Transcript Hydrodynamics Driven by High-Energy

What is radiation hydrodynamics?
•
The science of systems in which radiation affects the dynamics of
the matter
•
There are two radiation-hydrodynamic regimes
– Radiative-flux regime, where radiative energy transport is
essential (above ~ 30 eV)
– Radiative-pressure regime, where thermal radiation pressure
is dominant (above ~ 1 keV)
•
Today’s experiments are in the radiative-flux regime
•
Two rad-hydro phenomena: Marshak waves and radiative shocks
2003 HEDP Class
Inroductory Lecture
Page 1+42
Radiation waves develop when thermal
radiation diffuses into a medium
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They require that the medium be many radiation-mean-free-paths
thick (“optically thick”), and so usually involve high-Z materials
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Marshak waves: the penetration of radiation from a constant
temperature boundary into an optically thick medium.
•
Features for simple cases
– Depth depth time
– Shape constant in time
(“self-similar” structure)

•
These features remain approx.
true in more complex cases
2003 HEDP Class
Inroductory Lecture
From Drake, High-Energy-Density Physics,
Springer (2006)
Page 2+42
Hohlraums rely on Marshak waves to create
thermal environments at millions of degrees
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Put energy inside a
high-Z enclosure
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The vacuum radiation
field stays in
equilibrium with the
resulting hot surface
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One gets high
temperatures because
the Marshak wave
moves slowly,
penetrating few
microns
2003 HEDP Class
Laser spots seen through thin-walled hohlraum.
Credit LLNL
Hohlraum with experiment attached on bottom
From Drake,
High-EnergyDensity
Physics,
Springer (2006)
Inroductory Lecture
Page 3+42
Shock waves become radiative when
• Radiative energy flux exceeds incoming material energy
4
3
flux
Ts

o us /2 Upstream
preheated
downstream

•

Where post-shock temperature is RTs 

2( 1) 2
us
2
( 1)
2 us5
( 1)8
• Giving a dimensionless threshold Q  4

~ 5,000
4
R o 16( 1)

Material
Xe
Xe
CH
Density
0.01 g/cc
10-5 g/cc
0.01 g/cc
10 km/s
200 km/s
Threshold velocity
2003 HEDP Class

60 km/s
Inroductory Lecture
Page 4+42
The other key dimensionless parameter for
steady radiative shocks is “optical depth”
Optically Some experiments & astro
Larger density ratios
thick
Keiter Herrmann
Reighard Fleury
Z dynamic
Downstream
shocked
region
“LTE” shocks
Some astro
Recent French work
Bouquet, Michaut,
& collabs
Perhaps some shockclump interactions
Optically
Much astro
Blast waves in gasses
thin
Any experiments?
(photon starved upstream)
Optically
Optically
Upstream preheated region
thin
thick
2003 HEDP Class
Inroductory Lecture
Page 5+42
Geometry of optically thin (upstream)
radiative shocks
Cooling layer
Initial post-shock state
i,Ti,pi
Density
Downstream final state
f,Tf,pf
Temperature
Upstream o,To,po
Here we ignore ion-electron decoupling, which
occurs on a more-localized scale at the density jump
2003 HEDP Class
Inroductory Lecture
Page 6+42
The local fluid energy balance provides a
solvable system for steady shocks
  u 2 

 u   pu  FR


  2 

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Energy equation
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Assumptions:
1D, use   p / 1 , the momentum & continuity eqs.,

