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SECTION 3: SCIENTIFIC/TECHNICAL PROJECT DESCRIPTION
A. Introduction to dusty plasmas
A dusty plasma is a mixture of electrons, ions, and micron-size solid particles that are highly
charged [1-15]. It can also contain neutral gas [16-18]. Particles used by experimenters are
typically monodisperse polymer microspheres. These dust particles collect more electrons than
ions, causing them to become highly charged [19-24], with typically -104 e for a 5-µm sphere.
Dusty plasmas can sustain several kinds of waves, which have oscillatory motion that is
restored by collective electric forces. In particular, we distinguish two kinds of waves:
 The dust acoustic wave is the wave of interest for this proposal. It is a longitudinal wave
that is self-excited by flowing ions [25-43]. This wave is hydrodynamic in nature, and it
is sustained by a macroscopic electric field arising from charge separation of the dust as
compared to the electrons and ions. It does not intrinsically depend on the discreteness of
individual dust particles.
 The lattice wave can be either longitudinal or transverse [44-51]. This lattice wave arises
because of the microscopic electric interaction of individual dust particles, and it occurs
in the strong-coupling regime,   1. This lattice wave is not the subject of the present
proposal (although it is mentioned in a separate proposal by PI Bin Liu).
As an experimental system, a dusty plasma has several attractions for the study of waves.
Chief among these attractions is the ease of observing the wave using video imaging [52]. By
illuminating a dust particle cloud with a sheet of laser light, the experimenter can image a crosssection within the volume of the cloud. The collection of dust particles has very convenient
length scales (millimeters) and time scales (tens of milliseconds). Video imaging allows a
measurement of the particle number density, which is proportional to the light-scattering
brightness recorded in video images.
B. Objective of this project
We will perform experimental studies with nonlinear waves in dusty plasmas using the PK-4
instrument on the International Space Station (ISS).
We seek to answer fundamental physics questions in the following experiments. These
experiments have already been approved by the PK-4 Facility Science Team, and they have been
identified in the PK-4 Experiment Science Requirements (ESR) document as experiments
specifically for our Iowa group.
Main Experiment:
Synchronization of dust acoustic waves (DAWs):
1. Using sinusoidal manipulation:
Goal #1: Can we extend the physical understanding of oscillator synchronization, to the
case of propagating waves, using multi-wavelength experiments in dusty
plasmas?
Backup Experiment:
In addition to the Main Experiment above, we also have an alternative “Backup
Experiment.” The circumstances that would lead us to perform this Backup Experiment
include:



the PK-4 mission is extended.
our Main Experiment experiences a technical problem, or it has a negative scientific
outcome (its predicted phenomenon is not observed).
the phenomenon for the Backup Experiment is fortuitously observed early in the
mission, and we find that we can do the analysis quickly.
Dynamical phenomena in binary mixtures:
2. In a 3D dusty plasma with two sizes of dust particles, with a DC field:
Goal #2: Is the motion of dilute minority species particles governed by mobility?
C. Need for microgravity
(a)
(b)
1g
zero- g
Fig. 1 Sedimention due to gravity is avoided by microgravity conditions, enlarging the dust cloud, as shown in
these cross-sectional images of clouds of 3.4 m particles in the PKE instrument, under gravity (a) and
microgravity (b) conditions [G. E. Morfill et al., Proc. of 55th International Astronautical Congress, 2004.]. The
study of phenomena requiring finite distances, such as waves of multiple wavelengths or transport in 3D
systems, is enabled by this enlargement of the dust cloud.
This project requires microgravity because it is otherwise impossible to form a large 3D dust
cloud and observe wave propagation for a distance of many wavelengths. With gravity, a dust
cloud sediments to a few layers, Fig. 1(a) [53]. With microgravity, a much larger dust cloud can
be formed, as in Fig. 1(b) [53].
The dust acoustic wave studied here requires a 3D dust cloud. Under 1g conditions, 3D dust
clouds are very limited in size, as shown in Fig. 2(b), so that the dust acoustic wave propagates
only about two wavelengths before it reaches the boundary [39,43]. Microgravity conditions
enable experiments with a much longer 3D dust cloud, as in Fig. 2(a), which is an image from
the PK-4 Commissioning Experiment.
