Detailed Studies of Cold and Ultracold Ions in RF Traps

Christian Schneider, Steven J. Schowalter, Kuang Chen +3 more

Ethan Vale

M. Niranjan, Anand Prakash, S. A. Rangwala

We evaluate the performance of multipole, linear Paul traps for the purpose of studying cold ion–atom collisions. A combination of numerical simulations and analysis based on the virial theorem is used to draw conclusions on the differences that result, by considering the trapping details of several multipole trap types. Starting with an analysis of how a low energy collision takes place between a fully compensated, ultracold trapped ion and an stationary atom, we show that a higher order multipole trap is, in principle, advantageous in terms of collisional heating. The virial analysis of multipole traps then follows, along with the computation of trapped ion trajectories in the quadrupole, hexapole, octopole and do-decapole radio frequency traps. A detailed analysis of the motion of trapped ions as a function of the amplitude, phase and stability of the ion’s motion is used to evaluate the experimental prospects for such traps. The present analysis has the virtue of providing definitive answers for the merits of the various configurations, using first principles.

Roland Wester

Nikita Poljakov, M. Sameed, Stephen A. Jones

Abstract We simulated the motion of BaH + molecular ions which are sympathetically cooled by laser-cooled Ba + ions in linear quadrupole and octupole radiofrequency (RF) ion traps. By varying the trap voltages, we studied the velocity distribution of hydrogen atoms that would be produced upon threshold photodissociation of the BaH + . We found that hydrogen with a kinetic energy below 30 mK × k B can be produced in both traps, making the scheme suitable for loading hybrid ion–atom traps such as those used to synthesize and trap antihydrogen or study ultracold atom–ion collisions. We discuss how the dynamics of RF traps could be exploited to produce hydrogen below this limit by either driving the photodissociation transition with a small laser beam aligned along the axis of the trap, or by pulsing the laser during the micromotion turning points.

Henrik B. Pedersen, Henrik Juul, Frank K Mikkelsen +2 more

We describe the setup and characterization of a cryogenic multipole radio-frequency (RF) ion trap that enables the accumulation and cooling of mass-selected ions before injection into the SAPHIRA storage ring. To characterize the RF trap setup, we use ${\mathrm{OH}}^{\ensuremath{-}}$ anions and explore the threshold photodetachment cross section measured after storage in SAPHIRA as a probe of the rotational temperature. Beyond the temperature of the ion trap assembly, cooled to 6 K, the final rotational temperature of the ${\mathrm{OH}}^{\ensuremath{-}}$ ions is strongly influenced by the density of cooled He and the actual number of trapped ions while much less affected (possibly unaffected) by the time-varying field of the trap. To obtain rotationally cold ${\mathrm{OH}}^{\ensuremath{-}}$ ions, the RF trap must be operated with low He density and a low number of ions. High He densities, corresponding to a strong coupling of the trapped ions and He gas, lead to a significant rotational heating of the trapped ion ensemble, and the He density seems to limit the actual reachable rotational temperature. We demonstrate that cold ions in the RF trap remain cold for at least 30 ms in the SAPHIRA (300 K) storage ring at a base pressure of $\ensuremath{\sim}8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}\phantom{\rule{0.16em}{0ex}}\mathrm{mbar}$.