Transcript What is fusion? - UCLA Henry Samueli School of Engineering

What is fusion?
• It is combining two hydrogen
atoms to form helium
• It’s the opposite of fission, which is
splitting uranium atoms into smaller
pieces.
• Either nuclear process gives much
more energy than chemical processes
like burning gasoline.
Fusion is the energy of the sun
and the stars
The D-T reaction
Heavy hydrogen
D
T
a
Helium
Neutron
Deuterium
Tritium
n
This is not the cleanest reaction, but it’s the
easiest one to start with. The neutron causes a
small amount of radioactivity, 1000 times less
than in fission.
Advanced fuels would be
completely neutron-free.
Seawater is the fuel source
• Water contains one molecule of D2O for
every 6000 molecules of H2O.
• The cost of separating deuterium is
trivial.
• There is enough deuterium to supply
mankind for billions of years.
Accelerators would not work
D+
T+
Positive nuclei repel and will bounce off
T+
D+
Head-on collisions resulting in fusion are rare
We have to make a plasma
Electron (-)
Ion (+)
A plasma is a hot, ionized gas with equal numbers of ions and electrons. The
energy lost in non-fusion collisions remains in the plasma. Once in a while, there
is a fusion collision. This happens often enough if the plasma is dense enough
and hot enough.
How hot and how dense?
• Temperature 300,000,000 degrees!
• Density 1/10,000 of atmospheric density
• Net pressure is 4 atmospheres
Use smaller numbers:
1 eV (electron-volt)  10,000 K
300,000,000 K  30,000 eV = 30 keV
How to hold this plasma?
• No material wall can be used.
• The sun uses its large gravitational field.
• On earth, we have only electric and
magnetic fields (E and B fields).
• E-fields not good: pushes + and –
charges in opposite directions.
• Hence, we use magnetic fields.
We must make a “magnetic bottle”
What is a magnetic field?
The earth has a magnetic field,
which makes compasses work.
Iron filings show the field
of a horseshoe magnet
Coils can make B-fields
+
V
-
Permanent magnet
Electromagnet
How B-fields can hold a plasma
B
A magnetic bottle cannot be a sphere
B-field has to be zero at the poles
The simplest possible shape is a torus
The field lines can be
toroidal, like this one
Or poloidal, like these
The toroidal field is produced by
poloidal currents in “coils”
A
B
A combination: helical lines
`
`
PLASMA CURRENT
When the twist in the lines (the poloidal part) is produced by a
current in the plasma, the magnetic bottle is called a TOKAMAK.
Making a toroidal bottle work
Step 1: cancel vertical drifts with helical field
B
-
+
+
+
X
-
+
A
This is the first principle of toroidal
B
confinement
Step 2: Hydromagnetic instabilities
A) The Rayleigh-Taylor instability
PLASMA
PLASMA
E´B
Pressure
Pressure
Vi
Ve
·
MAGNETIC FIELD
(a)
·
Ve
·
---
+
E
+
+
+
MAGNETIC FIELD
(b)
Vi
·
Step 2: Hydromagnetic instabilities
B) the kink instability
WEAK FIELD
`
CURRENT
STRONG FIELD
Shear stabilization
Used to stabilize both R-T and kinks
The curvature effect
8
V
V
8
Convex curvature has a strong
stabilizing effect, but it cannot be
incorporated well in a tokamak.
Step 3: Microinstabilities
Plasma turbulence
Water turbulence
“Drift” waves were found to be the
cause of “Bohm diffusion”
B
These waves are driven only by the pressure gradient in the plasma.
It took several decades to solve this problem. During this delay, fusion got a
bad reputation.
The turbulence and fast loss rate have been eliminated by proper shaping of the
magnetic field.
Step 4: Banana orbits
“Neoclassical” diffusion
WEAK FIELD
STRONG FIELD
WEAK FIELD
STRONG FIELD
WEAK FIELD
STRONG FIELD
Magnetic islands
PASSING ORBIT
The plasma in a TOKAMAK is a gas that moves in these unusual ways.
Computer simulation
Design of TOKAMAKS had to wait for computers able to handle 3D simulations.
Mother Nature is helping us
1. Sawtooth oscillations
4
q=3
q (r)
3
q=2
2
q = 1.5
q=1
1
Unstable
sawteeth
0
0
2
4
6
r (cm)
8
10
Mother Nature’s helping hand
2. The H-mode (high confinement mode)
PLASMA PRESSURE
TRANSPORT
BARRIER
n, T
Pedestal
To divertor
0
MINOR RADIUS
This increases confinement by 2X and has been studied extensively.
The H-mode was discovered when powerful neutral-beam heating was used.
Mother Nature’s helping hand
3. Internal transport barriers
Learning from the H-mode, we have been able to produce
transport barriers inside the plasma
Mother Nature’s helping hand
4. Zonal flows
+ ++
- --
B
+ ++
- -++ +
+
+ +
- + + +
- -
Jupiter
Long turbulent eddies break themselves up into small ones.
Other beneficial effects in tokamaks
which arise naturally
• Bootstrap current (90% of tokamak
current can be produced by itself)
• Isotope effect (DT confined better than
DD)
• The Ware pinch (inward motion)
How far have we come?
100
Reactor level
Triple product
10
1
ALCATOR C
ALCATOR A
JT-60U JT-60U
JET
JT-60U
TFTR
JT-60U
DIII-D
JT-60U
JT-60U
TFTR
JET
JET
DIII
0.1
PDX
TFR
PLT
ST
0.01
T3
2-year doubling rate
0.001
1965
1970
1975
1980
1985
Year
1990
1995
2000
2005
Triple product Tn = Temperature x density x confinement time
Compare with Moore’s Law
Four large tokamaks
TFTR,
Princeton,
USA
JET,
European
Union
DIII-D,
General
Atomic,
USA
JT-60 U,
Japan
Inside the DIII-D
The D-shape, with divertor
The hot escaping
plasma is absorbed
by a “divertor”.
The tokamak scaling law
Ability to predict
The pressure law
The density law
Unsolved physics problems
ELMs (Edge Localized
Modes)
Disruptions
Fishbones
These cause sudden loss of plasma. Ad hoc suppression has
been devised, but no general solution.
ITER, the international tokamak
7 nations, > ½ world population
Site: Cadarache, France
Cost: 5B euros (construction), 5B euros (operation)
Construction underway
The time line
The aim of ITER is
to reach ignition,
when the alpha
particle products
of the DT reaction
can keep the
plasma hot
without external
heating.
Steps toward a reactor
1. Show a burning plasma in ITER
2. Simultaneously build machines to test
engineering concepts
3. Build a demonstration reactor DEMO
producing small but significant power
4. Build a 2000 MW fusion reactor
Major engineering challenges
• A material for the First Wall
• Energy handling by divertors
• Breeding tritium in Li blankets
Conclusions
• Progress has been remarkable on a very
tough problem
• The physics is understood well enough to
proceed
• The engineering has hardly started and
needs to be heavily funded
• There is an international will to solve both
climate change and energy shortage with
this significant step in human evolution.