(a) 0 g ISS
5 mm
(b)
1g
5 mm
Fig. 2. Size comparison of the 3D dust cloud in the PK-4
flight experiment (a), and ground-based experiments (b).
The PK-4 data are from the commissioning experiment
aboard ISS. (The image in (a) is split because the dust
column is so large that it requires two cameras.) The groundbased data are from our lab [39]. Both (a) and (b) show the
presence of a dust acoustic wave in the dust clouds.
Microgravity enables experiments with large number of
wavelengths.
Besides avoiding sedimentation, microgravity also enables the observation of the phenomena
driven by some weaker forces that are obscured by gravity. Under 1 g conditions, ion drag is an
order of magnitude smaller than the gravitational force. Under 0 g conditions, effects of the ion
drag force are easily seen, as they balance the electric force QE. The strength of this ion drag
force is not in proportion to the electric force, so that a force imbalance can act on one size of
particles, but not another [55-60]. Our optional mobility experiment will exploit this imbalance
by making a minority-size particle to drift through the center of a dust cloud, allowing a
detection of mobility-limited motion.
D. Experiment approach and PK-4 features used in this project
Our experiment requires a dust cloud that is three dimensional, can be imaged, and can be
manipulated. The PK-4 instrument meets all of these needs.
The PK-4 instrument [54, 61-72] is a multi-user facility provided by the European Space
Agency. It has been on-orbit in the International Space Station (ISS) since October 2014. Our
sketch of the PK-4 instrument, Fig. 3, shows its major scientific features. A U-shaped glass tube
is evacuated and filled at low pressure with neon (or argon) gas. This gas is partially ionized by
applying a high voltage to a pair of electrodes, called the active and passive electrodes. This
voltage can be either DC or a polarity-switched DC that alternates the direction of the electric
field. Dust particles are introduced by agitating dispensers, which are filled with several sizes of
polymer microspheres. When these microspheres (which we term dust particles) enter the
plasma, they rapidly gain a large negative electric charge. Due to this charge, these dust particles
are electrically trapped in the plasma. The dust particles do not touch one another, but maintain
an interparticle distance of about 0.2 to 0.5 mm.
TM
thermal manipulator
EM
EM electrode
RF1
RF2
movable RF coil
fixed RF coil
D1~D3 gas jet particle dispenser
D4~D6 shake particle dispenser
HV
high voltage with
polarity switching
Fig. 3 PK-4 flight model on the ISS. The U-shaped glass tube plasma chamber is powered via a pair of DC
electrodes mounted on the ends of two glass legs. Dust particles can be trapped in the straight portion between
the two legs, where the “positive column” of the gas-discharge plasma is located.
The main diagnostic on PK-4 is a pair of video cameras, which image the dust cloud. These
“particle observation cameras” are mounted on a movable optical stage, along with optics that
generates a sheet of laser light to illuminate a cross section within the dust cloud. There are two
cameras, and not just one, because the dust cloud is so long.
Manipulation devices on PK-4 include several features that we may use for our
synchronization experiment. These include function generators that can modulate voltages
applied to the DC electrodes, a third electrode termed the “EM electrode,” or a pair of radiofrequency (RF) coils. These function generators can be modulated at frequencies that are
convenient for our synchronization experiment.
The power supplies for operating the PK-4 plasma allow a choice of DC or polarity
switching. When operating with a DC high voltage, the dust cloud will tend to drift to one end of
the tube, where it will be lost to the vacuum port; this loss of the dust cloud can be stopped by
“trapping” the cloud. At the time we wrote this proposal, Core-Team members of the PK-4
Science Team were performing commissioning experiments to identify the best operating
conditions for trapping, for example by applying power to the RF coil to plug the dust cloud.
Operating with a DC high voltage is useful for certain purposes, including the strong selfexcitation of dust acoustic waves, which takes place due to a DC ion current driven through the
plasma. If we operate with polarity switching, we will use a duty cycle different from 50%, so
that there remains a net DC ion current that can excite the dust acoustic wave.
The feasibility of our Main Experiment, to study synchronization, is very promising:
 It is already demonstrated that PK-4 readily exhibits the dust-acoustic wave, when the gas
pressure is low enough. It has been observed many wavelengths, as is required for our
wave synchronization experiment. (See Fig. 2(a), where we show an image from
Commissioning Experiment of PK-4 on board the ISS.)
