Ultraviolet Induced Motion of a Fluorescent Dust Cloud in an Argon Direct Current Glow Discharge Plasma

Michael G. Hvasta Office of Science, SULI Program The College of New Jersey Princeton Plasma Physics Lab Princeton, New Jersey August 15th, 2007

Prepared in partial fulfillment of the requirements of the Office of Science, U.S. Department of Energy Science Undergraduate Laboratory Internship (SULI) Program under the direction of Andrew Zwicker in the Science Education Department’s Dusty Plasma Experiment (DPX) at Princeton Plasma Physics Laboratory.

Participant:

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TABLE OF CONTENTS

Abstract ……………………………………………………………………………. ……..3 Introduction

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Materials and Methods Results

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Discussion and Conclusions ……………………………………………………………..7 Literature Cited

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Acknowledgements

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Graphs and Figures

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ABSTRACT

Ultraviolet Induced Motion of a Fluorescent Dust Cloud in an Argon Direct Current Glow Discharge Plasma. MICHAEL GEORGE HVASTA (The College of New Jersey, Ewing, NJ 08628) ANDREW ZWICKER (Princeton Plasma Physics Lab, Princeton, NJ 08543).

Dusty plasmas consist of electrons, ions, neutrals and comparatively large particles (dust). In man-made plasmas this dust may represent impurities in a tokamak or in plasma processing.

In astronomical plasmas this dust forms structures such as

planetary rings and comet tails. To study dusty plasma dynamics an experiment was designed in which a silica (<5 μm diameter) and fluorescent dust mixture was added to an argon direct current glow discharge plasma. The fluorescent dust enables one to observe the entire three dimensional structure of the cloud when it is illuminated by an ultraviolet (UV) lamp. This lighting technique offers an advantage over laser scattering (which allows only two dimensional slices of the cloud to be observed) and is simpler than scanning mirror techniques or particle image velocimetry. Under typical parameters (P=150 mTorr, Vanode= 100 V, Vcathode= 400 V, Itotal < 2mA) when the cloud is exposed to the UV (100 watts, λ = 365 nm) the mixture fluoresces, moves ~2mm towards the light source and begins rotating in a clockwise manner (as seen from the cathode). By using a Charge-Coupled Device camera, dust clouds with diameters ranging from 6-10mm have been observed with particle rotational velocities in excess of 3

mm

/s near their periphery.

Particle velocities decrease towards the center of the cloud. By calibrating a UV lamp and adjusting the relative intensity of the UV with a variable transformer it was found that both translational and rotational velocities are a function of UV intensity. Additionally, it was determined that bulk cloud rotation is not seen when the dust tray is electrically floated while bulk translation is. This ongoing experiment represents a novel way to control and localize contamination efficiently in man-made plasmas as well as a pathway to better understanding UV-bathed plasma systems in space.

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INTRODUCTION

Dusty plasmas (sometimes referred to as complex plasmas) are comprised of electrons, ions, neutrals and comparatively large (micron sized) particles. Dusty plasma research has experienced a lot of attention and growth over the past several years. This widespread development can be attributed to the growing number of applications dusty plasmas have in both modern science and manufacturing. In earthbound systems, dusty plasmas are seen in plasma processing facilities and tokamaks. In these circumstances the dust is typically considered to be contamination, so it is important to learn how to control and limit the corresponding negative effects.

In space, astronomical dusty

plasmas create comet tails, planetary rings and are a major component of the interstellar medium. These dusty plasmas, outside of our protective atmosphere, are bathed in ultraviolet (UV) light and must be better understood if we are to comprehend the surrounding universe [1, 2, 3]. To this end, the aim of the Dusty Plasma eXperiment (DPX) is to investigate the dynamics of dusty plasma systems and their interactions with UV light. Laboratory dusty plasmas are typically illuminated with a two dimensional band of visible laser light. In this experiment, in order to study the three dimensional structure of the dust cloud, silica (the dust that was previously being used) was mixed with a fine fluorescent dust. The mixing of the dust mixture would allow the entire cloud structure to become visible when exposed to UV light. In addition to uniformly illuminating the entire cloud, the experiment serendipitously provided a method of making the cone shaped dust cloud move and rotate.

The following details the progress made in

characterizing and explaining this motion.

MATERIALS AND METHODS

The majority of our research was carried out in a 13.5” long cylindrical chamber with a 6” diameter. Figure 1 depicts the six-way cross configuration that was used. The dust tray consisted of a 1.9” x .25” stainless steel disc that was screwed onto a .5” diameter stainless steel shaft. The tray and the chamber were grounded. The electrodes

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in the experiment were made from .5” diameter stainless steel rods. The negatively biased cathode (V = -400 Volts) was positioned parallel to the Y-axis 2-2.5” above the dust tray. The positively biased anode (V = +100 Volts) was installed parallel to the Zaxis 1-1.5” above the dust tray. On top of the anode, near the tip of the electrode was another 1.9”x .25” stainless steel disc. This geometrical setup proved useful in creating distinct cone shaped clouds as seen in Figure 2.

