F. Naito

Beam commissioning of the J-PARC linac medium energy beam transport at KEK

The construction of the initial part of the J-PARC linac has been started at KEK for beam tests before moving to the JAERI Tokai campus, where J-PARC facility is finally to be constructed. The RFQ and MEBT (Medium Energy Beam Transport) have already been installed at KEK, and the beam test has been performed successfully. In this paper, the experimental results of the beam tests are presented together with simulation results with a 3D PIC (Particle-In-Cell) code.

Development of high-power ARES cavities

The ARES (Accelerator Resonantly coupled with Energy Storage) structure is a three-cavity system operated in the /spl pi//2 mode, where a HOM-damped accelerating cavity is coupled with an energy storage cavity via a coupling cavity equipped with a coaxial antenna damper to reduce the impedances of the parasitic 0 and /spl pi/ modes. Two high-power prototypes with different HOM-damping schemes have been developed, where one is with a quadrupole counter-mixing (QCM) choke structure and the other with grooved beam pipe one. Both prototypes have been tested with an electron beam of about 500 mA in the TRISTAN accumulation ring. This paper reports their RF structures together with some results of the high-power and high-current beam tests.

Tuning of the RF field of the DTL for the J-PARC

Tuning of the accelerating field of the DTL first tank for the Japan Proton Accelerator Research Complex (J-PARC) has been done. The first DTL tank consists of 75 full drift tubes, 37 post-couplers and 10 fixed tuners. The resonant frequency of the tank is 324 MHz. An uniform accelerating field has been achieved by the fine adjustment of the post-couplers and the fixed tuners. The field stabilization by the post-couplers against perturbations has been confirmed also. In order to achieve the stabilized-uniform distribution of the average field for each accelerating gap, the following techniques have been applied for the post-coupler tuning: (1) non-uniform insertion length of the post-coupler from the tank wall; (2) increment of the diameter of several post-couplers. The recipe of the fine post-coupler tuning is described.

HOM absorber for the ARES cavity

Two types of sintered SiC (silicon carbide) ceramics have been developed and applied as the HOM absorbers for the second prototype ARES cavity (ARES96). One is bullet-shape SiC ceramics and the other is a tile one. The prototype cavity equipped with these absorbers was successfully tested with an electron beam in the TRISTAN accumulation ring. High power tests of these absorbers were carried out using an L-band RF power source. The HOM absorber designs and the results of the tests are discussed.

Fundamental mode characteristics of ARES cavity under beam environment

Two types of ARES cavities were tested in the TRISTAN accumulation ring in KEK. In this paper, results relating to the fundamental mode in the accelerating cavity are reported. The status of the cavity, including the detuning frequency under beam loading and the output power from the coupling cavity damper, was measured at different beam current and with various bunch patterns. Its behavior agrees well with the calculation based on a coupled resonator model.

Coupling cavity damper for the ARES

The coaxial (WX120D) antenna damper has been developed for damping the parasitic 0 and /spl pi/ modes of the KEKB ARES cavity. It is installed in the coupling cavity of the ARES. The damper consists of a disk-type coaxial ceramic window and a cross stub angle. The damper has a broadband RF property in order to absorb the RF power of the parasitic modes. The cross angle is utilized for supplying the cooling water to the antenna. The damper was designed by using a computer simulation code without a cold model test. Finally the properties of the damper has been confirmed by the high-power and the beam test for the ARES cavity.

HOM characteristics of the ARES cavity

The ARES (Accelerator Resonantly coupled with an Energy Storage) cavity, which consists of three cells, has been developed for the KEK B-Factory, and two types of the ARES cavity have been constructed for the beam test. Both the types were installed in the KEK TRISTAN Accumulation Ring and beam-tested. The accelerating cell of the ARES has a HOM-damped structure to lower the impedance of HOMs in order to store a high current beam. The spectra excited by the beam (single bunch, 100 mA) were measured for both the cavities with scanning of the resonant frequency. Basically these results are consistent with those of calculations. The HOMs appear to be damped as expected. But the actual spectra for three cells are more complicated than only the accelerating cell. During this experiment, however, any beam instabilities due to the impedance of the cavities were not observed. This paper describes only the ARES95-one of the two types of the ARES.

An accelerator resonantly coupled with an energy storage (ARES) for the KEKB

In a large ring with extremely heavy beam loading, such as a B-factory, it is possible that the accelerating mode, itself, gives rise to a longitudinal coupled-bunch instability. In order to solve this problem, T. Shintake (1993) has proposed to attach a TE015-mode storage cavity to an accelerating cavity. It has subsequently been shown that the system can be put into practical use if a coupling cavity is added in between the two cavities. The three-cavity system, which is now referred to as an accelerator resonantly coupled with an energy storage (ARES), is under development for the KEKB.

The DTL/SDTL alignment of the J-PARC LINAC

The required alignment accuracy in the J-PARC linac is 0.1 mm in transverse direction. In the DTL/SDTL section, the fine alignment was carried out by using an optical alignment telescope along with the cavity installation. The displacement of the DTL by the unit tank connection was monitored by a laser tracker to obtain the tolerable displacement between unit tanks. The heights of cavities and magnets were compensated form the local settlement of the linac tunnel.

High power conditioning of the DTL for J-PARC

The high-power conditioning of the three DTL tanks for the J-PARC has been started in October 2006. The design rf peak-power levels for beam acceleration of the tanks are about 1.1 MW (DTL1), 1.2 MW (DTL2) and 1.0 MW (DTL3), respectively. As a result of the conditioning, we have achieved that the rf power levels are about 1.3 MW, 1.45 MW and 1.23 MW of which are 1.2 times the power levels of the desired one (the pulse length is 650 mus and the pulse repetition is 25 Hz). During the linac beam commissioning, the DTLs can keep the required rf power stable now.