Blomley, E.

Boltz, T.

Huttel, E.

Kehrer, B.

Mochihashi, A.

Müller, A.-S.

Schuh, M.

Teytelman, D.

English

KITopen

LONGITUDINAL BEAM MANIPULATION BY RF PHASE MODULATIONAT THE KARLSRUHE RESEARCH ACCELERATORA. Mochihashi∗, E. Blomley, T. Boltz, E. Huttel, B. Kehrer,A.-S. Müller, M. Schuh, Karlsruhe Institute of Technology , Karlsruhe, GermanyD. Teytelman, Dimtel, Inc., San Jose, USAAbstractIn electron storage rings, it is possible to manipulate thestored beams longitudinally by applying a phase modulationto the accelerating RF voltage. By choosing a proper modu-lation frequency, the electron bunch can be lengthened andthe Touschek beam lifetime can be improved accordingly.To understand the effect of such modulations on the beamfrom the beam dynamics point of view, we have performedsimulations for the beams under the RF phase modulation.The simulations were conducted for the Karlsruhe ResearchAccelerator (KARA) storage ring. In KARA, we have in-stalled the function of the RF phase modulation into the lowlevel RF system. The experimental results of the phase mod-ulation indicate the agreement with the simulation results.RF PHASE MODULATIONIn the RF phase modulation scheme, the modulation fre-quency is normally chosen as a harmonic of the synchrotronfrequency. For example, the quadrupole oscillation on thelongitudinal phase space can be excited if the modulation fre-quency is settled as the second harmonic of the synchrotronfrequency, and the bunch length can be increased because ofthe quadrupole oscillation mode. Several preceding studiesfor the phase modulation have been reported so far. Sakanakaet al. [1] reported that the RF phase modulation improved thebeam lifetime and suppressed the longitudinal coupled bunchinstability in KEK Photon Factory. In the reference [1], theyalso discussed the beam loading effect which can changethe effect on the phase modulation on the beam. Abreu etal. [2] discussed a theoretical model for the phase modula-tion and Landau damping effect which is accompanied bythe modulation effect. In the reference [2], they also dis-cussed the change in the longitudinal Hamiltonian due to themodulation frequency. By considering the results in thesepreceding studies, we mainly investigated the dependenceof the bunch lengthening on the modulation frequency andthe beam loading.SIMULATIONTo consider the effect of the phase modulation on thebeam, we have performed simulations based on the macro-particle tracking method. In the simulation, we have as-sumed the machine parameters of the KARA storage ring at2.5 GeV. Table 1 shows the machine parameters used in thesimulations.∗ akira.mochihashi@kit.eduTable 1: Machine Parameters of KARA 2.5 GeVParameters Values UnitsRF frequency 499.72 MHzHarmonic number 184Circumference 110.38 mRF Voltage 1.4 MVSynchrotron frequency 30.89 kHzNumber of RF sectors 2Number of cavities per 1 sector 2Momentum compaction factor 8.67×10−3Natural energy spread 9.08×10−4To consider the beam loading which occurs inside the RFcavities, we have included the beam-cavity interaction foreach cavity. In the simulation, we have used the measuredvalues of the quality factors and the shunt impedances foreach cavity, and to include the beam loading effect we haveapplied a similar method as discussed in [1]. In the calcula-tion of the beam loading, we have treated the transient beamloading effect which depends on the bunch filling pattern.In the following simulation results, we have used the bunchfilling pattern which is similar to the normal multibunchoperation at KARA; the filling pattern with 3 bunch trainsand 4 vacant-gap trains. The beam loading effect depends onthe tuning condition of the cavity, therefore we have settledand kept the tuning condition of all cavities under the opti-mum tuning; that is, the loading angles of all cavities havebeen always kept at zero. We have prepared 10000 macro-particles for 1 bunch and settled them on the longitudinalphase space with the equilibrium distribution initially. Thetracking calculations have taken place for twice the longitudi-nal damping time period. In the simulation, we have mainly3 tuning knobs to change the condition of the modulation;the modulation frequency, the modulation amplitude andthe beam current. In the following sections, we discuss thesimulation results when we changed these parameters.MODULATION FREQUENCYWe have swept the modulation frequency, which is aroundthe second harmonic of the synchrotron frequency fs of sin-gle particle, and analyzed the dependence of the rms bunchlength on the modulation frequency. In the longitudinalphase space the bunch follows the quadrupole oscillationmode due to the 2 fs modulation and the bunch length oscil-lates accordingly. We have estimated the effective rms bunchlength by taking the time-average of the bunch length oscil-10th Int. Particle Accelerator Conf. IPAC2019, Melbourne, Australia JACoW PublishingISBN: 978-3-95450-208-0 doi:10.18429/JACoW-IPAC2019-WEPTS016MC5: Beam Dynamics and EM FieldsD09 Emittance Manipulation, Bunch Compression and CoolingWEPTS0163123ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2019).