Last Post- Internship complete

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Well, it's been a very enjoyable and productive internship. I've learned a great deal, met many incredible people, and got to do useful research. The poster above (click here for larger image) summarizes the work I've done this summer, as well as where the project will go in the future.

If you've just found this blog, my first post contains a brief explanation of what I've been trying to accomplish, and the types of waves I've been looking at in dusty plasma. This blog's Picasa Web album contains all of the photos and videos of my lab and my experiment, and they are also scattered throughout this blog.

It's been fun participating in this program, and I highly recommend it to all eligible high school students that may be reading this. Thanks again to Princeton, the PPPL, and all of my mentors for making this all possible.

New nozzle

Well, the disc has yielded some interesting results, but not the ones I'd like. I'm able to form a stable cloud above the disc, but the negative charge that the disc acquires repels the negatively charged particles and prevents them from moving through the nozzle. We took out the plate yesterday and increased the hole size from 0.13 inches to 1 inch, and now I was able to get the cloud to pass through the nozzle; the conditions under which I can do this, though (higher pressure and high voltage) don't allow me to form dust acoustic waves. Otherwise, under conditions that allow me to form the waves, the cloud remains at a fixed distance above the plate, and moves up and down to maintain that distance when I raise and lower the plate.

My mentor and I had two ideas for moving forward. The first was that we could increase the nozzle's diameter even more; this would let us get the cloud in the nozzle more consistently, but at that point we may lose the compression which we hoped that the nozzle would provide. The second was that we could paint the disc black. We aren't sure what exactly this would do, but it was something that was present in the experiment we are trying to emulate. It's possible that the paint was only present there to minimize reflection of the laser; in that experiment the laser shined directly at the nozzle, while in ours it is parallel to it, so reflection isn't a problem. However, the paint would have electrically isolated the nozzle from the plasma; we are unsure of the effects that would cause.

We consulted my mentor's boss, and he suggested that since the problems might stem from the wide disc interfering with the plasma, we could try eliminating the rim by using a washer or something similar instead of the wide disc. We thought it was worth a shot, so I looked at the stuff we had around, and ended up using a copper gasket as the nozzle. Gaskets are an alternative way to seal chamber openings: instead of placing a rubber O-ring between two metal flanges compressing it with screws, a copper ring can be placed there; knife edges on the inside of the flanges press into the gasket as the screws are tightened, producing a much better seal. A copper gasket can only be used once, though, so we have a lot of used ones lying around.

I doubt we will get shocks from this immediately, but hopefully it will give some insight into where we should go. At the very least, it will eliminate one variable (rim size).

Even though I'm getting the results I want, I'm pretty happy with my construction; it's somewhat annoying to untie and retie the plate while trying to keep the fishing line in the grooves, but the rod itself works great. Without being able to raise and lower the plate under vacuum, it would have taken me several days to confirm that the cloud maintains a constant distance from the plate, compared to a minute with this assembly.

Construction finished

I haven't updated the blog over the past couple of days because nothing particularly interesting happened, as I was waiting for parts to ship and be machined. Yesterday the parts were finished, and I attempted to attach them all together, and found that it was a lot harder than I expected.We were using pretty thick fishing line, and it did not particularly like being wound in a tight spool. After considerable amount of frustration trying to secure the line to the rod with tape (a special tape designed for vacuum), I ended up gluing it to the rod with epoxy. Since the line had to go through the lower electrode, I had to leave the chamber open overnight while the epoxy set, which of course is not particularly good in terms of keeping the chamber free of dirt and vapor. Today, I tried to connect it all up again, only to have the epoxy break off half-way through winding it. I decided to toss the thick line and use a much thinner one instead (0.2 mm instead of 0.7), and was able to secure it with tape, tie it to the plate, and wind it up. I have to say, though, trying to simultaneously wind two 0.2 mm lines into two 6 mm grooves with your arms elbows-deep through a 6 inch hole in the chamber is neither easy nor particularly fun, especially when you are trying to avoid covering your hands in silica dust (I failed at that last part). However, I did eventually manage to set it up, and now I can raise and lower the plate as needed without breaking the vacuum. I can only hope now that the tape doesn't fall off and the plate doesn't shift it's angle. The good thing about the way I tied it, though, is that I don't need to unwind the line in order to change the plate's angle. Each of the two lines that comes down to the plate goes downwards through one hole, up through another, and is tied to itself, forming a triangle. By moving the knot up and down, I can change the height of that side of the plate, and moving the knot closer to one hole than the other shifts the angle of the plate in that direction.

