Hyperfine Interactions 132: 505 509, 2001. 2001 Kluwer Academic Publishers. Printed in the Netherlands. 505 First Measurements with the Gas Cell for SHIPTRAP O. ENGELS 1,L.BECK 1, G. BOLLEN 2, D. HABS 1,G.MARX 3,J.NEUMAYR 1, U. SCHRAMM 1, S. SCHWARZ 2,P.THIROLF 1 and V. VARENTSOV 4 1 Ludwig-Maximilians-Universität München, Garching/München, Germany 2 National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, Michigan, USA 3 Gesellschaft für Schwerionenforschung mbh, Darmstadt, Germany 4 V. G. Khlopin Radium Institute, St. Petersburg, Russia Abstract. SHIPTRAP is an electromagnetic transport and trapping system to provide very clean and cold beams of singly-charged recoil ions from the SHIP facility at GSI. The different components of the system are currently under development in Munich (gas cell and extraction RFQ) and GSI (Buncher RFQ and Penning traps) [1]. Design and manufacturing of the prototype buffer gas cell and the extraction RFQ based on a wide range of simulations have been completed. The results of these simulations together with the first measurements will be reported. Key words: gas cell, rf-ion guide. 1. Introduction The present state of ion trap technology allows the coupling of a trap system to radioactive ion beams of higher energies. Such a project is SHIPTRAP at GSI which is presently under construction to stop, cool and deliver heavy radionuclides for downstream experiments. The energetic radionuclides are delivered by the SHIP velocity filter [3]. The scientific case as well as a detailed description of the whole SHIPTRAP facility are given elsewhere [4, 5]. Stopping ions in a gas cell filled with buffer gas has been investigated for several years. The aim of most of these projects in the field of nuclear physics is the production of ions directly in the gas cell, by nuclear reactions of an energetic primary beam with appropriate targets placed inside the cell. This technique has been very successfully used in Jyväskylä [6], Leuven [7], and Mainz [8]. A new generation of projects has started recently in which the reaction takes place outside the gas cell. The reaction products are injected and stopped in a gas cell. Groups at ANL [9], RIKEN [10], NSCL-MSU [11], and Munich [12] are currently working in that field. The method of ion extraction from the gas cell differs for the cells in use or under development, depending on the boundary conditions. The concepts reach
506 O. ENGELS ET AL. from an extraction via gas flow and via gas flow combined with electrostatic fields to the extraction via gas flow plus a combination of electrostatic and rf-fields. After their extraction, the ions have to be separated from the gas and guided into a high vacuum region. This is done by an extraction RFQ, a longitudinally segmented rf quadrupole structure. This structure is similar to ion guides, emittance improvers and ion bunchers, described elsewhere [13 15]. 2. Setup In the SHIPTRAP project the gas cell design together with the extraction RFQ is expected to be the most critical part. Therefore a gas cell study program was initiated in Munich and a prototype gas cell has been designed for tests at the Tandem Accelerator in Garching and for first tests at SHIP. 2.1. CONCEPT The prototype of the gas cell for SHIPTRAP follows the concept of extracting the ions via the gas flow and guiding electrostatic fields. These two mechanisms can be simulated separately in first order, due to the special feature of a high transmission grid in front of the nozzle. This grid prevents a loss of ions in the defocusing field inside the nozzle and improves the focusing inside the cell due to a spherical shape. The extracted ions out of the gas cell are then guided via a linear RFQ into a good vacuum region. This extraction RFQ separates the ions from the neutral gas. 2.2. TEST SET-UP The prototype gas cell has a length and a diameter of 100 mm. The guiding field inside the cell is created by three electrodes and a spherical grid covering the supersonic nozzle with an inner diameter of 0.6 mm. The subsonic part of the nozzle has a conical shape to achieve gas velocities between the grid and the nozzle high enough to drag the ions through the nozzle [12]. The supersonic part of the nozzle creates a jet inside the extraction RFQ. In addition to this drag by the gas an accelerating field between the nozzle and the RFQ can be applied. The design of the extraction RFQ and the electronics profits from the work done at ISOLTRAP and GSI in the past three years [15]. Compared to the structures used there the extraction RFQ of SHIPTRAP is a short structure with 12 segments and a total length of 120 mm. The diameter of the rods is 11 mm with an aperture (diameter) of 10 mm. The ions are mass selective detected in a QMS followed by a secondary electron amplifier. This detector will be replaced by an MSP (Micro- Sphere Plate) to be able to detect the ions in rather bad vacuum conditions [16]. The test set-up is shown schematically in Figure 1. The pumping system consists of an oilfree backing pump (50 l/s) and two magnet beared turbo molecular pumps (400 and 1600 l/s). The larger one is to maintain
