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Modeling Astrophysical Phenomena in the Laboratory with Intense Lasers

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Astrophysical research has traditionally been divided into observations and theoretical modeling or a combination of both. A component sometimes missing has been the ability to quantitatively test the observations and models in an experimental setting where the initial and final states are well characterized. Intense lasers are now being used to recreate aspects of astrophysical phenomena in the laboratory, allowing the creation of experimental test beds where observations and models can be quantitatively compared with laboratory data. Experiments are under development at intense laser facilities to test and refine our understanding of phenomena such as supernovae, supernova remnants, gamma-ray bursts, and giant planets.
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102. We thank R. Caldwell, R. Cen, X. Fan, G. Huey, P. J. E.
Peebles, L. Wang, and members of the SCP team for
their contributions and collaborations on work de-
scribed here. Supported by NSF grant AST93-15368
(N.A.B., Princeton); U.S. Department of Energy
(DOE) grant DE-FG02-91ER40671 (P.J.S., Princeton);
the Physics Division, E. O. Lawrence Berkeley Na-
tional Laboratory of the U.S. DOE under contract
DE-AC03-76SF000098; and the NSF’s Center for
Particle Astrophysics, University of California,
Berkeley, under grant ADT-88909616 (S.P.).
REVIEW: EXPERIMENTAL ASTROPHYSICS
Modeling Astrophysical Phenomena in the
Laboratory with Intense Lasers
Bruce A. Remington,
1
David Arnett,
2
R. Paul Drake,
3
Hideaki Takabe
4
Astrophysical research has traditionally been divided into observations and theoretical
modeling or a combination of both. A component sometimes missing has been the
ability to quantitatively test the observations and models in an experimental setting
where the initial and final states are well characterized. Intense lasers are now being
used to recreate aspects of astrophysical phenomena in the laboratory, allowing the
creation of experimental test beds where observations and models can be quantita-
tively compared with laboratory data. Experiments are under development at intense
laser facilities to test and refine our understanding of phenomena such as supernovae,
supernova remnants, gamma-ray bursts, and giant planets.
Modern intense lasers produce energy
densities in submillimeter-scale
volumes that are far larger than
those produced by any other method. With
these highly versatile laser facilities, matter
can be prepared reproducibly in conditions
that are equivalent, in a rigorously scaled
sense, to those in large astrophysical sys-
tems such as supernovae, Herbig-Haro jets,
or giant planets. Examples of areas that can
be studied include strong shock phenome-
na, high–Mach number jets, strongly cou-
pled plasmas, compressible hydrodynamic
instabilities, radiation flow, photoevapora-
tion front hydrodynamics, and fundamental
properties such as opacities and equations
of state (EOS).
Nuclear fusion reactions are the funda-
mental energy source of stars, and their
cross sections quantify the individual reac-
tion probabilities, allowing the heat pro-
duction inside stars to be calculated.
Opacities are the fundamental atomic prop-
erties that govern radiation transport within
stars. Opacities quantify the probability
that an atom will absorb photons that pass
within its vicinity and consequently control
to a large extent the temperature profiles
of the interiors of stars. These fundamental
“input” quantities—cross sections and
opacities—are required in models of phe-
nomena such as stellar pulsations and su-
pernova light curves. The research re-
viewed here is aimed at probing astro-
physical dynamics directly—the “output”
of the models—by creating scaled repro-
ductions of the astrophysical systems in the
laboratory.
Supernovae
Core-collapse supernovae (SNe) represent the
dramatic endpoint in the life cycle of a star
(1–5). The final death throes of the star are
spent in a high-stakes “tug of war” pitting quan-
tum mechanical degeneracy pressure against
gravitational pressure. The outcome determines
whether the final state is a white dwarf, neutron
star, or black hole and is based on the strength
of the degeneracy pressure to withstand the
radially inward tug of gravity (6). Stars with
initial masses of 1 to 8 MJ(where MJcorre-
sponds to the mass of the sun) finish their
hydrogen burning while their cores are not yet
degenerate. They undergo core contraction,
which raises the core density and temperature
sufficiently to trigger He burning. These stars
subsequently lose mass effectively and end
their lifetimes as white dwarfs, with masses of
;0.6 MJ. White dwarfs are supported by the
pressure of the degenerate electrons in their
interiors; that is, it is the quantum mechanical
Pauli exclusion principle that prevents further
collapse. The maximum mass possible for a
white dwarf is the Chandrashekar limiting
mass, M
Ch
'1.4 MJ. More massive stars have
high enough temperatures in their cores to con-
tinue the nuclear fusion burning cycle up to Fe.
Once the core reaches Fe, the nuclear fusion
reactions no longer release net energy (because
the nuclear binding energy per nucleon is max-
imum in Fe, at nearly 9 MeV per nucleon), and
the thermonuclear fires are extinguished. The
mass of the Fe core continues to grow as the
surrounding layers burn their way to this ther-
monuclear end point until the Fe core mass
1
Lawrence Livermore National Laboratory, L021, Liv-
ermore, CA, 94550, USA. E-mail: remington2@llnl.gov
2
Steward Observatory, University of Arizona, Tucson,
AZ, 85721, USA. E-mail: darnett@as.arizona.edu
3
At-
mospheric, Oceanic, and Space Sciences, University of
Michigan, 2455 Hayward Street, Ann Arbor, MI,
48109–2143, USA. E-mail: rpdrake@umich.edu
4
Insti-
tute of Laser Engineering, Osaka University, Yamada-
Oka 2-6, Shita, Osaka 565, Japan. E-mail:takabe@
ile.asaka-u.ac.jp
SCIENCESCOMPASS
28 MAY 1999 VOL 284 SCIENCE www.sciencemag.org1488
exceeds ;1.4 MJ. At this point, there is no
longer sufficient heat produced in the core to
balance cooling by neutrino emission and pho-
tonuclear dissociation, and the core surrenders
to gravity, triggering a catastrophic gravitation-
al collapse that is over in a matter of seconds.