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOIFigure 1: Detuning curve of the effective rms bunch lengthunder the RF phase modulation. A beam current of 150 mAand a modulation amplitude of 50 mrad are assumed. Thehorizontal blue line corresponds to the natural rms bunchlength.lation. Figure 1 shows the "detuning curve" of the bunchlengthening under the 2 fs RF phase modulation. Here wehave assumed a beam current of 150 mA which is a typi-cal operation beam current at KARA and the modulationamplitude of 50 mrad for each RF sector. The phase differ-ence of the modulation at each sector has been adjusted sothat the beam has continuous modulation without any phasejump. As seen in Fig.1, the curve has its peak at a negativefrequency detuning condition. The negative detuning peakimplies that the RF potential well could be deformed effec-tively due to the phase modulation. Due to the deformation,the effective RF voltage could decrease and the synchrotronfrequency also could decrease, accordingly.MODULATION AMPLITUDEWe have performed the same simulation as for the caseshown in Fig. 1 but by changing the modulation amplitudefrom 50 to 200 mrad. Figure 2 shows the detuning curvesfor the modulation amplitudes of 50, 100 and 200 mrad. Asseen in the figure, the maximum rms bunch length increaseswhen the amplitude becomes larger, and additionally thepeak frequency of the detuning curve tends to move to nega-tive direction. This enhancement of the negative detuningcould come from the deformation of the RF potential well.Figure 3 shows the time-averaged longitudinal bunch pro-files for each condition of the modulation amplitudes. Herewe have picked up the bunch profiles at the maximum bunchlength conditions in Fig. 2, and taken the time-average for a25/ fs period with 100 sampling points. This figure clearlyshows that the bunch length becomes longer when the am-plitude is larger. The longitudinal profile keeps the originalsymmetrical Gaussian at smaller modulation amplitude con-dition, but it changes to non-Gaussian and asymmetric atlarger amplitude conditions.Figure 2: Detuning curves of the effective rms bunch lengthunder the RF phase modulation with different modulationamplitude of 200, 100 and 50 mrad. A beam current of150 mA is assumed in these simulations.Figure 3: Time-averaged longitudinal bunch profiles for eachcondition of the modulation amplitudes shown in Fig. 2.BEAM CURRENTTo discuss the influence of the beam loading on the ef-fect of the phase modulation, we have repeated the samesimulations in Fig.1 but varied the beam current. Figure 4shows the detuning curves for the beam current of 150, 100and 50 mA while keeping the other parameters fixed. Asseen in the figure, the bunch lengthening effect tends to beenhanced at higher beam currents. This tendency impliesthat the beam loading effect which occurs in the RF cavitiescould play an important role for the beam dynamics underRF phase modulation.EXPERIMENTSIn September 2018, we have installed the function of theRF phase modulation into our low-level RF system (LLRF,DIMTEL LLRF9/500). As KARA has 2 RF sectors and eachsector has its own low level RF system module, it is possibleto perform the RF phase modulation with different setups for10th Int. Particle Accelerator Conf. IPAC2019, Melbourne, Australia JACoW PublishingISBN: 978-3-95450-208-0 doi:10.18429/JACoW-IPAC2019-WEPTS016WEPTS0163124ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2019).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOIMC5: Beam Dynamics and EM FieldsD09 Emittance Manipulation, Bunch Compression and CoolingFigure 4: Detuning curves of the effective rms bunch lengthunder the RF phase modulation in different beam currentconditions of 150, 100 and 50 mA. The conditions are iden-tical to those in Fig. 1, except for the beam curerent.each RF sector. In the experiment, we have performed theRF phase modulation by using only one RF sector. From thesignal of the master oscillator, the LLRF generates the RFsignal for the pre-amplifier. The output of the pre-amplifiergoes to a 250 kW klystron which drives the two RF cavitiesin each RF sector. By using the LLRF system, we can settlethe phase modulation frequency and amplitude for each RFsector. The pattern of the phase modulation is a continuoussinusoidal wave. When the RF phase modulation is on, thebackward power from the cavities