The status of VEPP-2M collider is presented. Implementation of Round Colliding Beams (RCB) concept in the new collider VEPP-2M is outlined, potential advantages of RCB over the flat colliding beams are discussed. The main desing parameters and features of this VEPP-2000 collider are reported

Evstigneev, A.V.

Ganyushin, D.I.

Ivanov, P.M.

Koop, I.A.

Kuzminykh, V.S.

Lysenko, A.P.

Mezentsev, N.A.

Mityanina, N.V.

Nesterenko, I.N.

Otboev, A.V.

Perevedentsev, E.A.

Petrov, V.M.

Schwartz, D.B.

Shatunov, P.Yu.

Shatunov, Yu.M.

Skrinsky, A.N.

Valishev, A.A.

Volkov, V.N.

Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)

PROJECT OF VEPP-2000 ELECTRON-POSITRON COLLIDERYu.M. Shatunov, A.V. Evstigneev, D.I. Ganyushin, P.M. Ivanov, I.A. Koop,  V.S. Kuzminykh, A.P. Lysenko, N.A. Mezentsev, N.V. Mityanina, I.N. Nesterenko, A.V. Otboev, E.A. Perevedentsev, V.M. Petrov, D.B. Schwartz, P.Yu. Shatunov, A.N. Skrinsky,  A.A. Valishev, V.N. VolkovBudker Institute of Nuclear Physics, Novosibirsk, 630090, RussiaThe status of VEPP-2M collider is presented. Implementation of Round Colliding Beams (RCB) concept in the new collider VEPP-2000 is outlined, potential advantages of RCB over the flat colliding beams are discussed. The main design parameters and features of this VEPP-2000 collider are reported.PACS numbers: 29.17.+w1  STATUS OF VEPP-2M AND MOTIVA-TION FOR THE CONSTRUCTION OF A NEW COLLIDERSince the end of 1992 the  e e+ − collider VEPP-2M in Novosibirsk has been successfully running in the c.m. energy range from the threshold of hadron production up to 1.4 GeV. Since 1984 VEPP-2M is operating with the  five-pole superconducting  wiggler  with the  maxi-mum field  8B T= ,  which increases  the  beam emit-tance  by  a  factor  of  3.  The  integrated  luminosity  of about 50 pb-1 as collected with two modern detectors SND [1] and CMD-2[2] allowing precise measurements of most of the hadronic channels of  e e+ − annihilation. Together with 24 pb-1 collected at VEPP-2M in the pre-vious generation of experiments in 1974–1987, this in-tegrated luminosity is more than one order of magnitude higher than about 6 pb-1 accumulated by various experi-mental groups in Frascati and Orsay in the c.m. energy range from 1.4 to 2 GeV. Thus, there is a serious energy gap  between  the  maximum  energy  attainable  at VEPP-2M and 2 GeV in which existing data on e e+ −annihilation into hadrons are rather imprecise. Accurate measurements of hadronic cross sections in this energy range are crucial for better understanding of many phe-nomena in high energy physics. A recent decision to upgrade the VEPP-2M complex by replacing the existing collider with a new one, in or-der to improve the luminosity and at the same time in-crease the maximum attainable energy up to 2 GeV, will significantly broaden the potential of experiments per-formed  at  the  collider.  Following  modern  trends,  the new project was named VEPP-2000.2  ROUND COLLIDING BEAMSDuring  the  last  decade  at  BINP  the  concept  of Round Colliding Beams (RCB) [3] was proposed.The evident advantage of round colliding beams is that with the fixed particle density, the tune shift from the opposite bunch becomes twice as small as the tune shift in the case of flat colliding beams. Besides, the lin-ear beam-beam tune shift in the round beams becomes independent on the longitudinal position in the bunch, thereby weakening the action of synchro-betatron reso-nances.The main feature of the RCB is rotational symmetry of the kick from the round opposite beam; complement-ed with the  X-Z symmetry of the betatron transfer ma-trix between the collisions, it results in conservation of particle  angular  momentum. Thus,  the  transverse mo-tion becomes equivalent to a one-dimensional (1D) mo-tion. Resulting elimination of all the betatron coupling resonances is of crucial importance, since they are be-lieved to cause the beam lifetime degradation and blow-up.The above arguments in favour of RCB have been checked out by the computer simulations of the beam-beam effects in RCB option [4]. The simulations have also  demonstrated  stability  of  RCB against  the  “flip-flop” effect, similarly to conclusions from simple flip-flop models [5].3  VEPP-2000 PROJECT3.1  Collider OpticsOur approach to the new collider optics is based on the idea of round colliding beams [3]. The main princi-ples of round beam mode will be satisfied by placing SC solenoids in two Interaction Regions equipped with existing particle detectors (Fig. 1).Fig. 1. The VEPP-2000 collider layout.The  superconducting  solenoids  will  provide  equal *β -functions and rotate by  2/pi  the planes of betatron oscillations.  