When I first turned the laser on after installing the plate, I realized that I completely forgot to take into account the fact that the laser might not be perfectly centered. The laser was, in fact, somewhat off-center, and was missing the nozzle entirely; thankfully, it's position can easily be controlled (as I now discovered) by a few knobs. I can't help but think, though, that if we'd centered it earlier we may have been able to see a larger cloud, since the dust cloud is largest along the axis of the chamber.

The pump should bring it back to vacuum overnight, and tomorrow morning I'll be able to test the new setup for the first time. I'm not particularly optimistic; something tells me that the plate will drastically alter the geometry of the plasma (and thus the cloud) and won't allow us to put the nozzle in the middle of the dust-acoustic waves, or the plate will stop the dust from levitating higher than itself. Anyway, it's worth a shot, and I'm glad that the construction of it is finished.

Also, I finally have a good computer again; the old computer's motherboard got fried, but its hard drive was fine, so we swapped it out and now I have a working computer with all of my old data.

Faulty parts

The materials didn't come until 3:00 today, and when they did, we were told that the plastic we got for the rod is too soft to machine. We ordered a new rod made out of a different plastic, and it should come tomorrow; the time that it comes, and the availability of the machinist, will determine whether I will be able to assemble and install the device tomorrow or if I'll have wait until Monday. The latter would be pretty unfortunate. Yesterday I was able to get a cloud only 5 hours after the chamber was at atmospheric pressure, but of course I don't want to spend Monday pumping the chamber and baking plasma. I installed the plastic rod we had anyway, so just to see how it looks:

The red markers show the ports that I initially intended to use for the rod, on the same level as the top electrode. I'm a bit annoyed at whoever designed this chamber, though, since the conflat of that port (the thing attached to the opening) is too close to the large side ports to attach anything to. As far as I can tell, the only thing it could be useful for is for routing wires, because literally nothing else would fit in that space. And it's not just that one port; there are four symmetric ports with exactly the same problem (the pictures show two of them). I don't know much about vacuum chamber design, but it seems to me that they could have been moved just a few degrees around the chamber to avoid this problem altogether.

On the other hand, the plate will be suspended under the lower electrode anyway, and wouldn't be able to move above it anyway. In fact, the lower position for the rod is probably better, since a smaller distance from rod to plate will diminish the effects of swinging, not to mention that it will be much easier to attach when working through the front window. Not only that, but we couldn't find any Wilson seals (that's the thing the rod is passing through in the left picture, that allows it to turn while sealed) that would fit the small mini-conflats at the top; we'd need an adapter from normal-size down to mini, and we couldn't find enough of those either. The lower holes are the normal size (2⅛''), which is the standard size for our Wilson seals. Now that I think about it, there's really no benefit of using the top ports in the first place. That doesn't make me any less annoyed by the four nearly-useless ports, but maybe it was for the best.

Anyway, the rod we got was much more flexible that I would like, but it would have worked if we could machine it. Oh well. You can see that the rod is bending upwards from the force of the electrode in the second picture; we should be able to minimize that bending with the more rigid plastic and the grooves we will cut for the electrode. Despite the bending, though, our rod turned smoothly. I was also worried that plastic wouldn't maintain as good a vacuum through the Wilson seal, but it seems to be working.

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In other news, I went to a seminar today; it was a powerpoint presentation about how to give powerpoint presentations, so the presenter quite rightly skipped through it quickly and instead spent the time showing off the project her high-school intern was working on (and had been for a year prior to that). It was an air-plasma dialectric barrier discharge device; put more simply, it formed plasma arcs between your finger and the electrode when you were nearly touching it, thanks to some nifty voltage pulsing and a dialectric insulator between your finger and the high-voltage inner electrode. One of the interesting elements was that it formed in air (you could see the purple glowing arcs on your finger, and hear the buzzing from the many arcs); normally we work with low-pressure plasmas, since it's much easier to get arcing when there are fewer molecules to get in the way (hence why lightning, an air plasma, needs such a high current to form). The tricky thing is forming plasma at both a high pressure and a low current. The technology itself has lots of interesting applications, particularly with its interaction with organic matter. The presenter talked about the fact that the arcs sterilized whatever skin it was arcing to, and the intern's main research goal was to confirm that it could be used to apply a coat of cells to something, as in skin transplants and the like.