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Result

•
2 






1
 u   2 o   o  FR
 4 B  J R 
 
   
2 z 
z
 1     1    

3
o s
Where the radiation model is a transport model not a diffusion
model, with

2003 HEDP Class
T
B

4
T f4
JR 

Inroductory Lecture
Page 7+42
Cooling layers are optically thin
An example for the
thick-thin case
of our experiments
downstream
Density
(ratio to
Preshock)
From Drake, High-Energy-Density Physics,
Springer (2006)
 T-4/3 0 (approx. Xe conditions)
 = 4/3 here, so o = 1 mg/cc
Our experiments are in this regime but denser and faster
2003 HEDP Class
Inroductory Lecture
Page 8+42
Amy Reighard is leading experiments to study
such shocks for her Ph.D. thesis research
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Laser drive beams
launch Be piston into
xenon gas
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Piston drives a
planar shock
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Radiography detects
dense xenon
•
Gold grid provides
spatial fiducial
•
Parameters
–
–
–
–
1015 W/cm2
0.35 µm light
1 ns pulse
600 µm tube dia.
2003 HEDP Class
Inroductory Lecture
Page 9+42
In 1D simulations with HYADES, the shocked
layer grows thicker at nearly constant density
40 µm Be disk
At several times
~ 140 km/s
Simulation by Amy Reighard
2003 HEDP Class
Inroductory Lecture
Page 10+42
Data from Omega show a dense and apparently
uniform layer of shocked xenon
• 20 µm Be drive
disk
• Data at 14.6 ns
• Grid cells are 63
µm squares
2003 HEDP Class
Inroductory Lecture
Page 11+42
New phenomena become dominant in
relativistic HEDP
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We will discuss
–
–
–
–
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Relativistic electron motion in laser beams
Electron acceleration
Ion acceleration
Some other phenomena
“Relativistic” laser beams
– Quiver momentum
– Lorentz factor
I L 2
pe
ao 

mc
1.371018 W  2 /cm2
 r  1 ao2
 will introduce you to this technology
– Todd Ditmire
– Bill Kruer will explain the interaction physics

2003 HEDP Class
Inroductory Lecture
Page 12+42
Electrons in a light wave oscillate and drift
dp
e
 eE  v  B
dt
c
•
The force equation is
•
For an electron in a light wave
one can show

•
p2x
pz 
2me c
pz
For small laser irradiance the motion is a
drifting figure 8 with
px  cost and pz  cos2 t 
px

•

At very high laser irradiance the oscillations
become extremely anharmonic and the motion
is mainly along z with
vx  c
2003 HEDP Class
2
r
and vz  c 1
Inroductory Lecture
Enam Chowdhury,
Ph.D. Thesis
2
r
Page 13+42
To experience acceleration on a wake,
go wakeboarding or surfing
• A wakeboarder can surf the wake on a boat… gaining
momentum
http://www.nickandjulz.com/pro/photos/wake/
2003 HEDP Class
Inroductory Lecture
Page 14+42
To accelerate lots of electrons,
take them surfing too
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Electrons can surf the wake on a pressure pulse in a plasma
Plasma
Wake
Accelerated
electrons
Laser
pulse
SLAC beam
Electrons
Credit: LBL OASIS Group
•
Hogan et al., PRL 2005
The pressure pulse can be produced by one or more laser beams
or by an electron bunch. This is wakefield acceleration.
– Eric Esarey will introduce you to the alphabet soup of detailed
approaches
2003 HEDP Class
Inroductory Lecture
Page 15+42
To accelerate ions, repel them
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If you remove the electrons somehow, the remaining ions will
push each other apart
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How to remove the electrons:
Thermally
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–
–
–
–
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Hot electrons leave a surface plasma much faster than the ions do
This produces “sheath acceleration”
The electron “temperature” and the sheath potential increase with ao
The maximum ion energy is ~ 20 Z ao MeV
Ponderomotively, which means from the laser light pressure
– “Coulomb explosions” occur when a group of ions blows apart
– For a sphere