 The required ion flow, which is needed for self-excitation of the wave, is easily attained
in PK-4 using well-established operating conditions, with a DC current of typically 1
mA.


We have already performed extensive ground-based experiments to establish that the
phenomenon of synchronization can be studied using the proposed experimental protocol
and analysis methods.
The experimental parameters required in our task will be well within PK-4 capability.
Typical values include: a gas pressure of less than 50 Pa Neon to allow wave excitation, 1
mA current, DC operation or operation with < 50% polarity switching duty cycle, camera
operation at < 100 frames/sec, and formation of large clouds with particles of one size (or
two sizes, in the case of our mobility Backup Experiment).
PK-4 features used in this project
Imaging. Using the particle observation cameras with the illumination laser sheet, we can
observe a cross-sectional slab of a 3D particle cloud, Fig. 4(a).
For the purpose of our Main Experiment on wave synchronization, we will not require
particle identification, but will instead measure particle number density. We do not require
absolutely calibrated number density for most purposes, but instead only a relative number
density, which we will measure using the brightness within an image, the same method we have
demonstrated in our ground-based experiments with dust acoustic waves [35,37,39,43].
Nevertheless, if we do wish to report the absolute number density, we can do this using our
recently developed analysis method of extracting the 3D number density from 2D images; we
published this method in 2015 in Review of Scientific Instruments as part of our preparation for
PK-4 [73].
(a)
(b)
Fig. 4 (a) PK-4 image from Science Campaign 1, showing a cross-sectional image of the 3D dust cloud. (b)
At Iowa, our analysis of this image yielded these identifications of particles and their x-y coordinates; the
circles represent positions in one video frame, and the dots in the next consecutive frame. Velocities can be
determined by subtracting these consecutive particle positions. A particle “thread” is lost when a particle is
not found in the next frame due to motion in the 3 rd coordinate.
For our alternative Backup Experiment, we will use our PTV method to identify particles and
measure their velocities. We have already performed a feasibility test, using PK-4 flight data, to
verify that our PTV method will work with PK-4 images; these preliminary test results are
shown in Fig. 4(b).
Particles. Our Main Experiment requires just one size of particles. The particles will be
injected using the standard PK-4 operating methods with a shaker dispenser. Our Backup
Experiment, however, requires particles of two different sizes; if we perform that Backup
Experiment we will first fill the particle cloud with one size, using a shaker dispenser, and then
slowly introduce a few “minority” particles of a different size, which will migrate to the central
axis of the plasma. These minority-size particles will be introduced by a gas jet dispenser, which
was found in the first commissioning experiment to be capable of introducing small numbers of
particles, according to our conversation with a Russian Core-Team member.
Electrical manipulation. In our wave synchronization experiment, we will manipulate our
dust particles electrically to excite waves. We will test three ways of modulating the plasma
density or ion current:
 DC discharge current,
 EM electrode current,
 RF coil power.
In all three ways, we will apply the modulation using a function generator, controlled by the
experimental script.
The way that these manipulation devices affect the wave, to cause synchronization, is
generally global, and not local:
 The DC and EM electrodes are very similar to the manipulation electrodes we have
successfully tested in our ground-based experiments [39,43]. They are in effect like
large Langmuir probes, which can extract an ion current or electron current from the
plasma. This current extraction can be so large as to meaningfully modulate the
plasma density. This density modulation, in turn, has several effects: it can cause a
variation in the dust charge due to depletion effects, and it can cause a disturbance in
the large-scale electric ambipolar electric fields. The dust cloud is sensitive to all
these effects. By sinusoidally modulating such a manipulation electrode, we have
verified that we can cause synchronization of the dust acoustic wave, in our groundbased tests [39,43].
 The RF coil will have the opposite effect of introducing additional plasma density.
Increasing the current through the coil will cause an enhanced location ionization, and
an enhanced local plasma density. The ambipolar electric field will be altered by this
enhanced ionization, so that ions are pushed away from the RF coil region. The
frequency of the RF coil is high, but it can be modulated at a low frequency,
comparable to the frequency of the dust acoustic wave.
All three manipulation devices can be modulated sinusoidally, using the PK-4 function
generators, at an adjustable frequency and amplitude, as required for the synchronization
experiment proposed here.
