Laser Cooling Crystallizes Ion Beams
Beams of accelerated ions are important tools in the investigation of atomic structures. The problem is that the ions collide while being accelerated and while in a magnetic storage ring, which reduces the quality of the beam. Now researchers at Ludwig Maximilians Universität in Garching, Germany, have demonstrated that a laser cooling technique can transform an ion beam into a brilliant, crystalline state.
Detuned dye lasers simultaneously accelerate, cool and excite fluorescence in an ion beam. Researchers have used the Pallas tabletop ion storage ring to demonstrate the transition in an accelerated ion beam from a gaseous (top) to a crystalline (middle) state. The bottom image shows the ions at rest in the ring's symmetric fields. Courtesy of Ludwig Maximilians Universität.
Tobias Schätz, Ulrich Schramm and Dietrich Habs have constructed a tabletop ion-storage ring, called Pallas, with a diameter of just 115 mm. Pallas is a curved version of linear ion-confinement traps, which use highly symmetric electric fields.
With a stationary beam of magnesium ions, the field symmetry is sufficient to maintain a crystalline order, in which each ion maintains a fixed relationship to its nearest neighbors through electrostatic repulsion. But when the ion beam is accelerated -- typically using a dye laser -- and the temperature rises, velocity spread creates a gaslike beam.
The research team uses a counterpropagating frequency-doubled beam from another dye laser to cool the ions. The two laser beams, which have wavelengths of approximately 280 nm, are detuned to be near a Doppler-shifted ion-absorption line. This not only creates a narrow range of ion velocities, but also excites fluorescence, which is imaged with a photomultiplier tube or a CCD to quantify changes in the relative ion position.
With the relative velocity of the ions substantially reduced, the symmetry of Pallas' fields again dominates the ion distribution, creating a crystalline beam of high brilliance. Initially, Schätz created a linear string of ions traveling at 2800 m/s, but later efforts have produced crystals with higher-order symmetry, such as zigzag patterns and three-dimensional helices.
"These are the most brilliant ion beams possible," Schätz said. "Their density is only limited by their space charge, and, because of the low temperature of these beams, their energy spread is minimal."
Moreover, he said, the crystalline ion beams resist heating, enabling the researchers to transport them without cooling.
The researchers are bringing the technique to existing and new high-energy storage rings. They also will leverage the persistence of the crystalline state to perform high-resolution spectroscopy, where the spectral resolution improves as the number of revolutions increases.
The unique brilliance of the beams should open up a range of applications, from the precision doping of semiconductor structures to accurate tests of special relativity.
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