Physicists Slowed Bare Argon Nuclei From a Third the Speed of Light and Then Chilled Them With a Cloud of Electrons

Physicists at the Technical University of Darmstadt and the GSI Helmholtz Center for Heavy Ion Research have taken argon atoms stripped of every one of their electrons, slowed them from roughly 30% of the speed of light to a near standstill, held them in a trap, and then cooled them further using a cloud of electrons — a sequence that had never been completed before.

The result, published Aug. 1 in Physical Review X, solves a practical problem that has blocked a whole class of precision experiments. Highly charged ions are among the most useful objects in physics for testing fundamental theory. Strip an atom down to a bare nucleus and the electric fields near it become extraordinarily strong, which makes such ions exquisitely sensitive probes of quantum electrodynamics, of nuclear structure, and of whether the constants of nature drift over time. The difficulty is that the only way to make them in quantity is to run atoms through an accelerator, and that leaves them moving far too fast to study carefully.

"The production took place at about 30% of the speed of light, and before we could trap the ions, we first had to reduce their kinetic energy by a factor of about 10,000," said Dr. Simon Rausch, the lead author, who works in the research group of Professor Wilfried Nörtershäuser.

That factor of 10,000 is the heart of the achievement. The team decelerated the beam in stages and then confined the resulting ions in a Penning trap 387 millimeters long — a device that holds charged particles using a strong magnetic field combined with electric fields, so the ions circle in place rather than fly through. The ions remained stored for several seconds, an eternity by the standards of accelerator physics and long enough to run measurements on them.

Trapping was only half the problem. Ions arriving from an accelerator retain a spread of energies, and that residual motion smears out any spectroscopic measurement made on them. The Darmstadt group loaded a population of electrons into the same trap alongside the ions. The lighter electrons shed energy readily as electromagnetic radiation, and collisions transferred energy from the heavy ions to the electrons, which then radiated it away — the first time this electron-cooling technique has been observed working on highly charged ions inside a trap.

The group has since run user experiments with gold ions, which carry far more charge than argon and generate correspondingly more extreme fields. The work is described in S. Rausch et al., "Deceleration of Accelerator-Produced and In-Trap Electron Cooling of Highly Charged Ions" (DOI 10.1103/961c-j3p5).

What the technique opens up is a supply of slow, cold, heavily charged ions available on demand — the raw material for atomic and nuclear physics measurements, tests of fundamental symmetries, and studies of how such ions interact with solid surfaces.