Vol.25 No.3 2011
Laboratory Astrophysics
Progresses of Laboratory Astrophysics in China
ZHAO Gang*1 & ZHANG Jie2, 3
1
National Astronomical Observatories, CAS, Beijing 100012
2
Shanghai Jiao Tong University, Shanghai 200240
Institute of Physics, Chinese Academy of Sciences
3
The exciting discoveries in astronomy such as the
accelerating expansion of the universe, the atmospheric
composition of exoplanets, and the abundance trends of
various types of stars rely upon advances in laboratory
astrophysics. These new discoveries have occurred along
with dramatic improvements in measurements by groundbased and space-based instruments of astrophysical
processes under extreme physical conditions. Laboratory
astrophysics is an exciting and rapidly growing field
emerging since the beginning of this century, which covers
a wide range of scientific areas such as astrophysics, laser
and particle beam physics, plasma physics, atomic and
molecular physics, fluid dynamics, magnetohydrodynamics,
etc. Through the past years, laboratory astrophysics has
possessed a new experimental capability with the help of
existing and planned high-energy density facilities, such
as large lasers facilities, tokomak device and Z-pinch
generators, providing an opportunity to investigate a number
of related astrophysical phenomena in the laboratory. For
example, it is possible to carry out direct measurements
of complex opacities, the equations of state, strong-shockdriven turbulent dynamics, and photoionized plasmas in the
ZHAO Gang is a professor with the National Astronomical Observatories of the Chinese
Academy of Sciences, director of the CAS Key Laboratory of Optical Astronomy, and
former president of the Chinese Astronomical Society. As a pioneer in the field of Galactic
evolution and laboratory astrophysics in China, he has made major achievements including
implementing a systematic analysis of the chemical compositions and evolution of the Galaxy,
presenting for the first time an accurate formula for calculating the effect of neutral hydrogen
collisions, and applying the advanced techniques of spectral analysis to extra-planet studies.
He was among the first “Bairen” scientists of CAS and winner of the “National Science Fund
for Distinguished Young Scholars”, the second prize of the National Natural Science Awards
and the Hong Kong-based Ho Leung Ho Lee Prize for Scientific and Technological Progress.
ZHANG Jie is a Member of CAS, Member of the German Academy of Sciences Leopoldina,
Fellow of the Third World Academy of Sciences (TWAS), International Fellow of the Royal
Academy of Engineering, and President of Shanghai Jiao Tong University. He has devoted
years working at the Max-Planck Institute of Quantum Optics of Germany and Rutherford
Appleton Laboratory of UK, and has now become the top leading scientist in the field of strong
field physics, X-ray laser, and fast ignition of inertial confinement fusion. Professor Zhang’s
academic achievements were recognized by TWAS with the 2007 TWAS Prize in Physics;
by CAS with the 2007 Outstanding Science and Technology achievement Prize; by the
Chinese State Council with the Natural Sciences Prize in 2006; by the Hong Kong-based Ho
Leung Ho Lee Foundation with the Science and Technology Progress Award in 2006; by the
Government of China with the National Award for Outstanding Young Scientists in 1998, etc.
* Correspondence and requests for materials should be addressed to [email protected].
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laboratory. Unlike conducting astrophysical observations,
scientists can measure and control physical conditions in
the laboratory with much greater confidence. Some detailed
measurements of key physical parameters are significant not
only for the understanding of astrophysical phenomena but
also for theoretical breakthroughs under the framework of
traditional interpretations.
With recent advances in astronomical technology,
a number of large scale ground-based and space- based
telescopes have gone into operation. Their spatial resolution
is dramatically increased, and the spectral resolution is
considerably improved as well. These high resolution, high
signal-to-noise ratio spectra enable us to determine the
stellar structure, age, abundance and environments precisely.