This collapse is arrested only when the core
density reaches that of degenerate nuclear mat-
ter (;2310
14
g/cm
3
). The Fermi degeneracy
pressure, p
deg
;r
2/3
(where ris density), in-
creases sufficiently to stop the implosion, and a
spectacular nuclear rebound occurs whose
strength is determined by the EOS of bulk
nuclear matter. By a mechanism still debated,
this launches the powerful outward-propagating
shock wave (SW) that first “stalls” in the infall-
ing matter and then gets reenergized by convec-
tion and by energy deposition due to neutrinos
emitted from the core. Thus restarted, the SW
traverses the overlying layers and effectively
blows the star apart. Thus, the catastrophic end
of the massive star marks the spectacular be-
ginning of a core-collapse supernova (SN). This
explosive birth is observed as a bright flash of
ultraviolet (UV) light (3,7). If the core has a
mass larger than 2 to 3 MJ, the core collapse
continues to form a black hole.
The visual SN commences when the SW
breaks out through the surface of the star about
an hour after the core collapses (3). There is a
sudden increase in temperature to 20 to 30 eV
and luminosity, followed by a rapid drop in
both quantities, as the star expands and cools
adiabatically. About 30 min after SW breakout,
the luminosity approaches a constant value, as
the recombination front, which determines the
photosphere, moves inward in mass at a con-
stant temperature (for hydrogen) of about 6000
K. After some 20 to 40 days, the heat from the
radioactive core, heated by Compton scattering
of the g-rays produced from
56
Ni,
56
Co, and
44
Ti, reaches the photosphere, and the light
curve rises up in a broad secondary maximum.
Subsequently, the decay of the light curve is
monotonic in time at a rate determined by the
half-lives of the various radioactive nuclei that
serve as the heat source. The light curve con-
tains a wealth of information about the star and
its explosion. The luminosity varies directly
with the explosion energy per unit mass, E/M,
and is also proportional to the initial radius of
the star. For the same E/M, SNe from small
stars are not as bright, because more energy
goes into hydrodynamic expansion. The lumi-
nosity is on average inversely proportional to
the opacity, because lower opacity means short-
er radiative diffusion times. Finally, the light
curve time evolution is sensitive to the degree
that the core hydrodynamically mixes outward
into the envelope, bringing heat nearer to the
photosphere. The ability to quantitatively cal-
culate an SN light curve would allow the intrin-
sic brightness of the SN to be determined.
Comparison with the observed brightness
would give its distance, through the expanding
photosphere method (8,9). Together with spec-
troscopic measurements of its redshift, this al-
lows the Hubble constant H
0
to be determined
(10). There are several aspects to synthetic light
curve calculations that could benefit from lab-
oratory experiments, such as radiation flow,
opacities, and hydrodynamic mixing.
Exploding stars create a homologous ex-
pansion, where each radiating region resides
in a velocity gradient and sees plasma reced-
ing from it in all directions. For photons
emitted in one region to escape the star, they
have to pass through “windows” in opacity,
where the absorption probability is low. In
other words, the absorbing regions are always
redshifted relative to the emitting regions. To
be able to construct a synthetic light curve
requires that one calculate these “expansion
opacities.” Such calculations are complex,
and sophisticated opacity codes such as
OPAL (11) are indispensable.
Experiments are being developed to test
these difficult opacity calculations, focusing on
atomic transitions that have not been explored.
For example, in one experiment, a 25-nm-thick
iron foil was sandwiched between two C layers
and heated to ;20 eV with x-rays. The absorp-
tion spectrum near 730 eV was measured and
analyzed, comparing several different opacity
calculations (12). In another experiment, radia-
tion line transport was measured in an expand-
ing plasma (13). This experiment studied the
structure of a doublet in the aluminum spec-
trum, at a wavelength near 7.18 Å. The emis-
sion occurs from an optically thick plasma with
a substantial velocity gradient, so that emission
in one line is often absorbed and reemitted by
the other line at another location in the plasma.
The resulting line structure is complex but can
be reproduced by modeling only when this
expansion effect on the radiation transport is
taken into account. Hence, experiments are un-
der development to test opacity calculations,
both static and in expansion, relevant to SN
light curves.
A core-collapse SN is driven by a power-
ful SW, and strong SWs are the breeding
ground of hydrodynamic instabilities. Two
such instabilities seem particularly important:
the Rayleigh-Taylor (RT) and Richtmyer-
Meshkov (RM) instabilities. The RT instabil-
ity occurs when effective gravity (due to
acceleration) tries to pull a heavier fluid
through an underlying lighter one (for exam-
ple, large air bubbles under water or heated
gas from a powerful explosion in the atmo-
sphere). The RM instability is closely related,
with the role of gravity replaced by the inertia
from an impulsive acceleration due to an SW.
During the SW transit phase, the RM insta-
bility is triggered at each discontinuity in the
density profile of the star, that is, at the O-He
and He-H “interfaces.” After SW transit, hydro-
dynamic mixing continues because of the RT
instability, as the denser layers are decelerated
by the lower density outer layers. The outward
mixing of the higher density, radioactive core
material (for example,
56
Ni,
56
Co, and
44
Ti)
brings the radioactive heat source toward the
surface of the star. These explosion products
decay by the emission of g-rays, which Comp-
ton scatter off electrons in their vicinity. The
consequent reheating of the photosphere causes
the secondary maximum in the light curve. The
RT mixing induces this reinvigoration of the
light curve to start earlier, broadening the sec-
ondary maximum. Observations of the light
curve of SN1987A unambiguously showed this
broadening of the secondary peak, suggesting
enhanced transport from the core out to the
photosphere (1,2). Two-dimensional calcu-
lations of the development of the mixing at
the O-He and He-H interfaces with the SN
code PROMETHEUS (14,15) show that
spikes of denser oxygen and helium penetrate
outward into the less dense envelope of hy-
drogen, whereas bubbles of hydrogen move
inward relative to the average location of the
H/He boundary (Fig. 1A). This interpenetra-
tion occurs through the growth and nonlinear
Simulation
of SN1987A
A
t=12,557 s
Fig. 1. Mixing in SN explosion hydrodynamics.
(A) Image of simulated hydrodynamic mixing
from SN1987A at t512,557 s [reproduced
with permission from (14)]. (B) An image from
a laser experiment designed to measure this
hydrodynamic mixing of a l5200 mm ripple under scaled conditions at t535 ns [reproduced
from (53)].
