Katayama, T

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Particle Accelerators, 1992, Vols. 37-38, pp. 289-299Reprints available directly from the publisherPhotocopying permitted by license only© 1992 Gordon and Breach Science Publishers, S.A.Printed in the United KingdomBEAM EXPERIMENTS AT COOLERSYNCHROTRON TARN IITAKESHI KATAYAMAInstitute for Nuclear Study, University of Tokyo 3-2-1, Midoricho, Tanashi,Tokyo 188, Japan.( Received 3 December 1990)A cooler synchrotron, TARN II, has been in operation since the beginning of 1989. It aims at an energyof up to 1.1 GeV for protons and 370 MeV/u for heavy ions of q/A = 0.5, corresponding to a maximummagnetic rigidity of 5.8 T-m. An electron cooling device and a slow extraction channel have been preparedfor various beam experiments. In this paper, the status of TARN II is described, as are the recent resultsof beam experiments.INTRODUCTIONTARN II is an experimental facility for accelerator studies and atomic and nuclearphysics. In addition to the functions of beam acceleration and slow extraction, it isequipped for electron cooling!. This cooler synchrotron has a maximum magneticrigidity of 5.8 T-m, corresponding to a proton energy of 1.1 GeV. The main para-meters of the ring are shown in Table 1. The ring is hexagonal, with an averagediameter of 24.8 m. Its circumference is 77.76 m, or 17 times the extraction orbit ofthe injector cyclotron. It has 6 long, straight sections, each 4.2 m. They are used forthe beam injection system, an rf cavity, an electron cooling device, and a slowbeam-extraction system. It takes 3.5 s for the power supply to fully excite the wholemagnet system. The flat-top duration of magnetic field is variable and is long enoughfor beam cooling and extraction. The rf cavity can be tuned from 0.5 to 8.5 MHzand the power amplifier provides 5 kW, producing a gap voltage of 2 kV. The electroncooling system can cool an ion beam that has an energy of up to 200 MeV/u,corresponding to a maximum electron energy of 120 keY. It consists of an electrongun, an interaction region 1.5 m in length, and collector and electron guiding coils. 2The first attempt at beam injection was performed in December 1988, andalpha-particle beams of 28 MeV were circulated in TARN II. Subsequently, experi-ments in beam accumulation and acceleration have been performed for several daysper month, as were the studies of electron cooling. In September 1989, the first coolingexperiments were performed successfully with 20-MeV proton beam. Further experi-ments in beam acceleration and cooling are being performed presently, and areexpected to be finished by the end of 1990. After these accelerator studies, TARN II289290 T. KATAYAMATABLE 1Main Parameters of TARN II RingMaximum magnetic rigidityMax. beam energy protonions with qlA = 1/2CircumferenceAverage radiusRadius of curvatureFocusing StructureLength of long straight sectionSuperperiodicity acceleration modecooling modeRising time of magnet excitationRepetition rate (max.)Max. field of dipole magnetsMax. gradient of quadrupole magnetsRevolution frequencyAcceleration frequencyHarmonic numberMax. rf voltageUseful apertureVacuum pressure5.8 T.m1.1 GeV370 MeV/u77.76 m12.376 m4.045 mFBDBFO4.20m6.33.5 s to full0.1 Hz15.0 kG70 kGlm0.31-3.75 MHz0.62-7.50 MHz22kV50 x 200 mm210- 11 Torrwill be dedicated to experimental work on nuclear and atomic physics, as well as theinterdisciplinary sciences using heavy ion beams.2 MAGNET SYSTEMThe focusing structure of the magnet system is based on a focusing-defocusing(FODO) lattice, and the long straight sections are provided by inserting a 4.20-mdrift space between horizontally focusing quadrupole magnets at every unit cell(Figure 1). The entire ring is composed of six unit cells. For the acceleration mode,these cells are excited identically and the dispersion function' and the maximum f3xvalue can be kept small, around 10 m, which results in the large machine acceptance,400 n mm-mrad. Horizontal and vertical tune values, Qx and Qy, are around 1.75 inthis mode. On the other hand, to realize zero dispersion at straight sections' formomentum cooling, the superperiodicity is reduced from six to three by changingthe excitation current of the quadrupole magnets. In this cooler ring mode, themaximum f3x-value becomes large, around 16 m, and the acceptance is reduced to140 n mm-mrad. 3 Tunes are Qx == 2.25 and Qy == 1.25, respectively in this operationmode.The excitation of current for the .magnets are performed with four power supplies,one for the dipole magnets and three for the quadrupole magnets. The ramp shapeof the dipole field B is a trapezoid with a repetition rate of 0.1 Hz and a risetime of3.5 s. The currents of the three quadrupole-magnet power supplies of Q magnets aretracked, along with the bending-magnet current, within the tracking error of less than1 x 10- 4 with use of the self-teaching procedure in the control computer system.4COOLER SYNCHROTRON TARN II 291Extracted Beam~oTarg~ooElectron CoolingTransport LineInt.TARN!!SeptumMagnetAnalytical MagnetAnalytical Magnet'-_r ..