FOCUS
PLASMA
The ISS International
The Tamers of
Cold Chaos
Space Station hosts only
a select number of scientific
experiments – like those
and his staff at the MAX
PLANCK INSTITUTE
FOR
EXTRATERRESTRIAL
PHYSICS.
These physicists
whip cold plasmas consisting of charged microparticles
into line in order to study
their crystallization,
turbulence or flow properties through a nozzle.
The results of these studies
are relevant for applications in medicine and
the microchip industry.
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n a certain sense, a plasma crystal
is a legacy. When Gregor Morfill
launches into his story, his expression becomes deeply serious. “It all
really began on November 1, 1991,
although I didn’t realize it at the
time,” explains Morfill, Director at
the Max Planck Institute for Extraterrestrial Physics: “It was the day
my friend and colleague Christoph
Goertz was shot dead by a student in
Iowa.” Some time previously, Morfill
had discussed with Goertz the possibility of “taming” a plasma consisting of micrometer-sized plastic
spheres – physicists call it a complex
plasma – to become a crystal.
Today Gregor Morfill and his staff
use such plasmas to study how solids
melt, how liquids crystallize, how
they flow past an obstruction, creating turbulences in the process, and
how two liquids flow into each other.
“Until just a few years ago, the details of these processes at the individual particle level were still very
much a mystery,” says Morfill:
“Complex plasmas enable us to observe the individual particles and
therefore to investigate these phenomena for the first time at the most
elementary level.” These findings
may also have practical uses, such as
preventing turbulence in aerodynamics. In addition, the physicists in
Garching exploit their technology
for medical applications (see box,
page 41) and for improved efficiency
in microchip production.
However, complex plasmas and
plasma crystals not only serve as an
excellent model for fundamental
physical processes and offer the
promise of various technical applications; the new physics they present
also holds a fascination for physi-
2/2008
cists. This is due not least to the inherent contradiction in terms of
“plasma crystals.” In a plasma, matter appears in its least ordered state
– at least that was the conventional
wisdom. In a gas, the smallest particles – in the simplest case, atoms –
flit around independently and aimlessly. In a plasma, the particles have
also lost at least some of their electrons, causing positively charged
ions and electrons to swirl about in
confusion.
While matter can be broken down
into this form by a high voltage,
generally speaking, extremely high
temperatures are required, such as
those that prevail in the cores of
stars. It is inconceivable that a crystal could be grown in such an infernal heat. In crystals, matter is arranged with great precision; high
temperatures completely disrupt this
order. On the face of it, then, a crystal and a plasma would appear to
contradict each other.
FORCES AT PLAY IN
PLASMA CHAMBER
THE
Gregor Morfill and Christoph Goertz,
however, had an idea how they might
reconcile the two states: plasma particles ought to line up in a crystal
lattice even at room temperature
when the electrostatic forces between
the charged particles are greater than
the thermal energy of the particles
that swirl them about. “After Christoph Goertz’ death, at some point I
resolved to conduct experiments of
my own on the subject, despite the
fact that I am actually a theoretical
physicist,” says Morfill.
In principle, there should be two
ways of creating order in a plasma.
The first would be to cool it down to
P HOTO : A XEL G RIESCH
I
of GREGOR E. MORFILL
almost absolute zero (see box, page
46/47). The second is for it to consist
of large, cumbersome particles, which
do not dart about wildly like atoms
or molecules, even at room temperature. At the same time, these particles would have to be very highly
charged in order for them to interact
intensely among themselves – even
at moderate temperatures – and thus
inhibit their normally random motion. The researchers’ plan was to
create these conditions by means of
microparticles in a plasma.
“When it came to implementing
these ideas, I had a stroke of luck,”
says Morfill: “A colleague from the
DLR gave me an old vacuum chamber
that he no longer needed. And in Hubertus Thomas, I found a very talent-
A cage for a complex plasma: In a vacuum chamber, the
Garching-based physicists create cold plasmas from
microparticles and study them with the aid of laser beams.
ed doctoral student who was to set up
the experiment in Cologne.” Thomas,
however, began by completely unsettling his boss, calling Morfill from
Cologne to tell him that he must have
miscalculated. However, as Morfill
was taking another look at his equations, a form of mathematical feasi-
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Hubertus Thomas aligns the laser precisely: The beam, fanned out to a disk,
lights up the microparticles in the two-dimensional complex plasma.
