Simulating The Deceleration of Negative Ions in a Quadrupole Beam Guide Using Helium Gas

Abstract

Attohallen is a new facility, to be set up by the Department of Physics at the University of Gothenburg, that will be used for exploring the temporal evolution of entangled electrons emitted from atoms and molecules using attosecond chronoscopy and coincidental measurements. An important component of this facility will be a new anion source capable of delivering a high-purity and vibrationally cooled negative ion beam. Simulations of this anion source were performed using SIMION to characterise the behaviour of the components of the ion beamline. A key parameter is the minimum pressure in a quadrupole beam guide that is required pressures, such that the sum of the kinetic and thermal energies of different anion species are reduced to a distribution with full-width half maximum of less than 1 eV, while maintaining a high transmission rate. For masses in a range between 12 and 720 u, a maximal pressure of 10.5 Pa was required. Iterative simulations were used to optimise the electrode voltage to such that anions were effectively guided through the beamline, achieving a total transmission of _8%. These results have verified the feasibility of the proposed beamline design and yielded valuable insights into the expected gas loads and consequently the required pumping capacity for the planned experiments.