E. Our previous work on the PK-4 project
Our group has focused for several years on preparing for the PK-4 mission. PI John Goree
has participated in many meetings of the PK-4 Facility Science Team since 2008, as well as
workshops at CNES for planning the payload operations. Dr. Liu has performed computer
simulations since 2012 to test concepts for our PK-4 experiments, leading to six refereed
publications specifically for planning for PK-4. These publication topics include image analysis
methods [73], waves and shear-induced motion experiments [74,75], and mobility experiments
[58-60]. In these simulations, Liu used PK-4 parameters for charge, gas pressure, and particle
size.
Liu and Goree have also built the Iowa
ground-based model of PK-4, Fig. 5; this
model exactly copies the plasma tube for PK-4,
and it uses the same model of cameras for
particle observation.
Our laboratory also has other breadboard
plasma chambers, which we have used to
develop the experimental plans proposed here.
These tests include demonstrations of wave
synchronization [39,43]. The analysis methods
developed for those papers will serve us as we Fig. 5 Iowa ground-based model of PK-4.
analyze PK-4 flight data for 3D dusty plasmas.
F. Fundamental physics questions we seek to answer in our experiment
1. Main Experiment: Synchronization of dust acoustic waves (DAWs)
Goal #1: Can we extend the physical understanding of oscillator synchronization, to the case of
propagating waves, using multi-wavelength experiments in dusty plasmas?
Synchronization is a cooperative phenomenon widely observed in nature, from biological,
chemical and physical to social systems [76,77]. A signature of synchronization is a shift in the
frequency of an oscillator or wave. This frequency shift occurs due to a combination of
nonlinearity and external modulation. Oscillators were the original topic for studying
synchronization, starting with Huygen’s pendulum clocks in 1665 [76]. Other examples of
synchronized oscillators include heart pacemakers [78,79], chaotic laser array [80,81], and
interacting vortices within a superconductor [82,83]. The understanding of oscillator
synchronization has matured, with well developed theories such as the van der Pol oscillator
model [84]. Wave synchronization, on the other hand, is not a mature topic; it is not well
understood; it lacks suitable quantitative models. Waves are more complicated than point-like
oscillators because they oscillate not only in time but also in space. Waves are also complicated
by the effects of dispersion, i.e., the dependence of the wave’s speed on its frequency. Oscillators
have no effect comparable to dispersion, because they have no propagation speed.
Plasmas of all kinds support a wide variety of waves [85,86]. At large amplitude, these waves
become nonlinear, and exhibit phenomena such as synchronization [87-96]. Dusty plasmas are
particularly well suited for a study of wave synchronization because they meet four essential
requirements:
 a self-excited wave,
 nonlinearity,
 ease of modulation,
 diagnostics for wave motion.
The dust-acoustic wave (DAW), which is a lowfrequency compressional wave, somewhat like a
sound wave, is self-excited by an ion flow [25-43]. Its
frequency and wavelength are typically about 10 Hz
and 3 mm. The number density varies in this wave,
and can reach a large amplitude, as can be seen in
Fig. 6. Modulation of the dusty plasma is easily
accomplished by applying sinusoidal voltages to an
electrode. The diagnostic for detecting the wave
motion is video imaging, which provides the required
spatial and temporal description of the wave’s
amplitude as it propagates.
Fig. 6 Dust acoustic waves (DAWs) in a
PK-4 parabolic flight [Fortov et al., 2005],
Ground-based experiments, by our group [39,43]
for a cloud in a dc plasma at 70 Pa neon.
and other groups [31,33,36,42], have not only
The wave attains such a high amplitude
confirmed that synchronization of the DAW occurs,
(>50% modulation of number density) so
they have also revealed several new phenomena. The
that the wave is nonlinear.
discoveries of Ruhunisiri and Goree [39,43] include
our proposed research topics: branching and thresholding of a synchronized state, and a newly
observed nonharmonic synchronization state.
The need for microgravity arises from the very limited spatial extent of the ground-based
experiments. Due to sedimentation, the dust clouds in ground-based experiments have a finite
size, limited to about three wavelengths, as seen in Fig. 2(b). The observations of waves
propagating such a short distance are heavily affected by boundary conditions and
nonuniformities in the dust density. We can overcome these limitations by using microgravity
conditions to suppress sedimentation, allowing the DAW to propagate large numbers of
wavelengths in relatively uniform background conditions. Among all possible microgravity
platforms, PK-4 is ideally suited for this purpose: it not only meets all four requirements for the
study of wave synchronization – it meets them under very favorable conditions:
 The self-excitation of the DAW has already been confirmed in PK-4’s Commissioning
Experiment. See Fig. 2(a).
