Simulations for Ion Traps Buffer Gas Cooling
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1 Introduction 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
S. Schwarz National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, MI 48824, USA e-mail: [email protected] Schwarz, S.: Simulations for Ion Traps – Buffer Gas Cooling. Lect. Notes Phys. 749, 97–117 (2008) c Springer-Verlag Berlin Heidelberg 2008 DOI 10.1007/978-3-540-77817-2 4
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pull the ions through the buffer gas.1 If the axial DC potentials are chosen such that they form a potential well to accumulate ions, then this device is usually referred to as a linear Paul trap. By switching the DC voltages the ions can then be extracted as a cooled ion bunch. This type of ion accumulator has become a workhorse at many rare-isotope facilities (see, e.g., [1, 2, 3, 4]) due to its high efficiency and its ability to deliver excellent pulses and has been key to a number of successful high-precision Penning trap mass measurements [5, 6, 7]. The application of RF buffer gas coolers in this field is steadily expanding. They are now also used to increase the sensitivity of collinear laser spectroscopy [8] and they are planned to improve the resolving power of mass separators at existing and future ISOLfacilities [9, 10]. Earlier approaches to accumulate ions at a rare-isotope facility were made at ISOLDE/CERN [11] with a 3D Paul trap [12], which uses RF electric fields in all three dimensions to confine ions. An improved version of this
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