This  will  result  in  alternation  of  vertical and horizontal orientations of the planar betatron eigen-modes over each half-turn, which in turn will lead to their equal tunes and emittances. The optical functions of the round beam lattice are presented in Fig. 2.ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2001. №3. Серия: Ядерно-физические исследования (38), с. 6-8.6Fig. 2. Half period lattice functions. S=0 corre-sponds to IP.An essential advantage of the found optics is zero dispersion in the IRs, RF cavity, and injection straight sections.The chosen optics has another very useful feature. Variation  of  the  focusing  strength  of  the  solenoids changes *β  and the beam emittance in inverse propor-tion, at  a  fixed energy.  Changing the energy,  we can squeeze *β , conserving the maximum beam size at the solenoids,  thus  giving  a  possibility  to  tune  optics  for better  performance.  Apparently,  this  feature  provides the luminosity scaling at lower energies approximately as 2γ  (instead of 4γ  for the option with fixed *β ).The main parameters of the new collider are given in Table 1.3.2  InjectionThe  injection  of  beams  into  the  storage  ring  is planned to be done in the horizontal plane into the long drift opposite to the RF cavity. The inflector plates will be placed on the inner side of the vacuum chamber in the bending magnets at the ends of the drift. The advan-tage of such a scheme is independence of the injected beam trajectory on the solenoids field. This gives us an opportunity  to  test  different  options  of  optics:  usual round beams, “Mobius”, and flat beams with zero rota-tion of the betatron oscillation plane.The  BEP booster  is  capable  of  production  beams with the energy of up to 900 MeV. Thus, operation at lower energies will be continuous, with injection of the beam  at  the  experiment  energy.  In  the  range  from 900 MeV to 1 GeV the energy ranging from 900 MeV to the experiment energy is required.3.3   Chromaticity correctionThe chromaticity correction is performed by the sex-tupole families  Sx and  Sz, placed near the quadrupoles of  triplets,  where  the dispersion function is  non-zero. Another variant discussed implies a special correction of pole profiles of the horizontally focusing quadrupoles in the triplets.Table 1. Main parameters of the collider at E=900 MeVCircumference, m C 24.388RF frequency, MHz0f 172.0RF voltage, kV V 100RF harmonic number q 14Momentum compaction α 0.036Synchrotrone tune sν 0.003 Emittances, cm ⋅ rad xεzε52.2 10−⋅  52.2 10−⋅Energy loss/turn, keV 0E∆ 41.5Dimensionlessdampingdecrementszδxδsδ52.3 10−⋅52.3 10−⋅54.6 10−⋅Energy spreadεσ46.4 10−⋅xβ  at IP, cmzβ  at IP, cmxβzβ6.36.3Betatron tunes ,x zν ν 4.1, 2.1Particles/bunch ,e e− + 111.0 10⋅Bunches/beam 1Tune shifts xξzξ0.0750.075Luminosity/IP, cm-2 ⋅ s-1 maxL 321.0 10−⋅The scheme with only two sextupole families leaves the  problem  of  dynamical  aperture  unresolved,  this forces us to use an additional sextupole correction fami-ly  to  control  the  sextupole  perturbation  harmonics. These sextupoles are placed in dispersion-free regions: in the injection and RF cavity drifts, between the bend-ing magnets and quadrupoles. Application of these cor-rectors yields dynamical aperture of about σ14  (17mm inside the solenoid) which is still less than mechanical aperture. So, search for a better solution is in progress.4  TECHNICAL FEATURES4.1  Superconducting SolenoidsFocusing in the two interaction regions is performed by SC solenoids, installed symmetrically with respect to the  IPs.  