Fujimoto, M.

Hasegawa, J. H.

Hayashi, K.

Hosaka, M.

Katoh, M.

Mochihashi, A.

Takashima, Y.

English

KITopen

SUPPRESSION OF LONGITUDINAL COUPLED BUNCH INSTABILITYBY HARMONIC CAVITY IN UVSOR ELECTRON STORAGE RINGA. Mochihashi1,2,3,∗, J. Hasegawa1, M. Hosaka1,2,3, Y. Takashima1,2,31Graduate School of Engineering, Nagoya University, Nagoya, Aichi, Japan2Nagoya University Synchrotron Radiation Research Center, Nagoya, Aichi, Japan3Aichi Synchrotron Radiation Center, Seto, Aichi, JapanM. Fujimoto4, K. Hayashi4 and M. Katoh2,44UVSOR Synchrotron Facility, Institute for Molecular Science, Okazaki, Aichi, JapanAbstractIn the UVSOR electron storage ring [1], which is dedi-cated for a VUV synchrotron radiation light source, a lon-gitudinal coupled bunch instability (LCBI) is observed inmulti-bunch operation. To suppress the LCBI, we routinelyoperate a third harmonic cavity (HCV) in a passive mode. Byproperly tuning the HCV, the instability is almost completelysuppressed. Because of the lower beam energy (750 MeV)and brilliant beam emittance (17.5 nm-rad), the Touschekeffect becomes severe in the UVSOR. To guarantee enoughbeam lifetime, we also apply the HCV for lengthening thebunch. The suppression of the instability and increasingthe beam lifetime are crucial benefits by the HCV for theUVSOR. However, not only the origin of the LCBI but alsothe Landau damping effect by the HCV has not been under-stood systematically yet. We have noticed that one of theHOMs at the HCV itself could cause the LCBI and observedthe behavior of the instability, which strongly depends on thebeam current. From the experiment we have discussed thecause of the instability with the HOM theory. We have alsotried to observe synchrotron tune spread and discussed acompetition between the Landau damping and the instabilitygrowth.INTRODUCTIONThe UVSOR electron storage ring (UVSOR) is a VUV andsoft X-ray synchrotron radiation light source which is mainlyused for molecular and atomic science. The UVSOR hasstarted the beam operation on 1983 as a second generationsynchrotron light source, and since then, through twice majorupgrade and replacement of the accelerator complex, now theUVSOR has been operated as a third generation light sourceand been continuously operated stably to deliver high qualitysynchrotron radiation to the users. Not only the upgrade ofthe storage ring which is used for the light source, but alsofor the top-up operation the UVSOR facility had an upgradeto a booster synchrotron which is operated as an injector forthe storage ring. Due to the upgrade of the injector, nowthe UVSOR has kept enough stability for the beam current.However, the beam lifetime is still one of the main issue; itslower beam energy (750 MeV) and small emittance (17.5nm-rad) make the condition of the Touschek lifetime severe.∗ present affiliation : Karlsruhe Institute of Technology, Karlsruhe, Ger-many. akira.mochihashi@kit.eduMoreover, at present the UVSOR has a longitudinal coupledbunch instability (LCBI) in daily multi-bunch operation forusers.To guarantee enough stability on the stored beam, we haveroutinely operated a third harmonic cavity (HCV) in passivemode to lengthen the bunch and to suppress the LCBI withLandau damping. The frequency of the main rf acceleratingcavity is 90.1 MHz, so the operation frequency for the HCVis 270.3 MHz. In UVSOR, we have routinely operated theHCV system since 1994 [2] to suppress the LCBI. Becauseof several large upgrade and replacement of the acceleratorcomponents, it is supposed that the condition of the couplingimpedance in the storage ring has largely changed from theinitial condition, however, still we have observed a LCBIafter the upgrades. Even though we can suppress the LCBIby operating the HCV with proper tuning condition, we havenot yet had enough understanding for the origin of the LCBIin the upgraded UVSOR.Because over 20 years have passed since the fabricationof the HCV, sometimes we have minor troubles in the HCVbecause of its aging problem. Now we have started a plan tofabricate a new HCV system for the replacement. In the de-sign process, it is necessary to investigate how much Landaudamping effect the new HCV must have for enough suppres-sion of present LCBI. In the consideration, we have triedto make clear the origin of the main source of the LCBI inthe UVSOR by systematic beam diagnostics measurements,a theory of the coupled bunch instability and the Laudaudamping theory.LONGITUDINAL COUPLED BUNCHINSTABILITY IN UVSOR STORAGE RINGCause of Longitudinal Coupled Bunch InstabilityTo observe a dependence of the LCBI on the beam current,we have stored electron bunches in all of the RF buckets (uni-form filling) at the beam current of 300 mA which is equalto that in users operation. The HCV has been operated asa passive mode which is the same as in the users operation,and after tuning the HCV properly to suppress the LCBI,we have decreased the beam current intentionally by a beamscraper under the same tuner condition. By decreasing thebeam current we have observed the longitudinal beam mo-tion by a visible streak camera with a setup to observe theLCBI. Figure 1 shows a typical result in the experiment in9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW PublishingISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-THPAK031THPAK0313280ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2018).