SCIENCESCOMPASS
www.sciencemag.org SCIENCE VOL 284 28 MAY 1999 1489
evolution of the RT instability.
Laser-based experiments can generate
strong-SW–initiated nonlinear hydrodynamic
mixing conditions similar to those found in
SNe. In a set of experiments scaled to repro-
duce the hydrodynamics of the He-H interface
of SN1987A about an hour after explosion, a
strong SW was passed through an interface
separating dense “core” material (Cu) from the
lower density outer envelope (CH
2
)(16,17).
A two-dimensional (2D) sinusoidal ripple
(1D wave vector) was imposed at the inter-
face. The subsequent 2D growth due to the
RM and RT instabilities was measured by
x-ray backlighting. Spikes of Cu penetrating
upward into less dense CH
2
were observed as
a consequence of the RT instability (Fig. 1B).
This interpenetration was calculated in 2D with
PROMETHEUS, and the simulations repro-
duced the observations.
A theoretical look at the relation between
the hydrodynamics occurring in the SN com-
pared with that in the laboratory experiment
shows that a rigorous mapping exists. Con-
sider the He-H interface at 1600 s in the SN
and the Cu-CH interface at 20 ns in the laser
experiment. In both settings, the Reynold’s
number (the ratio of the inertial to the viscous
force) and the Peclet number (the ratio of the
convective to the conductive heat transport)
are large. Therefore, viscosity and thermal
diffusivity are negligible, and the dynamics
of the interface are well described by Euler’s
equations for a polytropic gas (18):
r
S
]v
]t1vz¹v
D
5p(1a)
]r
]t1¹z~rv!50 (1b)
]p
]t2ga
p
r
]r
]t1vz¹p2ga
p
rvz¹r 50
(1c)
where ris density, vis fluid velocity, tis
time, pis pressure, and g
a
is the adiabatic
index. These equations represent conserva-
tion of momentum, mass, and energy, re-
spectively. It is straightforward to show by
substitution that Eq. 1 is invariant under the
following scale transformation,
hSN 3ahlab (2a)
rSN 3brlab (2b)
pSN 3cplab (2c)
tSN 3a~b/c!1/ 2tlab (2d)
where h,r,p, and tcorrespond to character-
istic spatial, density, pressure, and time scales
and subscripts SN and lab refer to calcula-
tions of the SN and laboratory laser experi-
ment, respectively. When transformation 2 is
inserted into Eq. 1, the constants a,b, and c
cancel, and the dynamics described by Eul-
er’s equation are indistinguishable in the SN
and the laser experiment. Any insights gained
through the laser experiment apply directly to
the SN through the mapping described by Eq.
2. For example, the hydrodynamics illustrat-
ed in Fig. 1, A and B, are similar and can be
related through the SN-to-laboratory map-
ping of h,r,p,t, and acceleration gp/r
(Eq. 2) giving 10
11
cm to 50 mm, 8 310
23
g/cm
3
to 4 g/cm
3
, 40 Mbar to 0.6 Mbar, and
10g
o
to 10
10
g
o
, where g
o
corresponds to the
acceleration due to gravity at the surface of
Earth. These values were taken at times of
2000 s for the SN and 20 ns for the laboratory
experiment (18).
Supernova Remnants
Although SN explosions mark the end of
massive stars, they also mark the beginning
of their new lives as supernova remnants
(SNRs). Well-known examples of SNRs such
as the remnants of Tycho’s SN (19), Kepler’s
SN (20), the Cygnus loop (21), SN1006 (22),
and the Crab nebula (23) provide exquisite
visual testimony to their violent births. There
are several active areas of research regarding
the dynamics and evolution of SNRs that may
be better understood with laser experiments.
SW dynamics dominate the evolution of
SNRs. The rapidly expanding ejecta from the
SN drive an SW forward into the surrounding
medium, and a reverse SW forms where the
ejecta are decelerated by the accumulating,
shocked matter. The place where the ejecta
and ambient medium meet, called the contact
discontinuity, becomes hydrodynamically
unstable. Currently, the most actively ob-
served SNR is the young remnant forming
around SN1987A. This remnant consists of
the standard SN ejecta expanding into the
ambient medium, as well as a mysterious
inner and two outer circumstellar nebular
rings, which apparently existed before the SN
explosion. Various models have been pro-
posed for these rings, but as of yet no expla-
nation fully explains their origin. The SN
ejecta, however, are moving very fast (;10
4
km/s) compared with the nearly static (;10
km/s) inner ring, which has a diameter of ;1
light-year. It is expected that the ejecta-for-
ward SW system will impact the inner edge
of the inner ring within the next ;5 years.
This impact should launch a strong SW into
the ring, heating it to 100- to 300-eV temper-
atures, and cause emissions at wavelengths
from optical to x-ray. Observation of this
impact should shed light on the structure,
composition, and hopefully origin of the
rings. Recent images of the inner ring (24 –
26) show a rapidly brightening, localized hot
spot (upper right corner of Fig. 2A), suggest-
ing that perhaps the collision of the forward
SW with the ring has actually started. Spec-
tral imaging of Lyman-aradiation, which is
produced at the reverse SW, indicates that the
reverse SW has traversed about 80% of the
distance from the ring to the star (24).
Laser experiments can produce SW struc-
tures similar to those in a SNR, under well-
scaled hydrodynamic conditions (18,27–29).
Experiments have been developed in 1D to
reproduce the basic dynamics of SNR forma-
tion: fast moving SW-induced ejecta sweep-
ing into a surrounding low-density, static am-
bient atmosphere. This launches a forward
SW into the ambient medium and a reverse
SW into the stagnating ejecta (Fig. 2B), much
like the dynamics of SNR formation. Indeed,
the laboratory experiment can be modeled by
the self-similar model of Chevalier (30) de-
veloped to describe the 1D dynamics of
SNRs.