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I've also finally started my paper. I'm still not sure what the focus of my paper will be; it's either "here's how to generate a shock", or, "here's three ways how to not generate a shock", depending on the results, but of course a lot of it overlaps, so that's what I'm working on.

Fun with Inventor

The place we are ordering the parts from has a warehouse nearby, so materials normally ship within a day, but the person responsible for placing the order ordered the parts today, so I should have them by tomorrow. That said, I spent most of today designing the parts in Autodesk Inventor; I'm pretty happy to finally have an excuse to digitally model something that would actually be used. It was quite fun, although a lot of time was spent battling the lagging computer. I didn't bring my laptop to the lab, so I ended up installing Inventor on my secondary computer at the lab (since the primary broken one is being dealt with by IT). The computer, which was meant to be a server, has eight CPU cores and generally decent specifications, but its graphics card was made in 1998, which is hardly ideal for a graphical 3D application. In the end, it wasn't that much better than remote-desktopping into my laptop would have been. The design itself is fairly simple, as you might expect from a plate with a few holes and a rod with a few grooves:

The rod sits on top of the lower electrode; since they are attached through openings at the same height, the rod will have to push down on the electrode. Hopefully the friction there won't be too significant.

The current design for the plate. The green circle is the disk edge, the smaller blue circles are the relative size of the electrode, and the outer square is the window the disk has to fit through. I was planning on putting the small holes (for the fishing line) on the outside of the electrode, but there's not enough space there, so the line will have to go through the electrode.

I really love the whole paradigm around which Inventor works; instead of putting each piece directly where you want it to go, like in a traditional CAD or modelling application, in Inventor you just define the relationships between the elements to constrain them in certain ways. For instance, the small green circles you can see above (which define the fishing-line holes) can be freely moved around, while the other three holes will automatically mirror across axes, the dimensions recording the holes' positions will be updated, and the groove in the rod (where the fishing line will spool) automatically will move to position itself directly above the holes. Initially, I didn't have exact measurements for the electrode, but since I defined every part as a function of the electrode's dimensions, everything magically updated when I input its exact thickness and width. It's great. It really makes me want to take on a big project, some complex design, and model everything in Inventor. It's also really nice that Autodesk allows free access to their professional-level software for students.

Besides modelling, I spent a good deal of time hunting for a pair of elusive calipers; everybody wanted to use them but nobody knew where they were. We also successfully passed a safety inspection, and I'd spent some time tidying things up beforehand. That's pretty much it.

A new design

I tried rotating the probe above the lower electrode to position it directly above the cloud, but now with the greater distance between probe and cloud I was unable to see any movement of the cloud at all. If we were able to generate a pulse of more than 210V we would probably be able to induce a strong wave in the cloud, and maybe even a shock, but it would be hard to find a stronger waveform generator or put together a system for pulsing our standard power supply in the time I have left.

So, I'm done with the probe for now. I decided on a design for the nozzle that will let me control its height without opening the vacuum chamber. I made a little mock-up of it for my mentor:

It's pretty simple: the plate hangs by a few pieces of wire from a rod, which can be rotated to wind the plate up or down. The rod sticks through two openings of the chamber and is sealed by a Wilson seal that allows the rod to rotate while remaining sealed. In the actual design, the wire will be replaced by nylon line (fishing line), the metal tube replaced by a plastic rod (so as to be electrically isolated), and the plate will be a 1mm-thick steel disc. My mentor was skeptical of the design, saying that it would be better to just start with a simple construction that would support the plate from the base of the chamber, but I still feel that I would spend too much time adjusting it and moving the plate after each test if I needed to open the chamber each time.

There are a few problems with positioning the rod, since the top openings that I'd wanted to use are too close to parts of the chamber to attach Wilson seals to them (I'll add a picture tomorrow). My plan now is to put it in level with the bottom electrode, since the cloud should always be below that level anyway. We ordered all of the materials, which should come tomorrow or the day after, and then we'll have to get them machined. Grooves will need to be cut into the rod for the fishing line to spool into, the disc will need to be cut out of the sheet of metal we bought, and holes will need to be drilled into the sheet for the line and nozzle. We should be able to put it into the chamber by the end of the week, though hopefully sooner.

