Experimental Approach and Setup

What is the purpose of this experiment?

In order to understand how dusty plasmas are formed and affect regions in space and Earth's atmosphere, it is necessary to understand the charging mechanisms of the dust grains. It is the purpose of this experiment to investigate experimentally under what conditions do dust grains of various sizes and compositions become charged or discharged and theoretically model the observed interactions to compare with current models.

 Approach:

Many observational and experimental studies of dusty plasmas are currently underway. Laboratory experiments include focusing on the effects of immersing a particle in a plasma and measuring the affects of the interaction to studying waves. Data from the Voyagers, Galileo, and Cassini missions are also being analyzed to better understand dust and plasma interactions in planetary magnetospheres and rings, and comets. It is hoped that the work we do here will contribute to the many studies already in progress.

Although experiments in the past have studied the charging mechanisms of dust particles, our work uses a unique laboratory technique to address some of the problems that other experiments have encountered. We are using a technique known as electrodynamic suspension of particles. This allows us to suspend a single charged particle and then expose it to a controlled environment. The benefit of using this technique, is that the single particle can be exposed to one controlled environment repeatedly or it can be exposed to different environments measuring only the changed environmental conditions. Below, we briefly discuss the major components to the laboratory experiment.

 Particle Generator

We are using particles ranging from 0.1 to 50 microns in diameter of various materials, i.e. Aluminum oxide, kaolin, graphite. A particle is immersed in a liquid which travels down a column by a piston gently producing a pressure pulse. Once the liquid column encounters a tiny orifice, a stream of liquid is ejected. Due to turbulence and surface energy, the stream will break up into droplets. This stream enters a static electric field produced by a potential difference between the orifice and charging plate. The liquid is charged inductively and exits through a larger orifice in the charging plate to the balance. The liquid then evaporates off thus leaving the charged particle.

 Particle Containment

The charged particle is contained in an electrodynamic balance. An alternating potential is applied between a ring electrode and top and bottom electrodes. The potential and frequency range from 20 to 500 VAC and 50 to 300 Hz. The resulting time averaged electric field and the particles inertia, cause a charged particle to be confined to a null point of lowest total potential. If the alternating potential is not biased and the top and bottom electrodes are grounded, the null point becomes the geometric center of the balance. Under the influence of gravity, the particle will sag below the null point. An applied dc field between the top and bottom electrodes will balance the effect of gravity and position the charged particle at the geometric center of the balance. The magnitude of the dc field required to balance gravity is directly proportional to the mass to charge ratio of the particle. The balance is housed in a vacuum system. Once a particle is suspended, the system is then evacuated to less than 0.0001 torr.

Particle Environment

An electron beam is generated by a variable energy electron gun. The electron energy can be varied from 5 ev to 1000 ev. The electrons are directed into the electrodynamic balance through its cylinderical axis through the top electrode. The ultraviolet radiation will be generated by a deuterium lamp with a magnesium fluoride window to allow wavelengths down to 120 nm to be incident on the particle.

Measurement Tools

The mass to charge ratio of the particle is measured directly by knowing the dc potential across the top and bottom electrodes required to balance gravity. For this measurement, a geometric constant is required. The constant is due to the deviation from ideal hyperbolic surfaces of the ring and the top and bottom electrodes. It is measured by calibrating the electrodynamic balance electrode configuration with a known particle size. A polystyrene sphere is used for this calibration. A particle detection system using a video camera and automatic feedback are used to keep the particle at the geometric center of the balance.

The spring point instability is used to determine the size. Here the field strength is compared with the drag coefficent of the particle at an instability point. At this point, the resulting relation between the two parameters yield the size or diameter of the particle.

Drawing of Laboratory Set-up (Created by Ted Albritton)

A pdf version is also available.

Scanning Electron Micrograph of a 10 micron polystyrene sphere suspended in balance and then "plucked out" for studying.

(Scanning Electron Micrograph provided by Chris Cochrane)

Bill Nye the Science Guy made an appearance at Marshall Space Flight for one of his upcoming show on Comets and Meteorites. One of the segments will talk about our current experiment and how it relates to the study of comets. Visit Bill Nye the Science Guy's Lab Online.