Expectations are that the contact disconti-
nuity (the meeting point of the ejecta and
ambient plasmas) will be hydrodynamically
unstable, and 2D experiments have begun to
look at this. One of the driving motivations
for studying SNR physics relevant to
SN1987A is the long-awaited impact of the
SN blast wave with the inner circumstellar
SN1987A
Remnant
Hot spot
Ejecta
Ring
A
Fig. 2. Young SN remnant dynamics. (A) Observational image of the inner circumstellar ring of
SN1987A (http://antwrp.gsfc.nasa.gov/apod/ap980217.html) (image courtesy of the Supernova
Intensive Study Team; PI: Robert Kirshner). (B) Image from SW experiments designed to produce
similar, scaled regimes of strong SW hydrodynamics [reproduced with permission from (28)].
SCIENCESCOMPASS
28 MAY 1999 VOL 284 SCIENCE www.sciencemag.org1490
nebular ring. The interaction of the SW with
the ring is sure to be rich in 3D strong SW
effects. A laser experiment is being devel-
oped to elucidate the 3D nature of the inter-
action of a strong SW with a localized high-
density feature such as a sphere (31). The 3D
development strongly affects the interactions,
with azimuthal (3D) modes growing and en-
hancing the “shredding” of the sphere. A
similar 3D effect is likely for the interaction
of the SN1987A blast wave with the inner
ring and in SW-cloud interactions in general
(32).
Under the current conditions for the rem-
nant of SN1987A, the scale transformation
based on Euler’s equations described above
for the explosion hydrodynamics might be
applied again. For this to be relevant, one has
to consider whether the SW is radiative and
whether the ambient magnetic field localizes
the plasma. For the current conditions of
SN1987A, the plasma density is low enough
that the SWs are not radiative; that is, the
radiative cooling time scale (t
rad
) is long
compared with a hydrodynamic time scale
(t
hydro
): t
rad
/t
hydro
.. 1. Also, the ambient
magnetic field, B5;100 mG, is large
enough that the ion Larmor radius is much
smaller than spatial scales of interest. Hence,
the plasma can be treated hydrodynamically,
the dynamics can be treated again with Eul-
er’s equations (Eq. 1), and the same rigor-
ous scale transformation (Eq. 2) holds. For
the SNR-to-laboratory transformation corre-
sponding to the 1D experiment shown in Fig.
2B, we get 0.03 light-year mapping to 100
mm, 10
4
km/s to 60 km/s, and 1 year mapping
to1ns(18), where these values correspond to
times of 13 years in the SNR and 8 ns in the
laboratory experiment.
Spectral analysis of SW-induced astro-
physical emissions can yield the temperature,
density, degree of equilibration, ionization
state, and velocity of the SW. With an addi-
tional measure of the proper motion of the
SW, the distance to the emitting source can
also be determined. Such analysis of the SW-
induced emissions of hydrogen (Lyman b)
and ionized oxygen (O VI) from the remnant
of SN1006, which exploded in the year 1006
at a distance of 2 kpc (22), shows that the
plasma behind the SW front is not cooling
rapidly by radiation: t
rad
/t
hydro
.. 1. The
conclusion from this spectral analysis is that
plasma turbulence in the SW front is not
effective in producing temperature equilibra-
tion among the different ion species.
Gamma-Ray Bursts
Gamma-ray bursts (GRBs) are the greatest
enigma in contemporary astrophysics (33–37).
Detected at a rate of more than one per day
from random directions in the sky, GRBs typ-
ically have burst durations of a few seconds, at
photon energies of 0.1 to 10 MeV (Fig. 3A).
GRB distances remained unknown for the past
two decades, primarily because their radiation
in all wavelengths other than g-rays was unde-
tected. This changed recently with the determi-
nation of accurate positions (to within about 3
minutes of arc), obtained within hours of out-
burst by the BeppoSAX satellite. Optical spec-
troscopy of the light associated with the out-
burst, the “afterglow,” established that at least
some of the GRBs are at cosmological distanc-
es of several billion light-years (redshifts of
Dl/l51 to 3). To generate the observed
luminosities then requires total source energies
of ;10
53
ergs per burst. The rapid rise time and
rapid variability, Dt;1 ms, observed in some
bursts imply a source size, R
i
;cDt;10
7
cm;
that is, these tremendous total energies appear
to be emitted from very compact sources. The
observed photon energy spectra can extend to
;100 MeV, have a power-law shape (Fig. 3A),
and are fit with a simple functional form:
N~E!dE ,E2adE (3)
where Nis the photon number density at
energy E, with spectral index a;2. This
suggests that the source plasma is optically
thin to the radiation observed. (If the source
plasma were optically thick, the photons
would thermalize, and the observed spectrum
would have a Planckian, not a power-law
shape.) This presents a problem. When two
photons with energies E
1
and E
2
interact,
their center-of-mass energy is ;2(E
1
E
2
)
1/2
,
and the interaction can produce an e
1
e
2
pair
if (E
1
E
2
)
1/2
.m
e
c
2
, where m
e
represents the
rest mass of an electron (33). Denote the
fraction of photon pairs in a GRB satisfying
this condition as f
p
. The optical depth (OD)
for the gg 3e
1
e
2
process, varies as OD ;
f
p
/R
i
2
. Pairs are produced prodigiously, and by
Compton scattering, they would make the
plasma optically thick, thermalizing the pho-
ton spectrum. The observed spectra, howev-
er, are nonthermal, hence the “compactness
problem.” The fireball model was developed
to resolve this problem without introducing
“new physics.” In this model, the source cre-
ates a relativistically expanding fireball so
that the emission region is moving toward the
observer at relativistic velocities (33,36).
Consider a source of radiation moving toward
an observer at rest with a relativistic velocity
(V) characterized by a Lorentz factor (g
L
), g
L
51/(1 2V
2
/c
2
)
1/2
.. 1. The observer de-
tects photons with energy hn
obs
(where his
the Planck constant and n
obs
is the photon
frequency observed), whereas these photons
in the rest frame of the emission region have
energy hn
obs
/g
L
. Hence, at the emitter, the
fraction of photons with energies high
enough to produce e
1
e
2
pairs, f
p
, is reduced
by a factor g
L
22a
. Also, the emitting region
appears Lorentz contracted, so that in its rest
frame, the emission region is larger, with
R
i
;g
L
2
cDt. The result is that the OD for the
process gg 3e
1
e
2
now varies as OD ;
f
p
/g
L
412a
R
i
2
, which for g
L
.;100 resolves
the compactness problem. Through the blue-
shift boost, we observe the high-energy pho-
tons, but the emission region remains optical-
ly thin, giving the observed g-ray power-law
spectrum. The kinetic energy of the GRB
ejecta is assumed to be randomized behind
internal (“reverse”) SWs and emitted as high-
energy photons when the SW is at a radius of
r
int
5g
L
2
cDt510
12
to 10
13
cm, for g
L
5100
to 300. The “afterglow” is assumed to happen
from emissions behind the external (“for-
ward”) SW at a radius of r
ext
.;10
17
cm.
