Part 1 - MIT Club of Washington DC

A tutorial on Inertial Confinement Fusion (ICF): progress and
challenges
An attractive path to ICF that could lead to a practical fusion energy source
MIT Club of Washington DC
25 May 2015
Presented by Steve Obenschain
Laser Plasma Branch
Plasma Physics Division
U.S. Naval Research Laboratory
Work supported by DOE-NNSA
The Naval Research Laboratory
Navy’s Corporate Research Laboratory
2320 Federal employees/ 849 PhDs / $1.056B/yr
Advocated by Thomas Edison (1915)
Established by act of Congress in 1916
Startup in 1923
me
NRL Pioneered many advances:
U.S. Radar (starting in early 1920’s)
NRL developed radars “contributed to the victories of the U.S. Navy in the
battles of the Coral Sea, Midway, and Guadalcanal.”
GPS
Vanguard rocket and scientific package (2nd U.S. satellite)
1st reconnaissance satellite
Under cover of scientific research: Galactic Radiation and Background
(GRAB) satellite system.
NRL has a vigorous program in energy R&D
“The U.S. Department of Defense (DoD) consumed 889 trillion BTU of
energy in FY08…..Although this is less than 1.5% of overall U.S. usage, it
makes the DoD the single largest energy user in the country.”
Energy Sources
• Laser Fusion
• Methane Hydrates
Energy Storage
• Nanoscale Electrode Materials
for Batteries
Energy Conversion
• Photovoltaics
Power Delivery
• Superconductors
Fusion powers the visible Universe..
5
Can it provide clean plentiful energy on earth?
Nuclear Fusion -- the basics
Deuterium - D
need 100 million oC
+ confinement
neutron - n
+
Energy
+
+
Tritium - T
Fusion
Reaction
+
Helium - He4
D + T  He4 ( 3.45 MeV) + neutron (14.1 MeV)
this is the easiest fusion reaction to achieve
So what's so good about nuclear fusion as
potential energy source?
•Plentiful fuel
– Deuterium: from seawater
• Enough for billions of years!
– Tritium: bred from lithium
• Enough readily available lithium for 1000’s of years.
• Operation does not make greenhouse gasses
• Attractive advanced approach to nuclear energy
– Limited, controllable radioactive waste
– Could provide a good fraction of worldwide need for base-load electrical
power.
Magnetic Fusion Energy effort is centered on ITER
• First DT burn scheduled for ~2030
• 500MW fusion thermal power in
~15 min. pulses by ~2034.
• To be followed by DEMO a high
availability power reactor.
From http://www.iter.org/default.aspx
Basic principles of inertial confinement fusion
Deuterium Tritium plasma
• Temperature T (~10 keV)
• Density ρ
• Radius r
• Expansion velocity V
r
Plasma
v
Expansion velocity (v) ≈ (kT)1/2
Reaction rate = ρ2 RDT(T)
Available time t =r/v
We need a large fraction of the DT fuel to burn before it expands.
Fraction burned
≈ ρ2 x RDT (T) x t /ρ
≈ ρr x RDT (T)/v(T)
Large ρr allows large % of fuel to burn
But energy required and released scales as the mass - 4/3πρr3
Need to maximize the density ρ (~1000x solid density)
Inertial Fusion (via central ignition)
Lasers or x-rays heat outside
of pellet, ~100 Mbar
pressure implodes fuel to
velocities of 300 km/sec
ablator
Central portion of DT
(spark plug) is heated
to ignition.
(~100 Gbar, ~108 oC)
Hot
fuel
Thermonuclear burn
then propagates
outward to the
compressed DT fuel.
Cold
fuel
~ 2 to 4 mm
DT ice
foot
drive
Laser
Power
time
~ 3% of original
target diamter
• Simple concept
• Potential for very high energy gains (>100)
• Requires high precision in physics & systems
• Need to understand & mitigate instabilities
A heavy fluid supported by a lighter fluid is subject to
Rayleigh-Taylor Instability
Example: A glass of water turned upside down..
