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The Year in Science: Technology 1997

Plasma Mirror

By Fenella SaundersJanuary 1, 1998 6:00 AM


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For his work on nuclear fusion, physicist Michael Perry has built a very large laser. It’s beam packs a million billion watts—1000 times the power output of the entire United States. Of course, it lasts only a trillionth gadzillionth ck: real number pls of a second, which is why cities around the country don’t go dark when he does his experiments. But short as the blast is, it is long enough to obliterate lenses and mirrors used to focus the beam. What’s a physicist to do?

Last May, Perry and his colleagues at Lawrence Livermore National Laboratory in California announced that they had solved the problem by building a mirror out of hot electrically charged gas, also known as plasma. Perry starts by shining his laser beam onto a small mirror made of quartz. The laser vaporizes a thin layer of the quartz, which forms a cloud of plasma about the size of a half dollar. Electrons in the plasma, being lighter than ions, tend to move further out in front, forming a dense, highly reflective layer. The plasma forms so quickly—after absorbing only six percent of the laser beam—that there is still time left to reflect the remaining 94 percent of the beam. Plasmas have always been sort of tenuous, fuzzy things, like the corona of the sun, says Perry. But if you have a plasma with an abrupt interface to air, it will behave the same way as a metal mirror.

Once the laser ignites the little pellet of nuclear fuel, the resulting explosion destroys the plasma mirror. But that’s okay. The plasma mirror is so cheap to build, Perry can afford to replace it each time he runs his experiment. And having the plasma mirror there to direct the beam onto the target allows him to keep a second mirror——a large, rugged and expensively-built mirror designed to focus the beam—out of harm’s way.

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