Hirotaka Nakai

RF systems for the KEK B-Factory

This paper describes the design features and operational status of the RF systems for the KEK B-Factory (KEKB). Two types of new RF cavities have been developed to store very high-intensity beams with many short bunches. The design and performance of the cavities and other critical components, such as the input couplers and HOM dampers, are reported. The configuration of the RF systems is given and descriptions of various control loops are made, including a direct RF feedback loop and a 0-mode damping loop. The effects of transient beam loading due to a bunch gap on bunch phase modulations were simulated and measured. The development of a superconducting crab cavity, which is a component of luminosity upgrade strategy, is also presented.

Long term performance of the TRISTAN superconducting RF cavities

TRISTAN was operated with an average beam current of 12.0 mA at a beam energy of 29.0 GeV in 1990. The 32 5-cell superconducting RF cavities had been kept cold for about 6 months. No irrecoverable degradation of the cavity performance had been observed in the long-term operation. Changes of the cavity performance, problems, and improvements of the superconducting RF system are described.<<ETX>>

Upgrading of TRISTAN by superconducting RF system

The energy upgrade of TRISTAN by a superconducting RF system is progressing. Sixteen five-cell 508 MHz cavities made of niobium have been installed and started to operate in November 1988. Superconducting cavities have raised the energy of e/sup -/e/sup +/ to 30.4 GeV from 28.5 GeV with a normal conducting RF system. Construction, vertical tests, horizontal tests and three months of operational experience are described.<<ETX>>

Effect of macroscopic parameters of porous material on flow characteristics for superfluid helium thermomechanical pump

Flow characteristics of superfluid helium (He II) through a porous material for a thennomechanical pump (TMP) are not ideal in effect at a practical working point in employment such as cooling systems of detectors and superconducting magnets in space. If the He II flow through a porous element of a TMP were in the ideal state, there would be no dependence on geometry of the element. But the actual situation is not. In such non-ideal situations, some macroscopic parameters of the material as well as the geometry of a TMP element play important roles in a limitation of superfluid helium flow through a TMP element. Penneability of the porous element of a TMP is considered especially to discuss the flow characteristics of He II through a TMP element.

Cryogenic System for TRISTAN Superconducting RF Cavities: Description and Operating Experience

Publisher Summary A large scale helium refrigeration system was constructed and operated for about 7 years for superconducting RF accelerating cavities (SCC) in TRISTAN electron positron collider at National Laboratory for High Energy Physics (KEK). The 32 x 5 cell 508 MHz SCC in 16 cryostats were installed in TRISTAN electron-positron collider at KEK for further upgrading of the beam energy, from 27 GeV to 32 GeV. This chapter provides a short description of the main components of the cryogenic system together with the operating experience gained during the commissioning and 7 years operation. The total number of failures in the cryogenic system during about 7 years operation was 85. The experience in long operation of the large cryogenic system shows the compressor is the key component and the regular maintenance of the system including utilities is essential to attain the system reliability.

Transition to Turbulent State of Superfluid Helium Flow through Thermomechanical Pump Element

It has been found experimentally that there are two distinct flow regions in the flow characteristics of superfluid helium through the porous element of a thermomechanical pump (TMP). 1 One is the ideal flow region and the other is the turbulent one. However, it is not known yet how the flow turns turbulent. The concept of the critical heat input to TMP is introduced in this study to define the transition from the ideal flow region to the superfluid turbulent one. The temperature dependence and the geometrical dependence of the critical heat input are experimentally investigated and the results are discussed in this paper. This critical heat input gives one of the necessary information to design a TMP properly for the given specifications such as the required mass flow rate and the temperature of superfluid helium.

Magnetic Field Measurement of Superconducting Dipolemagnets with Harmonic Coil and Hall Probe

Magnetic field measurements and field analyses of 1-m long superconducting dipole magnets fabricated at the National Laboratory for High Energy Physics (KEK) have been carried out using a harmonic coil with the bucking scheme.1 Conditions of the data acquisition are optimized to achieve the accurate and efficient measurements. Not only in the steady state of the magnet excitation by constant currents, but also on the way the excite current increases until the magnet quenches the field measurements have been tried, and the results are discussed in this paper on the possibility of the “on-the-fly” measurement using a harmonic coil. Some results on the so-called remnant field of the magnets measured with a Hall probe are also described.

Flow Structure of Thermal Counterflow Jet

The flow structure of a thermal counterflow jet in otherwise quiescent He II is investigated by the application of the method of flow visualization and with a laser Doppler velocimeter. Flow visualization pictures show the evolution of the jet patterns from laminar to turbulent ones and the existence of a large scale entrainment into a jet. The LDV measurement gives the velocity distribution and the velocity fluctuation of the normal component at a point.