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FOR
P HOTOS : A XEL G RIESCH (2)
With such a tremendous charge, it is
not surprising that the microparticles
exert enormous electrostatic forces.
These forces are so great that they
are able to create order even within
the chaos of a plasma – but only if
the researchers keep them under control. Were they not to do so, the microparticles would simply fly apart,
since the plastic spheres are all negatively charged. Fortunately, the noble-gas plasma in which these microparticles are immersed generates a
potential well. The negative particles
are trapped inside this well, since the
MPI
TRAPPED INSIDE THE
POTENTIAL WELL
by six others. The researchers have
thus created maximum order within
a state of maximum disorder of matter. At the same time, they have also
opened up a vast new research field.
On the one hand is the new form of
crystal itself, and on the other hand,
the things it can be used to study,
such as the processes in which matter
solidifies or a crystal melts.
Freezing water or melting ice are
two of the most common everyday
physical phenomena. At the level of
individual particles, however, physicists still do not understand them,
because they are unable to watch the
water molecules during the melting
process. In the complex plasma,
however, this is possible. The physicists in Garching simply “light up”
the plastic particles with the light
from a fanned-out laser beam. The
twinkling particles can then even be
seen with the naked eye. For the sci-
FIG .:
ions, they strike the particles at a
much greater rate. Each microsphere
thus attains a powerful negative
charge, up to 10,000-fold. By comparison, the negative ions in cooking
salt have just one negative charge.
E XTRATERRESTRIAL P HYSICS
The particles initially whiz chaotically in all directions (left) before solidifying in crystal
form (right). In the liquid state (center), each particle generally already has six neighbors.
P HOTO : A XEL G RIESCH /
bility study for the plasma crystal, the
phone rang again. “We have the plasma crystal,” announced an exuberant
Hubertus Thomas. “Since then, I have
called him Doubting Thomas,” laughs
Gregor Morfill.
Taking up his boss’s ideas, Thomas
had first ionized argon gas in a vacuum chamber between two electrode
plates, one positioned an inch above
the other. This produced an ordinary
argon plasma, the kind that lights up
electric discharge tubes, sometimes incorrectly referred to as neon tubes. In
this argon plasma, Thomas scattered a
few micrograms of plastic spheres with
a diameter of around seven micrometers, reminiscent of fine dust.
As the resulting plasma contains
not only argon ions and electrons,
but also charged microparticles, physicists refer to it as a complex plasma.
In such a complex plasma, electrons
and argon ions adhere to the microparticles. Since the electrons are
substantially more mobile than the
positively charged noble-gas plasma
cancels out the negative charges of
the particles and holds them together
like putty. The crystallization of the
charged particles in this potential
well, however, is attributable solely
to the repulsing forces they exert
upon each other. These forces prevent
the particles from approaching each
other too closely, and drastically constrain their freedom of movement
within the potential well. As a result,
the particles remain stationary.
“With no intervention of any kind
on our part, the microparticles arrange themselves at intervals of approximately a quarter of a millimeter,” says Hubertus Thomas. Almost
every particle is regularly surrounded
entific studies, movies with CCD
cameras are recorded and analyzed
by computer. This allows the complicated process to be visualized and
viewed in comfort on a monitor.
Christina Knapek, a doctoral student in Morfill’s department, observes on a monitor how a two-dimensional liquid plasma crystallizes.
A special software program provides
her with a clearer picture of the disorder and the defects in the liquid
plasma. It places blue dots at points
where particles are surrounded by
seven, rather than the usual six
neighbors. A heptagonal defect of
this kind is almost always situated
adjacent to a pentagonal defect in
the crystal lattice, which
the program marks in red.
All other particles are
shown as arrows. The arrows show how the hexagons in which the neighbors of each particle are
grouped are oriented with
respect to each other. In a
perfect crystal, all arrows
point in the same direction, with neither red nor
blue dots among them.
When it displays a liquid plasma,
however, Knapek’s monitor is teeming with red and blue blots, and the
arrows point every which way. This
soon changes when the plasma begins to solidify. The red and blue
dots rapidly disappear, soon remaining only in lines along the boundaries that separate regions in which the
hexagons differ in their orientation.
“Look how these boundaries suddenly disappear,” says Christina Knapek,
pointing to a red-blue string of beads
that disappears that very instant. At
the same time, the arrows that, just a
moment before, were on a collision
course with those on the other side
of the boundary change direction.