All of the stainless steel shafts were

installed through Wilson-seals that allowed for the adjustment of their location and orientation. The walls of the chamber were painted black with a vacuum compatible paint that would increase the contrast between the dust cloud and its surroundings during observation. The research was performed with a 3:1 silica to fluorescent dust mixture. The dust was deposited on the dust tray after the chamber walls were washed with alcohol and the electrodes were cleaned with acetone. After a vacuum had taken the system down to ~10-6 Torr it would be backfilled with 150 mTorr of argon. By applying 1000-3000V to the anode an electrical arc down to the grounded dust tray would excite the dust into the plasma. Once the dust was excited into the plasma, electron capture would allow the dust grains to obtain a negative charge. The attraction between the charged grains and the anode would offset gravity and the particles would float within the plasma [4, 5]. The clouds would then be exposed to UV light from a 100 Watt lamp. This illumination would cause the 3:1 dust cloud to fluoresce, translate towards the light source and begin rotating in a clockwise manner (as seen from the cathode). Dust dynamics were captured using a Charge Coupled Device (CCD) camera. After filming, the camera would be rotated to face a ruler and, without touching the focus controls, its distance from a ruler would be adjusted until the etchings on the ruler were clearly in focus. Once the ruler was in focus the width and height of the frame could be determined. The resolution of the camera was 640 x 480 pixels so the width would be divided by 640 and the height would be divided by 480 to give us the dimensions of an individual pixel. A program called ImageJ was used to determine the number of pixels between two points which could then be converted into distance. Since each frame represented 1/30th of a second, the average velocity between two points could be obtained

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by dividing the distance by (n/30), where n is the number of frames between the two measurements.

RESULTS

There were primarily two series of experiments that were performed this summer. The first was designed to investigate the motion’s dependence on UV light intensity. The second series investigated the possibility of chamber wall out-gassing as the cause for cloud translation. The first series of experiments called for a way to manipulate the UV intensity within the chamber. A Variac variable transformer was used in the UV lamp calibration. At varying voltages a spectrometer would measure the relative intensity of the lamp. Conveniently, the UV intensity’s dependence on voltage was linear as seen in Graph 3. With this calibration curve the fluorescent cone-shaped dust clouds above an electrically grounded dust tray could be exposed to calibrated amounts of UV light. It was determined that the cloud’s rotational speed was linearly dependent on the amount of UV light. This can be seen in Graph 4. No rotation was observed if the cloud was suspended above an electrically floating dust tray. The cloud’s translation towards the light was also seen to be more extreme as the UV intensity was increased. Translation was unaffected by the grounding or floating of the dust tray. Clearly, the UV light intensity plays a key role in the dust cloud manipulation. The second series of experiments was needed to ensure that the observed effects were electrical in nature and not simply the paint on the chamber walls out-gassing and pushing the cloud towards the light. To this end another series of experiments was designed to see if neutral particles or some other form of out-gassing could provide a mechanism for cloud translation [6, 7]. The experiments began when the pressure in the chamber was at 1.5E-5 Torr and the chamber was isolated from the vacuum system. Over a period of 10 minutes, a pressure reading would be taken every minute.

Without any outside influence the

chamber would slowly leak or out-gas into the chamber to raise the pressure. This reading would become the standard against which the other tests would be compared.

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In the next step a focused beam of UV light was aimed at the dust on the tray and then the paint on the back wall of the chamber, opposite the UV lamp. The focused beam provided three times the intensity of regular UV lamp exposure. Pressure measurements were made in the manner described above. Graph 5 shows the results of this test and one can see that there is no difference in pressure within the chamber due to intense UV related out-gassing. Another step was needed to see if heating from the UV played a role in increasing the pressure within the chamber. The chamber was heated using the UV lamp in its regular position. Then a fan was used to cool the outside of the chamber in an attempt to offset the heating effects. Finally heating tape was wrapped along the back wall of the chamber and turned on to 100° F. Graph 6 shows the outcome of this experiment. Prolonged (10 min) UV heating provided little more than .1 mTorr increase in pressure. Heating via heating tape proved to be effective at increasing the pressure but on a scale that was practically unobtainable with the UV lamp. As a final test to the out-gassing theory the chamber was opened and the 3:1 dust mixture was replaced with pure silica. Upon repeating the same experiments with the UV lamp on the silica none of the same motional effects were seen.