However, the spectral analysis requires accurate atomic
data and the quality of these data still affects the precision
of line identification, diagnostics of plasma, abundance and
opacity of objects. There are many similarities between
the high energy density plasma in astrophysics and that in
plasma physics. To investigate high energy density plasma
with the scale of astrophysical objects in the laboratory,
one major method is to use modern laser experimental
devices in current laboratory astrophysical researches.
With the continued support of the National Natural Science
Foundation of China (NSFC), the Ministry of Science and
Technology (MOST) and the Chinese Academy of Sciences
(CAS), a research group led by Prof. ZHANG Jie from the
CAS Institute of Physics and Prof. ZHAO Gang from the
National Astronomical Observatories of CAS has made
significant breakthroughs in laboratory astrophysics in the
past years. In the following sections, we report the major
achievements of our recent experimental and theoretical
studies in laboratory astrophysics.
1. Modeling loop-top X-ray source and
reconnection outflows in solar flares in laboratory
Magnetic reconnection (MR) refers to the breaking and
reconnecting of oppositely directed magnetic field lines in
a plasma, and it is a process of energy conversion in plasma
physics. The model of MR is widely applied in astrophysics
including investigations on solar flares, star formation, the
coupling of solar wind with the Earth’s magnetosphere,
accretion disks, and Gamma-ray bursts. The MR process also
arouses wide interests in laboratory plasma physics, while its
topology can be constructed by different energy drives in the
laboratory, such as Z-Pinch and tokomak. There are many
pieces of indirect observational evidence for MR models
in astrophysical plasma, especially in solar plasma, among
which the loop-top X-ray source in solar flare is one of the
most famous. But so far, the explanation of loop-top hard
X-ray source is only qualitative and phenomenological and
lacks detailed and quantitative theoretical calculations, which
is directly due to limitations of astronomical observations.
Because of relatively low strength of magnetic fields, the
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Fig.1 Group photo of experimental staff from the NAOC and IoP's
laboratory astrophysics teams and the CAS Shanghai Institute of
Optics and Fine Mechanics, in front of Shenguang II laser facility.
plasma produced with traditional magnetic reconnection
devices cannot be compared with the MR phenomenon in
astrophysical objects. It is a breakthrough to use spontaneous
magnetic field of laser plasma to construct a MR topology in
the laboratory where the scaling law is quite easily achieved.
A mega-gauss magnetic field B could be generated in hot,
high-density plasmas by irradiating a solid target with highpower laser beams. During the laser pulse the magnetic field
is quasi-steady and approximately “frozen” in the plasma
expanding laterally.
Based on this quasi-steady state of the magnetic field,
our group reconstructs the topology of MR in the laboratory
using Shenguang II laser facilities of the National
Laboratory on High Power Lasers and Physics. We observed
the similar results of loop-top X-ray source in solar flares.
By applying the scaling law of magnetohydrodynamics,
we found that the physical parameters of the two systems
are strikingly similar. The work was published in Nature
Physics in 2010 with the referee’s comment reading “if the
physical processes observed in the laboratory are physically
the same as those observed on the Sun, and if the authors
can measure the physical quantities in the laboratory
experiment very accurately, those experimental results
are great discovery and will open important new field of
laboratory astrophysics”. The work was also highlighted by
Nature China and aroused interests from the community.
2. Measurements of the opacity of silicon at high
temperature and high density
We have measured the K-shell absorption spectra of
a silicon plasma for the first time. The experiments were
carried out using the Shenguang II laser facility. A silicon
dioxide foam was heated by thermal radiation emitted
from a laser-irradiated “dog bone” – like gold Hohlraum.
The backlight was produced by a gold foil irradiated by a
picosecond laser beam. By changing the delay time between
the two beams, the absorption spectra of the silicon plasma
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at different times have been measured. A gold cylindrical
Hohlraum target with a diameter of 1800 μm and length
of 1000 μm was used to generate a clean, near-Planckian
radiation field. Absorptions of the 1–2 transitions of Si
xii through Si vi were observed in the wavelength range
from 66 to 71 nm. The experimental results are simulated
with theoretical calculations under the assumption of local
thermodynamic equilibrium, a detailed level accounting
model was constructed to fit the experimental results
and a good agreement was achieved in general when the
effects of the oxygen in the SiO2 were taken into account.