~FIGURE 1 Layout of TARN II.292 T. KATAYAMA3 VACUUM SYSTEMA vaccum of better than 10- 10 Torr is required for the accumulation and accelerationof heavy ion beams, so the vacuum chamber and other elements are free of organicmaterials. The ramping rate of the magnetic field is as low as 0.45 Tis, so we wereable to make the vacuum chambers at the dipole and quadrupole magnets out of4-mm-thick SUS 316L steel. By sending current flowing directly through the cham-bers, we can bake them to 250°C. Details of the vacuum system are described inanother papers. Presently, the average vacuum pressure in the ring is several times10- 10 Torr; and the goal of 10- 11 Torr will be obtained after the chambers havebeen baked well.4 RF SYSTEMThe lowest injection energy was chosen to be 2.58 MeV/u for 2°Ne4+ (among thevarious ions from the sector-focused cyclotron), corresponding to the revolutionfrequency of 0.307 MHz in the TARN II ring. At the top energy of 1100 MeV forprotons, the revolution frequency is 3.5 MHz; thus the ratio of the lowest to highestfrequencies is around 11. The harmonic number was chosen to be 2, and the designedacceleration frequency is 0.6 MHz to 7.0 MHz. An, acceleration voltage of 2 kV withinthe acceleration period of 3.5 s is sufficient for a beam with an 0.5% momentumspread.The rf cavity is a single-gap, ferrite-loaded, two-quarter-wave coaxial resonator. Itcovers the frequency range from 0.6 to 8.0 MHz by changing the ferrite bias currentfrom 0 to 770 A. A power amplifier with a maximum output power of 5 kW canproduce 2 kV of accelerating voltage throughout the whole frequency range. 6The low-level rf electronics system is composed of a voltage-controlled .oscillator(VCO) and several feedback loops. Three memory modules store the ramp data: rffrequency, acceleration voltage, and bias current as functions of bending-magnet fieldstrength. At every 1-gauss increment, measured at the 25th dipole magnet, the dataare read from memories and converted into analog voltages through digital-to-analogconverters (DACs). The data are fed into a VCO, amplitude modulator, and biascurrent power supply, respectively. The error of bias current or equivalently thedegree of cavity detuning, is detected as the phase difference between the rf signalsat the grid and the plate of the final power tube. It is used for the correction ofresonance frequency of cavity via a hardware feedback loops (AFC). In addition, thesignals of horizontal beam position (~R) and of the phase. error (~¢) between beambunch and acceleration RF field, are fed back to the voltage controlled oscillator forthe correction of RF frequency. The output RF signal of this oscillator is fed to thedriver- and power amplifiers. 7During the injection period the VCO is phase-locked with the rf signal by usinga frequency synthesizer. The frequency and voltage in the injection period are finelyadjusted for maximum capture efficiency. In Figure 2, the block diagram of the rfsystem is shown. Triggering off the master pulse, the excitation of bending- andCOOLER SYNCHROTRON TARN IIMagnetic Field ofBending Magnet293B.BIIMaster...::S::.::t=ar:....::t:....f-+- -t--;--------.B Clock-=S:::..:t=a.:....:rt~l-_---4_--+--------.B Clock Stop___2-~~---_-_~-;---_-------.B C!_oc_k_-+--f~Reset~--~.J--------t--"!'----------MEMORY MODULEDACFrequency---::--~~------f---+----------'Bias Current---'!--~------+--+-----------Ace. Volta Jr--+-----~---+IFEED BACK CONTROL GATEInj. DON I OFFI' H(I a'l. . O~FF~__ilR L: ON H OFF--I I H"-----.;;...,..;.