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A prototype REBa2Cu3Oy (REBCO) sextupole coil was designed and fabricated for the chromaticity correction of an asymmetric–energy collider, named SuperKEKB. The REBCO sextupole coil consists of the six two-layer-rectangular coils, which are wound with a 4-mm-wide coated conductor and impregnated with the epoxy resin. The inner bore radius and coil length are 75 mm and 200 mm, respectively. The designed sextupole field gradient is 200 T/m2 at an operating current of 250 A. We conducted a series of the experiments on the REBCO sextupole coil after the fabrication. In this paper, we present the experimental results of the excitation test and magnetic field measurement at 4.2 K.

High-Q operation of a superconducting radio frequency cavity can reduce the power loss at the surface and is desirable for continuous-wave operation. To realize the high-Q operation, the surface resistance, which is a sum of the Barden-Cooper-Schrieffer resistance and residual resistance, needs to be reduced. The residual resistance has been found to originate primarily from the magnetic flux trapping in defects in the cavity during the cooling down process. A magnetometer called the “flux gate sensor” has been used to measure the ambient magnetic field; however, this is larger in comparison with the cavity and it is very expensive. Sensors based on the magneto-resistance effect, such as the Anisotropic-Magneto-Resistance (AMR) sensor, are smaller and much less expensive than a flux gate sensor. We examined the characteristics of the magneto-resistance sensors at room temperature and liquid nitrogen temperature. The results are reported in this paper.

International audience; The SuperKEKB collider has entered the physics production phase with the aim to reach its target luminosity of 6×1035 cm−2s−1. Vibrations of the accelerator elements, especially around the Interaction Point (IP), could limit the performance of the collider. Since 2018, a 24-hour monitoring system of vibrations has been put in place on both sides of the Belle II detector. Measurements and analyses of the spectral components of these vibrations and of the transfer function of the mechanical supports are presented, along with measurements of luminosity spectra from the LumiBelle2 detectors, showing good agreement in spectral peak frequencies for several of the vibration sources.

SuperKEKB is an electron–positron asymmetric-energy double-ring collider, which was built in Japan. It has been operated to explore new phenomena in B-meson decays. Hence, extremely higher luminosity is required. A collision scheme of low emittance with a large Piwinski angle called a “nano-beam scheme” has been adopted to achieve higher luminosity by squeezing the vertical beta function at the interaction point to be smaller than the bunch length. A “crab waist collision scheme” proposed by P. Raimondi et al. has also been adopted to improve the luminosity performance. The article presents an overview of the operation of the nano-beam and crab waist collision schemes at SuperKEKB.

Trapped magnetic flux in bulk superconductors reduces the quality factor Q in superconducting radio-frequency (SRF) cavities. However, the mechanisms underlying flux trapping and radio-frequency loss are not well understood. Detailed observation of the magnetic distributions is important for understanding such phenomena. Magnetic field mapping is useful for observing the magnetic field distribution around SRF cavities. Measuring the change in the magnetic field around the cavity elucidates the flux trapping behavior. Anisotropic magnetoresistive (AMR) sensors are inexpensive and small devices that can detect magnetic flux density. The magnetic sensitivities of AMR sensors need to be evaluated at liquid helium temperature for the magnetic field mapping of SRF cavities. In this study, a test stand was constructed to calibrate the magnetic sensitivities of AMR sensors in liquid helium, and 110 AMR sensors were tested using this stand. The magnetic sensitivities were evaluated systematically. A solenoid coil was used to control the uniform external magnetic field and to measure the magnetic sensitivity at low temperatures. All AMR sensors exhibited suitable sensitivities to the magnetic field around the SRF cavity. The variation in these sensitivities in all AMR sensors was ∼1%. The AMR sensors were found to have sufficient sensitivity for mapping the magnetic field around the exterior surface of the SRF cavity.

A fast luminosity monitor measuring the rate of the radiative Bhabha process at zero degree photon scattering angle based on diamond detectors was developed and successfully operated during the Phase-2 commissioning of SuperKEKB. The main purpose of this system, called LumiBelle2, is to provide: train integrated luminosity signals at 1 kHz with a relative precision better than 1% for luminosities higher than 1 0 34 c m − 2 s − 1 , input to a dithering feedback system designed to maintain an optimum horizontal overlap between the two colliding beams at the Interaction Point (IP), and bunch integrated luminosity signals at 1 Hz which are useful for machine tuning and beam parameters studies of the successive bunches along the train. In this paper, the design of the LumiBelle2 and Phase-2 results will be reported, including the evaluation of the single beam background, relative luminosity measurements and vertical beam size determinations at the IP using vertical offset scans.