Until now, such details of crystallization were a mystery to physicists.
Complex plasmas and plasma crystals, however, are the perfect means
to study them. The new forms of
matter deliver equally useful findings on the subject of turbulence.
OPEN WOUNDS
UNDER THE
PLASMA TORCH
When wounds fail to heal, doctors are often at a loss. They are frequently unable to help diabetics with
open sores on their feet, care patients with bedsores, or older patients recovering from major operations. In many cases, infections prevent the wounds from closing. Antibiotics are increasingly ineffective, since the pathogens that cause the inflammation have developed immunity. In addition, antibiotics
frequently have unpleasant side effects, and ointments must be applied to the wounds, which is painful
for the patient.
Plasmas might be a remedy. Their antibiotic action has long been known. Plasmas are already in use to
sterilize medical instruments. However, these plasmas are too hot to be used in treating patients. “Since
our plasmas are cold, I had the idea of using them for medical applications,” says Gregor Morfill. In the
past, however, the scientists at the Max Planck Institute for Extraterrestrial Physics had experimented
with their plasmas only at one-thousandth of atmospheric pressure – too low for them to be used to
treat wounds. Now, however, Bernd Steffes, one of the engineers in Morfill’s department, has developed a
plasma torch that operates at standard pressure and at around 30 degrees Celsius. In an opening the size
of a coin, the apparatus produces a cold plasma “flame” that is most effective at around two centimeters
below the opening.
The scientists have now established that the plasma flame kills various bacteria and fungi very effectively,
while at the same time being gentle on human tissue. At present, they are testing the apparatus in a
Phase II study on 105 patients, and have already brought great relief to some.
The view through the mirror (left photo) shows the bluish-glowing plasma in the torch. In the background, René Pompl (left) and Bernd Steffes discuss technical improvements. In Munich’s Schwabing
Hospital, Georg Isbary is already successfully using the method to treat patients (right photo).
“We still do not know exactly how the plasma works, and why it only acts on bacteria,” says René Pompl,
in charge of the work on plasma medicine. The plasma produces UV radiation and certain aggressive substances, such as ozone and hydrogen peroxide, but in both cases in doses too low to explain its antibiotic
effect. “It may be a charging effect,” says Pompl. It could be that mutually repellent electrical charges
build up on the cell wall of the bacteria, thereby tearing the cell wall apart. This is certainly suggested by
atomic force microscopic images of bacteria that were exposed to the plasma of the Garching torch for
three minutes. “The inner part of the cell appears to have leaked out,” says Pompl. The scientists have
even observed the same phenomenon on gram-positive bacteria, the cell wall of which is relatively robust. Since the walls of human cells are even more stable, this mechanism may explain why they do not
suffer harm in the plasma.
Physicists still have only a partial
understanding of how turbulence
arises in gases and liquids. Precisely
this issue, however, is of particular
interest in aerodynamics, hydrodynamics and, above all, nanofluidics.
Nanofluidics describes the behavior
of the minute quantities of fluid that
are used, for example, by chemical
laboratories on microchips the size
of a fingernail. In some applications,
nanodrops flow through ultra-fine
tubes, and it is found that their be-
havior differs fundamentally from
that of, say, water in a garden hose.
In order to explain this, Gregor
Morfill and his staff have studied, for
example, the turbulence in a stream
of liquid plasma, which in many
ways resembles water. They placed
an oval obstruction in the path of
the plasma flow, thus dividing it. The
area behind the obstruction filled
with liquid plasma and appeared as
motionless as the wake of a sailing
ship. The researchers focused on the
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PLASMA
boundaries of the two arms of the
divided plasma flow that flowed past
the calm wake of the obstruction. Directly behind the obstruction, the
particle stream still slipped smoothly
past the stationary particles. Gradually, however, the edge of the particle stream frayed, before finally
breaking up in larger eddies.
An experiment for space:
The high-speed camera (right)
captures up to 1,000 images
per second of the reddish-glowing plasma. The experimental
setup shown is in Garching, but it
is similar to the one on the ISS.
“In that case, we would have discovered something fundamentally new.”
While still searching for these laws
governing streams of individual particles, Morfill and his staff are investigating a closely related problem:
how small must the number of particles in a liquid actually be before
the liquid ceases to obey the laws of
hydrodynamics? In other words, at
what point are the phenomena in a
liquid no longer determined by the
cooperative properties (of the system), as physicists call them, but by
the properties of the individual particles? This question interests physicists in numerous areas, particularly
since they have begun experimenting with increasingly small systems
in nanotechnology.