Fig. 3. GRBs and relativistic plasmas. (A) Experimental g-ray energy
spectrum from GRB910601 [reproduced from (34)]. (B) Measured
electron energy spectrum from Petawatt laser experiments (280 J,
0.45 ps, ;10
20
W/cm
2
on 0.5 mm Au) [reprinted with permission
from (41)]. (C) Measured x-ray energy spectrum from experiments
with the Petawatt laser [reprinted with permission from (40), copy-
right 1998, American Institute of Physics].
SCIENCESCOMPASS
www.sciencemag.org SCIENCE VOL 284 28 MAY 1999 1491
Most GRBs show variability on time
scales much shorter than (typically one-hun-
dredth of) the total GRB duration (35). In the
fireball model, such variability comes from
internal (“reverse”) SWs, which convert a
substantial part of the directed kinetic energy
to internal energy. This energy is then radi-
ated as g-rays by synchrotron and inverse-
Compton emission of SW-accelerated elec-
trons. The GRB overall duration reflects the
duration over which energy is emitted from
the source. After internal SWs, the fireball
rapidly cools and continues to expand, driv-
ing a relativistic SW into the surrounding
interstellar medium gas. This external SW
continuously heats new gas and produces rel-
ativistic electrons that may produce the de-
layed radiation observed on time scales of
days to months, that is, the afterglow. So, a
relativistically expanding fireball produces
the rapidly varying, hard x-rays by internal
SWs and the longer lived slow “afterglow”
decay by the external SW.
Despite its qualitative successes, the
fireball model is incomplete. The cause of
GRBs is unknown but must be spectacular
because such great distances require enor-
mous energies for the burst to appear so
bright. The merger of a pair of neutron
stars, the core collapse of a failed SN, and
other exotic events involving black holes
and relativistic jets have been suggested
(33,34). Radiation escapes the fireball
only after it expands to radii many orders of
magnitude larger than the original source
size of ;10
7
cm. The g-ray emission oc-
curs when the source has expanded to a
radius of ;10
13
cm and the afterglow at
.10
16
cm. Hence, the observed radiation
does not provide direct information about
the underlying source. The predictions of
g-ray emission from the fireball involve the
interaction of plasma with SWs moving at
relativistic velocities and with magnetic
fields. The details of this interaction are not
understood. This superheated conglomerate
is thought to expand relativistically in a
fiery ball or jet of plasma, with copious
production of e
1
e
2
pairs. Explosion ener-
gies are estimated to be in the range of 10
52
to 10
53
ergs (approaching the rest mass
energy of the sun). Monte Carlo simula-
tions of the g-ray spectrum of a typical
GRB such as GRB0973 (38), with the use
of a model in which energetic electrons and
positrons from the fireball produce g-rays
through multiple Compton upscattering of
low-energy photons, qualitatively repro-
duce the observed GRB spectra and time
evolution. A related phenomenon is the
origin of ultrahigh-energy cosmic rays
(10
20
eV), which are thought to occur by
the Fermi acceleration mechanism at the
fireball wave front (35).
In experiments under development to
benchmark astrophysical codes for radiation
hydrodynamics, a radiative, high–Mach num-
ber jet has been created and characterized
(39). Here, the initial conditions were a hot
(;1 keV), high-velocity (;700 km/s) jet of
highly ionized Au plasma, where the radia-
tive cooling effects were large. Perhaps more
relevant are experiments under way with the
ultrahigh-intensity laser called the Petawatt
(40). Here, planar targets are irradiated by a
laser pulse (10
20
W/cm
2
), producing an ex-
panding high-energy density wave of hot
plasma, that is, a “laboratory fireball.” The
initial plasma temperature is thought to be
several megaelectron volts, the plasma is
relativistically hot, and electron-positron
pairs are created. For the highest intensity
shots, electrons have been observed up to
energies of 100 MeV, and positron energy
spectra have also been recorded (Fig. 3B)
(41,42). Perhaps most interesting in these
experiments is the observation of photo-
nuclear reactions. The energetic electrons
yield high-energy x-rays through bremsstrah-
lung (Fig. 3C), which excite the nucleus. The
nucleus deexcites by emitting a nucleon or in
the case of
238
U by fission. These reactions
can leave the nucleus in long-lived excited
states that can be counted after the fact by
g-ray spectroscopy. The exact laser-plasma
dynamics and subsequent plasma fireball
evolution are still being worked out. Howev-
er, what is clear is that plasmas have now
been created in the laboratory with a temper-
ature (T);1 MeV “thermal” component and
a higher energy tail (40–42). Substantial
e
1
e
2
production and excited nuclear levels
have been observed. Hence, aspects of the
underlying GRB fireball physics, such as rel-
ativistic plasma effects, are becoming acces-
sible in the laboratory.
Giant Planets and Brown Dwarfs
The “high-stakes tug of war” between quan-
tum mechanical degeneracy pressure and the
more familiar gravitational pressure was dis-
cussed in the section on SNe. A somewhat
more benign environment to consider strong
degeneracy effects is in the steady-state inte-
riors of the giant planets such as Saturn and
Jupiter and the newly discovered brown
dwarfs, (6,43– 45) as represented by the
phase diagram shown in Fig. 4A (46–48).
Here, because of their lower mass, M#0.08
MJ, these bodies never generate sustained
thermonuclear fusion as stars, and the degen-
eracy pressure and strongly coupled effects
dominate.
