Simulations of the HIE-ISOLDE radio frequency quadrupole cooler and buncher vacuum using the Monte Carlo test particle code Molflow+ M. Hermann ⇑ , G. Vandoni, R. Kersevan, C. Babcock CERN – European Organization for Nuclear Research, CH-1211 Geneva 23, Switzerland article info Article history: Received 3 April 2013 Received in revised form 2 August 2013 Accepted 22 August 2013 Available online 26 October 2013 Keywords: Vacuum simulation Molflow+ Test Particle Monte Carlo method HIE-ISOLDE ISOLDE Radio frequency quadrupole cooler and buncher abstract The existing ISOLDE radio frequency quadrupole cooler and buncher (RFQCB) will be upgraded in the framework of the HIE-ISOLDE design study. In order to improve beam properties, the upgrade includes vacuum optimization with the aim of tayloring the overall pressure profile: increasing gas pressure at the injection to enhance cooling and reducing it at the extraction to avoid emittance blow up while the beam is being bunched. This paper describes the vacuum modelling of the present RFQCB using Test Particle Monte Carlo (Mol- flow+). In order to benchmark the simulation results, real pressure profiles along the existing RFQCB are measured using variable helium flux in the cooling section and compared with the pressure profiles obtained with Molflow+. Vacuum conditions of the improved future RFQCB can then be simulated to val- idate its design. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction The High Intensity and Energy ISOLDE (HIE-ISOLDE) project aims at upgrading CERN ISOLDE and REX-ISOLDE facilities to in- crease the energy and intensity of the delivered radioactive ion beams. This is achieved by means of a new post-accelerating, superconducting linac, accompanied by a design study of the major subsystems and the target areas linked with the increased inten- sity of the proton primary beam from the future, upgraded injec- tors chain of Linac4 and Booster. In the framework of the HIE-ISOLDE design study, a new radio HIE-ISOLDE frequency quadrupole cooler and buncher (RFQCB), is developed [1]. A RFQCB lowers the temperature of an ion beam by reducing its energy dispersion and emittance. The beam cooling is achieved by collision with a buffer-gas in the confining field of a Paul trap. Beam optics and vacuum are here interlaced. Electrode form optimization with improvement of molecular conductances and increase of pumping speed aims at tayloring the pressure profile, increasing gas pressure in the Paul trap to enhance cooling – and reducing it at the extraction to avoid beam loss and emittance blow up while the beam is being bunched. At the same time, at the entrance of the device a trade-off be- tween acceptance and vacuum level has to be achieved. Both beam optics and differential pumping requirements give rise to a struc- tural complexity of the vacuum volume of the RFQCB, such that molecule distribution is best treated with direct, numerical simu- lation techniques [2,3]. Fig. 1 shows a scheme of the current RFQCB at ISOLDE. A direct simulation Monte Carlo, with the code Molflow+ [5], generating molecules one by one and tracking them from source to sink is applied here. Allowing for file import from CAD, Molflow+ permits to model ultra-high vacuum systems in the molecular flow regime for complex geometries. The code is written and main- tained by R. Kersevan and M. Ady (TE/VSC group at CERN). In order to benchmark the simulation results, real pressure pro- files along the present RFQCB, ISCOOL, are measured using variable helium flux in the cooling section and compared with the pressure profiles obtained with Molflow+. The simulations are in excellent agreement with the measure- ments. Vacuum conditions of the improved future RFQCB design may then be simulated to enhance pressure drop between the cen- tral cooling section and the entrance and exit electrodes. 2. Molflow+, a Test Particle Monte Carlo code Molflow+ is a numerical code for the calculation of molecular transmission probability and pressure distribution in vacuum sys- tems, in the molecular flow regime. In contrast to the viscous flow 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.08.061 ⇑ Corresponding author. Tel.: +41 762277735. E-mail addresses: mario.hermann@cern.ch (M. Hermann), giovanna.vandoni@- cern.ch (G. Vandoni), roberto.kersevan@cern.ch (R. Kersevan), carla.babcock@- cern.ch (C. Babcock). Nuclear Instruments and Methods in Physics Research B 317 (2013) 488–491 Contents lists available at ScienceDirect Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb