Simulations for Ion Traps Buffer Gas Cooling

Abstract

Buffer gas cooling of ions in traps and radiofrequency ion guides has become the method of choice to improve the quality of continuous ion beams and to provide cooled ion bunches with low emittance and small energy spread. The method is chemically unselective and has consequently been applied to manipulate ion beams ranging from He to the heaviest elements. Depending on the buffer gas type and pressure, cooling times can be as low as a few milliseconds. The transmission efficiency of today’s buffer gas coolers is approaching unity. These features make buffer gas coolers particularly useful at rare-isotope facilities, where speed, applicability, efficiency and good beam properties are essential to perform precision experiments with nuclei far from the valley of beta stability. Two types of gas-filled coolers/bunchers are currently in use at rare-isotope facilities. They are based on linear Paul traps and on Penning traps: First the externally produced ions are electrostatically slowed down to a few electron-volt energy and injected into the gas-filled ion trap. For deceleration, the central trap electrodes are typically operated at a pedestal voltage close to beam potential. Inside the trap the ions are slowed down by collisions with buffer gas molecules and accumulate in the effective potential well provided by the ion trap. Finally, the cooled ion cloud is ejected out of the trap by a strong electric field and the ion pulse is then reaccelerated to a desired beam energy. A radiofrequency (RF) multipole ion guide uses RF electric fields for the transverse confinement of ions and employs DC electric fields in the axial direction to