Getting a charge out of plasma!

This summer a cohort of students find themselves doing research at Gettysburg College.  Some are busy coding on computers, some are in chemistry labs making product after product, others are working with viruses hoping not to catch the virus themselves.  In Dr. Timothy Good’s lab we (Rikard Bodin and Neng Yin) find ourselves with grease on our hands, confusion in our heads, and hope in our hearts. Dr. Good runs the plasma lab at Gettysburg College. “Plasma Lab?!”, a voice in the back shouts, yes a plasma lab, where we study the 4th state of matter, plasma.  Plasma was first taught to me in 5th grade as “hot air”, and now I know that they will really tell kids anything just so that they have a fun fact to share at the dinner table. Because plasma isn’t just “hot air”. Plasma is ionized gas, containing atoms or molecules in the gaseous state that then have an electron stripped away, thus making it positively charged or ionized.  Ionized gas isn’t very stable so in order to maintain it you need a very high temperature which is where the explanation in elementary school came from, but the temperature is not a defining characteristic.

The first step in studying plasma is to make it.  Since plasma doesn’t exist around us we are forced to make our own which is where the main aspect of the summer comes in play.  In the plasma lab we have a small plasma chamber, the Pickets Charged Plasma Device (PCPD), which we use to make our plasma. This chamber needs to be vacuum sealed so that we can drop its pressure to 1 billion times lower than the atmosphere. Once we have established the vacuum we then add the gas of our choosing, which in our case is argon gas.  Once the gas is in the chamber we ionize it. This is done by running a current through filaments. When the filaments are sufficiently heated, they emit electrons. These emitted electrons are drawn in a current to the chamber walls and along the way collide with gas atoms, knocking other electrons off the gas atoms, thereby ionizing them. In order to make as much plasma as possible we have a permanent magnet array inside the chamber that redirect the electrons around so that they hit more atoms and create more ionized gas, increasing the plasma density.  

(Interior of plasma chamber before this year’s upgrade)

Summer 2018 research 5

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Summer 2018 research 4.jpg(Plasma chamber after adding new windows, fluorescence collection optics and optical fibers, as well as new Langmuir probe.)

Rikard

Our focus for the summer has multiple prongs, but the main focuses is on upgrading our apparatus in order to conduct experiments that investigate ion dynamics in double layers and plasma waves.  Experimenting in the PCPD, constructed in previous summer research campaigns, we already had the capability to create plasma and investigate it via a diagnostic tool called a Langmuir probe. The probe allows us to study the electron density, temperature and plasma potential but yields limited information about the ions.  One of our initiatives involved upgrading the chamber to allow us to conduct Laser Induced Fluorescence (LIF) spectroscopy on argon ions for the measurements of ion density, flow velocity and temperature. We procured and installed three new viewport windows on the vacuum chamber, while also adding new optical fibers for laser beam transport to the device and for transmitting the collected fluorescence to a remote detector.  We also performed maintenance on PCPD, repairing a pneumatic valve, installing a vacuum gauge and replacing filaments.

As well as actively working on upgrading the plasma chamber we have been working with the dye laser system in the lab, which if you have never gotten to play with an expensive laser, I do recommend it.  This ring dye laser is capable of creating a beam over a range of different colors by slightly changing angles in a birefringent filter, selecting which color resonates within the laser cavity. We start by optically pumping the dye laser with a green semiconductor laser and then can tune the dye laser from red-orange to yellow-green.  Doing this requires a lot of careful finicking with the mirrors as everything must be very accurately lined up to keep the laser working. Our aim is to create a laser beam with a wavelength of 611 nm that can be absorbed by argon ions in our plasma. After ions absorb the laser light, they emit fluorescence (LIF) at 461 nm that we collect with a lens telescope and detect with a photomultiplier tube.  

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(Rikard Bodin tuning the laser as well as a photo of the ring dye laser apparatus)

Neng

The Langmuir probe plays an essential role in the plasma experiment. It helps researchers to understand the behavior of plasma by collecting the charged particle (ion/electron) current as voltage is varied. My job at the beginning is to rebuild the Langmuir probe. Some of its parts have been old or broken and we required a longer probe shaft to reach the double layer region. One thing I learnt is that one should never overlook the tolerance which is labeled by factories. The Langmuir probe is made of a central conducting wire encased in layers of insulating ceramic and shielding metal tubes. When I was trying to insert one tube inside another, the tolerance gave me a hard time, although its precision  (tolerance) is two decimal places, as per the manufacturer’s’ standard specifications. Another prong has been to update the way we process our data from the Langmuir probe. We have developed programs in MatLab that analyze the probe’s I-V characteristic curves, employing the Druyvesteyn method to determine the electron energy distribution function and to calculate the electron temperature and plasma potential.