~-FIGURE 2 Timing chart of rf system. Triggered by the master pulse, the excitation of the magnets andthe ·dB/dt clock pulse begin. Feedback control gates and other timing signals are provided, with properdelays. A reset pulse restores the data in the memory modules to the initial values.294 T. KATAYAMAquadrupole magnets, dB/dt-clock, feedback control gates, and other timing signals,are generated with the proper delays.5 SLOW BEAM EXTRACTIONThe accelerated beams, such as protons, alphas, and Ne, will be slowly extractedfrom TARN II, for experimental use. The energy range of extraction is from 150 to350 MeVlu. The extraction scheme is designed to use third-order resonance, and theoperation point is moved from the normal (QH = 1.75, Qv = 1.80) to resonance(QH = 1.6667, Qv = 1.80) by changing the excitation of the quadrupole magnets. Asextupole magnet is used as an nonlinear field source. The ions whose betatronamplitudes become larger than 75 mm pass into an electrostatic septum, whichdeflects the ions by 5.8 mrad. The maximum voltage at the electrostatic septum is80 kV. The thin septum is composed of wires of 90 J.1m in diameter at 1.25-mmintervals. The effective thickness of the septum wires, seen by the ion beam, is 0.15 mm.The septum magnet is located one sextant downstream from the electrostatic septum,which deflects the ions another 85.2 mrad to extract the ions from the TARN II ring.Presently all the equipment is complete and the studies of beam extraction havestarted. 86 BEAM EXPERIMENTSTypical results of experiments on a-beam injection, accumulation and accelerationare as follows. At the exit of the cyclotron, the a-beam energy was 40 MeV and theemittance was measured at 15 n mm-mrad (horizontal) and 20 n mm-mrad (vertical),and the momentum spread was 0.2%. One third of the beam was transported fromthe exit of the cyclotron to the injection point of TARN II. The beam was multi-turninjected into the ring by two bump magnets excited with a repetition rate of 30 Hz.The decay times of the bump fields were set to approximately 40 J.1S, which corre-sponds to about 20 times of the revolution period of the beam in the ring. Thefrequency of rf field was set to a value corresponding to the harmonic number (2) ofthe beam circulation: 1.113 MHz for a beams at 40 MeV. Figure 3 shows the signalsof the pulsed arc of the ion source in the cyclotron, the discharge currents of thekicker magnet for the beam chopper, and the two bump magnets. Judging from thebeam signal of the electrostatic monitor, the gain of intensity of the circulating beamwas increased during about 14 turns, which corresponds well with the value expectedfrom the simulations.The lifetime of the beam was measured by the de.cay constant of the signal fromone of the electrostatic monitors (Figure 4). The e-folding lifetime was found to be75 s. Beam losses are dominated by scattering off the residual gas. This lifetime isroughly in agreement with the calculated result, subject to the condition that theaverage vacuum pressure in the ring was about 8 x 10- 10 Torr. The tune valueswere measured, with the rf knockout method, as 1.737 (horizontal) and 1.825 (vertical).COOLER SYNCHROTRON TARN II 295FIGURE 3 Pulse shapes of cyclotron arc, magnetic fields of kicker magnet and two bump magnets(from top to bottom). The bump fields are around 200 G.The acceleration tests were performed to the top energy of 240 MeV for Ct particles,which is presently limited by the capacity of the electric power station in the institute.Capacity improvements are under construction, and acceleration to the designed topenergy, 370 MeV/u, is expected in December 1991.In Figure 5, the excitation pattern of the bending magnet and the accelerated beamcurrent are shown. During the acceleration period, beam loss was kept almostnegligible with fine adjustment of ~¢ with and ~R feedback-loop gain control in thelow-level rf system. The number of accelerated particles was around 108 . Much•••••••••iiii=~~••••••~~~••liiiiiii=~.1il1iiii•••••••1I•••••••••=••••••••••••••••••••••••••••••••••••••••••••••••••FIGURE 4 Beam signals from the electrostatic monitor (5 dB/div). Horizontal scale is 7 s/div.296 T. KATAYAMAFIGURE 5 Upper: Excitation pattern of the bending magnet (1 s/div) and the intensity of acceleratedbeam current. Lower: Deviation of the closed orbit (L\R) during acceleration. Vertical scale is 5 mm/div.attention was paid to improvement of the sensitivity of the beam monitoring deviceand to noise reduction of electronic systems. In Figure 5b the detected signal of thebeam orbit, I1R, is shown, and one can see that the deviation of the closed orbitduring acceleration was controlled to within several mm.A strong phase-space compression of the ion beams, to be achieved throughelectron cooling, will offer new possibilities for accelerator technology and physicsresearch. Since the first success of the electron cooling experiments in September19899 , we have continued cooling experiments to examine the details. The electroncooling system is described in Ref 9. The cooling experiments have