Calculation of Particle Bounce and Transit Times on General Geometry Flux Surfaces. DOUGLAS SWANSON (Yale University, New
Haven, CT, 6520) DR. JONATHAN MENARD (Princeton
Plasma Physics Laboratory, Princeton, NJ,
8543)
Minimizing magnetohydrodynamic
(MHD) instabilities is essential to maximizing the plasma pressure
and the fusion power output from toroidal plasmas. One such instability
is the resistive wall mode (RWM). Plasma rotation above a critical
frequency has been observed to stabilize the RWM. The critical
frequency is predicted in some theories to depend strongly on characteristic
bounce and transit times particles take to complete orbits. Bounce
times are orbit times for particles with large magnetic moments
that are trapped poloidally in banana orbits. Transit times are
orbit times for particles with small magnetic moments that are
able to complete full poloidal circuits around the plasma. Previous
calculations of these bounce and transit times have assumed high
aspect ratio and circular flux surfaces, approximations unsuitable
for the National Spherical Torus Experiment (NSTX). Analytic solutions
for the bounce and transit times were derived as functions of particle
energy and magnetic moment for low aspect ratio and elliptical
flux surfaces. Numeric solutions for arbitrary aspect ratio and
flux surface geometry were also computed using Mathematica and
IDL and agree with the analytic forms. The solutions were found
to scale as the elongation at low aspect ratio, and as the square
root of the elongation at high aspect ratio. For typical values
of the parameters the bounce and transit times can differ from
the high aspect ratio, circular results by as much as 40%. Analytic
transformations to map the high aspect ratio, circular solutions
into the general geometry solutions are being investigated. Such
transformations could be easily incorporated into existing stability
codes such as MARS to refine models of RWM rotational stabilization.
Digital Lock-In Amplifier Based Ground Loop Monitoring System for Magnetically
Confined Plasma Devices. EDWARD CAMP (University
of Hartford, West Hartford, CT,
6117) HANS SCHNEIDER (Princeton Plasma Physics Laboratory, Princeton, NJ, 8543)
A Ground Fault
Monitor (GFM) system currently in use for the National Spherical
Tokamak Experiment (NSTX) utilizes analog lock-in amplifier hardware
to monitor vacuum vessel grounds to detect and help prevent current
loops that are potentially dangerous to personnel and equipment.
A ground loop fault is a condition where a loop is created between
two single point grounds, potentially resulting in some level of
current flow. Previous research has shown that a digital lock-in
amplifier would improve the performance and operability of the
current system. An automatic gain control system that uses multiple
level transmit signals could improve detection of both low and
high impedance faults, rather than compromising on a single transmit
level that would be clipped at low impedance faults and be buried
in noise at high impedance. This gain control program, along with
a digital lock-in amplifier, both created in LabVIEW, an integrated
visual programming language, produced an increase in usable sensitivity
from 250 ohms to 50,000 ohms in the current analog system to 2
ohms up to 70,000 ohms within 10% accuracy and up to 150,000 within
25% error in the LabVIEW program. This does not take into account
false alarms and nuisance trips, only the accuracy of the device
at quantifying loop faults. Improvements to this program could
be implemented by adding moving average filtering to improve calibration
data and increasing the number of discrete transmission levels
used by the program.
Effects of UV light on a fluorescent dust cloud. ENRIQUE
MERINO (Ramapo College of New Jersey, Mahwah, NJ, 7430) ANDREW
POST-ZWICKER (Princeton Plasma Physics Laboratory, Princeton, NJ,
8543)
Dusty plasma
research has applications ranging from microchip fabrication to
the study of planetary rings. Typically, the behavior of laboratory
dusty plasmas is studied by laser scattering techniques. A new
diagnostic technique developed at the Science Education Laboratory
at the Princeton Plasma Physics Laboratory uses a 100W ultra-violet
(UV) light to illuminate a fluorescent organic dust cloud created
in an argon DC glow discharge plasma. By using the UV light, the
fluorescent particles can be clearly seen during and after cloud
formation and their 3D properties analyzed. This technique has
been successfully used to study formation and transport of the
dust cloud. Observations have shown that after the dust cloud has
formed, the UV light causes rotation of the edge of the cloud (˜ 3mm/s),
while particles in the center of the cloud remain stable. Displacements
of several millimeters up and towards the UV light have also been
recorded by modulating the UV light. Through the use of a Langmuir
probe, changes in the charge of the plasma were recorded whenever
UV light was introduced. These changes occurred both in the presence
of a dust cloud and with a clean plasma as well. Although these
experiments were helpful in demonstrating that UV light had an
effect on the plasma, it is left as future work to determine the
effect of UV light on the dust particles themselves and propose
analytical models for the displacements experienced.