Magnetic shielding is used to reduce ambient magnetic field surrounding superconducting RF cavities in order to decrease surface resistance and achieve high quality value. The tolerance of the residual magnetic field can be as low as 0.1 μT. Therefore, it is necessary to choose a proper shielding material to reduce the magnetic field to within permissible range. In this paper, field measurements are carried out using a flux gate sensor inside the magnetic shield for the superconducting RF test facility nine-cell cavity system. The measurement results at room and cryogenic temperatures are reported. A modeling method for the actual structure of the shield is developed, and the simulation results are compared with the measured results.

SuperKEKB is an electron-positron circular collider with a 3-km circumference that aims to achieve a peak luminosity of 8 × 10 35 cm -2 s -1 using the so-called “nanobeam” scheme. The magnets in the entire interaction region and in the straight sections on either side of the interaction point have been replaced to realize collisions of nanosize beams. The magnet design and field measurement results are reported.

SuperKEKB, which is an electron-positron collider with a design peak luminosity of 8 × 10 35 /(cm 2 s), is under construction. The luminosity is 40 times higher than the world record set by its predecessor, KEKB B-factory. The design of SuperKEKB is based on the “nanobeam scheme,” which is a low-emittance lattice. To achieve the low-emittance beam, the chromaticity of X-Y coupling at the interaction point (IP) should be controlled. Skew sextupole magnets are found to be effective in reducing the beam size at the IP during the KEKB operation. A novel idea of tilting normal sextupole magnets to control the ratio of the skew to the normal field component has been proposed for the positron ring at SuperKEKB. This paper presents the design and the fabrication of the tilting sextupole magnet system.

Magnetic properties of Cryophy were measured at room and cryogenic temperatures using ring samples. Sample-to-sample variation in permeability was evaluated in addition to measurement error. Permeability dependence on the maximum annealing temperature was examined using ring samples annealed at 1100 °C and 1170 °C. From these data, Cryophy was chosen as magnetic shielding material for the superconducting cavities in the main linac section of the cERL (compact Energy Recovery Linac) at KEK. Magnetic properties of Cryophy are presented along with the results of simulation based on the data obtained. The result of the high-power test of the cERL main linac cavities is reported at the end.

The Superconducting RF Test Facility (STF) is a project at KEK to build and operate a test linac with high-gradient superconducting cavities. Two types of high gradient accelerating cavities, with design accelerating gradients of 35 MV/m and 45 MV/m, will be tested along with other essential technologies needed for the International Linear Collider main linac systems at the STF. In order to achieve the design accelerating gradients, the ambient magnetic fields inside the cavities must be well shielded, to a level of mG . The permeability of the shielding material shows a strong temperature dependence, which often results in a degradation of the shielding effect in a cryomodule when cooled down to the operating temperature. The magnetic field inside the magnetic shield for the STF cavities is measured along the STF beam axis at cryogenic temperature and compared with measurements at room temperature. The measurements at both temperatures are compared with simulation where the permeability curves of the shielding material obtained at room temperature, liquid nitrogen temperature and liquid helium temperature are used. The shielding effect dependence on temperature of the magnetic shield inside the STF cryomodule is presented along with a comparison between measurements and simulation.

Permanent magnets are superior to electromagnets in generating strong multipole magnetic fields. Their fields are sometimes stronger than those that can be generated by superconducting magnets with the same bore radius, when the number of poles are higher. Three fabricated multipoles including a quadrupole magnet for the ILC (International Linear Collider) final focus doublet, a quadrupole magnets as a spin filter for cold neutrons, and a sextupole magnet for neutron beam focusing are described.

The Superconducting RF Test Facility (STF) is being constructed at KEK. Two types of high gradient accelerating cavities, with accelerating gradients of 35 MV/m and 45 MV/m, will be tested along with other essential technologies needed for the International Linear Collider main linac systems at the STF. In order to achieve the design accelerating gradients, the ambient magnetic field inside the cavities must be well shielded, to the level of several mG. The earth's magnetic field in the STF tunnel, and the ambient magnetic fields inside the cryomodule and inside the magnetic shield were measured. The results are presented along with the permeability measurement data of several materials at 300 K, 77 K and 4 K, which will be used to estimate the shielding effect when the cryomodules are cooled down to the operating temperature in the future.

