An attractive path to ICF that could lead to a practical fusion energy source
Presented by Steve Obenschain
Laser Plasma Branch
Plasma Physics Division
U.S. Naval Research Laboratory
MIT Club of Washington DC
25 May 2015
Work supported by DOE-NNSA
A tutorial on Inertial Confinement Fusion (ICF): progress and
challenges
me
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
The Naval Research Laboratory
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
5
Fusion powers the visible Universe..
Can it provide clean plentiful energy on earth?
Deuterium - D
Tritium - T
+
+
Fusion
Reaction +
Helium - He4
neutron - n
Energy +
D + T He4 ( 3.45 MeV) + neutron (14.1 MeV)
this is the easiest fusion reaction to achieve
Nuclear Fusion -- the basics
need 100 million oC
+ confinement
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
From http://www.iter.org/default.aspx
• 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.
Basic principles of inertial confinement fusion
Deuterium Tritium plasma
• Temperature T (~10 keV)
• Density ρ
• Radius r
• Expansion velocity V
r v
We need a large fraction of the DT fuel to burn before it expands.
Expansion velocity (v) ≈ (kT)1/2
Reaction rate = ρ2 RDT(T)
Available time t =r/v
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)
Plasma
Inertial Fusion (via central ignition)
Central portion of DT
(spark plug) is heated
to ignition.
(~100 Gbar, ~108 oC)
Thermonuclear burn
then propagates
outward to the
compressed DT fuel.
~ 3% of original
target diamter
Lasers or x-rays heat outside
of pellet, ~100 Mbar
pressure implodes fuel to
velocities of 300 km/sec
Hot
fuel Cold
fuel
Laser
Power
time
foot drive
DT ice
ablator
~ 2 to 4 mm
• 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
Before
Glass of water
(Heavy Fluid)
Air
(Light Fluid)
During After
Example: A glass of water turned upside down..
An ICF pellet has a Rayleigh Taylor (RT) Instability: Pressure from the low density ablated material accelerates the high
density shell.
t1 = t0 + Accelerated
&
compressed
"Fuel"
ablated
material
laser
A
laser
t0 target
(section
of shell)
Ak (t) = Ako ek 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
Lawrence Livermore National Laboratory Pxxxxxx.ppt – Edwards, NRL, 3/18/15 15
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
NIF utilizes flashlamp pumped Nd:glass amplifiers
16
Nd:glass amplifier
Accommodates 8 30-cm
aperture beams
Near infrared λ = 1054 nm light from Nd:glass is
frequency tripled to UV and directed to target
https://str.llnl.gov/str/Powell.html
1 of 192 beams
OFFICIAL USE ONLY
OFFICIAL USE ONLY 2013-049951s2.ppt 17
NIF Laser Bay (1 of 2)
Photoshopped target bay all floors
Moses - IFSA, 9/9/13 2013-043921s1.ppt 18
NIF 6-m diameter target chamber
OFFICIAL USE ONLY
OFFICIAL USE ONLY 2013-043921s1.ppt Moses - IFSA, 9/9/13 19
Indirect Laser Drive (approach chosen for NIF)
Illustration from https://lasers.llnl.gov/programs/nic/icf/
Laser beams heat wall of a gold hollow cylinder (hohlraum) to ~300 eV and
resulting soft x-rays drive the capsule implosion.
Lawrence Livermore National Laboratory Pxxxxxx.ppt – Edwards, NRL, 3/18/15 21
The Challenge — near spherical implosion by ~35X
195 µm
DT shot N120716
Bang Time
(less than diameter
of human hair)
~2 mm diameter
The NIF indirect drive effort has greatly advanced the physics understanding of that approach
Spectrum
Backscatter
Streak Trajectory
Capsule shape
In-fight instability
Hohlraum performance
Wall motion
1D
3D
Shocks
R
time
time
DT hot spot shape
Picket drive symmetry
Stagnation
LEH size
Plasma conditions
• Relaxed laser uniformity requirements
• Higher mass ablation rate inhibits
hydro-instability.
