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Uncertain Principles

Physics, Politics, Pop Culture

Thursday, December 15, 2005

Seventeen Books

One of the latest "memes" going through those parts of blogdom that are prone to such things is the "Fifteen things about books" list. I think I saw this most recently from Scalzi. I was tempted to post something in response to his comments on Science Fiction vs. Fantasy, but in the course of trying to avoid writing a grant proposal review, I seem to have said most of what I wanted to say in his comments. Other than this: Damn but Gregory Benford sounds like a pompous ass in the essay that started this off.

Anyway, things about books. I started to do a list of facts about me and books, but that really wasn't going anywhere interesting, so instead, here are fifteen lines from fifteen books, chosen by the ultra-scientific method of looking over the bookshelves and grabbing thing that I thought highly enough of to want to quote from them. This seems like a reasonable post for a sloppy, icy Friday.

All the source books are fiction, some are better known than others, and they're ordered alphabetically by author to make it a little easier to identify them.

(Why seventeen? Because seventeen is the mystical number. And if I could remember which book that one came from, I would've replaced one of the above...)

Posted at 9:10 PM | link | follow-ups | 23 comments

Two Steps Back

In many ways, I got very lucky with the "Week in the Lab" series. I did choose a good week for it, in that I knew I had a decent chance of getting some stuff done that would make for reasonably interesting blogging. But I was lucky in that I managed to avoid any major catastrophes during that week.

Such as yesterday, when I came in to find that the cold-trap roughing pump I had borrowed from the accelerator lab had died. This by itself wouldn't be that big a deal, but I was using it at the time, as I was trying to regenerate a foreline trap that I had found on a shelf (a few tests this week suggest that I've got a problem with oil making it into the chamber). And when it burned out, it tripped a breaker.

That, too, shouldn't've been a big deal, because I had carefully dragged the pump over to plug it into an outlet far away from the rest of the apparatus. Sadly, thanks to a quirk in the lab wiring, the outlet I picked happened to be the same outlet that the other roughing pump was plugged into. Cutting the power to that pump stopped it, and vented the chamber, with my very expensive turbopump running.

For those keeping track, this is the second time that pump has been hit with atmospheric pressure while running, and it appears to have survived a second time (at least, it started back up yesterday, and pumped down in a reasonable way). If you're in the market for a turbopump, Adixen Vacuum Products (formerly Alcatel) makes a fine, hardy product.

Sadly, this kind of derails the project for the moment. I'm taking advantage of the down time to re-engineer a couple of things (adding a makeshift cold trap to the system, and buying a new foreline trap), but I also have another project that needs to be dealt with, and I was going to have to shift my efforts over there next week, anyway.

My student from this past summer is returning froma term abroad next week, and I hope to talk him into spending some time in the lab during the term (we'll see how that works out). Failing that, I'll schedule some optical-excitation-source work for a few of my lab days during the upcoming term, and we'll see what we can do. But for now, I'm going to be making a couple of entries in the boring and un-blogged series "A Day in the Office," as I've got a big stack of paperwork to deal with.

Posted at 8:27 AM | link | follow-ups | no comments

Tuesday, December 13, 2005

A Week in the Lab Index

Here are links to the posts in the "A Week in the Lab" series, collected in one place for handy reference. As I'll probably use this link for future references to the series, I'll provide a little context here, even though it will seem weird to people who've been reading for the last week.

The posts listed here are the result of an attempt at "Slow-Motion Experimental Physics Live-Blogging"-- I tried to document one full week of work in my research lab, to give people a glimpse of what it's like to do experimental atomic physics on a daily basis. It's not perfectly representative-- I picked a week that I had reason to believe would be more eventful than most, so there's less "And then I spent an hour trying to find the person who knows where the infrared viewer wandered off to..." than there might be in a week chosen at random. It did manage to cover a fair bit of the experience of life in the lab, though, so I'm pretty happy with it as a blogging exercise.

The original posts, in chronological order:

I enjoyed doing this quite a bit, but it was an awful lot of work. There may or may not be a second "Week in the Lab" in the future, but not for a while.

Posted at 8:19 PM | link | follow-ups | 3 comments

Stop! Bullet Time!

I injured my right hand playing basketball yesterday, which makes typing a bit of a nuisance (it's better this morning than last night, though my finger is still a little swollen). It's extra annoying, because we weren't even playing a real game, because of poor turnout-- we were just playing "21," which is one step up from "HORSE"...

Anyway, this means there won't be a thousand-word post on how much time I spent trying to find a spanner wrench yesterday. But I do want to note this nifty article on photography of shock waves, found via BioCurious (found, in turn, from a comment-thread link). They've got a bunch of very cool pictures of Matrix-like shock waves expanding away from bullets and the like.

Posted at 7:44 AM | link | follow-ups | 2 comments

Monday, December 12, 2005

A Week in the Lab: The Final Chapter

So, "A Week in the Lab" has come to an end. The experiment itself goes on, of course, but the week of blogging the experiment is at an end.

As physics, it wasn't terribly successful-- the experiment didn't succeed, after all. As a life-in-science blogging event, I think it worked pretty well. I got to cover a fair range of the experimental physics process, from the basic design stuff, to the nuts-and-bolts assembly, to the prelimanry calibration measurements, to the process of figuring things out from sketchy data, to the frustration of an incomplete experiment. I wouldn't call it the most successful week of my experimental physics career, but I think I might be happier with how this played out than anything else I've done on this blog. I'll have to look back at it again in a couple of weeks and see if I still feel that way, but at least at this early stage, I like the results.

(I hope it was interesting to read, or at least sort of illuminating. But in the end, it's all about meeeee....)

