Imaging of high-speed dust particle trajectories on NSTX

A.  L. Roquemore, W. Davis, R. Kaita, C. H. Skinner

*Princeton Plasma Physics Laboratory, Princeton New Jersey* , 08543

R. Maqueda

### Nova Photonics

N. Nishino

*Hiroshima University*, *Hiroshima Japan*

Imaging of high-speed incandescent dust particle trajectories in a
tokamak plasma has been accomplished on NSTX using up to three
high-speed cameras each viewing the same plasma volume from different
locations and operating at speeds up to 68000 frames per second with
exposures times varying from 2-200 micro seconds. The dynamics of the
dust trajectories can be quite complex exhibiting a large variation in
both speed (10-200m/s) and direction. Simulations of these trajectories
will be utilized to ascertain the role dust may play in future machines
such as ITER where significant dust production from wall erosion is
expected. NSTX has numerous view ports including both tangential as well
as radial views in both the mid plane and lower divertor. Several
vertical ports are also available so that a few specific regions in NSTX
may be viewed simultaneously from several different camera positions.
The cameras can be operated in the full visible spectrum but
near-infrared filters can be utilized to enhance the observation of
incandescent particles against a bright background. A description of the
cameras and required optics is presented and a brief description of the
software used to identify and track individual particles in 3D is given.

**I Introduction:**

Incandescent dust has been routinely observed on NSTX by visible fast
cameras that were originally employed to study specific plasma
processes. Only recently have fast cameras been employed with the
specific purpose of characterizing the dynamics of dust particles in
plasmas. This new interest is in response to the need to understand and
predict the consequences that large amounts of dust generated by wall
erosion will have on next-step machines. On NSTX, research into
measurement and control of dust is underway \[1\] with the goal of
projecting the results to ITER. The present study is aimed at providing
data on the dynamics of dust particles inside of a plasma to further
understand dust generation and mobility. The data will be used as input
to codes such as DUSTT\[ 2\], a dust transport code developed at UCSD.
To provide data for these models, visible cameras have been employed to
observe incandescent dust particles during their creation and to track
their trajectories inside the plasma. Because of the steep gradients in
plasma profile parameters, accurate coordinates of mobile dust particles
is necessary if meaningful predictions of dust migration are to be made.
At least two fast cameras are required to accurately track a particle
and provide reliable data to fit into predictive codes. A description of
the cameras is presented and the various views are shown. Example
trajectory data recently obtained with two cameras simultaneously, in
the divertor region of NSTX is presented .

# II Fast Camera capabilities

Dust particle trajectories can be quite complex. Some particles have
very high velocities, while others are literally suspended in one
location for long durations. Particles have been observed to make abrupt
180º changes in direction, others may initially have a constant velocity
and suddenly accelerate rapidly out of the field of view. Mid-flight
particle breakup is common and often spectacular. However, these extreme
behaviors are more of an abnormality constituting about 10% of the
observed particles. The majority of particles have a more docile
existence in that they move in either straight lines with constant
medium velocities or have some curvature that follows the plasma field
lines. Tracking these particles can be demanding, and requires a camera
with high sensitivity, fast frames rates and short shutter speeds.

Three cameras have been routinely employed to study dust behavior in
NSTX. Their capabilities vary greatly in framing speed and sensitivity.
The two fastest cameras are based on CMOS technology, which is known to
be more resistant to radiation damage than the more common CCD cameras.
None of the cameras are intensified and it is believed this may limit
the size of the particle that can ultimately be observed.

The camera having the greatest sensitivity as well as the highest
framing rate that is used for dust measurements on NSTX is the Phantom 7
manufactured by Vision Research and provided as part of a collaboration
between NSTX and Nova Photonics. It's primary mission is to measure fast
phenomena such as turbulence and MARFEs \[5\]. This camera is state of
the art in technology in that it has 12 bit monochrome bit depth, the
best on the commercial market. It provides frame rates of up to 120,000
frames per second (fps), though it is typically operated at 68,000 fps
with corresponding 128X128 pixel array and has enough on-board memory to
capture a full discharge on NSTX. Even at these fast frame rates, the
images are bright and small dust particles are easily visible. Small
scale deflections in the particle motion by phenomena such turbulence or
ELMS can be readily detected with this camera.

A second camera, the Photron Ultima SE, is provided as part of a
collaboration between the US and Japan. This camera has been used
extensively on NSTX as a divertor diagnostic to study Edge Localized
Modes (ELMs) \[3\]. The camera has the capability of operating at up to
40,500 fps, but for tracking dust, it is typically operated at less than
13,500 fps. Particles in the main chamber with speeds of 100 m/s will
only travel of order 1.5cm per frame over a path length of order 1-3
meters. The slower particles in the divertor are typically tracked at
2250 fps where 1-2 cm steps per frame are more than adequate to
determine particle velocities. The exposure time is the inverse of the
frame rate, so the slower rates are sometimes required to provide
adequate amounts of light to detect smaller particles. This camera is
immune to the ambient magnetic fields on NSTX and can be mounted
directly on a view port.