FIRST MEASUREMENTS WITH THE GAS CELL FOR SHIPTRAP 507 Figure 1. The SHIPTRAP test set-up consisting of the gas cell with the three electrodes (E1, E2, E3) to create the accelerating and guiding field and a nozzle covered by a high transmission grid, the extraction RFQ as a differential pumping section to guide the ions to a region with high vacuum and the QMS and detection section for a mass selective ion detection. The ions are injected radially into the gas cell. a pressure of 10 2 mbar at the position of the extraction RFQ, the 400 l/s pump is situated at the QMS and Detection section. In addition to the clean environment in the cell, the buffer gas is purified by a getter based system, suppressing the impurities in the gas to the level of <1 ppb thus preventing molecule formation. The whole setup can be heated up to 250. 3. Simulations After a wide range of simulations the design and manufacturing of the prototype of the gas cell is completed. The simulations covered the following topics: stopping of the ions in the gas (SRIM); drag of the ions via the electrical field in the cell towards a supersonic nozzle (SIMION); drag of the ions through the nozzle via the gas flow (VARJET, solving the full system of time-dependent Navier Stokes equations [17]). First stopping simulations indicate that, whatever gas is used, a majority (50 90%) of most radionuclides of interest could be stopped within a spheroid of 40 diameter and 120 mm length (for 232 Th at 100 kev/u in He at 100 mbar). To these values one has to add the horizontal and vertical SHIP-beam dimensions of 50 and 30 mm. These simulations imply the minimum dimensions for the innermost electrode of the final gas cell for SHIPTRAP. The ion motion caused by the electrostatic field was simulated extensively with SIMION. With a maximum voltage of 700 V at electrode 3 an average extraction time of 30 ms is expected. VARJET calculations showed that the gas velocities in the cell are negligible (<1 m/s), with the exception of the area close to the nozzle in the gas cell and between gas cell and RFQ. In order to determine the exact motion of the ions in this area one would need a combined code for gas- and electrodynamics. Such a
508 O. ENGELS ET AL. code is under development using the electrostatic field information of SIMION and the gasdynamic parameters of VARJET. Introducing the parameter G r,z = v ion;r,z v gas;r,z (1) K lead us to the following equations of motion for ions in a combination of gas and electric field: a r + q m (G r E r ) = 0, a z + q m (G z E z ) = 0, (2) where G r,z are the gasdynamic analogon of the electric field components E r,z, v ion;r,z and v gas;r,z are the radial and longitudinal components of the gas and ion velocity, respectively, and K is the ion mobility; a r,z are the acceleration components and q and m are the charge and mass of the ion. Beside the gas cell, the jet of the ion gas mixture expanding into the extraction RFQ is a critical point for the design of the cell and the RFQ. Therefore the gas flow and the cooling of the ions in the extraction RFQ were measured. 4. Measurements First measurements with the gas cell showed a clear dependence of the ion focusing towards the nozzle and the grid in front of the nozzle. The grid allows to achieve faster extraction times due to higher voltages applicable in the cell. These higher voltages lead to a stronger defocusing of the ions. However, this drawback can be compensated by the better focusing properties of the grid. Figure 2 shows as an example a comparison of a VARJET simulation and the measurement of the impact pressure in the RFQ. The pressure in the gas cell was 100 mbar of argon, with a pumping speed of 100 l/s at the position of the extraction RFQ. The pressure in the RFQ was measured with a pitot probe. The measured pressure is an impact pressure, caused by the gas jet impinging the probe. The Figure 2. Simulation and measurement of the impact pressure inside the RFQ.
FIRST MEASUREMENTS WITH THE GAS CELL FOR SHIPTRAP 509 simulation is in good agreement with the measurement with a discrepancy at short distances from the nozzle. This discrepancy is explained by a strong effect of the pressure probe on the supersonic jet in the vicinity of the nozzle exit. The outer nozzle diameter (6 mm) is comparable to the diameter of the probe (2 mm). Test measurements with laser ionized Cs in the gas cell were performed to optimize the voltages in the cell and the RFQ and to probe the extraction time. A voltage difference of 30 V was applied between the first and last segment of the extraction RFQ together with an rf-voltage of 340 V pp at 1 MHz in order to separate the ions from the gas and to guide the ions to a QMS for mass selective ion detection. 5. Outlook After completion of the tests at the Tandem Laboratory in Garching/Munich the prototype of the gas cell will be tested at GSI with a SHIP beam in the middle of 2001. In parallel, the final gas cell is under development and will be ready for the first tests end of 2001. The goal for the final version is to be able to stop all incoming ions and extract the ions in an average time of 30 ms. Acknowledgements Work supported by the BMBF with grant number 06LM973 and by the European Union (RTD project EXOTRAPS). References 1. Marx, G. et al., this issue, 463. 2. Bollen, G., Nucl. Instrum. Methods A 368 (1996), 675. 3. Münzenberg, G. et al., Nucl. Instrum. Methods 161 (1979), 65. 4. Dilling, J. et al., Hyp. Interact. 127 (2000), 491 496. 5. Proposal for SHIPTRAP: A capture and storage facility at GSI for heavy radionuclides from SHIP, GSI, 1998. 6. Dendooven, P., Nucl. Instrum. Methods B 126 (1997), 182. 7. Van den Berg, P., Nucl. Instrum. Methods B 126 (1997), 194. 8. Backe, H. et al., Phys.Rev.Lett.80 (1998), 920. 9. Savard, G., In: Proc. IGISOL-7, Mainz, 1999. 10. Wada, M., In: Proc. IGISOL-7, Mainz, 1999. 11. Bollen, G., private communication, 2000. 12. Engels, O. et al., Annual Report, GSI, 1999. 13. Ärje, J. et al., Phys.Rev.Lett.54 (1985), 99. 14. Lunney, M. D. et al., Internat. J. Mass Spectrometry 190 (1999), 153. 15. Herfurth, F. et al., submitted to Nucl. Instrum. Methods (April 2000). 16. Naaman, R. et al., Rev. Sci. Instrum. 67 (1996), 3332. 17. Varentsov, V. L. et al., Nucl. Instrum. Methods A 413 (1998), 447.