mainly beenperformed on a 20 MeV proton beam, which corresponds to an electron energy of11 keY. A typical electron-beam current is 0.4 A, and the strength of the solenoidfield is 600 G. The proton beam was injected into the ring by the multiturn-injectionmethod. The number of injected particles was about 107 . After adjustment ofCOOLER SYNCHROTRON TARN II 2975.1 seco secFIGURE 6 Time evolution of momentum spread of a bunched proton beam with cooling. Lower trace:Just after the injection. Upper trace: After 5.1 s. The horizontal scale is 15 kHz/div; the vertical scale is5 dB/div. The central frequency is 29.985 MHz.correction magnets, which compensate the effects on the ion beams due to thesolenoidal and toroidal magnetic fields of the electron equipment, we observed thecooling effect. The momentum spread of a stored proton beam was improved .fromthe initial value of 2 x 10- 3 to the final value·of 1 x 10- 4 with an e-folding coolingtime of several seconds. A typical time evolution of momentum spread (equivalently,the spread of revolution frequency) is given in Figure 6. Machine parameters duringelectron cooling are given in Table 2.TABLE 2Machine Parameters Relevant to Electron CoolingProton beam energyElectron beam· energyElectron currentInitial momentum spread, !:J.p/pFinal momentum spreadRF frequencyHarmonic numberHorizontal Pat cooler sectionVertical Pat cooler sectionMomentum dispersion at cooler sectionHorizontal betatron tuneVertical betatron tune20.26 MeV11.20 keV0.4 A2 x 10- 31 X 10- 41.578 MHz210.2m3.7m4.7m1.711.80298 T. KATAYAMAFIGURE 7 Longitudinal Schottky signal spectra. From the top, the numbers of particles in the protonbeam are about 3 x 107, 1 X 108 , and 3 x 108, respectively.Several interesting experimental results are obtained on the cooled beams. 1o First,in order to increase the beam intensity, electron cooling was applied during beaminjection. The strong phase-space compression due to cooling allows stacking, orrepeated multiturn injection. About 20 batches were accumulated, resulting in astored-beam intensity of 108 particles. This cooling-stacking method will be quiteuseful for accumulating beams of unstable nuclei or rare particles in the storage ring.Second, the Schottky spectra of the coasting proton beams observed by the spectrumanalyzer show a two-peak splitting structure within the peak. The splitting increaseswith the number of stored protons. This phenomenon is known to be caused bycollective motion of circulating particles, and the splitting is due to plasma waves,COOLER SYNCHROTRON TARN II 299which rotate with or against the beam direction 12. Third, a neutral beam is producedwhen cooling electrons are captured by the circulating proton beam. The neutralbeam carries a lot of important information concerning electron- and proton-beamproperties. From the horizontal and vertical distributions of neutral atoms, measuredat 4.5 m downstream of the cooling section, the beam emittance can be deduced tobe 1-2 n mm-mrad, which is much smaller than the initial values, of the order of100 n mm-mrad.ACKNOWLEDGEMENTThis work has been done with the collaboration with the TARN II group membersat the Institute for Nuclear Study, University of Tokyo.REFERENCES1. T. Katayama, in Proceedings of the 11th International Conference on Cyclotrons and Their Applications(Ionics, Tokyo, 1986), pp. 128-132.2. T. Tanabe et al., in Proceedings ofthe IEEE Particle Accelerator Conference., (Chicago, IL, USA, 1989).3. A. Noda, in Proceedings IEEE Particle Accelerator Conference. 1987, pp. 846-847.4. S. Watanabe et al., in Proceedings of the 7th Symposium on Accelerator Science and Technology (1990),pp. 249-251.5. A. Mizobuchi and K. Chida, in Proceedings of the 19th INS International Symposium on Cooler Ringsand their Applications (Tokyo, 1990).6. K. Sato et al., in Proceedings of the 11 th International Conference on Cyclotrons and their Applications(lonics, Tokyo, 1986), pp. 333-336.7. T. Katayama et al., in Proceedings of the 7th Symposium on Accelerator Science and Technology, 1990,pp.80-82.8. A. Noda, Institute for Nuclear Studies report INS-T-498 (1990).9. T. Tanabe et al., in Proceedings of the 7th Symposium on Accelerator Science and Technology (1990)pp. 201-203.10. T. Tanabe and T. Watanabe, Nucl. Instrum. Meth. A 278,241 (1989).11. T. Tanabe, Institute for Nuclear Studies report INS-T-498 (1990).12. V. V. Parkhomchuk and D. V. Pestsikov, Sou. Phys. Tech. Phys. 25, 7 (1980).

Beam experiments at cooler synchrotron TARN II

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