Funakoshi, Y

Kikutani, E

Koiso, H

Sato, A

Satoh, K

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Particle Accelerators, 1990, Vol. 31, pp. 141-146Reprints available directly from the publisherPhotocopying permitted by license only© 1990 Gordon and Breach, Science Publishers, Inc.Printed in the United States of AmericaAN ASYMMETRIC COLLIDER WITH THE TRISTAN MAIN RINGKOTARO SATOH, YOSHIHIRO FUNAKOSHI, HARUYO KOISO,EIJI KIKUTANI and ASAO SATONational Laboratory for High Energy Physics1-1 Oho, Tsukuba-shi, Ibaraki-ken, 305 JapanABSTRACT The paper describes two preliminary designs of the asymmetriccollider, 2GeV x 14GeV and 3.5GeV x 8GeV, with the TRISTAN main ring.The design goal of luminosity is 1 x 1033 cm-2s-1• While the total current issupposed to be limited by beam instabilities and accelerator hardware, it hasnot been examined whether the required total current can be stored.INTRODUCTIONAsymmetric colliders have been proposed for B-meson factories in laboratories havinglarge electron-positron storage rings, such as PETRA and PEp1,2. It is also reportedthat symmetric double-ring colliders proposed in PSI and Novosibirsk can be oper-ated as asymmetric colliders3,4. In KEK a design study is going on for proving thefeasibility of the asymmetric collider as a future project. One of the two rings is theTRISTAN main ring(MR) and the other is a new small ring(SR).This paper includes two preliminary designs of the asymmetric collider in theKEK site. The design goal of luminosity is 1 x 1033cm-2s-1 in both cases. Thetotal current will be limited by coupled bunch instabilities, heat-up problems of thevacuum chamber, short beam life due to the vacuum pressure rise and RF inputpower through couplers. The current limitation, ho~ever, has not been examined inboth designs. Neither the synchrotron radiation produced in the separation bend hasbeen taken into account. We need the collaboration with physics groups to finalizethe design.In the first design the small ring is housed in one of the existing experimentalhalls in TRISTAN with least engineering work on the existing concrete wall. Theenergy ofSRis 2GeV while MRis to be operated at 14GeV. One of the big problems ofthis design is the electron injection into MR with the 8GeV TRISTAN accumulationring(AR). In the end of each operation cycle of MR the stored current will be dampedbefore the deceleration to injection energy. It is foreseen that the filling time of MRwould be too long with the result of low integrated luminosity. In order to obtainhigh integrated luminosity this proposal requires a new 14GeV booster ring in the[1349]/141142/(1350] K. SArOH ET AL.TRISTAN tunnel.In the second design the energy of MR is chosen to be 8GeV, the maximumenergy of AR. Then MR is always staying at the colliding energy. In the end of theoperation cycle only the decrement of the stored current is refilled by AR. The energyof the small ring would be 3.5GeV. It is not necessary to construct a new boosterinjector. By the improvement of klystrons the linac would be able to inject positronbeams directly into SR at 3.5GeV. In this case, however, the small ring is no longerfitted into the experimental hall.2GeV x l4GeV COLLIDERThe size of SR is restricted by the area of the experimental hall(Fuji) and the RFharmonic number of MR. The circumference of SR is chosen as 1/32 of MR's. Thegeometry of SR and injection lines are drawn in Fig.l. Table I shows the machineparameters of the two rings. We adopt the flat-beam collision scheme instead of theround-beam collision. We choose a relatively small emittance ratio of 0.02. Supposethat the stored current in MR is 2mA per bunch, 640mA in total, and that theminimum (3;MR is Scm. If the maximum tune shift is 0.03 in both rings, the luminosityis expected to be 1.2 x 1033cm-2s-1 with the collision frequency of 32MHz.TABLE I Ring Parameters (2GeV x 14GeV Design){, = 1.2 x 1033cm-2s-1EnergyCircumferenceHarmonic numberNumber of bunchesCurrent per bunchTotal currentEmittanceEmittance ratio(3;(3;Beam-beam ~1JEnergy spreadBunch lengthEnergy lossfRFRF voltageSynchrotron frequencyCavity TypeNumber of cavitiesShunt impedanceBeam lossCavity lossTotal powerMR14Gev3018.lm51203202mA640mA2.4 x 10-7m0.022.5m0.05m0.037.7 x 10-42.0cm13.8MeV508.58MHz32MV,9.3kHzAPS 9-cell10675MO8.8MW1.5MW10.3MWSR2GeV94.3m16010224mA2240mA4.8 x 10-7m0.021.25m0.02Sm0.037.3 x 10-42.0cm0.254MeV508.58MHz2MV143kHzSingle-cell432MOO.57MW0.125MW0.7MWAN ASYMMETRIC COLLIDER WITH THE TRISTAN MR [1351]/143Fig 1. Geometry of the 2GeV x 14GeV collider.The insertion layout is found in Fig.2. The free space in the colliding sectionis 2m long. The nearest two pairs of quadrupoles are composed of sectors of perma-nent magnet. The separation bend has a relatively high field of O.6T. It producessynchrotron radiation with the 14GeV beam, which is to be a major noise source tophysics detectors. The effect of the radiation on the experimental detector, or themethod masking the radiation is not considered in the design.0.5mQC20.0n0§~t;l-0.50~-1.0-1.5a 2 4 8m 10Fig 2. Insertion layout of the 2GeV x 14GeV collider.The optimum emittance of MR is rather large compared with that in the originaloptics. In the example design the emittance is controlled by decreasing the phase144/[1352] K. SATOH ET AL.advance of the normal cell from the nominal value of 60 degrees to 30 degrees. Thepresent configuration of sextupole families is optimized for the nominal phase advance.Computer simulations using with