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.05 Beam Dynamics and EM FieldsD04 Beam Coupling Impedance - Theory, Simulations, Measurements, Code Developments3 beam current conditions. As seen in the figure, when thebeam current is large enough (300 mA) the LCBI is almostcompletely suppressed and the beam becomes stable. Inthe moderate beam current (100 mA), the LCBI starts andthe beam becomes unstable. In the small beam current (7.4mA), the LCBI stops and the beam becomes stable again.This behavior suggests a balance between the growth rate ofthe LCBI, Landau damping effect and the radiation dampingrate. In larger beam current condition, the rf voltage insidethe HCV is high enough to Landau-damp the LCBI becauseof the non linear rf field. When the beam current is smaller,the Landau-damping effect becomes no more sufficient andthe LCBI starts. In much smaller beam current the growthrate of the LCBI becomes smaller than the radiation dampingrate.Figure 1: The streak camera images at the beam current of(upper) 300 mA, (middle)100 mA and 7.4 mA. The HCVhas been operated in passive mode and the tuning conditionof the HCV has been fixed.To evaluate this, firstly we tried to find the origin of theLCBI in the UVSOR. In the consideration we have foundpossibility that TM020 mode at the HCV itself could causethe LCBI. From the simulation by Superfish code, we haveestimated the frequency of TM020 as 780 MHz, on the otherhand we have measured higher order mode (HOM) of theHCV by a network analyzer and found that one sharp reso-nance at 782.6 MHz. This resonance could be the TM020mode and couple with a longitudinal mode number of 11in the UVSOR. From the measurement by the network ana-lyzer and the calculation by Superfish, we have concludedthe loaded Q and the shunt impedance of TM020 in the HCVare 6500 and 270 kΩ; which is not negligible for the stabilityof the beam.To evaluate the origin of the LCBI, we have estimated agrowth rate of TM020 inside the HCV at the beam currentof 7.4 mA. From a theory of the longitudinal coupled bunchinstability [3] the growth rate is estimated to be 24.4 s−1 atthe longitudinal mode number of 11. On the other hand, theradiation damping rate of the UVSOR is 60.4 s−1 which hasthe same order of the magnitude as the instability growthrate. Figure 2 shows a spectrum data for the beam modeswhen the instability occurs. As seen in the figure, the mode11 (and mode 5 because the harmonic number of the UVSORis 16) is clearly enhanced in another mode spectral peaks.These results strongly imply that one of the causes of theLCBI in the UVSOR is HOM in the HCV itself. Here weemphasize that the HCV, though it could be one of the causesof the LCBI, is very helpful to lengthen the bunch stably andcan be used for improving the beam lifetime.Figure 2: Result of the mode spectrum under the longitudi-nal coupled bunch instability. Mode number 11 and 5 areenhanced in another mode spectral peaks.Landau Damping by Harmonic CavityWe have observed that the LCBI ceases when the beamcurrent is larger than 280 mA, which implies that the syn-chrotron tune spread increases due to the non linear fieldof the HCV and Landau damping becomes effective to sup-press the LCBI. To evaluate the effect due to the HCV fieldon the synchrotron tune spread, we have tried to measurethe synchrotron tune spread by using rf phase modulationmethod. We have applied phase modulation whose modu-lation pattern is pure sinusoidal wave to the main rf accel-erating cavity which has operation frequency of 90.1 MHz.We have adjusted the modulation frequency near from thesynchrotron oscillation frequency with proper modulationphase amplitude. Under the condition we have observedsynchrotron sideband spectrum around a higher harmonicsof the revolution frequency. By changing and scanning themodulation frequency, we have observed a dependence of thebeam response on the modulation frequency; this frequencydistribution contains information for the synchrotron tunespread. Strictly, the frequency distribution depends on thebeam response against the modulation, therefore, it does not9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW PublishingISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-THPAK03105 Beam Dynamics and EM FieldsD04 Beam Coupling Impedance - Theory, Simulations, Measurements, Code DevelopmentsTHPAK0313281ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2018).