Strongly coupled plasmas are typically
characterized by the dimensionless parame-
ter, G5(Ze)
2
/akT, where ais a characteristic
separation distance between ions, Ze is the
ion charge state, and kT is the temperature in
units of energy. In plasmas with G,,1,
thermal effects dominate and the plasma is
considered “ideal.” When G$1, the Cou-
lomb interactions become an equal player,
and the plasma enters the strongly coupled
regime, represented by the region to the right
and below the G51 line in Fig. 4A. When
G.178, the plasma becomes so strongly
coupled that the ions freeze solid into a crys-
tal lattice. Also, when the densities are high
enough or temperatures low enough that kT ,
«
F
, where «
F
5p
F
2
/2m
e
5(1/8)(3/p)
2/3
(h
2
/m
e
)n
i2/3
}r
2/3
is the Fermi energy ( p
F
is
the Fermi pressure and n
i
is the ion number
density), the plasma is called degenerate, and
is represented by the region to the right and
below the «
F
5kT line (Fig. 4A). Here,
electron degeneracy pressure becomes a ma-
jor part of the total pressure. The isentropes
for Jupiter and the brown dwarf Gliese 1229B
(45) (Fig. 4A) indicate that these bodies,
Fig. 4. The phase diagram and EOS experiments relevant to the giant planets and brown dwarfs.
(A) Theoretical phase diagram of hydrogen [reproduced from (46)] relevant to Jupiter ( J) and the
brown dwarf Gliese 1229B (G1229B). H
Hug
and D
Hug
are model hydrogen and deuterium Hugoniots.
(B) Measured compression (density) versus SW-induced pressure, that is, the measured principle
Hugoniot for cryogenic liquid D
2
[reproduced from (46)].
SCIENCESCOMPASS
28 MAY 1999 VOL 284 SCIENCE www.sciencemag.org1492
which are made up predominantly of H and
He, are both strongly coupled and highly
degenerate. Hence, the internal structure,
r(r), T(r), and to some extent the external
magnetic fields of the giant planets and
brown dwarfs are determined by the EOS of
degenerate hydrogen and helium at high pres-
sure, p51 to 100 Mbar. The EOS of strongly
coupled, degenerate plasma, however, is no-
toriously difficult to calculate from first-prin-
ciples theories, because of the complexity of
including quantum mechanical effects into
classical thermodynamic theories. Experi-
ments in this parameter regime are a vital
component in efforts to improve our under-
standing of Jupiter, the other giant planets,
and brown dwarfs.
The EOS of a material can be determined
by measuring its response to a known applied
pressure. Measurements of the EOS of cryo-
genic deuterium, D (an isotope of hydrogen),
at applied pressures ranging from 220 kbar to
3.4 Mbar have been made on the Nova laser
(46–48). In these experiments, the transition
of hydrogen from a molecular fluid insulator
phase to a monatomic metallic phase was
unambiguously observed. A departure from
the standard theoretical EOS models for hy-
drogen was found in the compressibility of
D
2
in this regime (Fig. 4B). The results were
consistent with a model that included the
potential energy sink caused by molecular
dissociation (D
2
3D1D). These results,
together with extensive results from gas-gun
experiments at lower pressure (49,50), have
implications for the composition and dynam-
ics of the outer layers of Jupiter, the other
giant planets, and brown dwarfs.
The pressure and temperature in the man-
tle of Jupiter near the surface are in the range
of 1 to 3 Mbar and a fraction of an electron
volt. Deeper in the interior, the pressure and
temperature increase, rising to 40 Mbar and a
couple of electron volts at the center (51, 52).
Near the surface, hydrogen exists as the mol-
ecule H
2
, but dissociates to H 1H and
ionizes deeper in the mantle. This transition
of hydrogen from insulator to conductor is
important, because conducting H in the con-
vective zone is thought to create the 10- to
15-Gauss magnetic field of Jupiter. One of
the fundamental open questions about the
interior of Jupiter is whether there is a sharp
boundary, a plasma phase transition (PPT),
between a molecular hydrogen mantle and a
monatomic hydrogen core at a radius of
;0.75 jovian radius (R
J
) and pressure of 3
Mbar. The regimes accessed by the laser and
gas-gun experiments represented on Fig. 4B
span this critical transition from mantle to
core of Jupiter and suggest that a sharp dis-
continuity between molecular (mantle of Ju-
piter) to monatomic (core of Jupiter) hydro-
gen does not exist. The experiments (46–48,
51, 52) suggest that on the jovian isentrope
molecular hydrogen probably begins to dis-
sociate at 400 kbar and dissociation continues
smoothly to completion at ;3 Mbar, with
metallization occurring right in the middle of
this region at ;1.4 Mbar and ;4000 K. It is
possible (52) that currents near the surface of
Jupiter, at radii out to 0.95 R
J
contribute to
the surface magnetic field, whereas previous-
ly it was thought that the magnetic field was
formed deeper in the interior at ;0.75 R
J
.
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SCIENCESCOMPASS
www.sciencemag.org SCIENCE VOL 284 28 MAY 1999 1493
... Alternatively, polarized positrons are usually produced via Bethe-Heitler (BH) process by hitting high-Z targets with circularly polarized γ photons [9,10] or prepolarized electrons [11]. These BH methods suffer low conversion efficiency of about 10 4 positrons (10 −6 nC) per shot, and thus high repetitions are necessary to meet the high-charge or high-density requirements of ILC (3.2 nC) and laboratory astrophysics [12,13]. ...
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Many works have shown that dense positrons can be effectively generated from laser-solid interactions in the strong-field quantum electrodynamics (QED) regime. Whether these positrons are polarized has not yet been reported, limiting their potential applications. Here, by polarized QED particle-in-cell simulations including electron-positron spin and photon polarization effects, we investigate a typical laser-solid setup that an ultraintense linearly polarized laser irradiates a foil target with micrometer-scale-length preplasmas. We find that once the positron yield becomes appreciable with the laser intensity exceeding 10^{24} W/cm^{2}, the positrons are obviously polarized. Around 30 nC positrons can acquire >30% polarization degree with a flux of 10^{12} sr^{-1}. The angle-dependent polarization is attributed to the asymmetrical laser fields that positrons undergo near the skin layer of overdense plasmas, where radiative spin flip and radiation reaction play significant roles. The polarization mechanism is robust and could generally appear in future 100-PW-class laser-solid experiments.
