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 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..

Can it provide clean plentiful energy on earth?5 -- the basics

need 100 million oC - n - D + confinement

+ Energy + + Fusion + - T 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 • Enough readily available lithium for 1000’s of years. • Operation does not make greenhouse gasses • Attractive advanced approach to nuclear energy – Limited, controllable – 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 • T (~10 keV) r Expansion velocity (v) ≈ (kT)1/2 v 2 • Density ρ Reaction rate = ρ RDT(T) Plasma Available time t =r/v • Radius r • Expansion velocity V

We need a large fraction of the DT fuel to burn before it expands.

2 Fraction burned ≈ ρ 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 Central portion of DT Thermonuclear burn of pellet, ~100 Mbar (spark plug) is heated then propagates pressure implodes fuel to to ignition. outward to the velocities of 300 km/sec compressed DT fuel. (~100 Gbar, ~108 oC)

Hot Cold ablator fuel fuel ~ 2 to 4 mm

DT ice

foot drive ~ 3% of original target diamter • Simple concept Laser • Potential for very high energy gains (>100) Power • Requires high precision in physics & systems time • 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 During After

Glass of water (Heavy Fluid)

Air (Light Fluid) An ICF pellet has a Rayleigh Taylor (RT) Instability: Pressure from the low density ablated material accelerates the high density shell.

t0 target laser (section of shell)

t1 = t0 +  Accelerated & compressed k t laser Ak (t) = Ako e "Fuel"

ablated Mitigation of RT: material A 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 program

National Ignition Facility Lawrence Livermore National Lab. Sandia National Lab

OMEGA Laser Facility Nike KrF Laser Facility University of Rochester, LLE 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

15 Lawrence Livermore National Laboratory 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 1 of 192 beams aperture beams

https://str.llnl.gov/str/Powell.html

16 OFFICIAL USE ONLY NIF Laser Bay (1 of 2)

OFFICIAL USE ONLY 2013-049951s2.ppt 17 PhotoshoppedNIF 6-m diameter target target chamber bay all floors

2013-043921s1.ppt Moses - IFSA, 9/9/13 18 OFFICIAL USE ONLY

OFFICIAL USE ONLY 2013-043921s1.ppt 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

21 Lawrence Livermore National Laboratory Pxxxxxx.ppt – Edwards, NRL, 3/18/15 The NIF indirect drive effort has greatly advanced the physics understanding of that approach

Hohlraum Backscatter performance Capsule shape Spectrum LEH size

Wall motion In-fight instability Stagnation

Plasma conditions

DT hot spot shape

Streak Trajectory 3D R Shocks

Picket drive symmetry time

time 1D But NIF so far has not achieved ignition with indirect drive, there is another way – laser direct drive

Indirect Drive • Relaxed laser uniformity requirements Hohlraum Capsule • Higher mass ablation rate inhibits hydro-instability. Laser • Less efficient illumination of target Beams • More complex physics • More challenging diagnostic access x-rays Direct Drive • Much more efficient (7 to 9 x) use of laser light. Capsule • Simpler physics • Much higher predicted performance () • 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 preheated ablator (lower density) DT ice  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

• 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) • Inhibits undesired laser-plasma instability Deeper UV • 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 Superior beam efficiency, and that is likely required to smoothing avoid deleterious cross-beam-energy transport.

Early time Nike zoomed focus Late time Nike focal profile 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 Initial pellet instability seeded by target imperfections

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" = OUT / laser power IN (Nuclear reactions in chamber “blanket” add 1.1× to target gain)

Target 1,430 Megawatts 572 Megawatts Gain = 130x (heat) ( electricity) Electricity Generator (40%) 430 Megawatts 143 Megawatts Power Lines KrF Laser (7% efficient) 10 Megawatts 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)

2,200 Megawatts Target 880 Megawatts Gain = 200x (heat) (electricity) Electricity Generator (40%) 737 Megawatts 143 Megawatts Power Lines KrF Laser (7% efficient) 10 Megawatts 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 Chain

Illuminated aperture imaged onto target

Laser profile in target chamber Experimental layout of Nike target chamber

x-ray streak camera neutron detector 12 beams for x-ray (1 of 3) Near UV/Visible lighters Streaked Spectrometer

Hard x-ray Spectrometer

target

44 high quality VISAR optical streak main beams 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 Richtmyer- Meshkov instability

Backlighter Laser Beams Main Laser Beams Rippled CH Target

Long Pulse (4 ns) Quartz Crystal Backlighter 1.86keV Target Si imaging

Magnification 15x

Amplitude 2D Image Time Streak Camera Time