The first camera to detect incandescent dust on NSTX is the Kodak Motion
Corder, an early model high-speed camera. This camera can operate at up
to 10,000 (fps) but in practice it is limited to much slower rates. As
the frame rate is increased to 1000 fps, the image is transferred to the
upper left hand corner of the pixel array requiring a wide-angle lens to
achieve a useful field of view. This limits the resolution and distorts
the edge of the image severely. It does however, have independent
exposure control that is typically operated at shutter speeds of 1/3000
second. This camera is most useful when mounted on a midplane port and
operated at 500 fps.

**III Viewing access to NSTX**

Six view ports provide access to the mid plane of NSTX. The toroidal
location and nominal field of view (fov) of each view port is shown in
Fig. 1. Some portion of a tangential cross section is obtained in each
view, limited by the angle of the view port and position of the
associated shutter. As can be seen, the coverage is near 50% of the
machine volume. However, only about 30% of the mid plane wall, where
creation of new particles is often observed, have overlapping views.
View port \# 1 is located directly adjacent to the impurity pellet
injector and views the interaction of pellets and dust with the plasma
boundary. Some initial data has been taken at this port and is the
subject of a future study.

Access to the lower divertor region is difficult because of the
mechanical support structure for the carbon based plasma-facing
component tiles. Development of a reentrant tangential view port in the
divertor region of NSTX was first proposed by one of the authors, N.
Nishino, for ELM and turbulence studies using the Photron camera
described above. The implementation of this view port is described in
Ref \[4\]. A complementary reentrant view port looks vertically downward
on the divertor from the top of the machine. It's view is somewhat
occluded because it looks through a radially-oriented gap in the tile
support structure. Its viewing region is centered on the tangency radius
of the view from the tangential divertor window and encompasses about 1
square meter of area. This is shown in Fig. (2). Traces of particle
trajectories taken from the vertical camera during a single discharge
are overlaid on Fig. 2 showing the general trend of particle motion in
the toroidal and radial directions. A dotted box encloses the region of
overlapping fov's obtained for the Photron camera from above and for the
Phantom 7 camera placed on the tangential divertor view. Each of the
traces that cross the dotted region were observed to have almost no
velocity component in the vertical direction as determined by the
Phantom 7 camera, and generally have a clockwise toroidal component
which is in the direction of parallel flow in the divertor at the outer
strike point.

The region of overlap has recently been extended inboard to encompass a
view of the center stack by using a more flexible fiber optic bundle
that allows a more angled view as compared to the fiber used in this
study..

# IV Viewing optics

Dust particles are visible while they are heated to incandescent
temperatures but their detection can be hampered by large amounts of
background light such as is always present around the strike points in
the divertor or around the RF limiters. It is helpful in these cases to
use a near-infrared filter to reduce the background light. Both cameras
used the near-infrared filter to obtain the traces in Fig. 2, 3 and 4.
Three of the viewing windows have reentrant designs that dictate the use
of fiber optic bundles. The bundles range from 1.3 -4 meters in length,
are very flexible and have a 8 X 10 mm cross section. Special reentrant
holders have been fabricated to aim the assembly of the bundle and
imaging lens at desired targets inside the vessel. The exit lens at the
end of the fiber bundle is closely coupled to the lens on the camera
with up to a 2 cm gap between the lenses to accommodate a filter wheel
if desired. Both lenses are usually telephoto lenses with the focus set
to infinity. This combination of telephoto lenses provides for control
of the image size by adjusting the zoom of each. If a filter (or filter
wheel) is used, placing it between two lenes focused at infinity has the
advantage that parallel rays of light are transmitted through the
filter. Since the transparency of narrow band interference filters is
shifted to shorter wavelengths as the angle of incidence moves away from
normal, a darkened ring around the outside of the image can be formed if
a filter is placed directly on the end of a camera without the use of
the fiber bundle. Full image capability is obtained when placing the
filter between the fiber bundle and the camera.

When the Kodak and Photron cameras are mounted directly on mid plane
view ports, filters are not used and the view is selected where the
background light is low.

