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KEKB Accelerator Physics Report KEKB Accelerator Physics Report Y. Funakoshi for the KEKB commissioning group KEK, 1-1 Oho, Tsukuba, Ibaraki 305-0801,Japan Abstract countered any harmful effects induced by the crossing an- gle except for some geometrical loss of the luminosity. Ta- 1 INTRODUCTION ble 1 summarizes the main design parameters of the KEKB. The KEKB B-Factory is an electron-positron double ring To realize the high design peak luminosity, the design beta collider aiming at the study of B meson physics with a de- functions at the IP were chosen as shown in Table 1. As sign luminosity of 1 × 1034/cm2/sec. The high design lu- shown later, the present values of the vertical beta functions minosity comes from the requirements of B meson physics are even smaller than the design value. The introduction of which studies very rare processes. Therefore, the luminos- the finite crossing angle seems to contribute to realize the ity is a parameter of overriding importance at B factory ma- small values of the vertical beta functions at the IP. chines. Another significant feature of the KEKB which dis- tinguishes it from conventional electron-positron colliders There are the other two issues related to the second and is that it is an energy asymmetric collider. This feature is third features. Although these are not dealt in this reports, also required by the physics motivations. The requirement these are not unimportant. The first issue is difficulty of of energy asymmetry inevitably leads us to a double ring optimizing machine parameters of the two beams for the collider. From the standpoint of machine design, this dou- beam-beam effect. Since we can choose the machine pa- ble ring feature enables a “high current-multibunch” ap- rameters of the two beams independently, it is not easy to proach like synchrotron light sources, which is vital to get optimize their combinations. At the design phase of the to a higher luminosity. KEKB, the energy transparency conditions were proposed. Here, we summarize the features of the B factories. However, the present machine parameters of the KEKB heavily break these conditions. This break is also brought • High current multibunch collider by the feature of the high beam currents as is shown later. • Energy asymmetric collider The second issue is difficulty of machine tuning. Since • Double ring collider the two rings are almost independent, we need careful tun- ing of the geometrical relationships between the two beams In this report, we try to show how these features restrict such as a beam orbit offset at the IP, a crossing angle, beam the machine performance (luminosity in this case) in the tilt at the IP, collision timing, waist points and others. Al- case of the KEKB. Of these features, the first one has been though the present KEKB can manage to keep these param- giving the most severe restriction to the KEKB. The his- eters under control, we need frequent tunings of these pa- tory of the KEKB has an aspect of tough struggles with rameters to keep the luminosity high [2][3]. various hardware troubles originated from the high beam currents [1]. Another severe restriction comes from the In addition to these features, the KEKB has another no- beam instabilities originated from the high beam currents. table feature that horizontal tunes are very close to the half Of the instabilities, the vertical beam blowup observed at integer resonance. It turned out that these tunes bring a the LER (Low Energy Ring) has been the most important higher peak luminosity, as is described in this report. in the KEKB. As is described in this report, the source of the blowup is believed to be the electron clouds which are formed by the photoelectrons and the secondary electrons. As is described later in this report, the specific luminosity of the KEKB decreases with decreasing the bunch spacing. LER HER Although we have not yet understood the true mechanism of Luminosity 1 × 1034/cm2/sec this phenomenon, this should also have some connection to Beam energy 3.5GeV 8.0GeV the high current multibunch feature. Beam currents 2.6A 1.1A The machine design of the KEKB was done with also Beta functions at IP (H/V) 0.33m/10mm considering the second and third features. One of the dif- Beam-beam parameters (H/V) 0.039/0.052 ficulties in the design phase was an IR design. To simplify Horizontal crossing angle ±11mrad the IR design and avoid deleterious effects of the parasitic crossing, the KEKB introduced a relatively large (horizon- tal) crossing angle of ±11mrad.Sofar, we have not yet en- Table 1: Main design parameters of the KEKB values of the luminosity as of May 26 2002. LER (3.5 GeV) Crab Cavitie Total integrated luminosity 