2
Emax ~ 40Z 2
2003 HEDP Class

ni
ro
 MeV
18
3 
10 cm 10m 
Inroductory Lecture
Page 16+42
One can create lots of new phenomena with
relativistic lasers
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Transmit light through high-density plasma
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Drill holes in dense plasma
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Make lots of electron-positron pairs
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Cause nuclear reactions
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Create GigaGauss magnetic fields
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And many more
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You will see some of these this week
2003 HEDP Class
Inroductory Lecture
Page 17+42
Part Two: The Toys
• Hardware
–
–
–
–
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J X B guns
Z pinch
High-energy lasers
Ultrafast lasers
Beams
• Codes
–
–
–
–
Eulerian
Lagrangian
PIC
Hybrids
2003 HEDP Class
Inroductory Lecture
Page 18+42
Marcus Knudson is a gunslinger
with ICE in his veins
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He will show you what one can learn by shooting “bullets” at
targets to create shocks and learn from what they do
– These bullets are called “flyer plates”
– One gun is the electric pulse generator of the “Z machine” at Sandia
•
•
•
The trick is to drive a current on one
surface of a thin conducting material
These bullets are called “flyer plates”
One can launch flyer plates this way with
velocities above 20 km/s
B
J
JXB
By arranging the currents to create gentle
compression, one can do Isentropic
Compression Experiments (ICE)
2003 HEDP Class
Inroductory Lecture
Page 19+42
Chris Deeney is a chef
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Much of his career has been cooking samples using “Z pinches”
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Today’s biggest x-ray barbecue is the Z machine at Sandia, when
run as a Z pinch (> 2 MJ of x-rays)
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Z pinches exploit the attraction between parallel currents
Cylindrical wire array
Inward J X B force
2003 HEDP Class
Implosion
Inward
acceleration
Inroductory Lecture
Stagnation
Shock heating &
Radiative cooling
Page 20+42
The action is at the center of a large though
compact structure
2003 HEDP Class
Inroductory Lecture
Page 21+42
Bill Kruer and Todd Ditmire are space rangers
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They spend their time zapping things with lasers
and analyzing what happens when you do
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High energy lasers amplify the light energy across a large area
then compress the beam(s) in space to create high energy density
vacuum
amplify
protect
irradiate
Smooth
(spatial filter)
2003 HEDP Class
Inroductory Lecture
amplify
Page 22+42
High-Energy lasers: big facilities; small targets
2003 HEDP Class
Inroductory Lecture
Page 23+42
Today’s workhorse in the US is Omega
Target chamber at Omega laser
2003 HEDP Class
Inroductory Lecture
Page 24+42
The National Ignition Facility will provide much
more energy
• > 1 MJ on target
• 192 beams
• LMJ in France will
be on the same
scale
18-wheeler cab and trailer
2003 HEDP Class
Inroductory Lecture
Page 25+42
Ultrafast lasers compress pulses in time as well
as space
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Amplify a long pulse over a large area
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Compress it to a small volume in time and space
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A “lambda-cubed” laser has a spot one wavelength in diameter and a
pulse one cycle long
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Grating pairs to stretch and compress the laser pulses in time
2003 HEDP Class
Inroductory Lecture
Page 26+42
Accelerators produce
high-energy-density beams
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Table from NAS report
2003 HEDP Class
Inroductory Lecture
Page 27+42
All the toys are worthless without good
diagnostics
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David Meyerhofer will discuss diagnostics
2003 HEDP Class
Inroductory Lecture
Page 28+42
Now we turn to computer codes
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These toys are essential to
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Evaluating long-term applications
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Designing present-day experiments
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Interpreting aspects of experiments that can’t be measured
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Connecting HEDP experiments with other systems, for example in
astrophysics
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There are many approaches; all have strengths and weaknesses
2003 HEDP Class
Inroductory Lecture
Page 29+42
Eulerian codes calculate in a fixed geometric
space
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The computational zones are defined in an Eulerian space