In our calculations, the radiative width and Doppler and
autoionization resonance broadening for each transition
have been included. Finally, the instrumental broadening
was applied after the absorption spectra were calculated.
As a silicon dioxide foam was used in the experiment,
the effect of the oxygen needed was taken into account in
determining ion distribution of the silicon. It was found
that good convergence can be obtained when the models
include levels with principal quantum numbers n' ≤ 8. The
plasma temperatures that we determined ranged from ~ 65
to 30 eV as time increased, while the densities decreased
from 80 to 18 μg cm-2. We also found that there are still
some discrepancies between the model calculations and
experimental results. The most likely reason for this is the
existence of temporal and spatial gradients due to the free
expansion of the silicon sample. In future experiments, the
density of the silicon plasma should be measured directly.
The sample should also be hampered in order to obtain
better homogeneity.
3. Soft X-ray and EUV spectroscopic measurement
in laboratory and their benchmark application for
astrophysical plasmas
Spectroscopy is a dominant source for observational
data besides imaging in astronomy, which plays an important
role in the studies of various astrophysical objects. In the soft
X-ray and extreme ultraviolet (EUV) wavelength regions,
a large number of high-quality spectra with high-resolution
are available by on-going space observatories via Chandra,
XMM-Newton and Hinode. The spectroscopic investigations
can give an insight into physics of stellar corona, solar
flares, coronal heating, supernova remnants, accretion of
stars or black holes, interstellar medium, galaxy, and intergalaxy medium and so on. Recently, imaging-spectroscopy
is available by Hinode in the EUV region by shifting the
incidence slit of the spectrometer. The recently launched
Solar Dynamic Observatory observes the Sun in seven EUV
channels with high time cadence. Next generation X-ray
observatories such as Astro-H and IXO will detect photons
with much higher efficiency up to three magnitudes higher
than existing ones and comparable/higher resolution. But
the analyses of the soft X-ray and EUV spectra obtained
Laboratory Astrophysics
Fig.2 Loop-top-like X-ray source and outflows observed in the
laboratory. (a) Magnetic reconnection model for the loop-top X-ray
source in a compact solar flare. (b) The pinhole X-ray image
observed forward of the Al foil target. Magnetic field lines are
illustrated based on the flux surface of the plasma bubbles. The
Al and Cu targets are the rectangles enclosed by white dotted
lines. The red arrows indicate outflow/jet directions. (c) X-ray
image with two laser spots separated by 400 μm and with a foil
thickness of 10 μm. The asymmetry of the laser intensity on the
Al target causes an imbalance of the laser spots as well as of the
magnetic fields B1 and B2, and further induces the inclination of
the upward outflow. The downward outflow impinges on the Cu
target and results in a hot X-ray source (cf. Zhong et al., Nature
Physics, 6, 984, 2010).
from these space missions are difficult due to their inherent
complexity besides the complex plasma conditions of
astrophysical sources. The present spectral models such
as MEKAL, Chianti, APEC and Cloudy extensively used
by the astronomical community are based upon theoretical
predictions of various atomic processes in the plasmas.
Moreover, most of atomic data are from calculations with
semi-empirical formulas and distorted-wave method. The
uncertainty of wavelength makes it difficult to confirm the
accurate velocity of accretion or outflow blocks by analysis
of emission lines. The uncertainty of line intensities will lead
to uncertain diagnostics and understanding of the radiation
mechanism of plasmas (collision, photon ionization, charge
exchange), and further the heating and cooling of plasmas.
Although theoretical modeling is an irreplaceable research
method for the spectroscopy over a wide wavelength band,
a benchmark measurement is very necessary for testing the
accuracy and uncertainty of these spectral models extensively
used in the astronomical community.