At the Max Planck Institute for Extraterrestrial Physics, Martin Fink is
addressing this problem by experimenting with a special complex plas-
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FOR
MPI
At the boundaries of order: The images show a plasma crystal (a), liquefied (b) by the
physicists in Garching, causing the order to be lost. The blue and red dots indicate heptagonal and pentagonal defects. The arrows show the distortions of the lattice. As the order is restored, the areas of differing orientation are realigned, and the defects disappear.
2/2008
P HOTO : A XEL G RIESCH
FIG .:
P HOTO : A XEL G RIESCH /
In their experiments, the scientists
made a surprising discovery. Three
mechanisms lead to the turbulence –
in equal measure: impacts between
two particles; individual particles
that penetrate deep into the static
plasma liquid; and finally, lumps or
eddies that become detached from
the edge of the flow. “We do not yet
know whether the individual particles are obeying the same laws here
as macroscopic liquids,” says Gregor
Morfill. Physicists regard macroscopic liquid streams, such as water in a
garden hose, as a “continuum,” and
describe them very successfully by
means of the laws of hydrodynamics. “I would be very surprised if
these laws could be extrapolated to
the level of individual particles,”
says Morfill. “If this were the case,
they would apply from a few nanometers to light-years.” Conversely,
he would find it no less exciting if
such extrapolation of the laws to the
smallest systems were not possible:
E XTRATERRESTRIAL P HYSICS
UNITY IN THE
GARDEN HOSE
ma jet. Astrophysicists generally use
this term “jet” to refer to a plasma
beam that is ejected many light-years
into space by a black hole. The plasma jets of the researchers in Garching are very modest in comparison:
they pass their plasma through a
constriction in a glass U-tube that
would fit comfortably on an ordinary
bookshelf. They begin with a stream
of single particles, then gradually allow the stream to swell. Single particles pass through the nozzle with no
noticeable change in their speed. As
soon as too many particles jockey for
position ahead of the nozzle, however, the stream becomes denser, and
on passing through the nozzle, attains a velocity that, in an airstream,
would be supersonic. This effect is
caused by the cooperative behavior
of the particles. “Our preliminary results suggest that as few as ten parallel streams of discrete particles
already exhibit this cooperative behavior,” says Martin Fink.
While Fink and his colleagues continue their hunt for the critical number, they will also seek explanations
for phenomena that they have observed for the first time in their experiments, such as non-linear waves
that form in the particle stream when
it is compressed in front of the nozzle. In practice, this research is difficult (and, in fact, in the laboratories
in Garching, virtually impossible) because gravity disrupts the physicists’
The idea of growing crystals from charged microparticles was developed by theoretical physicist Gregor Morfill. Today, these complex plasmas serve as models for numerous phenomena.
experiments. It exerts a substantial
force on the relatively heavy plastic
particles, and eclipses the more subtle
dynamic effects, not only in the plasma jets, but in the majority of their
experiments. What the researchers
need is weightlessness. For this reason, the tests on plasma jets have
been performed to date on parabolic
flights of an Airbus. In a steep dive,
the Airbus simulates free fall for
about 20 seconds. Tests in space with
permanent weightlessness, for example on the ISS International Space
Station, would be even better, of
course. But many scientists would
like to conduct their experiments
there, and accommodation in orbit is
at a premium – and it is priced accordingly. Also, plans for scientific
payloads on the ISS had long since
been made, and the first batch of experiments already selected. Knowing
this, the researchers in Garching did
not even attempt to propose their experiments being accommodated on
the ISS in the 1990s.
“But we had a stroke of luck,” says
Morfill. Shortly after his first publication on the subject of the plasma
crystal, the scientific journal NATURE
published an article by John Maddox, its editor at the time. Maddox
voiced his opinion that these experiments deserved a place on a space
shuttle more than most others. Short-
ly afterward, Morfill received a call
from DARA, the German Agency for
Space Affairs, asking him whether
he would like to submit an application for experiments under conditions of weightlessness, which he
then did. Two years later, the researchers were experimenting for the
first time on parabolic flights and in
a research rocket.
