Bongers, H

Ernhofer, S

Habs, D

Kester, O

Ratzinger, U

Rudolph, K

Schempp, A

Sieber, T

CERN Document Server

Design and Status of the Front Part of theREX-ISOLDE Linac?T. SieberaO. KesteraD. HabsaS. EmhoferaK. RudolphaH. BongersaA. SchemppbU. RatzingercaSektion Physik der LMU Muenchen, Am Coulombwall 1, D-85748 GarchingbInstitut fuer Angewandte Physik, Robert Mayer-Str. 2-4, D-60325Frankfurt/MaincGesellschaft fuer Schwerionenforschung, D-64220 DarmstadtThe accelerator for the REX-ISOLDE experiment at CERN [1] isa compact Linac, which will accelerate neutron rich nuclei withA/q=4.5 to energies between 0.8 MeV and 2.2 MeV (duty cycle:10%). Substantial components of the REX-ISOLDE Linac, suchas the mass separator, the 4-Rod RFQ, the IH-matching sectionand the IH-drifttube- structure, are developed, built up and testedat Munich. The design and status of these components and theresults of beam- and rf-power tests - especially with the RFQ andthe IH-structure - are presented in this paper.1 IntroductionREX-ISOLDE is an experiment with energetic radioactive beams at the RIB-facility ISOLDE at CERN [2]. The scheme of REX-ISOLDE is pursued in sev-eral steps fullling separated functions. Firstly the 1+ion beam from ISOLDEis accumulated, cooled and bunched in a Penning trap. In an electron beamion source (EBIS) the 1+ions are then converted to highly charged ions. In thenext step a separator selects the wanted charge-to-mass ratio. This bunchedlow-energetic beam of highly charged ions is rst accelerated in a 4-rod RFQto 0.3 MeV/u. In the following IH-structure and the three 7-gap resonatorsthe ions are further accelerated to a tunable nal energy between 0.8 MeV/uand 2.2 MeV/u.?work supported by the BMBF under contract 06 LM 868 I/TP:4Preprint submitted to Elsevier Preprint 30 March 2001SIS-2001-15004 September 20012 The REX-ISOLDE mass separatorDue to the energy spread of the ions which are expelled from the EBIS (E/Eof  5 10 3) a simple magnetic achromat with two 900dipoles is insucient,because the momentum spread p/p results in a deterioration of the(q=A)(q=A)resolution. We therefore employed a separator similar to a Nier-spectrometer[3]. From the known residual gas spectrum of the EBIS it is concluded thata mass separation with a q/A-resolution of about 1/150 is sucient to selectthe highly charged rare radioactive ions with intensities down to 103/s fromrest gas contaminants. The achromatic separator consists of an electrostaticand a magnetic bender. The resolving power of this system depends on theemittance of the injected beam. To provide a resolving power of 150, which issucient to separate all isotopes from residual gas ions, the EBIS emittancehas to be lower than 10  mm mrad at 20 kV extraction potential.All components of the separator are delivered to ISOLDE and ready for in-stallation, except for the electrostatic deector, which is currently completed.The buildup and alignment will start after the emittance measurements at theEBIS.3 The REX-RFQThe REX-ISOLDE RFQ is a progressive development derived from the GSIHLI-RFQ [4] and from the RFQ of the Heidelberg high-current injector [5]. TheRFQ tank is 3 m long and has a inner diameter of 0.32 m. The modulationof the four electrodes had to be optimized for A/q< 4:5 and for a specicinput and output velocity. In order to gain ecient adiabatic bunching andoptimum output emittances, we choose a rather low injection energy of 5keV/u. The lay-out of the RFQ is conservative and we therefore expect thatfor the maximum voltages between the rods even charge-to-mass ratios largerthan 1/6.5 can be accelerated. This will be important if heavier ions will beused in future experiments.For the design of the REX-RFQ the assumed normalized injection emittanceshave been x;y= 0.6  mm mrad. The nal transverse emittances are x;x0=0.68  mm mrad and y;y0= 0.67  mmmrad assuming 95% transmission. Theresulting emittance growth is only 11%. In longitudinal direction the phasespread and the energy spread are ' = 140and W=W = 1:5%.Low level frequency tuning and atness measurements resulted in an unatnessof the voltage distribution