The beam intensity of ion beams in RHIC is limited by a fast transverse instability at transition, driven by the machine impedance and electron clouds. For gold and deuteron beams we analyze the dependence of the instability threshold on beam and machine parameters from recent operational data and dedicated experiments. We fit the machine impedance to the experimental data

Blaskiewicz, M.

Cameron, P.

Fischer, W.

Montag, C.

Ptitsyn, V.

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- <  BROOKHB~EN NATIONxKL L A B O R A T O R Y  BNL-79903-2008-CP Analysis of intensity instability threshold at transition at RHIC W. Fischer, M. Blaskiewicz, I. Blackler, P. Cameron, C. Montag, T. Roser, V. Ptitsyn Presented at the 11" Biennial European Particle Accelerator Conference (EPAC 200%) Genoa, Italy June 23-27,2008 Collider-Accelerator Department Brookhaven National Laboratory P.O. Box 5000 Upton, NY 1 1973-5000 www. bnl.gov Notice: This manuscript has been authored by employees of Brookhaven Science Associates, LLC under Conhact No. DE-AC02-98CHI 0886 with the US.  Deparhnent of Energy. The publisher hy accepting the manuscript for publication acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscrivt, or allow others to do so, for United States Government purposes. This prepn'nt is intended for publication in a journal or proceedings. Since changes may be made before publication, it may not be cited or reproduced without the author's permission. DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government, Neither the United States Government nor any agency thereof, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or any third party's use or the results of such use of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof or its contractors or subcontractors. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. ANALYSIS OF INTENSITY INSTABILITY THRESHOLD AT TRANSITION IN RHIC* W. Fischert, M. Blaskiewicz, I. Blackler, P. Cameron, C. Montag, T. Roser, and V. Ptitsyn Brookhaven National Laboratory, Upton, NY, USA Abstract The beam intensity of ion beams in RHIC is limited by a fast transverse instability at transition, driven by the machine impedance and electron clouds. For gold and deuteron beams we analyze the dependence of the insta- bility threshold on beam and machine parameters from re- cent operational data and dedicated experiments. We fit the machine impedance to the experimental data. INTRODUCTION In RHIC all ions with the exception of protons cross the transition energy when they are accelerated to full energy. The beam intensity that can be accelerated through transi- tion is limited by transverse instabilities. Fast (compared to the synchrotron period of 100 ms) transverse instabili- ties were observed already in the first year of RHIC oper- ation [I]. The instability could be characterized as a sin- gle bunch phenomenon with growth times of 15 ms and 120 ms [2]. Data from button BPMs reveal frequencies up to 3 GHz [4] (also see below). The instability is driven by the machine impedance, and enhanced by electron clouds which form at transition because the bunch lengtb is short- ened. Since electron clouds lower the intensity instabil- ity threshold, beam losses occur first at the end of bunch trains (see Fig. I), and parts of the longitudinal distribution are removed (see Fig. 2). To stabilize the beam a yt-jump (Ayt = 1 in 30 ms) is implemented, sextnpole settings are carehlly controlled, and octupoles are turned on during the jump HI. o.w t 0 0 Figure 1: Transition transmission of Au bunches as a func- tion of the location in the bunch train. The ring was filled with 3 trains separated by 3 long gaps (Run-8 non- operational test ramp). Table 1: Comparison of Yellow Au beam transition cross- ing in Run-7 (2007, Au in Blue ring) and Run-8 (2008, d in Blue ring). Parameters are for Au beam in the Yellow ring. parameter Unit Run-7 Run-8 no of ramps for comparison .__ 60 60 Figure 2: Longitudinal profiles of deuteron (d) bunches of ramps with instability losses % 45 5 different intensities in Run4  before (top) and after (bot- ramps with detected coherence % 13 58 number of bunches _._ 103 95 average bunch intensity, before log 1.07 0.98 transition ... 22.8 26.6 gap voltage kV 150 150 synchrotron frequency, after Hz 8.9 6.7 bunch length, before (FWHM) ns 6.4 6.5 bunch length, after (FWHM) ns 10.3 7.1 octupole strength m-3 -5  -6...-12 'Work supported by US DOE under conhact DE-AC02-98CH10886. t Wolfram.Fisehe@bnl.gov tom) crossing transition (no octupoles). In Tab. 1 the main gold (Au) transition crossing param- eters are shown for the last 60 ramps in Run-7 (2007) [5] and Run-8 (2008) [6]. In both runs there was Au beam in the Yellow ring. The Blue ring had Au beam in Run- 7 and deuteron (d) beam in Run-8. In Run-8'a different lattice was used for the Au beam, which has higher phase advances and reduced IBS growth rates [7], also resulting in a higher y t .  