PLASMA JET
GLASS TUBE
IN A
A Russian research team learned of
the first successful experiments and
made contact with the MPE. The Russian Federal Space Agency, and in
particular Vladimir Fortov, a scientist
from the Institute of High Energy
Density in Moscow and, at the time,
Minister of Science and Technology
under President Yeltsin, supported
the project. As a result, experiments
by the physicists in Garching have
been conducted on the ISS regularly
since 2001. The current ISS “plasma
crystal laboratory” has a plasma
chamber the size of a shoebox. In
2010, a new laboratory (the third in
the series) is planned that will conduct research into the liquid state of
complex plasmas.
The team in Garching was among
the very first to have experiments
conducted on the ISS: the black metal container, not even hip-high, in
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1010
108
Deep-frozen
Plasma
Temperature [K]
JAN MICHAEL ROST
and his staff at the
MAX PLANCK INSTITUTE
FOR THE
PHYSICS
OF
COMPLEX SYSTEMS
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OF
P HYSICS
FOR THE
MPI
BOTTOM :
E XTRATERRESTRIAL P HYSICS / F IG .,
FOR
MPI
TOP :
F IG .,
P HYSICS (2)
FOR EXTRATERRESTRIAL
which they packed their plasma
chamber, the electrodes, lasers and
gas bottles and the rest of their
equipment, was transported to the
ISS on a Russian Progress rocket.
They were thus not affected by the
delays that hampered the launch of
the European Columbus laboratory
following the Columbia
Space Shuttle disaster. Experiments with complex
plasmas and plasma crystals
are all the more popular with
the cosmonauts and astronauts: in an interview with
the German magazine DER
SPIEGEL, Thomas Reiter emphasized how much they had
impressed him. And one of
the Russian cosmonauts even
preferred experimenting with
The manner in which turbulence is produced (small
the complex plasmas to re- image, top left) can be studied when a complex
laxing in front of the planned plasma flows around an obstruction. The laminar flow
then progressively frays out (image, bottom left).
recreation video.
P HOTO : A XEL G RIESCH / F IG .: MPI
Through the nozzle: At the constriction, the particle stream becomes denser and is accelerated.
The scientists, however, rapidly
progressed from basic processes such
as the crystallization of a complex
plasma to more exotic phenomena,
such as electrorheological liquids, or
ER liquids, as the physicists call
them. ER liquids solidify in a strong
electric field. Their uses include hydraulics and shock absorbers. They
consist of a neutral liquid and electrically active particles. The electrical
charges of these particles can be distorted in an electric field to form dipoles. The dipoles, in turn, arrange
themselves in the form of a rigid
network in which a negative pole is
attached to each positive pole.
To find out more about the magic
forces in ER liquids, the scientists in
Garching emulate them with their
By the time Jan Michael Rost, at the age
of 37, had assumed the job of Director
at the Max Planck Institute for the
Physics of Complex Systems in Dresden,
he had already accomplished an unusual
achievement: he had described what
happens when a helium atom loses its
two electrons. In and of itself, this is
nothing special, since helium is the second simplest atom in nature and has
thus been studied in detail. Rost, however, had succeeded in describing the
processes concerned in the same way as
a phase transition, for example from
solid to liquid, using the laws of classical
physics – in other words, without quantum mechanics, which physicists generally use to describe such processes. He
has remained true to this working method in his current research activities –
and with great success. Today, however,
Rost’s attention is directed, not at the
processes within a single atom, but at
the behavior of a large number of ionized atoms. These form – befitting of
the institute’s name – a truly complex
system.
“Complex systems lie on the boundary
between regularity and chaos,” is how
Rost paraphrases the subject of his research. They often behave unexpectedly,
on the one hand exhibiting a non-linear
behavior that is difficult to predict, and
on the other, being capable of organizing
themselves.
Plasmas constitute complex systems of
this kind. In traditional disciplines such as
astrophysics or fusion research, the typical behavior of a gas predominates; selforganization does not occur. And yet,
calculations performed by Rost and his
colleague Thomas Pohl predict that precisely this ought to occur in ultracold
plasmas.
“The theory is based on competition between two forces, both acting between
C OMPLEX S YSTEMS – T HOMAS P OHL
study plasmas at close
to absolute zero –
also in crystal form.