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.mean directly the tune spread itself. Here, however, as oneof the benchmark for the tune spread, we have performed themeasurement of the frequency response against the phasemodulation. Figure 3 shows a result of the frequency re-sponse due to the phase modulation for the synchrotronoscillation frequency. We have observed the frequency re-sponse for various beam current and analyzed the peak widthof the response. Figure 4 shows a summary of the FWHMfor each response peak. As seen the figure, it is clearly seenthat the width of the response peak tends to increase with thebeam current. Because the HCV has been operated with thepassive mode, the width increases with the beam inducedharmonic voltage inside the HCV.As one of the measure of the Landau damping, we haveestimated a parameter ξ [3] which is estimated from theimpedance value and gives a criterion for the stability of thebeam by the Laudau damping. Generally, the efficiency ofthe Landau damping depends not only on the width of thefrequency distribution of the beam but also the shape of thedistribution. Here, we only have estimated the efficiency ofthe Landau damping by a simplified stability criterion:|ξ | <1√3∆ω1/2, (1)where ∆ω1/2 represents a FWHM value of the frequencyspread for any kind of the distribution. Because ξ can becalculated from the impedance and is proportional to thebeam current, we can estimate the efficiency of the Landaudamping against the LCBI in our case. According to thetheory of the Landau damping, the |ξ | from the impedance ofthe HOM in the HCV, it is estimated to be |ξ |2π = 8.5 kHz/mAwhich corresponds to about 2.6 kHz at 300 mA. Accordingto the beam diagnostic experiment, the LCBI ceases over280 mA. The frequency width of the beam response underthe effect of the HCV in Fig. 4 has almost the same value asthe parameter of the stability criterion over 280 mA. Thisstrongly implies that one of the cause of the LCBI in largerbeam current condition is also the HOM in the HCV itself,and the HCV suppresses the LCBI with the Landau damping.SUMMARYAs we have mentioned, the response of the beam due tothe rf phase modulation could not reflect directly the fre-quency distribution of the beam; in our case, the particledistribution on the synchrotron oscillation frequency. Nowwe are proceeding some macroparticle simulation to eval-uate the response of the beam on the rf phase modulationincluding a transient beam loading inside the rf cavities. An-other simulation which can estimate the Landau dampingeffect under the condition of the HOM impedance from theHCV is also the next subject. We also plan to fabricate a newharmonic cavity in UVSOR storage ring. In its design phase,we take care not only to realize enough Landau dampingeffect by the HCV but also to avoid any critical HOM whichcan induce LCBIs.Figure 3: A result of a beam response against the rf phasemodulation around the synchrotron oscillation frequency.The abscissa corresponds to frequency difference from thepeak of the beam response.Figure 4: Dependence of the width of the beam responsepeak against the rf phase modulation on the beam current.The ordinate corresponds to FWHM value of the responsepeak.REFERENCES[1] UVSOR synchrotron facility,https://www.uvsor.ims.ac.jp/eng/index.html[2] K. Tamura et al., “Double RF system for Suppression ofLongitudinal Coupled Bunch Instability on UVSOR StorageRing”, Jpn. J. Appl. Phys., vol. 33 pp. L59-L62, 1994.[3] A. W. Chao, “Physics of collective beam instabilities in highenergy accelerators”, Wiley Interscience, 1993.9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW PublishingISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-THPAK031THPAK0313282ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2018).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.05 Beam Dynamics and EM FieldsD04 Beam Coupling Impedance - Theory, Simulations, Measurements, Code Developments

Suppression of Longitudinal Coupled Bunch Instability by Harmonic Cavity in UVSOR Electron Storage Ring

https://publikationen.bibliothek.kit.edu/1000088031/19886515

Suppression of Longitudinal Coupled Bunch Instability by Harmonic Cavity in UVSOR Electron Storage Ring

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