Mitt liv som Tobin

IT's been a busy week at LIGO for me.. taking data, installing electronics, soldering, assembling, testing, spectrum-analysing, programming, etc. The LIGO electronics shop here has all the oscilloscopes, multimeters, spectrum analysers, function generators, power supplies, cables and connectors of every sort, etc etc, to make any reasonable person happy.

This morning I stopped at the Columbia River Exhibition of History, Science, and Technology museum here in Richland, where for $2.50 a retired Hanford worker gave me a two-hour tour of interpretive displays and models of the Hanford Works. I quizzed a guy at work on interesting places to go see around here, and he was full of ideas. I ended up just driving up around Hanford Site to a place where there's a view of the reactors.

I drove out to the farming town of Othello just to have a destination... it started to snow, which at first was novel, but then very quickly I realized that there was a strong possibility that I'd get stuck out in the middle of nowhere. Was quite relieved to get to a larger highway which is plowed. I stopped in at LIGO to take care of a few things... the two people there were gearing up to be snowed in.

Back in Richland now... I have a flight out of here at 9:00 tomorrow.. to Salt Lake City to Chicago to Rochester. Rochester! Yay! Looking forward to seeing people I know again.

After a reasonably sumptuous breakfast at the hotel, Adrian and I drove out to the LIGO site. It's really closer than I had expected. The city of Richland, such as it is, peters out and quickly we're out on a straight stretch of 2-lane highway seemingly in the middle of nowhere. Steam is billowing up from somewhere a few miles off the road -- a reactor still running? The snow-covered desert is really quite pretty; the snowy hills blend right into the sky. At a bend in the Yakima river, we turn right onto the road that doesn't exist. A few miles later we're at the LIGO site, with a big sign out front with the LIGO logo.

There's a small collection of buildings at the "corner station" (the elbow of the "L"). One where we check in, where there are some offices too. Another has a tall roll-up door; this is some kind of assembly area. And the third building in the corner station itself. Inside everything is clean and well organised; it seems new.

The first half of the day was mostly the formalities (signing papers, reading safety information) and practicalities (getting keys, being shown around) of being introduced to LIGO, and the dozen-or-so people working on site. The Control Room is like Mission Control, with five projector displays on the wall (only the three forward ones currently in use), showing the current Interferometer status. Overhead, two large digital clocks display the time, one in UTC and one in local time. On two rows of desks there are numerous computer terminals, each with two large flatscreen displays. Almost all SUN Sparc machines. Closed-circuit TV displays show the area surrounding the LIGO center, and others show the LASER spots inside the interferometer. Most of the stations have the usual roller-chairs, but a few large inflated "fitness balls" are available too, lightening the atmosphere a bit.

At lunchtime we attended a meeting of the collaboration. Like at Big Physics labs everywhere, the meeting is done by teleconference on a sort of speaker phone system with PowerPoint slides projected onto the wall. The other LIGO observatory, in Livingston, Louisiana, is on the phone. As are, I think, the collaborators at MIT and Caltech. The meeting is, as meetings tend to be, very boring. They are planning when to do some big upgrades, planning for the next several years.

Daniel took Adrian and I into the clean-room environment that houses the corner station to get us acquainted. There's an automatic boot scrubber before we enter the 'garb' room, and then we put on booties and LASER safety goggles. Earlier Adrian had drawn for me a schematic of the LIGO observatory. The input laser is only a few watts, but the arms of the interferometer form resonant cavities, which build up a laser power on the order of kilowatts. The laser is infrared. We have proximity cards to enter the cleanroom area. We enter one-by-one, and the computer notes that we're now each "inside". Entering an area without using the proximity cards activates interlocks and automatically shuts down the lasers. The safety system is to protect people from lasers, and the cleanroom system is to protect the ultra-high-vacuum system and the clean optics from the dirt that rides in on people.

Once inside the clean area, the vacuum system of the corner station is visible. The beamlines are housed in a vacuum pipe about 1m in diameter. The optics systems are housed in large metal cylinders along the beamline that look like giant tin cans welded into the system. Beside the beamline are the usual rackmount systems familiar to any physics installation: crates containing VMW-bus backplanes, with cards sprouting patch cables. Analog signals are routed into digitizers. The digital signals are eventually stored in a sort of distributed shared memory, which is reflected between machines using gigabit ethernet over optical fiber. The air is clean and it's easy to breathe.

Daniel showed us around the electronics and we set about our mission. For various reasons, there is an interest in looking at a signal at 37.5 kHz -- noise tends to cancel out at this frequency, so it might be a good place to hunt for a signal. To gain access to this signal, software digital signal processing is used to shift this signal down to baseband, so that it can be stored at a reasonable sample rate. Our task is to verify the frequency response of some "whitening" (equalizer) boards, connect signals from the interferometer output photodiodes to this board, make sure the data acquisition (DAQ) is working properly, record data from this frequency range, and then look at it. The DAQ system seems to run on VxWorks (I believe the same OS that ran on the Mars rover).

I really have to brush up on my signal processing... FIR and IIR filters, Butterworth filter design, complex baseband signal representations / quadrature, control systems design. LIGO is really all about control loops: The main operating mode of the interferometer is to keep the interferometer output locked at a "dark fringe" (no light output; total destructive interference) by using actuators to move the mirrors; and the feedback signals to these actuators becomes the output signal of the detector. There are several resonant cavities, most of which are kept resonant by active control systems.

In between times, Adrian explains all about how LIGO works, and what we will be doing. ("And tomorrow, Dark Energy," he says. I don't know what Dark Energy has to do with anything...)