Before
Glass of water
(Heavy Fluid)
Air
(Light Fluid)
During
After
An ICF pellet has a Rayleigh Taylor (RT) Instability:
Pressure from the low density ablated material accelerates the high
density shell.
t0
laser
target
(section
of shell)
t1 = t0 + 
Accelerated
&
compressed
"Fuel"
laser
ablated
material
A
Ak (t) = Ako ek t
Mitigation of RT:
Minimize Ao (from target and drive imperfections)
Reduce ( t)
Laser-plasma instabilities that can scatter the laser light (a loss
mechanism) or produce high-energy electrons that heat the fuel too early
and thereby reduce compression.
DOE’s National Security Administration (NNSA) funds ICF research as
part of its stockpile stewardship program
National Ignition Facility
Lawrence Livermore National Lab.
OMEGA Laser Facility
University of Rochester, LLE
Z pulsed power facility
Sandia National Lab
Nike KrF Laser Facility
Naval Research Laboratory
NIF concentrates the energy from 192 laser beams energy in a
football stadium-sized facility onto few-mm-size targets.
Matter
temperature
>108 K
Radiation
temperature
>3.5 x 106 K
Densities
>103 g/cm3
Pressures
>1011 atm
Lawrence Livermore National Laboratory
15
Pxxxxxx.ppt – Edwards, NRL, 3/18/15
NIF utilizes flashlamp pumped Nd:glass amplifiers
Near infrared λ = 1054 nm light from Nd:glass is
frequency tripled to UV and directed to target
Nd:glass amplifier
Accommodates 8 30-cm
aperture beams
1 of 192 beams
https://str.llnl.gov/str/Powell.html
16
OFFICIAL USE ONLY
NIF Laser Bay (1 of 2)
2013-049951s2.ppt
OFFICIAL USE ONLY
17
Photoshopped
target bay all floors
NIF 6-m diameter target chamber
2013-043921s1.ppt
Moses - IFSA, 9/9/13
18
OFFICIAL USE ONLY
2013-043921s1.ppt
OFFICIAL
USE ONLY
Moses - IFSA, 9/9/13
19
Indirect Laser Drive (approach chosen for NIF)
Laser beams heat wall of a gold hollow cylinder (hohlraum) to ~300 eV and
resulting soft x-rays drive the capsule implosion.
Illustration from https://lasers.llnl.gov/programs/nic/icf/
The Challenge — near spherical implosion by ~35X
195
µm
DT shot N120716
Bang Time
(less than diameter
of human hair)
~2 mm diameter
Lawrence Livermore National Laboratory
21
Pxxxxxx.ppt – Edwards, NRL, 3/18/15
The NIF indirect drive effort has greatly advanced the
physics understanding of that approach
Hohlraum
performance
Backscatter
Capsule shape
Spectrum
LEH size
Wall motion
In-fight instability
Stagnation
Plasma conditions
DT hot spot shape
Streak Trajectory
R
Shocks
Picket drive symmetry
time
time
1D
3D
But NIF so far has not achieved ignition with indirect drive,
there is another way – laser direct drive
Indirect Drive
Hohlraum Capsule
• Relaxed laser uniformity requirements
• Higher mass ablation rate inhibits
hydro-instability.
Laser • Less efficient illumination of target
Beams • More complex physics
• More challenging diagnostic access
x-rays
• Much more efficient (7 to 9 x) use of laser
light.
Direct Drive
Capsule
• Simpler physics
• Much higher predicted performance (gain)
• Simpler target fabrication
• Advances in lasers (beam smoothing) and
target designs should provide needed
Laser Beams implosion symmetry.
Two developments that help enable symmetric direct
drive implosions.
1980’s Development & use controlled laser spatial incoherence
to achieve time-averaged smooth laser profiles on target.

Random Phase Plates – RPP (ILE, Japan)
Induced Spatial Incoherence – ISI (NRL)
Smoothing by Spectral Dispersion – SSD (LLE)
4
Laser intensity
log scale
Late 1990’s – Development of “tailored adiabats’ to reduce Rayleigh
Taylor instability at the ablation layer while maintaining high fuel
density.
ablator
DT ice

preheated ablator
(lower density)
DT ice
• Larger ablation velocity (VA= {mass ablation rate}/) suppresses RT instability.
• Can be accomplished via decaying shocks or soft x-ray preheat.