Does this mean I'm going to switch over to an all-"Week in the Lab" format from here on out? Um, no. It was way too much work to do-- it didn't cut into my lab time, obviously, but it took a huge chunk out of my everything else time. It was ok for a week, but I've got other things I need to do in the evenings, and I can't continue to devote that amount of effort to blogging about my lab activities (or anything else, really, so posting may be light for a bit).

I will probably post the occasional lab update in the future, but nothing quite so detailed. Not for a while, anyway.

Posted at 8:33 AM | link | follow-ups | 9 comments

Sunday, December 11, 2005

A Week in the Lab: What Now?

At the end of the day Thursday, I had a new detector that had been shown to detect stuff, and some reasonable theories as what the stuff was that it was detecting, so Friday was the day to go for optical excitation.

The day got off to a slow start because of snow. The forecast called for something like 6-10 inches, of which we got maybe 4-5. Still, it made the roads a little messy, and I went in a little bit later than usual to allow Facilities time to clear the parking lots.

The first thing I did was to check the detector current vs. stopping voltage for electrons produced by the lamp at three different pressures of gas in the chamber. I won't bother to display the graph here, because the curves were all essentially identical. This is a bit of a blow to one of my just-so stories from Thursday's data-- if the change in the voltage needed to stop the signal was due to electron collisions passing across the chamber, you would expect a change for electrons produced by the lamp as well-- but not a major blow. The change probably does have to do with energy loss due to electron collisions at higher gas pressures, but the collisions in questions are probably in the discharge region itself, where the pressure is a good factor of 100 higher.

Anyway, the experimental plan I devised was simple: set the screening voltage to the point where the signal goes to zero (to remove any electrons that might complicate the signal), turn the lamp on, and tune the 819 nm laser to the right wavelength, and look for a change. Any shift in the signal between having the laser blocked and unblocked should be due to metastables. To simplify the process a little further, I put a chopper wheel into the 819 nm beam, so that the laser was blocked and unblocked at a nice, regular rate without me leaning over the system to block and unblock the beam.

So, I set everything up, and ran the test, and saw... Nothing. Not a God damn thing. No change in the signal whatsoever, even when I ran the sensitivity way up on the detector amplifier. As far as I could see, there were no metastables at all being produced.

So, what's the problem? Well, there are three subsystems involved in the project, which gives us three possibilities for the source of the problem.

1) The Detector: As I noted in the last post, I don't really have any way of distinguishing between metastables and positive ions in the current detector scheme, which means I don't really know what sort of signal I should expect. It's possible (not terribly likely, but possible) that the signal from metastables alone is below my ability to detect in the current scheme.

There are two ways to attack this end of the problem. One is to do a little re-engineering of the detector, and see if I can arrange something that will screen both electrons and positive ions. I think I can do this without too much trouble, by re-arranging the existing wires. This will require re-opening the vacuum system, but such is life.

The other possible approach to the detector end of things is to look for the metastables optically. I have that system set up (see the earlier posts about lasers and lock-ins), but I haven't done anything with it yet. I can fire that up with the discharge source, and try to get a handle on the number of metastables produced, which would help me determine what sort of signal I should expect.

2) The Lamp: It's possible that the lamp isn't doing what it's supposed to do, and that I'm not getting any metastable atoms because there aren't enough 123 nm photons to do the first step of the optical excitation scheme.

This is a tricky problem to deal with, because I can't measure the spectrum of the lamp directly. I do have a photodetector inside the vacuum system that I can use to see if the lamp is producing photons in the 123 nm range, and whether they're being absorbed, but it generates currents in the nano-ampere range, and the only detector I have with that sensitivity is currently being used with the steel plate detector. We may have another amplifier lying around that I could appropriate, either in Physics or Electrical Engineering, but the people who know where to look weren't in on Friday because of the snow. I'll check with them on Monday, and see what can be done.

3) The 819 nm Laser: The laser I'm using in this experiment is a broad-band high-power diode, that puts out close to a watt of light over a bandwidth of 2 nm. I went with this sort of laser deliberately, because I don't want to have to stabilize the laser frequency if I can avoid it.

There are two ways the laser could be letting me down. First, the range of wavelengths that the atoms will actually absorb is much, much smaller than the bandwidth of the laser, so only a tiny fraction of that power (something like 1/1000) is actually useful. It might be that the laser just doesn't put out enough photons in the narrow range of interest. A back-of-the-envelope calculations says I ought to be OK, but I don't know for sure.

The other possible problem has to do with how the light is distributed over that 2nm range. It might be a smoothly varying curve, or it might be a collection of really narrow individual modes. If it's a smoothly varying distribution, then I only need to get the wavelength to within half a nanometer or so of the krypton absorption line for the system to work. If it's a thicket of little, narrow modes, then I would need to be very much closer to the exact frequency. I don't have a good way to distinguish between those possibilities at the moment, so it's conceivable that even though I think I've got the laser tuned to the right wavelength, I'm not getting any photons where I need them.

Both of these problems could be addressed by changing lasers. The diode that I'm currently using in the 811 nm grating-locked system can be run at 819 nm (it takes a bit of effort to get it there, but it can be done), so I could switch that laser over the being the excitation laser, and tune it right onto the correct wavelength for absorption. It would require using another laser for the 811 nm light, if I want to do optical detection as well, but I've got another grating-locked laser at 811 nm that I could use for that (it's the trapping laser for my main system-- all the stuff I've described this week has been done with a separate apparatus built just for this project).

In the end, I'm not lacking for options for a next step. The list above is in roughly the order of attack that I'll be using this coming week: first, the detector, then the lamp, then the laser. It's possible that none of those will work, or it may be that I've been doing something really stupid, and will find the mistake at 10:15 Monday morning. It's that sort of uncertainty that makes this process so much fun...

Posted at 12:17 PM | link | follow-ups | 1 comment

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