 The wave reaches nonlinear amplitudes as seen in Fig. 6, where the number density
varies from crest-to-trough by an order of magnitude.
 Several kinds of electrodes are provided with function generators that can drive varying
frequencies and amplitudes of modulation.
 The cameras provide the required diagnostics. The particle-observation cameras will be
the main diagnostic, while the glow camera will help as well in monitoring the
modulation of the background plasma conditions.
Our scientific goal is to:
 investigate three unexplained phenomena, which we recently discovered in our groundbased experiment. These phenomena are a nonharmonic synchronization state,
thresholding, and branching, as we explain below.
 provide the data required to develop and test theoretical descriptions of the spatiotemporal character of wave synchronization. We will obtain space-time diagrams of the
dust number density, which can be compared directly to solutions of nonlinear wave
equations for dust density.
Experimental plan:
In our first mission we will perform a preliminary experiment to test various modulation
schemes, and to define the required conditions and procedures:
 We will perform brief runs with three kinds of manipulation electrodes on PK-4: the EM
electrode, DC electrode (active and passive), and the RF coil. All three of these can be
driven by function generators at specified amplitudes and frequencies, as required for
synchronization experiments. We will identify the best of these three modulation devices
to use in the second mission.
 We will also perform a coarse-grained parameter search, to identify ranges of frequencies
and amplitudes to use in the second mission.
 This preliminary experiment will require roughly 10 minutes of observation time.
In our second mission, we will improve our experiment to generate our publishable data. The
experiment will use two kinds of scans:
 In our first scan, we will explore the
nonharmonic synchronization state and
other synchronization states. We will do
this by stepping the modulation frequency
over ~50 values, all at a fixed amplitude.
This scan will require about 2.5 minutes of
recording time, based on our ground-based
Fig. 7. Example space-time diagram, from our
experiment.
ground-based experiment [43]. A darker color
 In our second scan, we will investigate the
indicates a higher number density n for the dust.
branching and thresholding of the 1:1
synchronized state. Following the protocol we developed for our ground-based
experiments, this will be done by smoothly sweeping the frequency over a 15 Hz range in
an inner loop, and stepping the amplitude upward in an outer loop with 14 levels. These
sweeps will require 9.5 minutes.
In both missions, we will record image data
using the PK-4 particle observation camera. This
camera has a suitable resolution of 13.9 m/pixel,
almost the same as the 12.8 m/pixel resolution
in our ground-based experiment. While the PK-4
cameras can also operate up to 200 fps, we will
plan on a lower frame rate to conserve resources.
We estimate that 100 fps will provide a
sufficiently high Nyquist frequency to detect the
harmonics.
Analysis plan:
Fig. 8 Example data, from our ground-based
experiment [39], showing synchronization
states at wave frequencies that are a multiple of
rational number of the modulation. The
observed spectrum is below 50 Hz, so that
sampling at 100 fps would be adequate.
We will use analysis methods that we
developed for our previous ground-based
experiments. We start by preparing space-time
diagrams from the gray-scale images in each
frame from the particle observation camera. We average over the coordinate perpendicular to the
wave propagation, so that dust density data are reduced to two coordinates n(x,t). An example
space-time diagram is shown in Fig. 7. (Note that in these ground-based data the spatial distance
ranges only over two wavelengths, which we will greatly improve upon by using PK-4.) We will
use the space-time diagram to prepare our two main graphs: power spectra and Arnold diagram.
Using the data from our first scan, we will prepare power spectra graphs, as in Fig. 8. These
are prepared by Fourier transforming the time series of dust density, for a specified position in
the space-time diagram. This power spectra method is different from the Hilbert transform
method used by Williams [42], and correspondingly it is useful for different scientific purposes.
Our purposes are to identify the various nonlinear states, both harmonic and nonharmonic, and to
reveal small features such as sidebands. The signature of the nonharmonic state, in Fig. 8, is a
spectral frequency that does not vary linearly with modulation frequency.
