Impedance reduction by a SiC-loaded flange and its application to the J-PARC main ring septum magnet

Abstract

The beam power of the main ring of the Japan Proton Accelerator Research Complex (J-PARC) is currently being increased. For high-power beam realization, it is essential to suppress the beam instability that limits the beam power and to estimate and enact countermeasures against the beam coupling impedance of individual devices, which can affect the collective motion of the beam. Therefore, we promote the identification and the reduction of impedance. The fast-extraction (FX) septum magnet will be replaced with another magnet that copes with higher numbers of repetition cycles. Despite their different structures, both septa demonstrated a large impedance in estimates performed by the CST studio suite wake-field solver. As the new septum magnet has been completely manufactured, we considered reducing its impedance by installing additional components. These components must meet strict installation requirements: they must be easy to install on the septum magnet, must not protrude into the beam pipe connected to the septum magnet (to avoid interfering with maintenance), must not disturb the wide horizontal aperture of the septum magnet, and must not affect the magnetic field. The widely used taper impedance-reduction method would be effective but violates the second requirement above (i.e., protrusion into the beam pipe). We found that by attaching a copper plate and SiC to the flange of the septum magnet, we could effectively reduce the impedance while satisfying all installation requirements. The copper plate on the flange reduces the impedance below the cut-off frequency. Moreover, when SiC was loaded, the remaining impedance was three times lower than when using the copper plate alone. After applying this method to the new septum magnet, the maximum longitudinal impedance was reduced to 1% of the value without countermeasures, largely improving the beam stability condition. We also estimated the required thickness of SiC and the calorific value. This method saves space and is installed by simple attachment to the flange, regardless of the shape of the beam pipe. Therefore, it is also applicable to other devices.