# V 3D reconstruction of the trajectories

By having two images displaced by nearly 90 degrees, a good measure of
the trajectories in three dimensions can be obtained. NSTX utilizes a
measuring arm inside the vessel during outages, and many hundreds of
measurements have been taken of objects inside the vessel. By knowing
the spatial coordinates of the camera's view port and of several objects
on the far wall in the camera's fov, it becomes a geometry problem to
pinpoint the coordinates of a particle at a given time. An IDL program
has been written to convert camera pixels to an X,Y, Z coordinate
system. Coordinates from the two cameras can then be combined to yield
standard tokamak coordinates of R, Ø and Z. Velocities are then obtained
from the distances and framing rates of the cameras. Fig. 3 shows
averaged velocities of particles in the lower divertor obtained using
this method. The velocities are for the particle traces described in
Fig. 2. The tangential view was used to identify particles that had
little or no vertical velocity component, at least in the region of
overlapping views. Corrections for lens distortion have not been made in
these measurements as it requires a vessel entry and will have to await
the next outage. Fig. 3 shows that the majority of particles in the
lower divertor have velocities between 20-60 m/s with some slower and
some faster particles observed. Tracking of dust particles in the main
chamber simultaneously with two cameras has recently been achieved and
is presently being analyzed. These particles tend to have higher average
velocities as compared to the divertor particles and will be reported on
in the future.

The IDL tracking program locates the pixel with the greatest intensity
in a user-selected square around the particle and assigns a coordinate
to the pixel. The program also provides a measure of the relative
intensity between each frame. Fig. 4 shows the relative brightness of
the particle for each frame in the particle traces of Fig. 2. The
general trend is for a rapid increase and then equally rapid decrease in
brightness. The increase most likely occurs as the particle heats up.
The decrease in brightness occurs as the particle either cools down as
it leaves a region of high heat flux or diminishes in size due to
ablation. Other competing processes are also present, for instance
changes in opacity of the plasma due to turbulence or ELMs. This is a
topic that can only be answered by modeling.

# VI Sources of Dust

Dust is produced in a variety of circumstances. Giant ELMs produce
prolific amounts of dust in the divertor region, as do disruptions.
Initial conditioning of the RF antennas also injects dust into the
plasma. Operating the Coaxil Helicity Injection experiments often
produces a shower of particles from the gap between the inner and outer
divertor, usually at the end of pulse. MARFEs on the center stack can
produce dust particles during their formation. During normal operation
when the strike points are first forming on the divertor plates dust is
often visible. Any loss of confinement, even at the end of a normal
discharge can cause hot spots to be formed that eject a localized plume
of dust particles into the vessel. Also, dust is ejected from the
passive plates in normal operation whenever the plasma is operated with
a small outboard gap between the plasma boundary and the wall. An
example of dust creation is shown in Fig.5 directly after a giant ELM
during a high elongation double null discharge. The outer strike point
is on the inner divertor plate. The particles are formed near the center
stack and rotate toroidally in a clockwise direction. As their velocity
increases, the centrifugal force causes the particles to move radially
outward. One encouraging observation is that when a surface is well
conditioned it will produce very little visible dust. For instance,
during long pulse operation, only the first 100 ms of the first pulses
of the day will produce significant amounts of debris around the
divertor strike points. As operation continues during the day, the
amount of dust decreases significantly as long as large ELMS are
avoided.

Future work will include replacing the Kodak with a more useful camera
and adding an additional viewport at the mid plane of Bay G.

# VII Acknowledgements 

The authors would like to thank Tom Holoman for fabricating all of the
camera mounts and filter wheels used in the work and to A. Pigarov and
S. Krasheninnikov for encouraging these measurements. This work is
supported by US DOE Contract No. DE-AC02-76CH03073

## **References**

\[1\] C. H. Skinner et al, Review of Scientific Instruments 75, 4213
(2004)

\[2\] ^1^ A.Yu. Pigarov, S.I. Krasheninnikov, T.K. Soboleva, T.D.
Rognlien, Physics of Plasma 12, December, 2005.

\[3\]N. Nishino, Symposium on Fusion Engineering, Knoxville Tenn, (2005)

\[4\] A.L Roquemore et. al. Rev Sci Instrum 75, 4190 (2004)

\[5\] R. Maqueda to be published, JNM (2006)

**Figure Captions:**

Fig. 1 Plan view of NSTX showing overlapping camera fov's for various
toroidal locations.

Fig. 2 Overlapping camera views in the lower divertor region o f NSTX.
The overlaid traces are actual trajectories taken from the vertical
view.

Fig. 3 Velocities of particles in the lower divertor from two sequential
discharges.

Fig. 4 Emission intensity along the particle path for each trace in

> Fig.2

Fig. 5 Sequence of exposures from the Phontom 7 camera showing the
cascade of particles created after a Large ELM event. Each frame is
displaced in time by a few hundred microseconds.