To study the possibility of HTS applications in the design of magnets for particle accelerators, we investigated the trapped field distribution in a bulk superconductor. An array of Hall probes, calibrated at liquid nitrogen temperature, was used to monitor the trapped field distribution in a hole (10 mm in diameter) in a rectangular bulk superconductor (15 mm thickness) as a function of time. The bulk superconductor was placed in an external field created by a dipole magnet, where the field strength was varied up to about 1.5 T. The decay process of the trapped field was monitored using the Hall probe array. The decay speed was observed to be dependent on the initial external field applied, the distance from the edge of the material and the trapped field strength. The results are summarized in this report

Field trapping and shielding experiments were performed using high-T/sub c/ bulk superconductor with a cylindrical hole in it. An array of Hall probes was used to measure the field inside the hole as a function of position and applied field. The trapping of magnetic field in the hole was accomplished by cooling the bulk superconductor in the presence of an applied field and then gradually reducing the applied field to zero. Shielding experiments were carried out by cooling the superconductor in the absence of a magnetic field and then gradually applying an external field. This paper describes the results of these experiments.

KEKB has been operated since December, 1998, to deliver BB pairs for a physics detector. In this paper,we describe the commissioning procedure of KEKB.

KEKB is a two-ring electron–positron collider with asymmetric energies of 8 and 3.5 GeV to study CP violation in B meson decay. In KEKB, there are 21 types of magnets; about 1600 in total. About 430 dipole and quadrupole magnets were recycled from TRISTAN, the preceding program. All quadrupole magnets are equipped with vertical and horizontal steering dipole magnets. The number of steering magnets is about 1700. There are 212 sextupole magnets, and all of them are fixed on remotely controlled movers to adjust their positions to the beam passage. All main dipole magnets have back-leg coils to steer beams precisely. All quadrupole and sextupole magnets are equipped with correction coils to have a capability for beam-based alignment. Also one-turn coils are installed as well to each magnetic pole of the main magnets to monitor the magnetic flux in the case of trouble. The magnetic field in all magnets was measured and its quality strictly checked. After field measurement, the magnets were installed and precisely aligned. A cooling water system and a power supply system for these magnets were constructed. Magnet design was started in 1994, and construction of the two rings was completed in November 1998. The parameters of the magnets and the construction of the KEKB magnet system are described. Some of the problems experienced during this construction work are also presented.

Magnetic shielding is a key technology for superconducting radio frequency (RF) cavities. There are basically two approaches for shielding: (1) surround the cavity of interest with high permeability material and divert magnetic flux around it (passive shielding); and (2) create a magnetic field using coils that cancels the ambient magnetic field in the area of interest (active shielding). The choice of approach depends on the magnitude of the ambient magnetic field, residual magnetic field tolerance, shape of the magnetic shield, usage, cost, etc. However, passive shielding is more commonly used for superconducting RF cavities. The issue with passive shielding is that as the volume to be shielded increases, the size of the shielding material increases, thereby leading to cost increase. A recent trend is to place a magnetic shield in a cryogenic environment inside a cryostat, very close to the cavities, reducing the size and volume of the magnetic shield. In this case, the shielding effectiveness at cryogenic temperatures becomes important. We measured the permeabilities of various shielding materials at both room temperature and cryogenic temperature (4 K) and studied shielding degradation at that cryogenic temperature.

A novel scheme for the focusing of high-energy leptons in future linear colliders was proposed in 2001 [ P. Raimondi and A. Seryi , Phys. Rev. Lett. 86 , 3779 ( 2001 ) ]. This scheme has many advantageous properties over previously studied focusing schemes, including being significantly shorter for a given energy and having a significantly better energy bandwidth. Experimental results from the ATF2 accelerator at KEK are presented that validate the operating principle of such a scheme by demonstrating the demagnification of a 1.3 GeV electron beam down to below 65 nm in height using an energy-scaled version of the compact focusing optics designed for the ILC collider.