• Less efficient illumination of target
• More complex physics
• More challenging diagnostic access
But NIF so far has not achieved ignition with indirect drive, there is another way – laser direct drive Indirect Drive
Laser
Beams
x-rays
Hohlraum Capsule
Direct Drive • Much more efficient (7 to 9 x) use of laser
light.
• Simpler physics
• Much higher predicted performance (gain)
• Simpler target fabrication
• Advances in lasers (beam smoothing) and
target designs should provide needed
implosion symmetry.
Capsule
Laser Beams
4
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)
DT ice
preheated ablator
(lower density)
DT ice
ablator
Late 1990’s – Development of “tailored adiabats’ to reduce Rayleigh
Taylor instability at the ablation layer while maintaining high fuel
density.
• Larger ablation velocity (VA= {mass ablation rate}/) suppresses RT instability.
• Can be accomplished via decaying shocks or soft x-ray preheat.
Laser intensity
log scale
NRL is the world leader in high-energy electron-beam pumped krypton fluoride (KrF) lasers
• 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)
Nike 60-cm aperture amplifier
Provides the deepest UV light of all ICF lasers (λ=248 nm)
Use of KrF light has many advantages for direct drive
Deeper UV
• Inhibits undesired laser-plasma instability
• Higher efficiency implosions.
• Less laser energy required to obtain
ignition and high yield
Superior beam
smoothing
• 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.
Nike focal profile
Nike
zoomed
focus
Early time
Late time
Shock Ignited (SI) direct drive targets
Low aspect ratio pellet helps mitigate
hydro instability Peak main drive is 1 to 2 × 1015 W/cm2
Igniter pulse is ~1016 W/cm2
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.
* 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)
0.4 mm
Initial pellet
Imploded pellet
(magnified scale)
138 x
energy gain
Snapshots of high resolution 2-D simulation of implosion
Simulation
shows growth of
instability
seeded by target
imperfections
2 mm
0.2 mm 0.1 mm
The target has to release enough energy to power the reactor… AND produce electricity for the grid
KrF Laser
(7% efficient)
Electricity
Generator
(40%)
Target
Gain = 130x
Power Lines
10 Megawatts
430 Megawatts
143 Megawatts
1,430 Megawatts
(heat) 572 Megawatts
( electricity)
Target "Gain" = Fusion power OUT / laser power IN
143/572 = 25%
Recirculating power
(Nuclear reactions in chamber “blanket” add 1.1× to target gain)
Higher target gain increases power to grid and reduces %
of power needed to operate the reactor.
KrF Laser
(7% efficient)
Electricity
Generator
(40%)
Target
Gain = 200x
Power Lines
10 Megawatts
737 Megawatts
143 Megawatts
2,200 Megawatts
(heat) 880 Megawatts
(electricity)
Target "Gain" = Fusion power OUT / laser power IN
143/880 = 16%
Recirculating power
(Nuclear reactions in chamber “blanket” add 1.1× to target gain)
Nike krypton-fluoride laser target facility
NRL Laser Fusion
Nike Target chamber
56-beam 3-kJ
KrF laser-target facility
Target chamber optics
60 cm aperture amplifier
Nike laser Chain
Illuminated
aperture imaged
onto target
Laser profile in target chamber
12 beams for x-ray
lighters
44 high quality
main beams
target
back and side
lighters
imaging crystal
optical streak
camera
x-ray streak
camera
x-ray framing or streak camera
Experimental layout of Nike target chamber
side-on refractometer
VIS
AR
neutron
detector
(1 of 3) Near UV/Visible
Streaked Spectrometer
Hard x-ray
Spectrometer
Monochromatic x-ray imager coupled with streak camera
revealed an oscillatory behavior of ablative Richtmyer-
Meshkov instability
Streak Camera
Quartz Crystal
1.86keV imaging
2D Image
Main Laser Beams
Tim
e
Magnification 15x
Backlighter Laser Beams
Backlighter Target Si
Rippled CH Target
Long Pulse (4 ns)
Time
Am
pli
tud
e
