The hydrogen atom is composed
of a single proton and a single electron. One must supply about
2 x 10**-18 Joules of energy (13.6 eV) to pull the two apart or
"ionize" the hydrogen. Those who have traveled in Europe
and were inclined to read a candy bar label know that a typical
candy bar provides the hungry snacker about a mega-Joule, or 10**6
Joules. Evidently, one could ionize a lot of hydrogen with a single
Kit-Kat! On the other hand, one can see how hard it really is to
ionize matter by considering the temperature to which one would
have to heat hydrogen to ionize a significant fraction
of it. One's first estimate might be that the gas has to be heated
until there is approximately 13.6 eV of energy per atom -- that's
about 150,000 degrees Kelvin. However, because the atoms in a gas
have a distribution of energies (with a few high-energy members
that are very effective at ionizing other atoms in collisions),
and because the frequency of ionizing collisions depends on the
density of the gas, it turns out that nearly complete ionization
of a modest density hydrogen gas in thermodynamic equilibrium is
obtained when the average energy per atom is only about 1 eV, corresponding
to a temperature of about 10,000 K. This is still pretty hot for
typical terrestrial conditions, of course.
Matter that has been ionized is called
'plasma'. Because we live on Earth, where matter is rarely heated
to the point of ionization, plasma is less familiar to us than
everyday condensed matter. It is, however, a very interesting
state of matter to study, for fundamental reasons as well as for
many applications. Because the constituent elements of a plasma
carry net electric charge and can therefore interact over long
distances via electromagnetic fields, plasma dynamics is a far
richer area of study than ordinary fluid dynamics.
(In a fluid composed of non-ionized atoms, each atom is electrically
neutral, so that interactions occur mainly only when atoms collide.)
In fact, the complexity of plasma dynamics is so great that progress
toward understanding and actually predicting the behavior and
properties of flowing, turbulent plasma was limited until recently.
What has changed? Several decades of
development of the fundamentals of plasma theory and steady experimental
efforts to produce, measure and characterize plasma in the laboratory
have recently been augmented by tremendous advances in supercomputer
simulations. Algorithmic advances (including the development of
simplified but rigorously correct nonlinear equations appropriate
for specific situations) and advances in raw computational power
have contributed roughly equally to advances in plasma simulation.
As noted by a National Academy review in December, 2002, plasma
physicists have achieved "notable advances in understanding
and predicting plasma performance" in the last decade. The
report continues, "Of particular note is the ongoing effort
to develop a fundamental understanding of the complex turbulent
processes that govern the confinement of hot plasmas in magnetic
fields." Scientists at the University of Maryland have been
at the forefront of this effort for many years. Recently hired
Assistant Professor William Dorland is the leader of an international
scientific team that is developing theory and simulations of hot,
magnetized plasma turbulence, with a specific focus on collisionless
dynamics.
Why might one be specifically interested
in collisionless, magnetized plasma dynamics? These apparently
exotic conditions arise in diverse situations. Fluctuations in
the diffuse interstellar plasma which fills much of the space
in our galaxy have been observed and are thought to be associated
with collisionless, magnetized plasma turbulence. Dorland's research
group has support from NASA and NSF to predict key signatures
of these fluctuations, hopefully enabling more detailed characterizations
of the interstellar matter and fields.
In a second astrophysical problem,
there is a collisionless,
magnetized plasma accreting onto a supermassive, compact object
near the center of our galaxy (a situation that is likely typical
of so-called 'bulge galaxies'). Characteristics of the turbulence
in this plasma directly affect the X-ray luminosity of this accretion
flow. New X-ray satellite telescopes (the USA's Chandra and its
European counterpart) are collecting observational data from this
region of space -- making now an excellent time to be in the business
of predicting observable features. Key mysteries that are being
addressed relate mainly to the dynamical basis of so-called 'two
temperature' flows.
Dr. Dorland's group is attacking both
astrophysical problems with
simulation technology they developed to support the international
controlled thermonuclear fusion program. Plasma turbulence is
a central scientific problem in the quest for controlled thermonuclear
fusion, because it is turbulence that limits the performance of
existing experiments. To understand why, it is necessary to review
the basics of thermonuclear fusion.
To achieve fusion, one must induce
positively charged nuclei of light elements to get close enough
together to allow the (attractive) strong force to take over from
the (repulsive) electromagnetic force. Under such conditions,
the nuclei will spontaneously fuse, releasing usable energy in
the process. The fusion of almost any two nuclei that appear before
iron in the periodic table will result in the release of energy
-- the reaction is exothermic. (Elements beyond iron in the periodic
table have exothermic _fission_ reactions.)
The electromagnetic repulsion felt
by two nuclei is very strong, and is proportional to the product
of the charges of the nuclei. It is easy to see, therefore, that
hydrogen nuclei should be the easiest nuclei to fuse in the laboratory.
In fact, it turns out that fusing 'heavy hydrogen' (specifically,
the isotopes deuterium and tritium) is the easiest reaction to
induce of all. In the magnetic confinement fusion program, plasma
composed of heavy hydrogen is heated to approximately 250 million
degrees, at which point typical collisions result in exothermic
fusion reactions. This has been achieved in the laboratory --
but at a greater cost than expected. The problem is that turbulence
in the super-heated plasma causes rapid cooling. More precisely,
turbulence induces energy diffusion which gets stronger very rapidly
as the plasma gets hotter. As a result, one must supply a lot
of energy to get the fusion reactions going. The ultimate performance
of a given experiment, measured by the ratio of self-heating to
applied heating, is limited by and exquisitely sensitive to the
details of the underlying turbulence.
Dr. Dorland and his group are working
on the development of the theory and of first-principles simulations
of this turbulence. Continuing progress requires maintaining the
existing capability to perform and understand state-of-the-art
simulations on the fastest parallel supercomputers available.
Their calculations have profoundly affected the direction of the
international magnetic confinement fusion program over the last
decade. The present focus of their research is on conclusively
identifying theoretically predicted signatures of the turbulence
in experiments around the world, and on helping design new experimental
configurations which should be much less affected by turbulence.
__________________________________________
Dr. Dorland is an assistant professor
in the University of Maryland Department of Physics working with
plasma physics. If you have any questions for Dr. Dorland, you
may contact him at bdorland@physics.umd.edu. |