In other news, my primary computer (at the lab) died today. First all of it's USB devices (mouse and keyboard) stopped working, and when we tried to reboot it, it gave us a blue screen of death. We've been unable to start it since; it doesn't even respond to the power button. My secondary computer is still functional (though it's not very comfortable to work at), and all of my videos and images were backed up on Dropbox, but most of my recent MATLAB code is only on that computer. We should (hopefully) be able to recover it even if the computer is dead, but it's good that I'm not going to be analyzing any data for a while until we get the new setup running.

The end of one chapter, the beginning of another

It looks like I've reached a dead-end in my search for shocks with the method I've been using. I've tried a number of things with the waveform generator, but other problems are arising, along with lots of unanswered questions. At the maximum wave we can generate, the cloud barely budges, its movement hardly detectable when the cloud is filled with dust acoustic waves. The movement is also only visible by the movement of the edges of the cloud; its not large enough for us to be able to detect any wave on top of the acoustic ones. Furthermore, the cloud moves radially away from the probe. This would seem to be the expected behavior, if only it had acted that way before. When I was first generating waves in the cloud from pulses to the probe (before the leak was fixed), the waves (which were then large) traveled vertically downwards; neither I nor my mentor had any idea why they traveled that way regardless of the probe's position. However, since the acoustic waves we generate travel in the same direction, we had hoped to be able to use the pulse to compress the wave and produce the shock. Now that the pulse travels radially, we can't do that, since a radial pulse (given our geometry) travels mostly perpendicular to the direction of the acoustic waves. Now that I think about it, it might be possible to twist the probe around to position it directly above the cloud, making the pulse travel downwards through the cloud; still, the pulse's small amplitude means that it will be even less visible among the dust acoustic waves.

 Another mystery (though somewhat less interesting and relevant) is the question of what determines the repulsion of the cloud from the probe. When I first began working here, the probe was grounded (unintentionally) by an oscilloscope which was meant to measure the pulse that was generated. In this case the probe strongly repelled the cloud, likely due to becoming negatively charged when bombarded by the plasma's electrons. When we disconnected the ground, the probe no longer repelled the cloud to any significant degree, and the cloud hardly reacted to the probe's movement. Now, it acts the same as it did before when grounded, but repels the cloud a small amount when not grounded. Moving the probe into the cloud now results in a circular space around the probe with no dust. My mentor suggested that the probe, when floating (not grounded), would attract the dust as it acquired a voltage from the electric field, but that produces more questions than answers. As my mentor said, I could spend the rest of my time here just studying the factors controlling that repulsion.

 So, the "combine acoustic waves with pulses" idea seems to have led us nowhere. Another method we could use to produce a shock is placing a nozzle in the path of the dust acoustic wave; this would compress the wave as it passes through, creating a shock behind the nozzle. Other experiments have successfully produced shocks in this manner, though with a different setup that ours. My job now is to figure out how to make and position such a nozzle inside our vacuum chamber. Ideally, I want a small opening in a disk (or between plates) that could be positioned in the center of our cloud and later adjusted. My mentor suggested attaching it to supports connected to the grounding plate or chamber base, but it would also have to be attached in a way that would allow fine adjustments when needed. Another problem with this kind of setup is that it would be impossible to modify without opening the vacuum chamber, and so we'd be limited to doing one experiment a day when we want to change the probe. Initially placing it in the cloud could also take several days, as it would probably be a matter of trial and error, with each trial separated by a full day of pumping down the vacuum. Furthermore, we'd be unable to change any parameters of the experiment (electric field, pressure), since those changes would move the position of the cloud and misalign (?) it with the nozzle.

 The best solution, of course, would be to make it adjustable from outside without breaking the vacuum seal. Though my mentor said that such a solution would probably involve more time spent engineering it than would be saved on the experiment, I have a few ideas I'm thinking about to make it work. One of these is to suspend the disk from above using some string or non-metallic wire, which could allow me to pull the wire in or let it out from above through a vacuum seal. That has its share of problems, too, but that's what I'm thinking about now.