Contact: Adam Marcus
marcusa@umich.edu
(734) 647-7046
University of Michigan
ANN ARBOR---In work fundamental to the nature of atoms, as well as some
stars and big planets, University of Michigan scientists have measured
how matter changes under extreme pressure.
Using a high-resolution femtosecond laser---which can deliver a trillion
Watts of power in 1015 seconds---Prof. Don Umstadter and his colleagues
were able to watch how and when electrons and atoms organize themselves
in the super-dense environments similar to those found in fusion
reactors, white and brown dwarf stars and Jovian planets such as
Jupiter and Neptune.
The work, which appears in the May 18 issue of Physical Review Letters,
is the first experimental confirmation of earlier predictions about the
behavior of atoms in these conditions.
Moreover, the fact that Prof. Umstadter's group was using a relatively
inexpensive, table-top laser demonstrates that this type of high-
intensity research can be done by graduate students on college campuses.
In most phases of matter, electrons---the negatively charged particles
which swarm around atomic nuclei---lead an orderly existence, confined
to particular orbits, or "shells" around atomic nuclei depending on how
much energy they have. The particles can change orbits by shedding or
receiving energy, usually in the form of light, but they generally have
a strong affinity for their particular nucleus. Previously unverified
mathematical and computer models, however, suggested that electrons in
a super-dense plasma exist mostly in a free-for-all state, a particle
sea unassociated with any given nuclei.
In their experiment, Umstadter, a professor in the Department of Nuclear
Engineering and Radiological Science and in the Department of Electrical
Engineering and Computer Science, and his colleagues in the Center for
Ultrafast Optical Sciences heated solid carbon (as one would find in a
pencil lead) with 100-femtosecond pulses, converting it into a dense
soup of charged atoms, or ions. As the matter expanded and cooled, the
electrons played a game of musical chairs, joining up with available
nuclei at random. When they chose an atom, the electrons lost energy
in the form of easily detectable X-rays. In fact, the X-ray emission is
so regular that it can be read as a signature to determine precisely
when the electrons have settled into their new state, or phase. And by
measuring the X-ray emission signatures, the researchers were able to
identify when and under what conditions of temperature and density the
settling sea literally parted and the matter changed phase.
In addition to illuminating the timing of phase changes, the experiment
also sheds light on the conditions under which electrons are bound to
atoms, and thus, on the conductivity of matter.
Whereas the electron sea freely conducts electricity, the bound state
is non-conductive, because there are no free electrons to carry current.
"By just changing its density, an insulator can be made into a
conductor, or vice versa," says Umstadter.
Umstadter says femtosecond lasers allow for the first "clean measure-
ments in a system that's usually never clean." In the astrophysical
realm, researchers can use the data to help calibrate their measurements
of the conditions in the interior of planets or collapsed stars. And it
should also be of help in designing laboratory fusion reactors, where
such measurements of the structure of this unusual state of matter are
made difficult by the smearing-out of information by rapidly changing
conditions.
Funding for the work came from the U.S. Department of Energy, Division
of Basic Energy Sciences.
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