With this the two beams crossed yt at dif- ferent times in Run-8 allowing for a better radius control in the Yellow ring. To avoid modulated long-range beam- beam effects during the ramp the Yellow rf frequency is locked to the Blue rffrequency, and both the Blue and Yel- low rf frequency control also have input from the radial loop, If transition in Blue and Yellow is crossed at the same time, it is difficult for the Yellow frequency control to sat- isfy both conditions, lock to the Blue frequency (which also responds to the Blue radial error) and respond to the Yellow radial error. In Run-7 45% of the last 60 ramps showed transition losses like those shown in Fig. 1, in Run-8 only 5%. A coherence monitor (fast orbit rms, see Ref. [2]) detected coherent beam motion in 13% ofthese ramps in Run-7, and 58% in Run-8. For most of the ramps analyzed in Run4 the coherence monitor was monitoring the last bunch in the train, usually most vulnerable because the electron cloud density reaches its maximum at the end of the bunch train. In Run-7 the coherence monitor was monitoring the most intense bunch. In Run-8 an additional transition monitor was available, confirming that some bunches were unstable on almost every ramp [8]. In Run4 more attention was paid to avoid emittance growth due to instabilities out ofconcems over mismatched beam sized at store that could reduce the Au beam lifetime (the d beam has IBS growth rates an order of magnitude smaller than the Au beam). This led to fewer bunches with reduced bunch intensity (Tab. I). A test ramp in Run4 with the same number of bunches, the same bunch pattern, and the same bunch intensity also showed the transition losses that were observed in Run-7 (Fig. 1). With the different operational parameter sets for Run-7 and Run-8, and only a single ramp for direct comparison, no discernible difference in the intensity instability thresh- old at transition can be established. INSTABILITIES DRIVEN BY THE MACHINE IMPEDANCE In Run-8 a test ramp was done with only 20 bunches of varying deuteron bunch intensity and zero octupole strength at transition to determine the instability thresh- old under these conditions. The bunch spacing of 630 ns, 6 times larger than in operation, suppressed the electron cloud formation at transition, so that the instability would be driven by the machine impedance only. Figure 3 shows the transition transmission as a function of the bunch in- tensity, with an instability threshold of 0.55 x 10%. The longitudinal profiles, shown before and after transition in Fig. 2, can be fitted to a Gaussian profile with an rms bunch length of ut = 2.35 ns. We used the code MOSES [9] to fit an impedance to the observed instability threshold, assuming a transverse mode-coupling instability (TMCI). A TMCI was also ob- served in the SPS [Il l .  For the case without tune spread in the beam, a broadband impedance of 2 MWm shows mode coupling for the observed intensity instability thresh- old (Fig. 5). This compares with a broadband impedance However, since the fastest observed rise time (15 ms) of 3-5 MWm determined in an earlier measurement [lo]. is short compared to the synchrotron period (= 100 ms) the instability in our case may be better described by beam breakup rather than mode coupling. Figure 4 shows a dif- ference signal from a button BPM showing an unstable bunch with a frequency of 2 GHz. Figure 4 Difference signal from a button BPM showing an instability with a frequency of 2 GHz. The horizontal scale spans 11 ns. (Run-8 non-operational test ramp with d.) lb ("4 Figure 5: MOSES [9] output showing mode coupling for Ib = 0.7 mA, corresponding to 0.55 x 10" dibunch, the instability threshold found experimentally without oc- tupoles and zero chromaticity. A broadband impedance with 2 MWm and Q = 1 is used. -5 , , 120 -6 -. .I - .  - g -6 : -9 ; -1, - ; -10 ~ - g .12 -Yalwa<Np,e Nrnplbu .,.nYti*mocd S d C ,  20 -33  . Y I U ~ ” . , a l i ~ u ~ ~ s l u l y u ~ m d u ~ l i  Dgh, ,<de> .>.I 0 %  9450 9500 PSI0 9600 9610 - . * ~ n l g i ~ H . B I : Y ” . Y ~ , a ~ , , . n ~ ~ , ~ ~  ofwarr@gbt S d C I  M.