May 28, 1998 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 trillions 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 measurements 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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