(PhysOrg.com) -- When tasked with the challenge of transporting individual ions through a sluggish sea of gas in a matter of milliseconds, a land-locked East Lansing physicist came up with a very West Coast solution – surfing. But instead of bobbing in the ocean, failure to catch a wave results in losing the opportunity to study extremely rare ions found nowhere else on Earth.
The National Superconducting Cyclotron Laboratory (NSCL) produces rare isotopes – atoms with weird numbers of neutrons that can disappear in the blink of an eye – for physicists to study in order to understand the basic workings of the universe. They do this by accelerating stable nuclei up to half the speed of light and smashing them apart. Though fast beams of the rare isotope products can be studied in many experiments, they need to be slowed down for others.
During the last decade, NSCL pioneered gas cell stopping technology where fast beams of rare isotopes were slowed to a virtual standstill through interactions with helium gas. Once slowed, however, the challenge became getting them out of the gas to study before they decayed.
“The helium gas inside the device is like honey to ions,” explains Georg Bollen, Experimental Systems Division Director for the Facility for Rare Isotope Beams (FRIB), the $615 million Department of Energy project that will subsume NSCL. “The ‘honey’ is sticky and rapidly slows down the isotopes. Then you have to find a way to drag them through it and out.”
Earlier devices used an electrostatic gradient to produce a type of electric hill for the atoms to descend towards the extraction point. The technique works, but it requires a cumbersome number of electrodes with several connections and electrical circuits. So Bollen devised an electronically simpler scheme that moves the isotopes through the gas in much the same way that a wave in the ocean moves a surfer.
The technique uses a traveling electric field that creates a wave along the electrodes. Meanwhile, another electric field pushes the ions down towards the waves to ensure that they catch one, while yet another alternating electric field – this one at radio frequencies – keeps the isotopes from crashing into the floor of the device. When balanced, the system pushes the ions to a perfect level to be propelled by the travelling electric wave.
In short, unlike a surfer at Waikiki Beach, the waves are so large that it’s nearly impossible for an ion to miss its wave.
The concept remained theoretical until recently, when nearly a year’s worth of effort by graduate assistant Mandie Gehring and research associate Maxime Brodeur proved that it could work. After successfully getting a high enough radio frequency to keep the ions off the floor, creating new closely spaced electrode designs, and getting the system balanced and working together, they had success. The wave system achieved excellent transport efficiency over distances of 35 centimeters at gas pressures up to 240 mbar, which had never been done before. What’s more, the electronics are much simpler than previous methods, using only four connections.
The next step is to continue developing the technology for use in the next generation of gas stopping devices, such as cryogenically cooled gas cells and the Cycstopper currently under construction at NSCL, a circular deceleration device inspired by the cyclotrons that accelerate the ions.
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