In this paper, the linear and second order optics corrections for the KEK Accelerator Test Facility (ATF2) final focus beam line are described. The beam optics of the ATF2 beam line is designed based on a local chromaticity correction scheme similar to the ILC final focus system. Beam measurements in 2012 revealed skew sextupole field errors that were much larger than expected from magnetic field measurements. The skew sextupole field error was a critical limitation of the beam size at the ATF2 virtual interaction point (IP). Therefore, four skew sextupole magnets were installed to correct the field error in August 2012. By using the four skew sextupole magnets, the predicted tolerances of the skew sextupole field errors of the ATF2 magnets were increased. Furthermore, analyzing field maps of the sextupole magnets identified the source of the skew sextupole field error. After the field error source was removed, the IP vertical beam size could more easily be focused to less than 65 nm.

The ATF2 project is the final focus system prototype for ILC and CLIC linear collider projects, with the purpose to reach a 37nm vertical beam size at the interaction point. In the nanometer beam size regime, higher order multipoles in magnets become a crucial point for consideration. The strength and rotation angle of the ATF2 QEA magnets were reconstructed from measurements done in IHEP in the past and compared with more recent ones from KEK. Based on a sensitivity study, we report on the analysis of possible strategies to mitigate the effects of the measured multipoles. A suggestion is given which will benefit the ATF2 present commissioning to reach the nominal beam size, and also to facilitate the implementation of the reduced β optics in the future.

In order to achieve the high luminosity required at the proposed International Linear Collider (ILC), it is critical to focus the beams to nanometer size with the ILC Beam Delivery System, and to maintain the beams' collisions with a nanometer-scale stability. To establish the technologies associated with this ultra-high precision beam handling, a special beamline has been designed and built as an extension of the existing extraction beamline of the Accelerator Test Facility at KEK, Japan. The ATF provides an adequate ultra-low emittance electron beam that is comparable to the ILC requirements; the ATF2 mimics the ILC final focus system to create a tightly focused, stable beam. There are 37 magnets in the ATF2, 29 quadrupoles, 5 sextupoles and 3 bends. These magnets had to be acquired in a short time and at minimum cost, which led to various acquisition strategies; but nevertheless they had to meet strict requirements on integrated strength, physical dimensions, compatibility with existing magnet movers and beam position monitors, mechanical stability and field stability and quality. This paper will describe how 2 styles of quadrupoles, 2 styles of sextupoles, one dipole style and their supports were designed, fabricated, refurbished or modified, measured and aligned by a small team of engineers from 3 continents.

We have measured and corrected chromatic $X\mathrm{\text{\ensuremath{-}}}Y$ coupling at an interaction point to improve the luminosity of KEKB. We have measured the beam position of betatron oscillations induced by the kicker using turn-by-turn beam position monitors. A phase space structure reconstructed by the beam position provides us not only the Twiss parameters but also information regarding $X\mathrm{\text{\ensuremath{-}}}Y$ coupling. We have also determined chromatic $X\mathrm{\text{\ensuremath{-}}}Y$ coupling using the measured $X\mathrm{\text{\ensuremath{-}}}Y$ coupling at each momentum deviation from the designed beam energy. Skew sextupole magnets are used to correct the chromatic $X\mathrm{\text{\ensuremath{-}}}Y$ coupling.

An orbit feedback system around the interaction point (IP) has been developed and successfully employed at KEKB for more than 6 years. The purpose of the system is to maintain an optimum geometrical relationship of orbits of two beams at the IP and to prevent a luminosity degradation due to orbit drifts. The feedback system is based on orbit measurements around the IP rather than a direct measurement of the luminosity. Owing to the system, the luminosity degradation due to the orbit drifts is suppressed to around or less than 1%.

A superconducting RF test facility (STF) is being constructed at KEK for R&D oriented toward the International Linear Collider (ILC). Under KEK's guidance, Hitachi, Ltd. has manufactured cryostat components at its factory and completed final assembly in the STF Linac Building at KEK. The STF cryomodule consists of two cryostats, each about six meters long based on the TESLA-TTF-type III design. Each cryostat can house four 9-cell cavities, each one meter in length. This report outlines the flow of manufacturing from the factory to final assembly, and describes the manufacturing of equipment, assembly precision, and other matters.

We summarize the machine operation of KEKB during past one year focusing on progress for this period.

We plan to install two superconducting crab cavities into the rings at January, 2006. In our plan, we will install one crab cavity per one ring into the NIKKO straight section where the cryogenic infrastructure is already operated for the superconducting accelerating cavities. For crab crossing, we have to enlarge the horizontal beta function(200m for HER) and have to adjust the horizontal phase advance between the interaction point and the crab cavity. In this paper, we will report the lattice modified for the crab crossing and the study results about the single beam dynamics.