The physicists in Garching are now passing streams of complex plasmas through a
nozzle in a glass U-tube, as presented by Martin Fink, in front of the test apparatus.
the plasma particles,” explains
study processes in the interior of
Rost. A plasma is normally so hot 106
atoms at time resolutions of a trilthat the atoms have lost one or
lionth of a second and less, this is
more of their electrons, thus bea very long time,” says Rost. In
coming positively charged ions.
fact, the researchers photograph
104
That is why the electrical
the plasma with an ordinary, albeit
Coulomb force acts between
very fast, CCD camera, and are
them. At the same time, owthus able to track its development.
102
ing to the high temperature,
Particularly fascinating for Rost is
the ions move at high velocity.
the fact that this technology proIn contrast to plasmas consistvides ready access to plasma phe100
ing of microparticles, the kinomena at the atomic level. “It
netic energy in normal plasmas
could be said that we are studying
consisting of atomic ions is much
plasmas in slow motion, under a
-2
10
greater than the Coulomb energy,
magnifying glass,” says Rost. The
even at room temperature. The
results can be transferred to other
ions thus flit about in a disorplasmas, which is one of the main
10-4
derly manner. If the temperature
reasons for the research.
of the plasma is now lowered, the
The experimenters in France and
kinetic energy is reduced prothe US have already cooled the
-6
10
gressively until, at some point, it
plasma down so much that it bereaches the level of the Coulomb
gins to adopt behavior similar to
100 103 106 109 1012 1015 1018 1021 1024 1027
energy.
that of a liquid. Should they suc-3
Density [cm ]
The electrical forces now gain the
ceed in lowering the
Plasmas exist at very different temperatures and densities: The
upper hand. They ensure order, in
temperature even fursolar system is several million degrees hot, and extremely dense. Ultrathe first instance between neighboring
ther, the ions will ations, as is typical for liquids. According cold plasmas exist barely above absolute zero, and are very sparse.
tach themselves to
to the calculations performed by Rost
nested spherical shells.
and his colleagues, a plasma crystal
This, at least, is what Rost’s calcushould be formed as soon as the Coulations predict. The physicists inlomb energy reaches a value of pretend to ascertain whether this
cisely 174 times the kinetic energy.
does in fact occur by X-raying the
Plasmas can be cooled to this level by
plasma cloud. The light should
a combination of laser cooling and
then generate an annular pattern
magneto-optical traps, which are emin a camera.
ployed in many areas of low-temPlasma crystals are not toys; they
perature physics to freeze atoms and
enable classical theories of the bemolecules. The experiments, the recihavior of gases under extreme
pe for which is supplied by theorist
conditions to be examined. The
Rost, are being conducted at the Univibration behavior of plasmas, for
versity of Paris in Orsay, and at Rice
example, observed in many other
University in Texas and the University
experiments, can be studied on the
of Maryland.
atomic scale – a dream of many
In a magneto-optical trap, a gas conphysicists.
sisting of atoms is first captured and
In addition, Rost’s results have atcooled in a number of stages. Among
tracted the interest of researchers
the phenomena exploited by physiat the CERN European Laboratory
cists for this purpose is that, when
for Particle Physics, whose aim is
atoms are excited by a laser beam
to produce antihydrogen from
Looking at a crystal ball: At very low temperatures, the ions of
and then emit a photon again, they
an ultracold plasma of positrons
a plasma solidify and arrange themselves on spherical shells.
experience a recoil. Should the recoil
and antiprotons. Some findings
act against their direction of motion, they
could also be exploited by research on the
bombardment, the plasma heats up abruptare braked and cooled down.
FLASH free-electron laser at DESY in Hamly to almost 1 Kelvin, still substantially
When the gas cloud, not even one cubic
burg. When it impacts upon matter, FLASH
colder than the temperature of a convenmillimeter in volume and suspended in
generates extremely short-lived plasmas
tional plasma. In the magneto-optical trap,
magnetic fields, has cooled to around a
that behave in the same way as ultracold
the temperature of the plasma also drops
millionth of a degree above absolute zero,
plasmas in slow motion.
again rapidly.
a laser pulse is shot into it. In this case,
Essentially, however, Jan Michael Rost and
The miniature plasma cloud survives only
rather than cooling the cloud further, it
his colleagues are conducting basic refor a very short time. It expands and drops
heats it up briefly within a small volume.
search. Perhaps, says Rost, his findings on
out of the magnetic trap, since the latter is
During the bombardment, some atoms lose
the dynamic behavior of the ions from
capable of trapping atoms, but not ions.
their electrons, which in turn shoot further
chaos to order could be transferred to
Nevertheless, the physicists have around a
electrons off the surrounding atoms. This
completely different areas altogether, such
millionth of a second in which to study the
electron avalanche creates a plasma in the
as social behavior. That, however, is a difultracold plasma cloud, which is about 0.2
heart of the ultracold gas. Under the laser
ferent story.
