equations
Strong parallel transport :
V||/V┴ ≈ 106
gives the formation of magnetic surfaces
Fast processes (10-8 – 10-7 s)
•Plasma pressure becomes a
function of magnetic surfaces
p=p(ψ)
where ψ – poloidal magnetic flux
•Equilibrium installed
p = jxB
MAST in UK
Magnetic confinement problems
Magnetically confined Fusion plasmas
suffer from:
1) Instability - difficult to confine a high density and
temperature plasma with low magnetic fields.
2) Turbulence - limits the confinement time for a
given sized machine.
3) Power loading - high volume to surface area ratio
means power loading on surfaces is high.
4) Neutron activation - materials must withstand
high neutron fluxes
Examples of plasma behaviour: sawteeth
TS
•Oscillations of plasma parameters
•Name from SXR trace
• Shown by most tokamaks
• appear when q<1
• 2 very different time scales
(crash ~ µs, ramp ~ 10’s ms)
What is the sawtooth?
Kadomtsev model
Examples of plasma behaviour:
Plasma heat and particle losses
•Collisional transport
•Fluctuations driven transport
δn/n ~ δT/T ~ eδ/T < 50%
δBr/B ~ 10-4
It is commonly accepted: the enhanced transport is the
result of fluctuations
Flux due to the electrostatic fluctuations :
Γ=< δn δv >~< n c δE/B> ~< n c δ/L /B> ~< n/L cT/(eB)>
Flux due to the electromagnetic fluctuations:
Γ=n/B < δv|| δBr >
Bohm’s like scaling law.
Can the fluctuations be suppressed ?
Examples of plasma behaviour: H-mode
• Upon exceeding a critical
heating power (PL-H) transition
from L-mode to H-mode
occurs
• Transition occurs with
spontaneous formation of an
Edge Transport Barrier (ETB)
– thin, situated at edge of plasma,
just inside of the scrape off layer
(region II on picture)
Examples of plasma behaviour: H-mode
• Drop in D radiation indicating
decrease in particle flux with
formation of transport barrier
• Many evidences of fluctuation
suppressing overall the plasma
volume
• No commonly accepted complete
physical model
Dα line intensity
H-mode and
ELMs:
movie from MAST
shot
plasma current
H-mode
50 ms
Dα - signal
Examples of plasma behaviour: Modes
with Internal Transport Barrier (ITB)
JT-60 1994
The role of q-profile
Ne
• An ITB is in essence similar to the
H-mode ETB, however it is not
restricted to the edge
– ITBs can be formed at almost
any point in the plasma
• ITBs can dramatically increase
plasma performance
• ITBs are obtained by manipulating
plasma’s q-profile
– produce regions of weak or
reversed magnetic shear
q’=rdq/dr
• ITBs form at min q or on a rational q
close to the minimum
Ti
Te
q
When?
Fusion
Power
Pulse
duration
Q
1997
16MW
~1 second
<1
2015-2020
500-700MW
<30 minutes
>10
~2050
~3000-
~1 day
~50
4000MW
The next step - ITER
• To demonstrate integrated
physics and engineering at ~GW
level at min. cost
• Superconducting coils, powerplant-level heat fluxes, “nuclear”
safety
• Its design is realistic, detailed
and reviewed like no other
fusion device
• Negotiations in final phase for
ITER
• Unofficially Europe now made a
decision to built ITER in South
France
Fusion economics
ITER cost
6 billion $(1989) per 500-700MW
thermal energy
(over $1billion was spent for project design)
Usual fission
power plant
$0.7billion per thermal gigawatt
Oil fuel
exploration and development
(not for production)
$100 billion in last year spent
by 30 top oil firms
To summarize the reviewed problems …
The tokamak device picks up many physical problems together:
Plasma confinement:
•Different kinds of instabilities
•Plasma transport across the magnetic surfaces
•Disruptions
Diagnostics
External Heating
Tokamak-reactor problems (challenge for ITER):
•Power exhaust
•Neutrons