A series of measurements were carried out to investigate the dynamic beam-beam effects at KEKB. The beam-beam kick and the coherent beam-beam tune shift were measured by comparing the beam parameters between a colliding bunch and a noncolliding one. We found that collisions occur with a horizontal orbit offset to obtain the maximum luminosity, and that this reproducible offset appears to be attributed to an electron cloud effect, which might play an important role in the beam-beam interaction. The horizontal beam size estimated from the beam-beam kick slightly decreased as the beam-beam parameter increased. The horizontal emittance estimated from the beam-beam tune shift was higher than an unperturbed emittance, and agreed with the calculated emittance using a higher-mode tune. The ratio of the square-root betatron function between two locations in the ring was measured as a function of the beam-beam parameter. These experimental results would provide useful information on dynamic beam-beam effects.

A series of measurements were carried out to investigate the dynamic beam-beam effects. The beam beam kick and the beam-beam tune-shift were obtained by comparing the beam parameters between a colliding bunch and a non-colliding one. The horizontal beam size at the interaction point estimated from a beam-beam kick curve slightly decreased as the beam-beam parameter increased. The horizontal emittance estimated from the beam-beam tune shift was larger than an unperturbed emittance and agreed with calculated emittance using a higher-mode tune. A ratio of the beta function between two locations in the ring, however, showed a big disparity between measured values and a calculation.

We report measurements of the properties of the $D_{sJ}^+(2317)$ and $D_{sJ}^+(2457)$ resonances produced in continuum $e^+ e^-$ annihilation near $\sqrt{s}=10.6 \mathrm{GeV}$. The analysis is based on an $86.9 \mathrm{fb^{-1}}$ data sample collected %at and 60 MeV below the $\Upsilon(4S)$ resonance with with the Belle detector at KEKB. We determine the masses to be $M(D_{sJ}^+(2317)) = 2317.2 \pm 0.5(\mathrm{stat}) \pm 0.9(\mathrm{syst}) \mathrm{MeV}/c^2$ and $M(D_{sJ}^+(2457))=2456.5 \pm 1.3(\mathrm{stat}) \pm 1.3(\mathrm{syst}) \mathrm{MeV}/c^2$. We observe the radiative decay mode $D_{sJ}^+(2457) \to D_s^+ \gamma$ and the dipion decay mode $D_{sJ}^+(2457) \to D_s^+ \pi^+ \pi^-$, and determine their branching fractions. No corresponding decays are observed for the $D_{sJ}(2317)$ state. These results are consistent with the spin-parity assignments of $0^+$ for the $D_{sJ}(2317)$ and $1^+$ for the $D_{sJ}(2457)$., Comment: 5 pages, 6 figures; Added content, updated and paper submitted to PRL

We have measured beta functions using single kick closed orbit at KEKB. The measured beta functions were compared with the model lattice and gradient errors were extracted from the result in a beta beat. The gradient errors were also obtained by changing strength of steering magnets and measuring beam positions as the second method. The methods of the error estimation and preliminary results are reported.

The first beam collision study was done in the KEKB double ring collider. Beam collision conditions were found by searching beam-beam deflection curves in a trial-and-error method. From these curves, beam sizes at IP and a luminosity was estimated.

The commissioning of the KEKB B-Factory storage rings started on Dec. 1, 1998. The two rings both achieved a stored current of over 0.5 A after operating for four months. The two beams were successfully collided several times. The commissioning stopped on Apr. 19, taking a 5-week break to install the Belle detector.

A model quadrupole magnet for a future linear accelerator has been designed, fabricated, and tested at KEK. The model quadrupole is a superferric superconducting magnet with a design field gradient of 57 T/m. The design field gradient is obtained by saturating the iron yoke. Superferric magnets are considered to have the advantage of simplicity, less sensitivity to the position of the superconductor coils, and are therefore thought to be easier to construct and less expensive than usual superconducting magnets. However, if the iron is heavily saturated, the magnetic field becomes more sensitive to the coil positions. The magnetic properties of the magnet were measured with a harmonic coil, and its field gradient, effective length, and multipole components were compared with a 3D simulation. The results are presented and discussed in this paper.