THOMAS BÜHRKE
millimeters in size. “For physicists who now
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Training for space: Alexej Ivlev and his colleagues tested the experimental
setup for the ISS comprehensively before packing it in a black metal drum.
On the ISS, lifting the plasma experiment
is merely a finger exercise for Sergej Krikalev.
Microparticles form a line as soon as the researchers apply a voltage of 50 V to the
complex plasma. In the process, they learn something about electrorheological liquids.
complex plasma – a project to which
Alexej Ivlev has also turned his attention. Together with Gregor Morfill
and a number of colleagues, Ivlev
drew up a test procedure that was
implemented by Thomas Reiter on
board the ISS. Between the two electrodes at the top and bottom of the
plasma chamber, the German astronaut prepared a “liquid plasma” – a
relatively thin noble-gas plasma and
microparticles that moved around in
the same way as liquid particles. Reiter then applied an alternating voltage to the electrodes.
VACUUM CLEANER FOR
PLASMA PROCESSES
On the monitor, which also formed
part of the experimental setup in orbit, he then observed how the microparticles neatly lined themselves
up in rows – that is, one-dimension48
MA
X
P
L ANCK
R
E SE ARCH
al solid bodies constituting a preliminary stage of solidified plasma.
What Reiter witnessed perfectly
matches the predictions of the Garching-based physicists: the negatively
charged microparticles are too heavy
and sluggish to follow the alternating field, the orientation of which
changes rapidly. The situation is different for the positive noble-gas ions:
they flow back and forth between the
electrodes, always following wherever the negative electrode happens to
be. The highly negatively charged
plastic particles, however, also influence this back-and-forth movement
of the ions: they act as a lens for the
stream of positive ions, and focus it
on a point downstream. “The alternating voltage now causes the focus
of the positive ions to jump continually from one side of the microparticles to the other,” explains Alexej Iv-
2/2008
lev. This means, however, that
positive charge centers can always be
found between adjacent microparticles. These charge centers link the
microparticles to form chains. In the
absence of an external voltage, the
ions would simply arrange themselves in a sphere around the negatively charged microparticles.
Now that the researchers have become veritable virtuosos in their
handling of the test object, they are
seeking to exploit the know-how for
technical purposes – in order to tackle complex plasmas, where they are
nothing but a nuisance. Specifically,
wherever industry works with plasmas, for instance in the etching of
conductors and transistors onto
chips, the manufacture of solar cells,
or the finishing of glass and textile
surfaces. During these processes, a
quantity of fine dust is created that
forms a complex plasma, the particles of which grow continually via
attachment of atoms or molecules (or
both). To date, the industry has not
been able to find an effective means
of preventing the unwanted grains
of dust from contaminating the surfaces, chips and other materials, rendering them useless. The result has
been major economic losses.
The plasma physicists in Garching
now aim to use an electric field to
wipe the dusty plasmas away. In the
P HOTOS : A XEL G RIESCH (2)
PLASMA
P HOTOS : A XEL G RIESCH ( LEFT ) / MPI FOR E XTRATERRESTRIAL P HYSICS ( RIGHT )
F IG .: MPI FOR E XTRATERRESTRIAL P HYSICS
FOCUS
Clear the stage for microparticles: In the future, the physicists
in Garching plan to direct the particles by means of the strip electrode.
future, this will be a task for Ke Jiang, a doctoral student. Ke Jiang has
just put a large new plasma chamber
into service. It is large enough that
an open copy of this MaxPlanckResearch magazine could be placed inside – with room to spare. The electrode of this plasma chamber consists
of a strip electrode – metal strips each
measuring the width of a finger. By
means of these strips, Ke Jiang can
direct microparticles within a cloud
of complex plasma, pass large waves
through the charged particle cloud,
or simply brush it off the plate.
Whether this plan will work, and
Yangfang Li is also involved in the
experiments involving the strip electrode.
whether it will be of actual use to the
industry, will be the subject of his research in the coming years. “This
project is still in the early stages,”
says Gregor Morfill. Should it bear
fruit, his research would certainly
help the industry save a great deal of
money.
PETER HERGERSBERG