Using data from our second scan, we will prepare an Arnold tongue diagram, as in Fig. 9.
This is a graphic representation to indicate a binary condition, whether synchronization occurs or
not, as two modulation parameters are varied.
Our purposes in using the Arnold diagram will be
to investigate branching and thresholding, which
are visible in the example of Fig. 9 from our
ground-based experiment. At amplitudes below
the threshold level, synchronization does not
occur — using PK-4 we will determine whether
this is due to frictional dissipation or whether it is
merely an artifact of the small sample volume in
ground-based experiments. For the branching
Fig. 9 Example Arnold diagram, from our
phenomenon, we will test two explanations:
ground-based experiment [39], showing
either the branches are part of the same 1:1
synchronization states as shaded regions in a
synchronized state, or they indicate a merging of
parameter space for the external driving
amplitude and frequency.
two different synchronized states.
Theoretical support for the experiment:
A theoretical effort will be undertaken to meet a need for the experiment: providing a
suitable theory for synchronization of propagating waves, to compare to the experiment. The
theory effort has two components:
 The largest effort will be analytic theory, carried out at NRL, under separate funding,
under the leadership of Dr. Ganguli. Details are provided in Ganguli’s proposal.
 A complementary effort will be numerical simulations performed at the University of
Iowa, under the funding proposed here. These simulations at Iowa will be done by
Professor Abhijit Sen, assisted briefly by a postdoc. Two models will be used:
(i) A hybrid fluid model. The dust, electrons and ions will be considered as each being a
continuum, sharing the same volume. The dust will be modeled with full equations of continuity
and momentum, so that its density and flow velocity will be fully evolved in space and time. To
provide for the disparate time scales, electrons and ions are assumed to have Boltzmann
distributions. One of the outputs of the model is the spatio-temporal evolution of the dust number
density, which is the same quantity obtained in the experiment using cameras. This will allow a
direct comparison of the model to the experiment. The dust momentum equation will have an
additional electric field term (that has both spatial and temporal variation) to represent the
external driver, mimicking the modulation in the experiment. The dust dynamical equations will
be cast in the form of first order convective differential equations and solved using the flux
corrected scheme as developed by Boris et al [97]. The electron and ion density contributions
will be updated by solving the Poisson equation at each time step to determine the electrostatic
potential. Such a full-scale fluid simulation with appropriate boundary conditions will provide a
realistic tool to interpret the experimental results and also help validate the fKdV model that will
be developed at NRL under separate funding. It should be mentioned that our hybrid model can
be efficiently solved by using the LCFCT code [97] and has been employed for a number of
studies in the past [98,99]. As a validation of the Sen et al. model, this hybrid code has been used
successfully already for ion acoustic precursor solitons [100].
(ii) Molecular dynamics simulation. The equations of motion will be integrated for point
particles interacting through Yukawa potentials. The purpose is to account for the effects of
discreteness of the dust particles. The lighter electron and ion species will be incorporated
through a shielding factor in the interaction potential – namely the Yukawa potential, i.e., the
‘dressed particle’ approach of Joyce et al. [101,102]. The simulations will be carried out using
the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) [103] open source
code, initially in a two dimensional periodic system and later extended to a fully threedimensional system. The code has been successfully employed in the past to study nonlinear
excitations in a dusty plasma medium [104]. These simulations will help provide some
parameters for the dust equation of motion in the hybrid fluid model.
The theoretical effort will be coordinated in two ways. First, all the theory will be done in
close coordination with experiment, so that the theory includes the proper physical processes,
parameter values, and boundary conditions. This will be accomplished by having Prof. Sen carry
out his work while at The University of Iowa. Second, the two theoretical efforts, analytic and
numerical, will be coordinated; this will be done by frequent communication between Prof. Sen
at the University of Iowa and Ganguli’s group at NRL. This communication will come easily,
because Ganguli and Sen already correspond frequently on other projects.
2. Backup Experiment: Mobility in a DC field
We next describe an experiment available to us,
as an alternative, in the unlikely case that problems
arise and our Main Experiment is found to be
impractical. This experiment was approved July
2014 by the PK-4 Facility Science Team for
inclusion in the Experiment Science Requirements
document.
Goal #2: Is the motion of dilute species
particles governed by mobility?