Accelerator design at SuperKEKB Yukiyoshi Ohnishi∗, Tetsuo Abe, Toshikazu Adachi, Kazunori Akai, Yasushi Arimoto, Kiyokazu Ebihara, Kazumi Egawa, John Flanagan, Hitoshi Fukuma, Yoshihiro Funakoshi, Kazuro Furukawa, Takaaki Furuya, Naoko Iida, Hiromi Iinuma, Hoitomi Ikeda, Takuya Ishibashi, Masako Iwasaki, Tatsuya Kageyama, Susumu Kamada, Takuya Kamitani, Ken-ichi Kanazawa, Mitsuo Kikuchi, Haruyo Koiso, Mika Masuzawa, Toshihiro Mimashi, Takako Miura, Takashi Mori, Akio Morita, Tatsuro Nakamura, Kota Nakanishi, Hiroyuki Nakayama, Michiru Nishiwaki, Yujiro Ogawa, Kazuhito Ohmi, Norihito Ohuchi, Katsunobu Oide, Toshiyuki Oki, Masaaki Ono, Masanori Satoh, Kyo Shibata, Masaaki Suetake, Yusuke Suetsugu, Ryuhei Sugahara, Hiroshi Sugimoto, Tsuyoshi Suwada, Masafumi Tawada, Makoto Tobiyama, Noboru Tokuda, Kiyosumi Tsuchiya, Hiroshi Yamaoka, Yoshiharu Yano, Mitsuhiro Yoshida, Shin-ichi Yoshimoto, Demin Zhou, and Zhanguo Zong

KEKB has been operated since December, 1998, to deliver B ¯ B pairs for a physics detector. In this paper,we describe the commissioning procedure of KEKB.

The machine commissioning of KEKB started in December 1998 and its operation was terminated at the end of June 2010 to upgrade KEKB to SuperKEKB. In this paper, we summarize the history of KEKB and show the achievements made there.

Magnetic shielding is a key technology for superconducting RF cavities. The tolerance of the ambient magnetic field depends on factors such as the operating RF frequency and acceleration gradient, but can be lower than 1 μT. Conventional permalloy is often used for shielding low magnetic fields such as the earth's magnetic field. However, it is not well known how much the shielding effects degrade at cryogenic temperatures where superconducting cavities are operated. Some high Ni-content alloys, which are reported to maintain high permeability at cryogenic temperatures, are commercially available at present and are used for magnetic shielding of superconducting cavities at many laboratories. In this study, the magnetic properties of these materials are measured at room and cryogenic temperatures. The permeability dependence on the heat treatment conditions is studied by using one of the high Ni-content alloys. The effects of mechanical strain are also examined at room temperature. Evaluation of the magnetic shielding for the superconducting cavities of cERL is also discussed in this paper.

The next-generation B-factory “SuperKEKB” is now under construction. To achieve a 40-times higher luminosity than KEKB, which held the world's record luminosity of 2.1 × 1034 cm-2s-1, many items are being upgrade, and new challenges for the “nano-beam” scheme are being undertaken. In this paper, we introduce the major efforts of the magnet system in the main rings.

Permanent magnet quadrupoles (PMQ) and sextupoles (PMSx) have been fabricated and tested. Their effective strengths are variable. The PMQ has been developed as a final focus magnet for a linear collider. There is enough space to put Gluckstern's 5-ring singlet structures in each incoming beam line. The magnetic center offsets and higher harmonic components can be adjusted by adjusting the positions of the magnet pieces with screws on holders. The measured integrated gradient can be reduced down to less than 1% of the maximum strength. The PMSx has been developed to focus pulsed cold neutrons for a small angle neutron scattering (SANS) experiment. The strength is modulated by a so called double ring structure, where the inner ring is fixed and the outer ring is rotated. A torque canceller can reduce the torque to rotate the outer ring by one third of its original magnitude. The focal length of the fabricated triplet lens is less than 50 cm for Very Cold Neutrons (VCN). It is demonstrated that VCN-SANS can shorten a SANS beamline, which usually has a few tens of meters, down to 2~3 meters.

KEKB has achieved the peak luminosity of 4.1 x 10{sup 33} cm{sup -2} sec{sup -1} and the present capability for delivering integrated luminosity is 224 pb{sup -1}/day. This paper describes (1) the recent performance and the problems of KEKB and (2) the comparison of beam-beam simulations with experiments at KEKB.

A low beta lattice with /spl beta/x*//spl beta/y*=100/1 cm has been realized in the KEKB colliding rings. The basic optical parameters have been measured and compared with the model.