Experimental Studies of Electrode Biased Compact Toroid Plasmas in the Magnetic
Reconnection Experiment. ELIJAH
MARTIN (North Carolina State University, Raleigh, NC, 27609)
DR. MASAAKI YAMADA (Princeton Plasma Physics Laboratory, Princeton, NJ, 8543)
Compact Toroid
(CT) plasmas such as Field-Reversed Configurations (FRC’s) and
Spheromaks are known to exhibit a global instability known as the
tilt mode, where the magnetic moment of the CT tilts to align itself
with the external magnetic field, as well other non-rigid body
instabilities. Possible tilt stabilizing mechanisms for these instabilities
include external field shaping, nearby passive stabilizers, and
plasma rotation. This research focuses on reducing the growth of
the tilt instability by introducing toroidal rotation in spheromaks
formed in the Magnetic Reconnection Experiment (MRX). Rotation
is introduced by the use of interior and exterior electrodes; the
result is a Jbias x
Binternal torque
on the CT plasma which in turn leads to toroidal rotation of the
CT plasma. In order to power the bias electrode a 450 V 8800 µF
capacitor bank capable of delivering up to 450 amperes was constructed
along with the required control and triggering circuitry. Solid
state switches allow for fast turn on and turn off times of Jbias. The bias current and the voltage drop across the electrodes
are measured using a current shunt and voltage divider respectively,
and the resulting flow in the CT plasma is measured with a Mach
probe. Internal arrays of magnetic probes and optical diagnostics
will be used to parameterize the performance of the CT plasma during
bias. Construction and testing of all necessary components and
diagnostics is complete; preliminary experiments were designed
such that the resistivity of the plasma could be determined. It
was found that a typical CT plasma has a resistivity of 34.1 ± 3.6
ohm m, a leaky capacitor model of the CT plasma was applied to
determine the resistivity theoretically. A theoretical resistivity
of 4.9 x 10-3 ohm
m was calculated based on conditions of a typical CT plasma. The
strong disagreement between experimentally and theoretically determined
values is hypothesized to be due to non-optimal control of CT plasmas
formed in MRX. The focus of future research will be optimizing
control of the CT plasma; agreement between the model and experiment
will then be studied as well as experiments designed to induce
toroidal rotation.
Formation and Transport of a Fluorescent Dust Cloud: a New Diagnostic Technique
in Dusty Plasma Research. ENRIQUE
MERINO (Ramapo College of New Jersey, Mahwah, NJ, 7430) ANDREW
POST-ZWICKER (Princeton Plasma Physics Laboratory, Princeton,
NJ, 8543)
Dusty plasma
research has applications ranging from microchip fabrication to
the study of planetary rings. Typically, the behavior of laboratory
dusty plasmas is studied by laser scattering techniques which give
a 2D slice of the dust cloud or by the use of 3D particle image
velocimetry methods (PIV). Although these diagnostic techniques
allow for the study of cloud formation, they require extensive
computer processing or simulations to study the formation processes.
A new diagnostic technique has been devised to study 3D behavior
and cloud formation, without the need for the complicated and usually
expensive methods mentioned above. A fluorescent organic dust is
used to create a cloud in an argon DC glow discharge plasma, illuminated
by a 100W ultra-violet (UV) light. By using UV light, the fluorescent
particles can be clearly seen during cloud formation and their
3D properties analyzed. One question remaining, however, is whether
the UV light perturbs the plasma by changing the local charge balance
or actively changing the dust cloud charge by photoelectric emission.