tends to increase. Becauseof the device interlock, the backward power practically lim-its the maximum modulation amplitude. At the momentwe have performed the experiment with fixed modulationamplitude of 3 degree (52 mrad).The experiments took place with the normal multibunchfilling pattern which is similar to that used in the simula-tions. To measure the bunch length we used a visible lightstreak camera (Hamamatsu, C5680). By performing thedual sweeping mode, we measured both the bunch lengthand its change due to the phase modulation. On the streakcamera image, it was clearly visible that the longitudinalbunch structure changed because of the phase modulation.To observe the change clearly, we set the time range of theslow sweeping axis (horizontal axis) to 100∼200 µs to coverseveral dozens of synchrotron oscillation periods. From onestreak camera image taken by single shot measurement, weanalyzed the effective rms bunch length by dividing the im-age into horizontal segments which have 5 pixel width foreach and analysing the bunch length for each segment. Afterthe analysis of the segmentation, we evaluated the bunchlength statistically. To get better quality of the statistics,we took 100 single shot data for each experimental condi-tion. Figure 5 shows the detuning curve of the effectiverms bunch length under the 2 fs phase modulation. As seenin the figure, the width and the shape of the peak of thedetuning curve seems to be quite similar to the peak fromFigure 5: The experimental result of the detuning curve ofthe effective rms bunch length under the RF phase modu-lation. The error bars for the measured bunch length areinside the plotted circles. The synchrotron frequency fromthe bunch length measurement and calculated momentumcompaction factor is 30.47 kHz in this experiment.the simulation. In the experiment, we have also measuredthe bunch length without the RF phase modulation to getthe information about the synchrotron frequency. From thebunch length measurement, the calculated value of the mo-mentum compaction factor and the energy spread, we haveestimated a synchrotron frequency of 30.47 kHz for the con-dition of Fig. 5. This means that the detuning curve from theexperiment does have the positive detuning peak from the2 fs frequency. The reason of the discrepancy between thesimulation and the experiment is not yet clear. One possi-bility would be an underestimation of the fs value. Becausethe bunch length measurement took place during the RFphase modulation experiment, the beam had a high beamcurrent in a multibunch filling. Such a condition could leadto the bunch lengthening because of the ring impedance,and the lengthened bunch can cause the underestimationof the synchrotron frequency. A precise measurement ofthe synchrotron frequency, such as a measurement in singlebunch mode, would be the next subject to be done.OutlookNow we continue to do the experiment to understand thebeam dynamics of this modulation scheme in-depth. In par-allel, we investigate the practical effects on the beam by themodulation to make use of this scheme for the accelerator op-eration; improvement of the beam lifetime and suppressionof beam instabilities.REFERENCES[1] S. Sakanaka, M. Izawa, T. Mitsuhashi, and T. Taka-hashi,“Improvement in the beam lifetime by means of an rfphase modulation at the KEK Photon Factory storage ring ”,Physical Review Special Topics - Accelerators and Beams, vol.10th Int. Particle Accelerator Conf. IPAC2019, Melbourne, Australia JACoW PublishingISBN: 978-3-95450-208-0 doi:10.18429/JACoW-IPAC2019-WEPTS016MC5: Beam Dynamics and EM FieldsD09 Emittance Manipulation, Bunch Compression and CoolingWEPTS0163125ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2019).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI3, pp. 050701, May 2000. doi:10.1103/PhysRevSTAB.3.050701[2] N. P. Abreu, R. H. A. Farias, and P. F. Tavares, “Longi-tudinal dynamics with rf phase modulation in the Brazil-ian electron storage ring”, Physical Review Special Topics- Accelerators and Beams, vol. 9, pp. 124401, Dec. 2006.10.1103/PhysRevSTAB.9.12440110th Int. Particle Accelerator Conf. IPAC2019, Melbourne, Australia JACoW PublishingISBN: 978-3-95450-208-0 doi:10.18429/JACoW-IPAC2019-WEPTS016WEPTS0163126ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2019).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOIMC5: Beam Dynamics and EM FieldsD09 Emittance Manipulation, Bunch Compression and Cooling

Longitudinal Beam Manipulation by RF Phase Modulation at the Karlsruhe Research Accelerator

https://publikationen.bibliothek.kit.edu/1000099074/44343929

Longitudinal Beam Manipulation by RF Phase Modulation at the Karlsruhe Research Accelerator

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