“mker Figure 6: Average beam emittance at the beginning of d- Au stores for a number of fills during which the Yellow octupole strength and beam intensity was changed. SUMMARY With somewhat different operational parameters for Au beams in Run-7 andRun-8 no discernible intensity instabil- ity threshold can be established for the observed fast trans- verse instabilities at transition. From a test ramp with deuteron beams and no octupoles a broad band impedance of 2 MWm was found to re- produce the intensity instability threshold assuming trans- verse mode coupling. This compares with a transverse impedance of 3-5 MR/m found in an earlier mcasure- ment [lo]. During operation in Run-8 the octupole strength was in- creased to create a tune shift of np to 0.01 at an rms beam size. With this instability losses could be suppressed and the average emittance growth reduced. However, the large tune spread is a concern for the single particle stability and even with the large tune spread some bunches are still un- stable. PARAMETER DEPENDENCE OF INSTABILITIES IN OPERATION To increase the instability threshold in operation the scx- tupole and octupole settings at transition can be changed as well as the bunch pattern if fewer than the maximum number of bunches arc used. To avoid bad-tail instabilities the sextupole families are set such that the chromaticity is negative before and pos- itive after transition. The chromaticity measurement near transition is difficult and the exact time ofthe zero crossing is not hown. In previous runs it was found that crossing zero chromaticity earlier, presumably before transition, is advantageous. During a couple of ramps in Run-8, a transverse instabil- ity 250ms after transition was observed by the coherence monitor. The chromaticity is anchored in two “stepstones” 2 s before and after transition, and interpolated linearly in- between. The chromaticity set points increase by 10 units between these two stepstones, resulting in a chromaticity slope dE/dt = 2.5s-’. To overcome the instability, the Chromaticity set points before and after transition were in- creased by one unit, thus shifting the zero crossing time 400ms earlier. On subsequent ramps, no instability was observed on the coherence monitor; however, the resulting beam emittance at store was larger than before the chro- maticity change. In Run-7 a pattern with 2 long gaps in the train was used, in Run-8 a patlern with closer to uniform distribution of the gaps. From simulation a smaller electron cloud density is expected for the latter case [12]. With the number of bunches close to the maximum in both cases the difference is not expected to be large. During Run4  the octupole strengths in the Yellow ring were increased at transition to suppress the instabilities. Figure 6 shows the octupole strength together with total An beam intensity at transition and the average emittance of the beginning of the store. The average emittance is cal- culated from the luminosity and the beam currents and av- erages over both transverse planes of both beams. For Au beam with a normalized emittance of 12 m m r a d ,  an oc- tupole strength of -10 m-3 creates a tune shift of 0.0093 at therms beam size. In Fig. 6 one can see that the increase of the octupole strength around fill number 9550 led to a reduction of the initial store emittance and that the decrease of the octnpole strength before fill number 9600 increased the emittance again. However, even with the large tune spread coher- ent beam motion was still detected in more than half of all ramps (see Tab. 1) indicating that at least a few bunches were still unstable. The octupole strength was not increased further over concerns of single particle losses due to the large tune spread. With a few exceptions for bunch inten- sities below 0.94 x lo9 An ions no coherent beam motion was detected, between 0.94 and 1.03 x lo9 bunches were either stable or unstable, and above 1.03 x lo9 bunches were generally unstable. ACKNOWLEDGMENTS The authors are thankful to T. Satogata for help with the data processing, and to E. Metra1 and G. Rumolo for help with MOSES and discussions on instability issues. REFERENCES [ I ]  C. Montag et al., Phys. Rev. ST - Accel. Beams 5 084401 [2] M. Blaskiewicz et al., PAC’O3, pp. 3026-3028 (2003). [3] F. Zimmermann, Phys. Rev. ST - Acqel. Beams 7 124801 [4] V. Ptitsyn, private communication (2007). [5] A. Drees et al., PAC‘07, pp. 722-724 (2007) [6] C.J. Gardner et al., WEPPOl I,  these proceedings. [7] V. Litvinenko, private communication. [8] P. Cameron, private communication. 191 Y.H. Chin, CERN/LEP-TH/88-05 (2005). (2002). (2004). [IO] S.Y. Zhang et al., EPAC’02, pp. I 112-1 I I4 (2002). [ I  I ]  G. Arduini et al., PAC’O7, pp. 41624164 (2007). [ 121 W. Fischer and V. Ptitsyn, BNL C-.WAP/295 (2007). [I31 M. Blaskiewicz et al., EPAC‘OZ, pp. 1488-1490 (2002). 

Analysis of intensity instability threshold at transition in RHIC.

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Analysis of intensity instability threshold at transition in RHIC.

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