tracking programs are necessary to analyze thesextupole correction in the high-emittance optics together with the chromatic effectproduced by the strong quadrupoles in the colliding insertion.In the present design conventional cavities are assumed in both MR· and SR.In MR 10 nine-cell APS cavities, which are the same type as used in TRISTAN, areemployed. In SR we adopt 4 single-cell cavities with the same acceleration frequencyof MR. The parameters of the cavity are referred to the re-entrant cavity in KEKPF.It is believed that the total current is limited by coupled-bunch instabilities excitedby higher-order modes in RF cavities. We have not examined the stability of therequired stored current both in the two rings. If the estimated total current is muchless than the required we would change the present RF design. In order to storeso many bunches with large current it is definitely required to control or suppressthe coupled bunch instability. Practical methods are the development of efficientfeedback systems to damp the coherent motion of bunches, and the development ofcavities with least higher-order mode resonances, such as a single-mode cavity.One of the big problems is the electron injection into MR with the 8GeV AR.Keeping the stored current during the deceleration from 14GeV to 8GeV is supposedto be hardly practical. The stored current would be damped after each operationcycle. Then the electron filling time would be too long with the result of low integratedluminosity. Installation of a new 14GeV synchrotron in the TRISTAN tunnel wouldsolve the problem. MR would have no need to change energy, staying at 14GeV, andonly the decrement in the stored current is filled by the injection. It is also expectedthat coupled-bunch instabilities are less effective on the 14GeV beam than on thepresent 8GeV injected beam.3.5GeV x 8GeV COLLIDERThe design of the 3.5GeV x 8.0GeV option has been done on the basis of that ofthe 2GeV x 14GeVoption. The candidates of the circumference of SR are 1/20 and1/16 of MR's considering the beam energy and the RF harmonic number. Of thesecandidates, 1/16 MR's is chosen to reserve enough space for RF cavities.The geometrical view of SR is shown in Figure 3. Table II shows the machineparameters for the collider. Some assumed parameters are chosen as the same valuesas those in the first option; the emittance ratio as 0.02, the attainable current inMR as 2mA per bunch, 640mA in total and the maximum tune shift as 0.03. Asmaller fJ;MR of 3cm is obtained because the smaller energy asymmetry enables thefinal focusing Q magnets to be stronger than those of the first option. The designluminosity is 1.1 x 1033cm-2s-1 with the collision frequency of 32MHz.AN ASYMMETRIC COLLIDER WITH THE TRISTAN MR [1353]/145TABLE II Ring Parameters (3.5GeV x 8GeV Design)£, = 1.1 x 1033cm-2s-1EnergyCircumferenceHarmonic numberNumber of bunchesCurrent per bunchTotal currentEmittanceEmittance ratiof3;f3;Beam-beam ~l/Energy spreadBunch lengthEnergy lossfRFRF voltageBeam lossMR8Gev3018.1m51203202mA640mA2.3 x 10-7m0.021.5m0.03m0.031.2 x 10-32.2cm5.4MeV508.58MHz14.2MV3.5MWSR3.5GeV188.6m3202061mA1220mA2.7 x 10-7m0.021.25m0.025m0.039.4 x 10-41.6cm1.3MeV508.58MHz5MV1.6MWThe layout of the colliding insertion is found in Figure 4. The free space in thecolliding section is 1.9m long. The separation bend is 2m long and has a considerablyhigher field(1.4T) compared with that of the former option. The critical energy of thesynchrotron radiation emitted by the 8GeV beam in the bend is 59.6keV and is 20%lower than that by the 14GeV beam in the first option. The radiation power fromthe 8GeV beam in the bend is, however, 80% higher than that from the 14GeV beam.Fig 3. Geometry of the 3.5GeV ring.146/[1354] K. SATOH ET AL.m 0I2I3I4I5I6I7I8I9Im0.00.20.40.6IPQCD QCFSeparation BendQCl QC20.81.01.2Parameters related to separation bendBend filed (Tesla)Bend length (m) 2.0Bending angle (mrad) 240 105critical energy or photon (keV) 11.4 59.6QC3SRFig 4. Insertion layout of the 3.5GeV x 8GeV collider.The optimum emittance of MR is 2.3 x 10-7m and almost the same as that ofthe 2GeV x 14GeV design. To realize this rather large value, the phase advance ofthe normal cell is changed from the nominal value of 60 to 50 degrees, in additionto the use of 1.4T wiggler magnets installed in the symmetry region of the ring.Furthermore, the number of the bending magnets is halved in such a way that everyother bending magnet is removed from the lattice used in the present MR. It isexpected that the sextupole field becomes lower with wider dynamic aperture, sincethe dispersion in the sextupole magnets becomes larger. Any tracking study, however,has not been performed to examine the dynamic aperture in this scheme. The removalof the bends is also helpful to make additional space for vacuum pumps, which arenecessary for the sufficiently low vacuum pressure to guarantee an enough beam life.In this design, the bunch current and the number of bunches in MR are assumedto be the same as those in the 2GeV x 14GeV design. The coupled bunch instabilityis still the main difficulty for the attainment of the design luminosity.REFERENCES1. H. Nesemann et al., EPAC(1988), F. Willeke, talk at Workshop onB-Factories and Related Physics Issues in Blois(1989).2. S. Chattopadhyay, these Proceedings.3. K. Wille, these Proceedings.4. A. Zholents, these Proceedings.

An Asymmetric Collider With The Tristan Main Ring

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