Fig. 10 Our simulation on the mobility of a
When a different size of dust particle is single particle in a 3D dusty plasma, under PK-4
introduced into a dusty plasma, under microgravity conditions [58]. Shown here are coordinates of
conditions, it typically settles at the center or edge particles in a portion of the dust cloud.
of the dust cloud [56,57,105-109]. We will exploit this separation tendency by introducing a very
small number of different-size particles (projectiles) into the center of a pre-existing dust cloud
(target). In the presence of a DC field, we expect that the projectile particles will drift slowly
through the target, with an average velocity that we will measure. This average velocity will be
the result of a balance of a net force (difference of electric and ion drag forces) and a drag due to
fraction of particles
Coulomb collisions with the target. Microgravity is essential for this experiment because
otherwise, in a 2D ground-based experiment, the differently sized projectile particles will
separate from the target, while in 3D they can remain along the central axis of the plasma.
This experiment differs from the “lane formation” experiments planned by other PK-4
experimenters because our projectiles will be dilute. We will inject so few projectiles that one
projectile will not influence the motion of another, as is the case with lane formation [56,57].
To prepare for this experiment, and to establish theoretical predictions for comparison to
experiment, we have performed and published 3D simulations [58-60]. We used PK-4
conditions, and tracked the motion of a differently sized projectile as it drifted through a target
cloud of particles, Fig. 10.
Our scientific goal is to:
 verify the projectile drift is mobility-limited.
The experimental plan’s first step will be to form a steady dust cloud (target) with no waves
or flows. We require a DC field, with the dust cloud trapped by an application of the RF coil or
other means. In the second step, we will intermittently inject small numbers of particles (<
100/second) of another size (projectiles). The projectiles will be imaged as they drift down the
axis of the plasma. We will plan the time requirements using our MD simulation data; our
preliminary estimate is 10 minutes of camera time.
Our analysis method will start with particle identification to track particles at least two
frames. (Our analysis of early PK-4 flight data indicates that we can do this typically for up to
100 frames, as shown in Fig.
11.). We will then obtain the
Fig. 11. Our analysis of early
1
drift velocity, calculated simply
data from the Science
as the average velocity for
Campaign 1 experiment on
PK-4 aboard ISS, showing
particles. If we find that the
that we can track individual
drift velocity is constant, this
dust particles for typically up
will suggest that motion is
0.1
to 100 video frames. The
mobility-limited. As a check,
vertical axis of this graph is
we
can
compare
the
the fraction of particles that
can still be identified uniquely
instantaneous
fluctuations
after the indicated time has
(about the average velocity) to
elapsed.
the fluctuation levels observed 0.01 0
200
400
frame number
in our MD simulation.
G. Relationship to other proposals
The present proposal is related to a proposal “Understanding the Frequency Synchronization
Physics in PK-4 Experiment” submitted by PI Gurudas Ganguli, of the U.S. Naval Research
Laboratory (NRL). Ganguli’s proposal is theoretical. It will provide theoretical support for the
present proposal, which is mainly experimental.
The present proposal is scientifically separate from another proposal, “Three-dimensional
dusty plasma experiments,” submitted by Bin Liu, with Goree as a co-I. The scientific goals of
the two proposals are different. The two proposals also require that two separate flight
experiments be performed, because they need different operating conditions.
H. Broader Impacts
K-12 outreach task
 The PI will give a presentation at an
additional class in an elementary school.
This will be done as part of the “Hawkeyes on Science” program, which provides
equipment and a truck. The goal is to
generate excitement about science. This
presentation will be given to one class, in
addition to the presentations that the PI
will give for his current DOE grant. The
presentation will center on sedimentation
and buoyancy, and how the International
Space Station allows scientists to avoid
these conditions.
Interdisciplinary research
 This research has an overlap with
nonlinear dynamics as well as other
plasma physics areas
Graduate education
 A graduate student will be supported for
participation in the experiment and data
analysis.
Goree, on a K12 outreach, May 2014. Two
classes of fifth graders to discover, with hands-on
experiments how cool gases sink while warm
gases rise due to buoyancy. Fifth graders respond
well and show their excitement, as seen here.
Broad dissemination of results
 Research results are disseminated in journals, conferences, a website, and a Twitter account.
When possible, the peer reviewed journals used will be open-source.