79.8 /fb (planning) Peak luminosity 7.25 ×1033cm2/sec HER (8 GeV) Integrated luminosity / shift 139.9 /pb Wiggler BelleIP vities Tsukuba Daily integrated luminosity 387.0 /pb Sper caNikko Integrated luminosity/7days 2524 /pb KEKB Monthly integrated luminosity 8.01 /fb TRISTAN tunnel B-Factory Table 2: Record values of the luminosity as of May 26 2002. ARES ca Beam viti Transport Fuji es (LER) Lines Wiggler (HER) 4 MACHINE PARAMETERS AND vities Oho FEATURES OF THE MACHINE ARES ca Table 4 shows a parameter list of the KEKB at the record peak luminosity. This table tells some characteristic fea- e+ Linac tures of the KEKB. (3.5 GeV) e- J-Linac The present KEKB is filled with beams at every 4th RF e+ target (8 GeV) bucket. In the design[4], the number of bunches was as- sumed to be around 5000 which means that every RF bucket J-Arc is filled with particles (except for some abort gap). In the present KEKB, the specific luminosity is decreased when the number of bunches is increased from the every 4th RF Figure 1: A schematic view of the KEKB. bucket case by reducing bunch spacing. Although we tried longer bunch spacing, 4 RF bucket spacing (∼8nsec) is the 2 OVERVIEW OF MACHINE best choice at the present KEKB. The other parameters are chosen under this restriction of bunch spacing. COMPONENTS It is notable that the bunch currents of the present KEKB Fig. 3 shows a schematic view of the KEKB. The KEKB are much higher compared with the design values particu- is composed of an injector linac and two rings. The two larly in the HER. This is also a consequence of the bunch rings were constructed in the existing TRISTAN tunnel and spacing restriction. To compensate this unusually high have the same circumference of about 3016m. Positrons bunch current to some extent, the horizontal emittance of of 3.5GeV are stored in the low energy ring (LER). For the HER is enlarged compared with the design. On the other beam acceleration, we use specially developed damped hand, the LER bunch current is not so high as the HER. Un- cavities named “ARES” in the LER. The high energy ring til very recent operations , the luminosity did not increase (HER) stores 8 GeV electrons. In the HER, superconduct- with higher LER beam current than some threshold current. ing damped cavities are also used in addition to the ARES It is believed that this luminosity saturation with the LER cavities. In the LER, wiggler magnets are also used to beam current arose from the beam blowup due to the elec- equalize the radiation damping time to that of the HER. As tron cloud. In this situation, the LER beam current was lim- is already mentioned, the two rings cross horizontally at the ited by the electron cloud instability in the sense that the IP with a half crossing angle of 11mrad. To recover the lu- luminosity did not increase with a higher LER beam cur- minosity loss due to this crossing angle, installation of crab rent. However, as a result of cumulative installations of the cavities is planned. However, a budget problem prevents us solenoid winding in the LER, the single beam blowup from from installation of the crab cavities in the immediate fu- the electron cloud is not visible with the present maximum ture. beam current. The scrubbing effect of the chamber wall possibly contributed to suppress the blowup. The present 3 PRESENT PERFORMANCE beam current limitation comes from the heating problem in the IR region. Fig. 3 shows a history of the KEKB luminosity. The top The horizontal and vertical beta functions at the IP has row shows a history of a peak luminosity. As is seen in the been determined by a trial and error method. The vertical figure, the improvement of this year is remarkable. The sec- beta functions are much lower than the design values. ond row shows a history of a daily integrated luminosity. Another feature of the KEKB is that the working points The third row shows a history of peak beam currents of a are very close to the half integer resonance as is seen in day. The bottom row shows a history of an accumulated lu- the table. These horizontal tunes make the horizontal emit- minosity by the Belle detector. Table 1 summarizes record tance large and the horizonal beta functions small to a large Figure 2: History of the KEKB luminosity. extent. This large emittance compensates the large bunch LER HER currents and contributes to stabilize the beams against the 18 24 ε (nm) beam-beam effect. As is seen in the table, both of the x (18) (18) horizontal and vertical tunes of the both rings are located ∗ ∗ 0.59/0.0062 0.63/0.007 β /β (m) above the half integer resonance, while the vertical tunes x y (0.33/0.010 ) (0.33/0.010 ) are above the integer resonance in the design.
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