along the rods below 1.5%, an Rp value of 145km and a quality factor of 4050. The power tests have shown a rather shorttime for conditioning up to 80 kW and a very good rf-coupling (reection:-35dB). The Rp-value of 145 km could be veried by X-ray spectroscopy and2Fig. 1. left: Energy Peak of the REX-RFQ at 28.35 kW rf-power (electrode voltage:37.3 kV). right: Transmission at dierent electrode voltages.beam measurements. During the beam tests with He1+, the specications of atransmission close to 100% and an energy spread of 1:5% could be fullled(see Fig. 1).4 The matching section between RFQ and IH-structureThe very special beam dynamics concept of the IH-structure requires a rathersmall phase spread of the ion bunches and a converging beam in both trans-verse directions at the entrance of the IH-resonator. For the matching of thebeam from the RFQ into the acceptances of the IH-structure, a section con-sisting of two magnetic quadrupole triplet lenses and a rebuncher has beendesigned. The rebuncher is a three gap split ring resonator with a peak volt-age of 30 kV, which has to match the phase spread of 15oto the phase spreadacceptance of 10oof the IH-structure. Measurements at the power resonatorresulted in a quality factor of 3700 and a shunt impedance of 11.9 M/m.In order to reach the peak voltage, 0.3 kW rf-power will be required. Thematching section is currenty in operation at the test beam line at the Garch-ing accelerator laboratory and will be delivered (together with the RFQ) toISOLDE after bunch- and emittance- measurements behind the rebuncher.5 The IH-structureThe Interdigital-H-type (IH)-structure is an ecient drift-tube structure withspecial beam dynamics (Combined Zero Degree Structure, KONUS [7]). TheIH-structure of REX-ISOLDE is a short version (1.5 m, 20 gaps) of similarstructures like e.g. 'tank 1' of the lead LINAC at CERN [6]. A new feature3of this accelerator is the possibility to vary the nal energy by modicationof the gap-voltage distribution via capacitive plungers. The maximum energygain of about 0.9 MeV/u corresponds to 4.05 MV eective resonator voltageor 5.04 MV absolute voltage. Assuming a shunt impedance of 300 M/m, thetotal peak power consumption will be 60.5 kW.0 5 1 0 1 5 2 01 0 01 5 02 0 02 5 03 0 03 5 0 m e a s u r e d  g a p - v o l t a g e D e s i g n  g a p - v o l t a g e  f o r  1 . 2  M e V / ugap-voltage in kVg a p - n u m b e r0 5 1 0 1 5 2 05 01 0 01 5 02 0 02 5 03 0 0 m e a s u r e d  g a p - v o l t a g e D e s i g n  g a p - v o l t a g e  f o r  1 . 1  M e V / ugap-voltage in kVg a p - n u m b e rFig. 2. Design voltage distributions for 1.1 and 1.2 MeV/u in comparison to thecurrently measured distributions at 101.28 MHz.A comparison between the reference gap voltage distribution given by parti-cle dynamics calculations and the measured distributions at the model andthe power resonator have shown signicant discrepancies to calculations withMAFIA within 2%. Thus for the power resonator slight changes of the un-dercut geometry and of the drift tube structure have to be done in order toincrease the resonator frequency which is currently to low for operation in the1.1 MeV/u mode (see Fig. 2). The low level measurements of the resonatorparameters in the 1.2 MeV mode showed a shunt impedance of 330 M/m anda quality factor of 16500. During the rst phase of operation at REX-ISOLDE,the IH-structure can cover an energy range of 1.15 to 1.2 MeV/u.References[1] D. Habs et al., Nucl. Phys. A616 (1997) 29c.[2] B. Jonson et al., Nucl. Phys. News 3 (1993) 5.[3] R. Rao, D. Habs, O. Kester, K. Rudolph and T. Sieber in: Proc. of the EPAC98,Stockholm, June 22-26, 1998.[4] J. Friedrich et al. in: Proceedings of the IEEE PAC 1991, p.3044.[5] C.-M. Klener et al. in: Proceedings of the EPAC 1992, p.1340.[6] D. Warner et al., "CERN Heavy-Ion Facility Design Report", CERN 93-01 (1993)[7] U. Ratzinger, Il Nuovo Cimento 106A (1993) 15834

Design and status of the front part of the REX-ISOLDE Linac

Design and status of the front part of the REX-ISOLDE Linac

Abstract

Similar works

Full text

Available Versions

CERN Document Server