NRL is the world leader in high-energy electron-beam pumped
krypton fluoride (KrF) lasers
Nike 60-cm aperture amplifier
•
•
•
•
Gas laser verses solid-state Nd:glass used in NIF (easier to cool)
Electron beam pump versus flashlamp light with glass
Operates in deeper UV
56 beams extract energy with Nike (more beams & fewer amplifiers than with
glass)
Use of KrF light has many advantages for direct drive
Provides the deepest UV light of all ICF lasers (λ=248 nm)
Deeper UV
Superior beam
smoothing
• Inhibits undesired laser-plasma instability
• Higher efficiency implosions.
• Less laser energy required to obtain
ignition and high yield
• Much more uniform target illumination.
• Focal zooming that is desired to increase
efficiency, and that is likely required to
avoid deleterious cross-beam-energy
transport.
Early time
Nike
zoomed
focus
Nike focal profile
Late time
Shock Ignited (SI) direct drive targets
Pellet shell is accelerated to sub-ignition velocity (<300 km/sec), and ignited
by a converging shock produced by high intensity spike in the laser pulse.
Low aspect ratio pellet helps mitigate
hydro instability
Peak main drive is 1 to 2 × 1015 W/cm2
Igniter pulse is ~1016 W/cm2
* R.
Betti et al., Phys.Rev.Lett. 98, 155001 (2007)
High gain is obtained with both KrF (λ=248 nm) and frequency
tripled Nd:glass (λ=351 nm) lasers with direct drive shock ignited
targets with focal zoom.
“Shock Ignition”
Direct Drive (248 nm)*
“Shock Ignition”
Direct Drive (351 nm)*
“Shock Ignition”
Direct Drive (351 nm)
No zoom
* 2 focal diameter zooms
during implosion
Simulations predict ignition and high energy gain with a
529 kJ KrF direct drive implosion (1/3 of NIF’s energy)
Snapshots of high resolution 2-D simulation of implosion
Simulation
shows growth of
instability
seeded by target
imperfections
Initial pellet
2 mm
138 x
energy gain
Imploded pellet
(magnified scale)
0.4 mm
0.2 mm
0.1 mm
The target has to release enough energy
to power the reactor…
AND produce electricity for the grid
Target "Gain" = Fusion power OUT / laser power IN
(Nuclear reactions in chamber “blanket” add 1.1× to target gain)
Target
Gain = 130x
1,430 Megawatts
(heat)
572 Megawatts
( electricity)
Electricity
Generator
(40%)
430 Megawatts
143 Megawatts
Power Lines
10 Megawatts
KrF Laser
(7% efficient)
143/572 = 25%
Recirculating power
Higher target gain increases power to grid and reduces %
of power needed to operate the reactor.
Target "Gain" = Fusion power OUT / laser power IN
(Nuclear reactions in chamber “blanket” add 1.1× to target gain)
Target
Gain = 200x
2,200 Megawatts
(heat)
880 Megawatts
(electricity)
Electricity
Generator
(40%)
737 Megawatts
143 Megawatts
Power Lines
10 Megawatts
KrF Laser
(7% efficient)
143/880 = 16%
Recirculating power
Nike krypton-fluoride laser target facility
NRL Laser Fusion
Target chamber optics
60 cm aperture amplifier
Nike Target chamber
56-beam 3-kJ
KrF laser-target facility
Nike laser Chain
Illuminated
aperture imaged
onto target
Laser profile in target chamber
Experimental layout of Nike target chamber
x-ray streak
camera
12 beams for x-ray
lighters
neutron
detector
(1 of 3)
Near UV/Visible
Streaked Spectrometer
Hard x-ray
Spectrometer
VISAR
target
44 high quality
main beams
optical streak
camera
imaging crystal
back and side
lighters
x-ray framing or streak camera
side-on refractometer
Monochromatic x-ray imager coupled with streak camera
revealed an oscillatory behavior of ablative RichtmyerMeshkov instability
Rippled
CH Target
Backlighter
Laser Beams
Main Laser Beams
Long Pulse (4 ns)
Quartz
Crystal
1.86keV
imaging
Backlighter
Target Si
2D Image
Streak
Camera
Time
Time
Amplitude
Magnification 15x