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Les instabilités hydrodynamiques sont des phénomènes néfastes à l’obtention des conditions nécessaires à l’allumage des cibles en Fusion par Confinement Inertiel. Lorsque la capsule est accélérée, toute perturbation spatiale du front d’ablation peut croître exponentiellement sous l’influence de l’instabilité de Rayleigh-Taylor. Avant cette phase d’accélération, l’évolution de l’amplitude du front d’ablation est déterminée par les instabilités de Richtmyer-Meshkov et Landau-Darrieus. Ce travail présente au lecteur des études analy-tiques et numériques en géométrie plane permettant de décrire ces phénomènes complexes.Des expériences sur les installations OMEGA-EP et NIF ont été menées avec des mousses de faibles densités irradiées en attaque directe pour étudier l’évolution du front d’ablation sous l’influence de Ces instabilités. Parmi ces mécanismes, l’instabilité de Landau-Darrieusn’a encore jamais été observée au front d’ablation. Afin de déterminer les conditions expérimentales permettant de la mettre en évidence, un nouveau modèle analytique a été développé pour décrire l’écoulement au sein de la cible. Sous certaines hypothèses, ce modèle permet en effet d’étudier la stabilité du système à travers un modèle perturbatif linéaire et de calculer les effets de l’instabilité de Landau Darrieus sur le front d’ablation. Afin de comparer et de valider ce modèle, des calculs ont été réalisés avec le code d’hydrodynamique radiative FLASH sur des durées de plusieurs dizaines de nanosecondes.Comparé à d’autres modèles de la littérature et aux résultats des simulations, le modèle et les simulations numériques permettent de construire une stratégie expérimentale permet-tant pour la première fois l’étude de l’instabilité de Landau-Darrieus au front d’ablation.Pour accroître les chances d’observation, la taille de la zone de conduction en face avant de la cible doit être plus petite que les longueurs d’onde d’interface étudiées. Nécessitant des conditions expérimentales particulières et peu explorées jusqu’à maintenant, des expériences préliminaires ont ainsi été réalisées afin de valider nos outils numériques et notre modélisation. Les résultats d’expériences récentes réalisées sur le NIF confirment la signature d’un nouveau comportement du front d’ablation, ouvrant ainsi de nouvelles perspectives pour l’étude de l’instabilité de Landau-Darrieus
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Extreme-ultraviolet pulses can propagate through ionised solid-density targets, unlike optical pulses and, thus, have the potential to probe the interior of such plasmas on sub-femtosecond timescales. We present a synthetic diagnostic method for solid-density laser-generated plasmas based on the dispersion of an extreme-ultraviolet attosecond probe pulse, in a pump–probe scheme. We demonstrate the theoretical feasibility of this approach through calculating the dispersion of an extreme-ultraviolet probe pulse propagating through a laser-generated plasma. The plasma dynamics is calculated using a particle-in-cell simulation, whereas the dispersion of the probe is calculated with an external pseudo-spectral wave solver, allowing for high accuracy when calculating the dispersion. The application of this method is illustrated on thin-film plastic and aluminium targets irradiated by a high-intensity pump pulse. By comparing the dispersion of the probe pulse at different delays relative to the pump pulse, it is possible to follow the evolution of the plasma as it disintegrates. The high-frequency end of the dispersion provides information on the line-integrated electron density, whereas lower frequencies are more affected by the highest density encountered along the path of the probe. In addition, the presence of thin-film interference could be used to study the evolution of the plasma surface.
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The Weibel instability is investigated theoretically and numerically under three scenarios: counterstreaming electron beams in background plasma, an electron–positron beam and an electron–proton beam in background plasma. These models occur widely in laboratory and astrophysical environments. The Weibel instability growth rates are determined numerically from the corresponding cold-fluid dispersion relations, which are confirmed with two-dimensional particle-in-cell simulations. The maximum growth rates for the counterstreaming beams in background plasma are an order of magnitude smaller than the maximum growth rates for the beams cases in the same range of density ratios and beam energies. The maximum growth rate for the electron–positron beam case is shown to be at most a factor $\sqrt {2}$ greater than the electron–proton beam case with similar dispersion behaviours. A non-monotonic relation is found between the maximum Weibel instability growth rates and the electron–positron beam energy, suggesting that increasing beam energies does not entail an increase in the Weibel instability growth rate.
Preprint
The interaction between the supersonic jet and background can influence the process of star formation, and this interaction also results in a change of the jet's velocity, direction and density through shock waves. However, due to the limitations of current astronomical facilities, the fine shock structure and the detailed interaction process still remain unclear. Here we investigate the plasma dynamics under different collision states through laser-driven experiments. A double-shock structure is shown in the optical diagnosis for collision case, but the integrated self-emitting X-ray characteristic is different. For solid plastic hemisphere obstacle, two-layer shock emission is observed, and for the relatively low-density laser-driven plasma core, only one shock emission is shown. And the plasma jets are deflected by $50 ^{\circ}$ through the interaction with the high-density background in both cases. For collisionless cases, filament structures are observed, and the mean width of filaments is roughly the same as the ion skin depth. High-energy electrons are observed in all interaction cases. We present the detailed process of the shock formation and filament instability through 2D/3D hydrodynamic simulations and particle-in-cell simulations respectively. Our results can also be applied to explain the shock structure in the Herbig-Haro (HH) 110/270 system, and the experiments indicate that the impact point may be pushed into the inside part of the cloud.
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A point X-ray source located 9' NE of the center of SN 1006 has been spectroscopically identified as a background QSO, with a redshift of 0.335. The object is moderately bright, with magnitude V = 18.3. If its ultraviolet spectrum is typical of low-z quasars, this object will be a second source (after the Schweizer-Middleditch star) to use for absorption spectroscopy of material within SN 1006. Absorption spectra provide a unique probe for unshocked ejecta within this supernova remnant and can possibly solve the long-standing problem of "missing" iron in the remnants of Type Ia supernovae.
