New boundaries: experiments verify ion behavior in plasmas
fluorescence setup in Noah Hershkowitz's lab
ince 1949, scientists have assumed that ions in a plasma must achieve
a velocity known as the ion-sound, or Bohm, velocity before they flow
out of the plasma. Back then, academic and theoretical physicist David
Bohm showed that the ions must drift at a certain velocity based on
electron temperature and ion mass—even if the ions have a zero
temperature in the bulk plasma.
But what scientists didn’t know is how
the ions sped up. Bohm suggested it was the result of weak electric
fields that formed in the plasma away from the thin boundary, or sheath,
at the plasma’s edge. Normally, that sheath is positively charged,
while the rest of the plasma is electrically neutral.
In the theoretical literature describing the
phenomenon, says Professor Noah
Hershkowitz, the region in which ions
are accelerated is often called the presheath.
Although determining the details of the plasma
potential profile near boundaries is one issue critical to basic understanding
of confined plasmas, until recently, no one had experimentally verified
Bohm’s theory and subsequent models of presheaths, says Hershkowitz.
“For a long time, I’ve been interested
in the question of if you have a plasma, and if you have various kinds
of geometries and various kinds of sources, how does the plasma potential
get from one place in the plasma to the other,” he says.
He and graduate student Lutfi Oksuz recently
concluded long-awaited experiments that verified K.U. Riemann’s
more recent theory about weakly collisional plasmas, in which ions collide
with the neutral species. As their primary diagnostic of plasma potential,
they used hot-wire emissive probes, which only emit electrons if they
are biased below the plasma potential.
The two determined that a weakly collisional
plasma’s potential profile consists of a presheath, where ions
accelerate, a transition region of the sheath in which most plasma electrons
are reflected, and an electron-free region of the sheath. The two published
their results in the Sept. 30, 2002, Physical Review Letters.
The evidence holds significance for a number
of applications, says Hershkowitz. In semiconductor processing, ions
falling through the sheath pick up a lot of velocity as they travel
through the sheath. Collisions in the presheath are the source of velocity
perpendicular to this direction. “And since the parallel velocity
is much larger than the perpendicular, you can etch vertical structures,”
he says. “So the presheath ion-scattering provides the ultimate
limitation on how vertical structures can be etched.”
In a fusion device, loss of plasma at a limiter
or diverter determines both the heating and particle recirculation at
that point and provides basic limits as to how much current researchers
can extract, what density level they can achieve, how long materials
last, and more. “The details of what happens near the sheath can
determine what happens to a satellite in space: what the electric fields
near the satellite look like, when particles are emitted by photoemission
or by interaction with the surroundings, how that plasma is lost and
so on,” he says. “Understanding what’s going on is
an important thing to do.”
Having verified the ions’ speed at the
sheath and determined the plasma’s potential profile, Visiting
Professor Greg Severn, Hershkowitz and graduate students Eunsuk Ko and
Xu Wang conducted a second experiment with more than one ion species
in the plasma.
In that case, researchers assumed each ion species
would fall out at its own Bohm velocity, he says. Experimenting with
a plasma made up of equal amounts of argon and helium ions, the group
learned that the heavier argon ions fell out faster at the boundary
than they would have if they’d been the only ion species in the
The group, which published the results in the
April 11, 2003, issue of the Physical Review Letters, used laser-induced
fluorescence (LIF) with a diode laser to scan the ion velocity and determine
the ion drift. Hershkowitz hopes to begin a second phase of the experiment
in which he and his students will add another ion species such as xenon
and use LIF to determine the velocities of both species.
Hershkowitz says both experiments dispelled
long-standing assumptions that weren’t true. “It’s
a question that we felt should be answered and 50 years is a long time
to not know that something is really correct or not correct,”