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.
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