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We describe and report progress on the joint U.S. and German campaign to map the X-ray emission from the entire Cygnus Loop with the {ital ROSAT} High Resolution Imager. The Cygnus Loop is the prototype for a supernova remnant that is dominated by interactions with the interstellar medium and supplies fundamental physical information on this basic mechanism for shaping the interstellar medium. The global view that these high-resolution (FWHM{approximately}10{sup {prime}{prime}}) observations provide emphasizes the inhomogeneity of the interstellar medium and the pivotal nature of cloud{endash}blast-wave interactions in determining the X-ray morphology of the supernova remnant. While investigating the details of the evolution of the blast wave, we also describe the interstellar medium in the vicinity of the Cygnus Loop, which the progenitor star has processed. Although we do not expect the X-ray observations to be complete until 1997 September, the incomplete data combined with deep H{alpha} images provide definitive evidence that the Cygnus Loop was formed by an explosion within a preexisting cavity. {copyright} {ital 1997} {ital The American Astronomical Society}
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We present a high-resolution X-ray image of SN 1006: a mosaic of four ROSAT HRI images that together cover the entire supernova remnant. A well-defined shock front is observed surrounding the remnant shell. Comparison of the X-ray and radio images shows a close correspondence in general morphology, especially for the brightest features. Along the northeast limb of the shell, where the remnant is brightest in both bands, the X-ray/radio correspondence is striking in its detail. The data are consistent with a model in which the X-ray emission in this region results primarily from synchrotron radiation by the same population of shock-accelerated electrons that produce the radio emission. New optical images show Hα emission over a much greater portion of the remnant than has previously been reported. Newly discovered faint filaments in the south, like the much brighter ones along the northwest limb, delineate the primary shock as it propagates into the interstellar medium. In the northwest, a narrow X-ray peak traces just inside the crisp optical filaments and provides a laboratory for observing the effects of a fast, nonradiative shock propagating nearly perpendicular to our line of sight. The calculated profile of the thermal X-ray emission from a rapidly ionizing postshock plasma closely matches the observed profile, supporting a model in which nonequilibrium thermal processes dominate the X-ray emission in the northwest. The measured displacement of 8'' between the optical filaments and the X-ray peak, combined with previously measured proper motions for the filaments, indicates a timescale of only ~120 yr for X-ray heating and ionization behind the shock. In addition to the narrow optical filaments around the rim of SN 1006, we find faint, diffuse Hα emission within the remnant shell to the southwest. This emission probably represents nonradiative sheets viewed obliquely.
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We present results of the first measurements of density, shock speed, and particle speed in liquid deuterium compressed by laser-generated shock waves to pressures from 25 to 210 Gpa (0.25 to 2.1 Mbar). The data show a significant increase in D2 compressibility above 50 Gpa compared to a widely used equation of state model. The data strongly suggest a thermal molecular dissociation transition of the diatomic fluid into a monatomic phase.
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Standards of living in the industrialized world and human population have grown to the extent that they can no longer be maintained without the profligate fuel consumption that fostered them. Unfortunately, global petroleum production will begin to decline during the early decades of the 21st century. Finite exploitable petroleum resources dictate that growth in human population will soon be reversed. With a permanent 2.4% annual decline of global oil consumption, the world's oil reserves will last forever. Acceptance of such economic sacrifice, if accompanied by a voluntary decrease in human population, could provide an adequate fuel supply and thus adequate living standards for all future generations. -from Author
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Observations of SN 1987A are reviewed. The characteristics of the supernova's evolution are outlined. The detection of neutrinos from SN 1987A is discussed, emphasizing the theoretical significance of the neutrino observations. Consideration is given to the characteristics of the supernova progenitor, the neutron star in SN 1987A, and the detection of optical pulsations.
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Supernovae of Type II occur at the end of the evolution of massive stars. The phenomenon begins when the iron core of the star exceeds a Chandrasekhar mass. The collapse of that core under gravity is well understood and takes a fraction of a second. To understand the phenomenon, a detailed knowledge of the equation of state at the relevant densities and temperatures is required. After collapse, the shock wave moves outward, but probably does not succeed in expelling the mass of the star. The most likely mechanism to do so is the absorption of neutrinos from the core by the material at medium distances. Observations and theory connected with SN 1987A are discussed, as are the conditions just before collapse and the emission of neutrinos by the collapsed core.
Article
The Nova laser [E. M. Campbell, Laser Part. Beams 9, 209 (1991)] was used to shock-compress liquid deuterium and obtain new principal Hugoniot measurements of density and pressure between 0.3 and 2.1 Mbar. In this pressure-density region, deuterium is predicted to transform from a molecular insulating fluid to an atomic conducting fluid. Nova data show a rapid increase in density from 0.6 g/cc at 0.3 Mbar, to 1 g/cc at 0.6 Mbar, suggestive of such a transition. The observed sixfold compression near 1 Mbar is larger than predicted by many widely used equation of state models.
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Laboratory studies of hydrodynamic effects driven by a flowing, expanding plasma of high-energy density and high Mach number are reported. The flowing plasma is the ejecta from matter accelerated and heated by an ablative shock. X-ray backlighting diagnoses the structure produced when this plasma impacts low-density foam. We observe the forward shock driven into the foam and the stagnated ejecta which drives a reverse shock into the flow.
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As luminous events that can be physically modeled, supernovae provide an attractive route to the value of the Hubble constant. The modeling involves radiation transport through matter undergoing homologous expansion with velocity gradient on the order of 10−6 s−1. For supernovae of type Ia, which are thermonuclear disruptions of mass accreting or coalescing carbon–oxygen white dwarfs, one wants to be able to calculate the light curve (luminosity in some optical passband versus time), which is powered by the radioactivity decay chain 56Ni↠56Co↠56Fe. For all kinds of supernovae, including those of types II, Ib, and Ic, which result from the gravitational collapse of the cores of massive stars, the goal is to accurately calculate the emergent ultraviolet–optical–infrared spectra, as a function of time. Local-thermodynamic-equilibrium (LTE) light-curve calculations for type Ia supernovae by Ho¨flich and co-workers, and our spectrum calculations based on a fully relativistic non-LTE radiative transfer code, are described. The associated radiative transport needs are discussed.