Rich Morin :: tchotchkes

One Man's Rubbish Is Another Man's Gold

This story may appear to be about physics, but it's actually more about convergence, cooperation, serendipity, etc. In any case, I'll try to keep the physics details under control.

SLAC (Stanford Linear Accelerator Center) gets its name from a two-mile "linear accelerator". Electrons and positrons are "injected" at one end, moving at almost the speed of light. As they move down the line, they are passed by high-energy microwaves (which are moving at light speed). Each passing wave gives each particle a bit of a push.

By the time a particle has reached the end of the line, it has gained a lot of energy (and a tiny increment of speed - see Einstein :-). This means that it does interesting things when it runs into any of several types of "targets" at the far end. These things are studied diligently, producing many papers and occasional prizes for the scientists at SLAC.

Going back to the start of the line, we see that there is a need for fast-moving electrons and positrons, often in large numbers. Generating these "on the fly" isn't practical, so SLAC built SPEAR, a "storage ring" where large quantities of particles could be held.

The ring is actually a polygon, composed of many straight line segments and shallow-angle "corners". At each corner, a set of magnets bends the stream of particles to point down the next segment. As a side-effect of this bending, hard "synchrotron" radiation is given off. The ring's designers saw this as an undesirable side-effect, resulting in difficult engineering and safety issues.

Meanwhile, however, some other scientists realized that this radiation could be really useful for other research. For instance, biologists and materials researchers could use it to examine the internal structures of matter. So, it was suggested that SLAC build some "ports" into the ring, allowing controlled amounts of radiation to emerge. After looking into the matter thoroughly, SLAC did so.

This turned out to be a fabulous success. Great science was done, many papers (and occasional prizes :-) resulted, and life was good. Except for one thing. The storage ring, although critical to the researchers using the incidental radiation, wasn't under their control. Instead, it was under the control of the linear accelerator folks, whose needs might not match those of the radiation users. Also, adding new ports was an expensive and time-consuming process.

Finally, a new ring (SPEAR 3) was built, specifically optimized for the needs of the hard-radiation users. It had lots of ports (and room to add more), and a design that optimized it for radiation production:

... a modern, low emittance 3 GeV ring optimized for producing high flux and high brightness photon beams at the Stanford Synchrotron Radiation Laboratory. SPEAR 3 will have an emittance of 18 nm-rad, reduced from 160 nm-rad, and a 500-mA beam current, increased from 100 mA. A water-cooled copper vacuum chamber will be thermally stable and, together with mode-damped RF cavities, will assure multibunch beam stability. Beam lines will see one to two orders of magnitude increase in beam flux density and brightness after they have been upgraded for the higher beam power.

-- Design of the SPEAR 3 Light Source; R. Hettel, et al

In summary, an undesirable side-effect of a piece of support equipment has been turned into an extremely useful research facility, because of the creativity, flexibility, and tenacity of the SLAC scientists. The new facility, with "one to two orders of magnitude increase in beam flux density and brightness" will surely be a significant improvement.

As Dr. Pasteur observed, "Chance favors the prepared mind."

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One Man's Rubbish Is Another Man's Gold in Science , Technology - posted at Sat, 13 Mar, 10:34 Pacific | «e» | TrackBack