Summer 2018 research 3

 

(Above is the probe inside the chamber with violet argon plasma and below is how the probe looks like outside the chamber)

 

Summer 2018 research 2(Neng Yin working on Langmuir Probe analysis using Matlab programs in the Plasma Research Laboratory at Gettysburg College.)

The final prong to our summer research was to go to West Virginia University where we can take our own data at their Plasma lab.  Dr. Good has been a collaborator with Dr. Earl Scime for a number of years. They were kind enough to allow us and Dr. Evan Aguirre, a recent graduate of WVU, into their lab where we could conduct double layer research and take our own data.  While at WVU, we collected probe I-V characteristic curves at a series of radial locations in the HELIX device. We carried out this radial scan for a couple different gas pressures, while also recording the LIF spectrum to measure ion flow. We are trying to correlate the gas pressure to the velocity of ion beams accelerated by current free double layers.   Our hypothesis is that the coupling of RF wave energy in the plasma source to electrons is altered by enhanced collisions with neutral gas atoms.

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(The HELIX/LEIA plasma apparatus at WVU.  Plasma created in the smaller tube, HELIX, in the front flows into the expansion chamber, LEIA at the back.)

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(Dr. Aguirre, Neng, and Rikard at the control panel in the Scime Plasma Research Laboratory at West Virginia University.)

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IMG_1216(A view down the axis of the HELIX/LEIA plasma apparatus; Dr. Good in the window reflection.)

West Virginia University has a very nice and very large Helicon plasma device, which works a little differently to ours since it uses waves launched by an RF antenna to create an argon plasma.  Using this device we can take some very good LIF and probe data under conditions in which ion beams are accelerated into LEIA by a double layer electric field structure at the juncture of HELIX/LEIA. The LIF data is measured in LEIA while we also take Langmuir probe data upstream in HELIX.  We are varying the neutral argon gas flow rate into the apparatus in order to investigate how collisions with neutral atoms alter the ion beam acceleration and the electron energy deposition profile.  After returning to Gettysburg College, we analyzed the LIF and  Langmuir probe data to yield some very interesting results such as the electron temperature and plasma potential  profiles shown below.

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(Example DAQ control panel showing: LIF data at top left, Langmuir probe I-V characteristic curve below it,  control settings at bottom and plasma conditions listed on the right.)

(Ion Beam Velocity slowing with increasing neutral gas flow; results from laser induced fluorescence spectral data taken at WVU.)

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(Electron Temperature radial profiles for increasing neutral argon flow rate; Langmuir probe data from WVU.  Note that the temperature falls on the edge and rises on axis as the flow rate increases, altering the profile.)

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(Plasma potential radial profiles for increasing neutral argon flow rate; Langmuir probe data from WVU.)

Rikard

I’ve really enjoyed my experience researching plasma physics this summer.  Most labs or experiments are just coding or letting experiments run and then waiting but mine has been very hands-on.  I’ve gained a lot of machining experience as well as problem solving practice as Dr. Good and I tackle problems head on.  There has been plenty of waiting for parts but the process has been invaluable. Plasma physics is a great field to get into, if you like physics, it encompasses such a wide range of physics topics such as mechanics, quantum mechanics, optics, thermodynamics, and electricity and magnetism.  That means that I have been actively practicing and learning all of those fields.

This summer has been full of highs and lows from when things do work to when something either breaks or doesn’t work and requires maintenance.  Research is all about high hopes and walking through the fire. I learned that everything that can go wrong will go wrong, and that it’s best to accept that and take the blows on the chin.  I have to say I like research.

Neng

Designing and building experimental devices like a Langmuir probe can always make me satisfied. In this lab, it truly provides me opportunities to work on the circuits and use my soldering skills. It does like what you did in the curriculum labs, like following instructions written by upper class students. What I experienced in this plasma lab involved a learning process. There are no clear instructions or procedures but instead we employ the research papers by former students and found in the published literature.  Rather than receiving direct instruction, we always learned by ourselves. We made mistakes and the data did not come out nicely at first, and then we studied it and tried to figure out what causing the problem. Later we made some improvements and tested them to great success. Finally, we shared the happiness of accomplishment. I believe this is the substantial reason that continuously encourages me study physics.

In this summer, I profoundly experienced how hard the research is for scientists. I would like to picture it as one walking in a vast expanse of desert without map and compass but trying to find a way out.

Acknowledgements

This work is supported by the Gettysburg College X-SIG program through the Dickson Fund. We would also like to thank Professor Earl Scime for the opportunity to perform some of this plasma physics research in his laboratory at West Virginia University supported by NSF award PHYS 1360278.

 

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