Irradiation of ultra-intense lasers on target and electron
beam ion trap (EBIT) can generate hot plasmas with
temperatures from 10 eV to several keV, which are typical
temperature values of coronal plasmas. We performed
the soft X-ray (silicon) and EUV (iron) spectroscopic
measurement on Jiguang II ultra-intense laser facility at
the CAS Institute of Physics and the EBIT facility at MaxPlanck Institute for Nuclear Physics. About 50 silicon lines
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with high accuracy of 0.05nm were obtained in our laser
experiment. By scanning the electron energy of EBIT, twodimensional spectra as the function of electron energy and
photon energy were obtained with an EUV spectrometer
with high efficiency of collection, from which blending
emission lines from different charge stages were resolved
clearly. And several emission lines were identified for the
first time in the laboratory. Collision and radiation modeling
reproduced these laboratory measurements well for the
low- and high-density plasmas. In turn, the laboratory
measurement benchmarked the theoretical model.
Additionally, we diagnosed the electron density of these
experimental plasmas by the spectroscopic method, which
was consistent with the results of other groups. In this work,
we noticed the dependence of the overlap factor between
the ion cloud and electron beam on the effective charge of
ions for the first time: the benchmark work for diagnostic
line ratio by using He-like spectroscopy is not valid for
B-like spectroscopy. We also applied this benchmark
model to systematically analyze the soft X-ray emission
lines of highly charged silicon, sulphur and iron in coronal
observations, and tens of emission lines and blending
contamination were identified for the first time.
4. Electronic structure and radiative opacity of the
metallic elements in hot and dense stellar material
As one of our theoretical achievements on radiative
opacity, we calculated the electronic structures of the
metallic elements in hot and dense stellar material with an
average-atom scheme, which was designed to consider the
environmental influence in a mixture. It ensured that all
kinds of atoms had the same temperature, the same chemical
potential, and the same electron density at the boundaries
between the atoms, and that the electrical neutrality
within each atomic sphere was satisfied by using a selfconsistent field calculation. Opacities, which are strongly
environmentally dependent, for stellar materials with
solar composition and relative abundance were calculated
with the calculated electronic orbitals and excitation cross
sections. We showed clearly that contributions from both
bound-free and bound-bound transitions between the
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states of the metallic elements played significant roles,
although their relative abundances are much less than those
of hydrogen and helium. Comparison was made for the
Rosseland mean opacity, which had been calculated for
a variety of temperatures and densities of a model solar
material, between ours and those of OPAL and LANL
(Los Alamos National Laboratory Astrophysical Opacity
Library) with good agreement for quite large ranges of
temperature and density. Our approach was a simple and
effective way to generate opacities for a variety of models
for stellar materials with densities above 0.1 g cm -3 .
Because of the continuous nuclear reactions in a star, stellar
material keeps changing during its evolution. Considering
the simplicity and efficiency of the present scheme, it is
possible to calculate the opacity instantaneously along
with the change in compositions and the thermodynamic
environment and also to include much heavier elements
without prohibitive difficulties. It would also be possible
to combine it with stellar evolution models. However, the
accuracy of the model still needs improvement before the
results are applied to the calculations of stellar properties
such as solar oscillations.
Laboratory astrophysics is a newly established and
rapidly growing cutting-edge interdisciplinary science as
a combination of astrophysics, plasma physics and atomic
physics. The experiments in laboratory astrophysics
based on recent developments of large laser facilities
may bring breakthroughs and solve some key problems in
astronomy. Therefore, governments of many developed
countries are investing heavily in laboratory astrophysics.
For instance, the US astronomy and astrophysics decadal
survey recommended the establishment of a Laboratory
Astrophysics Program funded by both the NASA and the
NSF. The CAS started to support laboratory astrophysics
research in 2000, and we have made a number of
significant progresses in this challenging field mainly
based on new advanced femto-second and long pulse
ultra-intense laser facilities. With newly developed
facilities such as Shenguang III and US NIF, we may have
more opportunities to acquire a better understanding of
our universe through experimental and theoretical studies
via laboratory astrophysics.