Computer simulation and visualization of supersonic jet for gas cluster equipment A. Ieshkin a , Y. Ermakov b , V. Chernysh a , I. Ivanov a , I. Kryukov c , K. Alekseev d , N. Kargin d , Z. Insepov e,f,n a Faculty of Physics, Lomonosov Moscow State University, Leninskie Gory, Moscow 119991, Russia b Scobeltsyn Nuclear Physics Research Institute, Lomonosov State Moscow University, GSP-1, Leninskiye Gory, Moscow 119991, Russia c Institute for Problems in Mechanics, Russian Academy of Sciences, prosp. Vernadskogo,101, Block 1, Moscow 119526, Russia d National Research Nuclear University «MEPhI», Kashirskoye shosse 31, Moscow 115409, Russia e Purdue University, 500 Central Drive, West Lafayette, IN, USA f Nazarbayev University Research and Innovation System, Kabanbay Batyr Avenue 53, Astana, Kazakhstan article info Article history: Received 30 March 2015 Received in revised form 1 June 2015 Accepted 13 June 2015 Available online 20 June 2015 Keywords: Cluster ions Flow visualization Supersonic nozzle abstract Supersonic nozzle is a key component of a gas cluster condensation system. We describe a flow visualization system using glow discharge with annular or plane electrodes. The geometric parameters of a supersonic jet under typical conditions used in a gas cluster ion beam accelerator are investigated. As well numerical simulations were performed. Dependence of inlet and ambient pressures and nozzle throat diameter on the shock bottle dimensions is described for different working gases. Influence of condensation rate on shock bottle axial size is discussed. & 2015 Elsevier B.V. All rights reserved. 1. Introduction Recent decades, gas cluster ion beams (GCIB) have been under extensive study. GCIB are widely used in practical applications, such as precise surface polishing and etching, ultra-shallow implantation, ion-assisted deposition of thin films, probing in SIMS technique [1–3], as well as in investigations of fundamental properties of matter [4,5]. The usual way of obtaining gas clusters is adiabatic expansion of a working gas through a supersonic nozzle, so that expanded gas becomes cold enough for clusterization. Information on properties of the jet below a nozzle is essential for optimization a nozzle and skimmer geometry and the distance between them. The typical structure of a supersonic jet shown in Fig. 1 is described in [6,7]. It is shown in Fig. 1, that the gas expanding to the ambient pressure p 1 forms a “shock bottle” configuration. Expansion waves reflect from the jet boundary as weak compres- sion waves and form an oblique shock. Behind the oblique shock, the flow is supersonic as well, but its Mach number is less than in the jet core. In the core, the flow is isentropic, and ideal gas equations are correct for it. In the low-temperature media of the expanding gas condensation starts and clusters can exist. Then, passing through the normal shock, which is called Mach disc, entropy increases: the gas gets warm and clusters are likely to dissociate. To prevent this dissociation, a skimmer is used. It cuts the Mach disk, penetrates into the core of the jet and evacuates clusters before they collapse. The skimmer should be distant enough from the nozzle in order to let the clusters grow. On the other hand, it should not disturb the shock bottle structure. So, knowing the form and the structure of the flow is essential for optimizing cluster ion sources. The picture given attributes to a jet from a sonic nozzle, i.e. a nozzle with only a converging part. However, a supersonic nozzle consisting of converging and diverging parts is usually used to generate GCIB [1,3]. Unfortunately, for such a nozzle we could not find any information on a jet structure under extensive clusteriza- tion. Typical ways of observing gas flows are rather complicated techniques such as schlieren-photography, electron-excited lumi- nescence or Raman scattering [6–8]. Besides, these techniques demand additional equipment and substantial changes of the Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/nima Nuclear Instruments and Methods in Physics Research A http://dx.doi.org/10.1016/j.nima.2015.06.026 0168-9002/& 2015 Elsevier B.V. All rights reserved. n Corresponding author at: Purdue University, 500 Central Drive, West Lafayette, IN, USA. E-mail address: zinsepov@purdue.edu (Z. Insepov). Nuclear Instruments and Methods in Physics Research A 795 (2015) 395–398