Y. Morita

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.

Development of a High Performance Transfer Line System

High performance transfer lines are key components for design and construction of cryogenic systems for superconducting magnets and cavities. We have developed two types of high performance transfer lines; main transfer lines with liquid helium flow and cold gas return dual path and sub-transfer line with single path, all helium lines in these are guarded by 80K liquid nitrogen cooled thermal shields. We have adopted the aluminum molding made by extrusion for the 80K thermal shields. The structure of the transfer lines was designed to be easy to assemble. The performance of these transfer lines were tested. The heat loss of the main transfer lines at straight section are 0.04W/m and 0.06W/m for supply and return line, respectively.

A Status Report on the Development of 5-cm Aperture, 1-m Long SSC Dipole Magnet at KEK

The design and construction of a series of eleven 5-cm aperture, 1-m long SSC model dipole magnets KEK#1-#11 was started in May 1990. We have paid much attention to the design of magnet end part and developed new-type end spacers1 which have constant perimeter profiles to minimize internal stress of the cables at the coil end. Except for length, these short magnets have the same baseline design and features as the long magnet. We have performed the quench test, ramp rate quench dependence, and AC loss measurement in 1.8K superfluid helium cryostat.3

Development of J-PARC MR Main Magnets Power Supplies for High Repetition Rate Operation

J-PARC aims at achieving a MW-class proton accelerator facility. One of the promising solutions for increasing the beam power is to fasten the repetition rate of MR from current rating of 2.5 sec to 1 sec. However, in this scheme, the increase of output voltage and the power variation on the electric system are serious concerns for main magnets. At the same time, current ripple reduction is required in order to increase the beam quality for the hadron experiments. We have been developing power supplies which have potentials to solve these problems and plan to replace the current power supplies with them. The new power supply system has following features. The number of power supply is twice so that one power supply drives half number of loads compared with the current system. The power supply is consists of some choppers which are wired in series. As is shown in Figure 1, the energy recovery scheme based on the capacitive energy storage is introduced [1]. Parallel connected choppers increase the equivalent switching frequency so that the switching ripple is possible to be reduced by the filter. This paper introduces the power supply system of J-PARC MR main magnets for high repetition rate operation, and also reports design and test results of prototype power supply we developed.

Cryogenic System for KEKB Superconducting RF Cavities

KEKB (KEK B-Factory) is a double-ring electron-positron collider used for studies of CP-violation and other topics on decays of B mesons. Because of the high beam current of KEKB, installation of superconducting cavities has been proposed for the KEKB, and 4 niobium single-cell superconducting cavities were installed in the KEKB ring in the summer of 1998. For cooling of these cavities, an existing cryogenic system with a capacity of 8 kW at 4.4 K and helium transfer lines, which were constructed and used for TRISTAN superconducting cavities, were reused. In this system newly developed, high performance, sub-transfer lines cooled with 80 K liquid nitrogen thermal shields were chosen and constructed to connect the cavity cryostats with the main transfer line. The operation of the complete system for the commissioning of KEKB was initiated at the end of 1998. This paper describes the cryogenic system and the operating experience gained during the commissioning.

Quench Characteristics of 1-M-Long SSC Model Dipole Magnets

A series of fifteen 5-cm-aperture, 1-m-long SSC model dipole magnets with various types of end design and cable have been designed and fabricated at National Laboratory for High Energy Physics (KEK).1,2,3 The ramp-rate-dependent quench tests of the magnets KEK#3 to #15 were performed in a 4.2-K vertical cryostat. A ramp-rate-dependent test of the magnet KEK#6′ was also performed in 1.7-K pressurized superfluid helium. Special ramp tests so called “heating” and “cooling” experiments4 were also performed on the magnet KEK#10, as well as heat induced quench tests using the spot heaters installed in midplane of inner coils of the magnets KEK#7′ and #15 and in the splice part of the magnet KEK#13Y.

Test Demonstration of Magnet Power Supply with Floating Capacitor Method

The Japan Proton Accelerator Research Complex (J-PARC) aims at achieving a MW-class proton accelerator facility. We plan to increase the beam power by shortening the repetition period of the Main Ring (MR) from the present period of 2.5 sec to 1 sec in the future. In this scheme, there are serious concerns regarding the main magnets. One involves the increasing output voltage, and the other is related to the power variation of the electric system. We propose an innovative floating capacitor method to produce a high output voltage and suppress the power variation with capacitor energy storage for addressing these concerns. Nevertheless, the driving power supply used with this method needs to establish control of the floating capacitor voltage. We developed and introduced recovery control of the floating capacitor voltage for each accelerator cycle. We also confirmed that the tracking error can be corrected by iterative learning control with the floating capacitor method. In this article, the magnet power supply with the floating capacitor method is described, and test results achieved with the mini model power supply are presented.

