LIGO diary--report #2

by Wm. Robert Johnston
last updated 1 August 2001

Wednesday, 11 July 2001: The beginning of this week I completed the requirements to join the ranks of Basic Trained Personnel in regard to laser safety. This allows me to enter the LVEA, the large room containing the laser and vacuum assemblies of the end station (the corner of the "L" shape). On Monday I submitted to a two-hour eye exam, complete with flash photos of the retina of each eye. (The ophthalmologist, viewing these photos, was able to deduce that I was born prematurely.) This baseline eye exam is required of all LIGO personnel that work around the laser, so that should eye damage from laser light ever occur, it can be established.

On Tuesday I had my first tour of the LVEA. In the large room (like a warehouse) the tunnel and associated assemblies are laid out on the ground in a "+" shape. The laser is at the end of the "east" branch. The laser used is infrared, meaning the laser light is invisible to the human eye--but direct or indirect exposure can still carry enough energy to damage the eye. The laser travels in the vacuum tube to the center of the "+" shape, the location of the beam-splitter mirror. This mirror splits the beam into two beams, one down the X-arm ("west" branch of the "+" shape) and one down the Y-arm ("south" branch of the "+" shape). Between the beam-splitter mirror and each arms' end station mirror, the laser beam is trapped, so to speak. A fraction of the light from each returning beam is directed from the center of the "+" shape to the "north" branch, where the recombined laser beams mutually interfere. This is what provides information on changes in the relative lengths of the arms, allowing (someday soon!) detection of gravitational waves (GWs). (The four branches form right angles, but not exactly N-S-E-W.)

The tunnels are 1.2 meters in diameter, but the LVEA contains much larger chambers for equipment such as described above. The vacuum tubes in the LVEA are actually a series of large roughly spherical chambers perhaps 3 meters across. Each one can be sealed off independently, allowing maintenance of a component in a single chamber without repressurized and subsequently depressurizing the whole 8 kilometers of tunnel. This saves time: removing the air from the whole tunnel takes a few days. LIGO's vacuum assembly, with a total volume equal to a cube about 20 meters on a side, will operate at one-one trillionth of normal atmospheric pressure. This is among the largest vacuum chambers in the world.

The LVEA room is a clean room environment. Before entering, we have our shoes brushed, put booties over our shoes, and don laser goggles (these reduce the transmission of the specific infrared light from this laser). Others working directly on tube assemblies must also wear gowns, hair covers, and face masks.

So late Tuesday morning I was put to work. Along a row of cabinets from the laser to the beam-splitter mirror they have stray radio signals in some of the cables. I used a directional radio antenna and a signal analyzer to map the intensity of the radio emissions along a 50-foot path parallel to the tube assembly (most of the distance from the laser to the beam-splitter mirror).

Today (Wednesday) I accompanied the electronics specialist as he experimented with grounding one of the cables at various locations. The current proposal is to install a proper ground for this cable, then check and see if the stray emissions remain.

Various presentations on GW-related science and technology are interspersed during the work week. Thus far I have been to these presentations:

Weather has cleared up; it has been mostly sunny (and hot) since Saturday. Today we actually have a slight breeze. Incidentally, LIGO is located in a forested area of Louisiana, surrounded by pine and other trees. (We can't explore the neighboring woods, though, because of snakes and hunters.)


Image credits: Wm. Robert Johnston, © 2001 (top/middle); LIGO-Livingston, 2001 (bottom).

© 2001 by Wm. Robert Johnston.
Last modified 1 August 2001.
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