In fact, initial observations show that particles in the back of
the dust cloud experience a displacement towards the UV light,
while particles in the front move away from it. Velocities ranging
in the order of 1.5 mm/sec to 3 mm/sec were recorded for different
areas of the cloud. Future work will use a Langmuir probe to separate
changes in plasma parameters from changes in dust charge.
Laser Induced Fluorescence Motional Stark Effect Control & Data Acquisition
Application. PATRICK MALONEY (Carleton College, Northfield, MN, 55057) JILL FOLEY (Princeton Plasma Physics Laboratory, Princeton, NJ,
8543)
The motional
Stark effect (MSE) is a standard technique for measuring spatially
resolved magnetic fields in plasma experiments. These fields are
found with a beam of neutral hydrogen atoms which when traveling
through a magnetic field perceive a Lorentz electric field in their
own frame. The resulting electric field causes observable line
splitting in hydrogen’s spectral lines, which is proportional to
the electric field. The polarization of the lines gives the field
direction. However, while MSE is extremely effective for fields > 1
T, it can be difficult to measure weaker fields as a consequence
of spectral line overlap. A dominant source of the overlap is from
different emission lines being red and blue shifted across finite
sized collection optics, an effect referred to as geometric broadening.
To counter this effect, a method using Laser-Induced Fluorescence
(LIF) to excite a single atomic transition at a time in the hydrogen
beam has been proposed. Using this technique, the exact energy
and thus wavelength of incoming photons is set by the laser, allowing
geometric broadening effects to be completely ignored. Already
the combination of MSE and LIF has been able to measure magnetic
fields on the order of tens of Gauss in neutral gases but has yet
to be tested on plasma. The purpose of this project has been to
implement a data acquisition and control system with LabView for
the MSE-LIF development laboratory, including the new Spiral Antenna
Helicon High Intensity Background (SAHHIB) experiment, a plasma
test bed for LIF-MSE. The resulting program displays and records
a variety of measurements including pressures at critical points
in the vacuum system, laser and RF power characteristics, and most
importantly, time dependent LIF signals used to find magnetic field
magnitude and direction. The program is useful because previous
measurements were primarily taken by hand making the collection
of experimental parameters tedious. The data acquisition and control
application developed to study LIF-MSE may also be employed for
studying different instabilities in the helicon plasma source SAHHIB.
Should the LIF-MSE device on SAHHIB prove successful, it will eventually
be employed at the National Spherical Torus experiment (NSTX).
Using LabVIEW for Complete Systems Control of an ECR Thin Film Deposition System. BRANDON BENTZLEY (The College
of New Jersey, Ewing, NJ,
8628) ANDREW POST-ZWICKER (Princeton Plasma Physics Laboratory, Princeton, NJ, 8543)
Interest in studying
an electron cyclotron resonance (ECR) deposition system is fueled
by ECR’s better deposition rates and precision relative to traditional
deposition systems; however, the ECR deposition system is extremely
complex and requires that its gas flow control, magnets, 2.45 GHz
source, and other components all work in concert. Operating the
system requires an experienced user to constantly compensate for
the dynamics of the system, such as argon gas pressure and magnetic
field strength. A method of computer automation, such as LabVIEW,
permits the system to operate itself, allowing for less experienced
operators, reproducible conditions, and a safer working environment.
LabVIEW, in conjunction with National Instruments hardware, sends
and receives voltage signals and serial commands in order to control
microwave power, magnet current, target bias voltage, vacuum and
compressed-gas valve position, chamber pressure, and robotics commands.
The VI takes many factors into account simultaneously, such as
chamber pressure, ion current and spectroscopic data, in order
to make decisions about the system state. LabVIEW was found to
produce easy to manage, consistent, and reproducible conditions
by simplifying complex procedures, such as system startup routines
and robotics commands, to a click of a button, by compensating
both accurately and quickly for changes in plasma conditions, and
by checking the state of the system in order to prevent system
malfunction.