A High Power Test Method for Pattern Magnet Power Supplies with Capacitor Banks

In J-PARC Main Ring, upgrade towards the beam intensity of 750 kW is planed. To achieve this, synchrotron repetition period must be shortened from the period of 2.48 s to about 1s with new power supply for the main magnets. We are considering and developing a new power supply with large capacitor banks. This capacitor banks are needed to reduce the power variation at the main grid for the future operation with shorter repetition period. However, it is very difficult to perform the test of the new power supply at its rated power before its installation. This is because the power supplies for the J-PARC MR main magnets handle too much power to be tested in factories or test benches. We suggest a test method using two capacitor banks for the power supply test. In this method, two choppers and small inductive load are connected between two capacitor banks. By controlling the energy flow to go and return between the two capacitor banks in this setup, the received power and inductive load can be very small. In this article, the details of the control method and the results of the test experiment using mini-model power supply are described. 1 . J-PARC MAIN RING アップグレードと 新主電磁石電源 茨城県那珂郡東海村にある J-PARC Main Ring (以下 J-PARC MR)は、大強度陽子ビームを 30 GeVまで加速 する陽子シンクロトロンで、これにより加速された大強 度陽子ビームは長基線ニュートリノ実験および原子核 ハドロン実験に利用されている。現状では最大 230 kW (ニュートリノ利用運転時)のビーム強度であるが、こ れを 750 kW にアップグレードする計画が進行中であ る。アップグレード最大の目玉は、ビーム取出しサイク ルの高繰り返し化で、現状 2.48 秒の繰り返し周期を 1 秒程度まで短縮する。これを達成するためには主電磁石 電源の置き換えが必須であり、以下に J-PARC MRの高 繰り返し新主電磁石電源が達成すべき課題を述べる。 高出力電圧 現行の主電磁石電源では、現状の 2.48秒 の繰り返し周期を大幅に短縮する事は不可能であ る。インダクタンス負荷を高速で励磁するために は、高電圧が必要であり(V = LdI/dt)、J-PARC MR 主電磁石ファミリ一つを 1秒繰り返しで励磁 するには、1電源あたり最大約 6 kVが必要である。 一方、現行電源の定格出力電圧は 3 kV程度である。 したがって、現行電源の二倍の定格出力電圧の電 源が必要である。 交流系統における電力変動の抑制 現行電源の方式で は、主電磁石のエネルギーは交流系統へ直接回生 される。このため、現行 2.48秒繰り返し時の電力 変動幅は 60 MVAを超える。これは、1秒繰り返し では 140 MVA相当になることを意味するが、この 値は電力会社による許容電力変動を遥かに超える と考えられる。したがって、出力電圧を上げるた めに現行の電源と同じものを倍の台数投入すると ∗kurimoto@post.j-parc.jp いう単純なやり方は成り立たず、何らかのエネル ギー貯蔵システムを J-PARC敷地内に設け、そこへ エネルギー回生する電源が必要である。 2 . コンデンサバンクを用いたエネルギー回 生 Figure 1: Conceptual schematic of a power supply with a capacitor bank. 以上に述べた要求から、我々はコンデンサバンクをエ ネルギー貯蔵装置とした電源構成を J-PARC新電源とし て検討、開発中である [1][2][3]。回路構成は Fig. 1に示 すように、整流器とチョッパの二段構成で、DCリンク コンデンサを大容量化しコンデンサバンクとする。これ により、磁気エネルギーのやり取りはチョッパを介して コンデンサバンクと電磁石の間で行われ、系統からの受 電は損失分のみとなる。

AC loss measurement of superconducting dipole magnets by the calorimetric method

AC losses of superconducting dipole magnets were measured by the calorimetric method. The magnets were model dipole magnets designed for the SSC. These were fabricated at KEK with 50-mm aperture and 1.3-m overall length. The magnet was set in a helium cryostat and cooled down to 1.8 K with 130 L of pressurized superfluid helium. Heat dissipated by the magnet during ramp cycles was measured by temperature rise of the superfluid helium. Heat leakage into the helium cryostat was 1.6 W and was subtracted from the measured heat to obtain AC loss of the magnet. An electrical measurement was carried out for calibration. Results of the two methods agreed within the experimental accuracy.

Test of copper-braid-stabilized bus lines for superconducting dipole magnets

A high cryogenic stability suprconducting bus-line has been developed to connect a superconducting dipole magnet with a full length of 13 m to a current lead approximately 2 meters from the magnet. The superconducting bus-line is made of NbTi strand cables for magnet use soldered to copper braid. The copper braid has a large surface area to improve cooling efficiency and increase cryogenic stability. Three kinds of bus-line are prepared on experimental basis: a bare superconducting cable, a superconducting cable joined copper braid with a thin layer of solder, and one made by filling the inside of copper braid with solder. Cryogenic stability tests confirmed that a bus-line equipped with a copper braid provides twice the cryogenic stability as a bare superconducting cable.