Each  solenoidal  block  consists  of  a  main solenoid which is longitudinally divided into two parts, and a compensating solenoid with reverse field to adjust longitudinal field integral and focussing. Such a scheme gives an additional possibility to control the  *β  value by feeding only one half of the main solenoid at lower energies.The solenoid coil is divided into three sections: in-ner  section  has  thickness  30  mm  and  is  made  of SnNb3  wire  1.23  mm in  diameter  (50% Cu +  50%SnNb3 ); middle section has thickness 20 mm and is wound with a NbTi  wire 1.2 mm in diameter (48%Cu+52% NbTi ) and outer layer has thickness of 10 mm, ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2001. №3. Серия: Ядерно-физические исследования (38), с. 7-8.7made  of  NbTi wire  0.85  mm  in  diameter  (48% Cu+52% NbTi ). To feed this three-section coil we plan to use two power supply units. Connection scheme implies that the current in the outer section is the sum of cur-rents in the inner ones. The distribution of currents in the sections is: inner section - 145 A, intermediate sec-tion - 167 A, outer section - 312 A. The peak magnetic field is 12.1 T.Magnetic flux is closed by the iron return yoke lo-cated together with all the coils in a common LHe cryo-stat. Aperture of the coil is 50.0 mm. The inner tube of the helium vessel is a part of the collider vacuum cham-ber. A nitrogen vapour cooled liner is envisaged to pro-tect the surface of the helium cryostat from heating by synchrotron radiation.4.2  Dipole Magnets, Quadrupoles and SextupolesConstrained  VEPP-2M  complex  area  restricts  the machine dimensions leading to necessity of using strong dipole magnets. To achieve the beam energy of 1 GeV guiding field  of  2.4  T is  required.  The design of  the BEP booster ring magnet [6] which works at this field level is intended to be used. Magnet bending radius is 1400 mm, the gap is 40 mm. Number of coil turns is 10. At maximum current 9.5 kA the power consumption is 100 kW/magnet.New lattice will include 5 families of quadrupoles with maximum gradient  of 50 T/m and 3 families of sextupoles. Inscribed circle diameter of quadrupoles and sextupoles  is  40  mm.  Chromaticity  correction  sex-tupoles (two families) are located between quadrupoles of the triplets. Similar 5 kW power supply units will be used to feed the coils in the quadrupole magnets and in the sextupoles. All other low-current coils of the closed orbit  steering  and  gradient  correction  coils  in  the quadrupole  magnets  will  be  powered  using  existing power supplies.4.3  RF SystemBeam revolution frequency is 12.292 MHz. The ac-celerating RF frequency was chosen at 14-th revolution frequency  harmonic  i.e.  172 MHz.  With  accelerating voltage of 100 kV the bunch length is about σ = 3 cm at the energy of 1 GeV. Energy loss per turn is 64 keV, and with colliding beam currents of 1.02 ×  A the pow-er delivered to the beams is 12.8 kW. The so-called sin-gle-mode cavity is proposed to be used to ease suppres-sion of  coherent  instabilities,  see Fig.  3.  Two coaxial damping loads are foreseen to absorb the energy from high-order modes excitation. The fundamental mode is isolated from the upper load by the tunable choke.Fig. 3. Cross-section of the cavity. The locations  of HOM.4.4  Vacuum SystemHigh vacuum pumping of the experimantal straight sections will be performed by the internal tube of the LHe vessel. For this purpose it is planned to make slits in the nitrogen cooled liner which protects the LHe sur-face from heating by the synchrotron radiation. In the rest regions combined ion-pumping and getter pumping are intended to be used. Average vacuum in the ring at the working currents should be higher than 810− torr.5  CONCLUSION− Experimental testing of RCB at VEPP-2000 should verify  predictions  on  extremely  high  attainable space charge parameters for the round beams.− The  machine  tune-up  procedures  will  be  worked out for implementation of such a non-conventional optics.− The  efforts  and  expenses  needed  to  build VEPP-2000 are moderate and so this work can be carried out within the next year.REFERENCES1. M.N.Achasov  et  al.,  Preprint  BudkerINP  98-65, Novosibirsk, 1998.2. R.R.Akhmetshin et al., Preprint BudkerINP 99-11, Novosibirsk, 1999.3. V.V.Danilov et al., “The Concept of Round Collid-ing  Beams”  //  Proc.  EPAC'96,  Barcelona,  1996, p.1149.4. A.N.Filippov et al.// Proc. 15th Int. Conf. High En-ergy  Accelerators,  Hamburg  (Germany),  1992, p.1145.5. A.V.Otboyev and E.A.Perevedentsev // Proc. 1999 PAC, New York (1999).6. V.V.Anashin  et  al.,  Preprint  BudkerINP  84-114, Novosibirsk, 1984. ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2001. №3. Серия: Ядерно-физические исследования (38), с. 8-88

Project of VEPP-2000 electron-positron collider

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Project of VEPP-2000 electron-positron collider

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Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)