X-rays speed softly, carry a big blast.

X-rays speed softly, carry a big blast

For defense scientists developing new X-ray generators, bigger is better. Researchers last month shot past the previous record for most powerful X-ray yield using a machine that squeezes a gas into an X-ray-emitting plasma. On its fifth try after some new modifications, the machine, called Saturn, blasted out 14 trillion watts of X-ray energy in 40 billionths of a second -- expelling more than 30 times the amount of energy in all the electricity consumed in the United States at that moment.

The blast was big but "soft" -- meaning most of the X-rays fell into the weaker end of the X-ray energy range. With these lower-energy rays, the researchers rely on strength in numbers to create record-breaking blasts, and they find it much easier to get a huge yield of soft X-rays than hard, explains M. Keith Matzen, who led the work at Sandia National Laboratories in Albuquerque, N.M. The Sandia scientists need both soft and hard X-rays for Saturn's major applications--weapons testing and, ultimately, the development of X-ray lasers for potential use in advanced weaponry, microscopy and holography.

Saturn in its original mode works in much the same way as a dental X-ray machine, shooting electrons at a metal sheet. The bombarding electrons reshuffle the electrons in the metal atoms, converting their energy to X-rays (SN: 10/31/87, p.276). The device can produce an X-ray dose equal to a million dental X-rays -- though even the most power-crazed dentists probably couldn't fit the 96-foot-diameter cylindrical apparatus into their offices.

While the original mode yielded mainly hard X-rays, the Sandia scientists recently modified the device to work by a second process, known as "gas puff z-pinch," which generates soft X-rays. In this mode, explains Matzen, the X-rays come from a cylinder of gas about an inch long and an inch across, positioned in the center of the vast machine. To trigger X-ray production, Matzen's group must send a huge, 10-million-amp current through the gas cylinder -- a task that required months of research to accomplish, he says. The current tears electrons from gas atoms, changing the gas to a sea of charged particles known as a plasma. It also generates a magnetic field that rapidly squeezes the plasma into a millimeter-thick filament. In this implosion, called a z-pinch, the plasma heats up to 10 million [deg.]C, at which point it emits X-rays.

Sandia researchers plan to use the powerful machine for blasting X-rays at various weapons to see how well they hold up. Saturn's pulses simulate the X-rays that weapons might receive from nuclear explosions in space, says James E. Powell of Sandia. Another plan, and Matzen's chief interest, is to use Saturn to develop an X-ray laser. No one yet has achieved a laser with a wavelength short enough to fall into the X-ray range, says Matzen. The shorter the wavelength of a laser, he explains, the greater the power source needed to drive it -- and Saturn's soft blasts might someday provide a source potent enough to bring Sandia to that goal.