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GSI-UPGRADE-ACC-03 GSI SCIENTIFIC REPORT 2009 Introduction ThehighchargestateinjectorHLIattheUNILACprovidesmainlybeamsforthesuperheavyelementresearch (SHIP,TASCA).Highaveragebeamintensities,i.e. high dutyfactors,areprerequisite. FedbyanECRsource,the HLImeetstherequirementswell. Additionally,itisalso usedforrareisotopebeams. TheHLI–linaccomprisesaradiofrequencyquadrupole (RFQ)andanIHstructure. Whilethehighchargestate oftheionsfromtheECRsourcefacilitatestheacceleration,theexistingRFQwasfiguredouttobeabottleneck concerninghighdutyfactoroperation. Thereforeit was decidedfouryearsagotoreplaceitbyanewRFQ.This measureispartofanupgradeprogramincludingasecond, new28-GHzsuperconductingECRionsource[1]. DesignofthenewRFQ ThenewRFQhastoaccelerateionswith A/q ≤ 6in cwoperation. Thedesigngoalwastoenablethiswiththe existingrfpoweramplifier(≤70kWaveragepower). For A/qupto8.5thedutycycleislimitedto50%.Moredetails aregivenintab.1.TheRFQstructureisshowninfig.1. Figure1:NewRFQduringfinalassembly(courtesyofG. Otto,GSI). 148 ∗ p.gerhard@gsi.de Property Value Injection/extractionenergy[keV/u] 2.5/300 RFfrequency[MHz] 108.48 A/q(cw/max.)
GSI SCIENTIFIC REPORT 2009 GSI-UPGRADE-ACC-04 Measurement of Space Charge Driven Resonances in the UNILAC Alvarez DTL L. Groening 1 , I. Hofmann 1 , W. Barth 1 , W. Bayer 1 , G. Clemente 1 , L. Dahl 1 , P. Forck 1 , P. Gerhard 1 , D. Jeon 2 , M.S. Kaiser 1 , M. Maier 1 , S. Mickat 1 , T. Milosic 1 , D. Uriot 3 , and S. Yaramyshev, 1 1 GSI, Darmstadt, Germany; 2 ORNL, Oak Ridge, TN, U.S.A., 3 CEA/Saclay, Gif-sur-Yvette, France Last years experiments aimed to the investigation of resonant transverse emittance growth for high intensity beams. The driving force is the self field of the beam's space charge. The first resonance is driven by similar beam envelope phase advances in the transverse and longitudinal dimension [1]. First theoretical investigations date back to 1992 and avoiding this resonance is a design rule for all high intensity linac projects. Experimental confirmation was not achieved world-wide due to the leak of diagnostics, machine control, and beam matching. For a given experimental set-up the longitudinal phase advance �l is constant and the resonance is probed by varying the transverse phase advance �� using the quadrupole strength. Figure 1 plots the measured transverse emittance at the DTL exit as a function of the phase advance ratio. Additionally, results from computer simulations are shown being in good agreement with the measured data as well as with the underlying theoretical concepts. A detailed description of the experiment can be found in Ref. [2]. Norm. Transv. rms Emitt. (mm mrad) 0.40 0.35 0.30 0.25 0.20 Experiment DYNAMION TraceWin 0.0 0.5 1.0 1.5 2.0 Initial Depressed Tune Ratio � || / �� Figure 1: Emittance as function of phase advance ratio. The second resonance is purely transverse and it is driven by the octupolar component of the polynom describing the beams transverse charge density. The corresponding field term scales as an octupole, namely � r 3 . This field component acts to the periodically focused beam as a perturbation and it is resonantly excited if the transverse phase advance becomes equal to 360°/(3+1), i.e. 90°. This resonance has been predicted about three decades ago [3]. Recent simulation studies [4] revealed that this resonance dominates the well-known envelope instability occurring at 90° as well [5]. Moreover, they revealed that this resonance might be measured using the UNILAC's Alvarez DTL. Figure 2 shows the measured emittance at the DTL exit as a function of the transverse phase advance. The predicted resonant increase at about 90° was observed and GSITemplate2007 has been confirmed with three different simulation codes. A characteristic feature of an octupolar resonance is the formation of four arms in the phase space distribution. This four-fold filamentation was measured as well at the UNILAC and it is displayed in Fig. 3. The described experiments (see [6] for details) are the first successful validation of resonant emittance growth in a linear accelerator world-wide. Since resonant phenomena along a linac are tiny perturbations, it requires a very stable and reliable operation of the UNILAC. The same applies to the diagnostic components. Norm. Transv. rms-Emitt. (mm mrad) 0.40 0.35 0.30 0.25 0.20 Experiment DYNAMION PARMILA TraceWin 0.15 40 60 80 100 120 Transverse Phase Advance (deg) Figure 2: Emittance as function of transverse phase advance. Measurement Simulation Horizontal Vertical Horizontal Vertical Horizontal Vertical (a) �� = 63° (b) �� = 83° (c) �� = 102° ± 15 mrad I / I max (%) 0 - 5 5 - 10 10 - 20 20 - 40 40 - 100 ± 15 mm Figure 3: Phase space distributions at DTL exit. References [1] I. Hofmann et al., PRL 87, 034802 (2001). [2] L. Groening et al., PRL 103, 224801 (2009). [3] I. Hofmann et al., AIP Conf. Proc. No. 642, p. 248, (2002). [4] D. Jeon et al., PRST-AB 12, 054204 (2009). [5] I. Hofmann et al., Part. Accel. 13, 145 (1981). [6] L. Groening et al., PRL 102, 234801 (2009). 149
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