DISCUSSION AND CONCLUSION

The linear dependence on UV light intensity in the first series of experiments coupled with the lack of convincing evidence for out-gassing in the second series leads one to conclude that the causes the observed motion are due primarily to UV photoionization and the corresponding electrical effects [8]. In the immediate future, dust location on the tray will hopefully answer whether the grounded dust plays a role in cloud translation. It is theorized that rotating the dust tray through the Wilson-seal with only half of the tray coated in dust will cause variations in the cloud’s translation. Other future experiments must include looking more closely at the electric field via Langmuir probe. Additionally, attempts to measure the amount of current (if any)

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that flows through the dust tray during cloud rotation will be useful in determining if there are crossed-fields forces causing the observed rotation.

LITERATURE CITED

[1]

Edward Thomas Jr., and Michael Watson, “First experiments in the dust plasma experiment device,” in Physics of Plasma, Vol. 6, October 1999, pp. 4111-4117.

[2]

Li-Wen Ren, Zheng-Xiong Wang, Xiaogang Wang, Jin-Yuan Liu and Yue Liu, “The dust acoustic solitary waves in dusty plasmas: effects of ultraviolet radiation,” in Physics of Plasmas, Vol. 13, September 20, 2006, pp. 1-5.

[3]

Edward Thomas Jr., “Observations of high speed particle streams in dc glow discharge dusty plasmas,” in Physics of Plasmas, Vol. 8, Janurary 2001, pp. 329333.

[4]

V. Land and W.J. Goedheer, “Can we use uv light to control dust charging? An investigation using particle-in-cell/Monte carlo simulations,” Institute for Plasma Physics Rijnhuizen, the Netherlands, www.rijnh.nl.

[5]

A. Barkan, N. D’Angelo and R.L. Merlino, “Charging of dust grains in a plasma,” in Physical Review Letters, Vol. 73, December 5, 1994, pp. 3093-3096.

[6]

Phil Danielson, “Sources of water vapor in vacuum systems,” in R&D Magazine, September 2000.

[7]

Marshal Dhayal, Morgan R. Alexander and James W. Bradley, “The surface chemistry resulting from low-pressure plasma treatment of polystyrene: The effect of residual vessel bound oxygen,” in Applied Surface Science, Vol. 252, September 15, 2006, pp. 7957-7963.

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[8]

Victor Land and Wim J. Goedheer, “Manipulating dust charge using ultraviolet light in a complex plasma,” in IEEE Transactions on Plasma Science, Vol. 35, April 2007, pp. 280-285.

ACKNOWLEDGEMENTS

This summer’s research has been a wonderful educational experience. A special thanks to Andrew Zwicker whose humor and guidance kept the lab bright and productive, Andy Carpe whose technical expertise and tireless efforts to find the right parts allowed the experiments to grow, Nick Guilbert whose attention to detail forced me to become more refined in my scientific thinking, Brandon Bentzley whose personal example inspired me to apply to PPPL and James Morgan whose efforts let me forge strong friendships during my time here at Princeton.

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GRAPHS AND FIGURES

Figure 1: A diagram of the experimental setup used in DPX. The Z-axis is directed out of the plane of the page.

Figure 2: Three cone shaped clouds forming just above the dust tray.

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UV Intensity vs Voltage y = 0.0169x - 1.2685 R2 = 0.9974

Relative Intensity

1.2 1 0.8

Ultraviolet Intensity

0.6

Linear (Ultraviolet Intensity)

0.4 0.2 0 105

115

125

135

Volts

Graph 3: A graph depicting the UV lamp's linear dependence on voltage. The voltage was adjusted with a variable transformer.

Average Particle Rotational Speed y = 1.1962x - 0.1618 R2 = 0.9812

Velocity (mm/s)

1 0.8 0.6

Average Particle Rotational Speed

0.4

Linear (Average Particle Rotational Speed)

0.2 0 0.4

0.5

0.6

0.7

0.8

0.9

1

Relative UV Intensity

Graph 4: A graph depicting the average rotational velocities' linear dependence on UV intensity.

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Pressure (torr)

Pressure vs Time (Focused UV) 4.50E-04 4.00E-04 3.50E-04 3.00E-04 2.50E-04 2.00E-04 1.50E-04 1.00E-04 5.00E-05 0.00E+00

Focus on Tray Focus on Back Wall No UV

0

5

10

15

Time (min)

Graph 5: A graph depicting no appreciable change in pressure when an intense UV beam if focused on different parts of the chamber.

Pressure vs Time

Pressure (Torr)

6.00E-04 5.00E-04

No UV

4.00E-04

Close UV - No Fan

3.00E-04

Close UV - Low Fan

2.00E-04

Close Fan - High Fan Rear Heating - No Fan

1.00E-04 0.00E+00 0

5

10

15

Time (min)

Graph 6: Heating from the UV lamp raised the pressure only slightly. Using the heating tape increased the pressure further but through heating elements that operated well beyond the capabilities of the UV lamp.

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