Modelling the Differential Pumping of the TASCA Gas-Vacuum System

The scientific program at the TASCA gas-filled separator [1] requires the highest possible UNILAC beam intensities. Presently, they are of the order of one particle microampere and a significant increase is in preparation. To make use of these intensities a windowless differential pumping system (DPS) was installed and a computer program was developed to simulate vacuum conditions at various sections along the beam line as a function of parameters like gas composition and pressure in the separator, pumping capacity and the geometry of the collimator; see Fig. 1 for the program interface. The goal was to maintain a stable pressure in the separator (typically about 1 mbar He) and get a rapid pressure drop over a short distance to obtain 10 mbar in the beam line. The system consists of three pumping sections and our design strongly profited from the experience obtained at RITU [2]. Our first section will have a 1070 m/h (volume flow rate, N2) Roots pump with a 62 m/h backing pump (test and comparisons with simulations were performed with smaller pumps). The second section has a 400 L/s (N2) turbomolecular pump with the specific feature that it can operate at a relatively high pressure coupled to a 32 m/h backing pump. The third section consists of a 1400 L/s (N2) turbo and a 20 m/h backing pump. One envisioned configuration, which is depicted in Fig. 1, has collimators of 10x140, 13x300 and 16x300 (inner diameter x length / mm). Calculations with the DPS simulation program show a pressure drop of about a factor 100 for each section; see Fig. 1. The program is written in LabVIEW and was tested experimentally with different gases. A good agreement between calculated and measured pressures was obtained. The program calculates the pressure and flow through collimators in a static regime of selected gases based on the following equations: 1. For viscous flow: Qv = (πD∆P)/(256ηL), with η: viscosity of gas at t=20C, D: collimator diameter [m], L: collimator length [m], P: pressure [Pa].