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Strengths
– Adaptive grids are straightforward
– Can follow swirling motions
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Weaknesses
–
–
–
–
Trouble following material boundaries
Large diffusion
Must start with very large box
Resolving shock waves can be hard
Credit Kifonidis et al.
2003 HEDP Class
Inroductory Lecture
Page 30+42
Lagrangian codes track the motion of mass
• Features
– Fixed mass in each zone
– Zone boundaries can move
• Strengths
– Keeps materials separate
– Follows shocks well
– Complex physics models
straightforward
• Weaknesses
– Material cannot swirl
– Not readily adaptive
Simulation by Laurent Boireau
2003 HEDP Class
Inroductory Lecture
Page 31+42
Various modern codes combine both
Lagrangian and Eulerian features
Diverging Instability Experiment
•CALE (LLNL, Omar Hurricane)
2003 HEDP Class
Supersonic Jet Experiment
•RAGE (LANL, Bernie Wilde)
Inroductory Lecture
Page 32+42
Particle In Cell (PIC) codes track particles or
superparticles
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Simulate motion of actual or
representative particles with
correct mechanical equations
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Evolve electromagnetic fields
based on Maxwell’s
equations using particle
properties
•
Strengths
Collisionless shock
driven by ultrafast laser
– Exact simulation
•
Weaknesses
– Limited space and time
– Collisions are approximate
•
Chuang Ren will discuss PIC
codes Friday
2003 HEDP Class
Inroductory Lecture
Credit: Louis Silva
Page 33+42
Part 3: The applications
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Inertial fusion
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Experimental Astrophysics
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Accelerators
2003 HEDP Class
Inroductory Lecture
Page 34+42
These cool toys were developed for inertial
confinement fusion (ICF) research
2003 HEDP Class
Inroductory Lecture
Page 35+42
ICF is exciting but also a tough challenge
• Take a mm-scale cryogenic capsule filled with DT
• Implode it
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–
–
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At 300 km/s using giant lasers or Z pinches
So gently that the fuel stays frozen
Without letting instabilities rip it apart
Possible ignite it with a relativistic laser
• Get an energy gain of > 100 from the fusion burn
• Applications
– Defense
– Power generation
2003 HEDP Class
Inroductory Lecture
Page 36+42
In ICF one first compresses the fuel using an
ablatively driven implosion
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This is necessary to avoid blowing up the lab
ICF fusion
Capsule
An ablatively driven implosion
Fuel layer is first compressed by shocks.
Then the shell is accelerated inward by high-pressure, low-density corona.
Stagnation creates a central hot spot surrounded by cold dense fuel
2003 HEDP Class
Inroductory Lecture
Page 37+42
After compression, one has to make the ICF
fuel burn by fusion
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Let the central hot spot be
much smaller and rapidly
ignite the compressed fuel
Two approaches
Design the central hot spot
so it ignites the fuel
Options: lasers, particles, slugs
This is the traditional approach
This is called fast ignition
Riccardo Betti will tell you about it
Max Tabak will tell you about it.
Rick Freeman will discuss
particle transport, also essential.
2003 HEDP Class
Inroductory Lecture
Page 38+42
Some of us are using these new tools to create
experimental astrophysics
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New tools enable new science, and
create new sciences
Astronomy: the human eye and brain
Spectroscopy enabled
and created astrophysics
e.g., Hubble
diagram
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High Energy Density facilities are
new tools ….. Dmitri Ryutov will
describe some of the astrophysical
applications
2003 HEDP Class
Inroductory Lecture
Page 39+42
Others are using these tools to create the next
generation of particle accelerators
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Eric Esarey will discuss this
Credit:
2003 HEDP Class
Inroductory Lecture
Page 40+42
In you want a better foundation in HEDP
•
Come next summer to the second offering of
Foundations of High Energy Density Physics
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A thorough introduction to the foundations of this subject
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Taught by one lecturer (me) to provide a continuous discussion
with common notation based on a book
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A two week course
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The 28 students last year were strongly enthused
– Otherwise I would not be doing this again!
– Contact [email protected]
2003 HEDP Class
Inroductory Lecture
Page 41+42
High-energy-density physics is exciting!
2003 HEDP Class
Inroductory Lecture
Page 42+42