Innovations in RF Cavity Development for High-Gradient Acceleration at J-PARC
This document outlines the advanced requirements for RF cavities in high-energy accelerators, emphasizing the need for broadband capabilities, high gradients, and large apertures to accommodate beam excursions. Notable developments include the use of Magnetic Alloy (MA) cavities at KEK for J-PARC, demonstrating significant performance with large permeability and high Curie temperature. The MA cavity achieved noteworthy voltage levels in various modes and was designed with specific dimensions to ensure optimal shunt impedance and cooling efficiency. Additionally, the design considerations for gradient magnets and coil arrangements are discussed to enhance magnetic field distribution.
Innovations in RF Cavity Development for High-Gradient Acceleration at J-PARC
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Presentation Transcript
Part II Hardware R&D
Requirements of RF cavity • RF must be • Broad band • Frequency sweep of a factor. • High gradient • Make it fast acceleration possible. • Large aperture • Especially in horizontal to accommodate orbit excursion. • A few MHz to have large longitudinal acceptance • RF cavity with Magnetic Alloy has been developed at KEK for JPARC cavity.
Characteristics of Magnetic Alloy (MA) • Large permeability ~2000 at 5 MHz • High curie temperature ~570 deg. • Thin tape ~18 mm • Q is small ~0.6 Q can be increased with cutting core if necessary.
mQf (shunt impedance) • A mQF remains constant at high RF magnetic RF (Brf) more than 2 kG • Ferrite has larger value at low field, but drops rapidly. • RF field gradient is saturated.
Development of MA cavity for JPARC • Direct water cooled test cavity. • Achieved • 100 kV/m for CW mode • 220 kV/m for burst mode
MA core for 150 MeV FFAG • Wide aperture in horizontal, ~1m. • Outer dimension is 1.7m x 0.985 m x 30 mm
Cavity assembly Number of cores 2~4 Outer size 1.7m x 1m Inner size 1m x 0.23m RF frequency 1.5 - 4.6 MHz RF voltage 9 kV RF output 55 kW Power density 1 W/cm^3 Cooling water 70 L/min
Measured cavity impedance Sufficient shunt impedance in the frequency range of operation. Frequency (MHz)
Gradient magnets • Three ways to realize a gradient magnet. • Large gap inside, small gap outside • Main coil plus trim coil on flat gap • Cos[nq] like magnet
Tapered gap • Gap(r) is proportional to 1/B(r) • Easiest • Fringe field has wrong sign. • g/r should be constant to have similar fringe field effects
Fringe field of tapered gap • Inner radius has longer fringe field. • Gap is longer • Coil width is constant • Focusing action at the edge is not constant.
Return Yoke Free Magnet • Magnetic flux of triplet magnet
Pole face winding • Gap height is constant. Field strength is varied with coil arrangement.
Variety of coil winding Three ways to put trim coils. (Top: shape of winding, Center: global and local k, Bottom: fringe field)
Coil winding for spiral magnet • The same idea for spiral sector magnet.
Cos[nq] like (conceptual) • Similar to a superconducting magnet • Schematic diagram
Superconducting magnet with multipole combination(design example) Field as a result of multipole combination.
Current distribution Instead of many multipoles, a couple of well Shaped current distribution. Design example Field shape
Flux distortion due to neighboring material • Inevitable in a compact machine
Injection • Electric deflector and kicker in POP • Magnetic deflector and electric septum for multi-turn injection in 150 MeV • Fast kicker for one turn injection in PRISM • Continuous orbit shift with induction acceleration in KART
