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内容記述 |
LIPAc (Linear IFMIF Prototype Accelerator), an accelerator designed to provide a 125 mA proton and deuteron beam up to 9 MeV in CW (Continuous Wave), is under construction and commissioning in Rokkasho, Japan. One of the main part of the RF (Radio Frequency) system of LIPAc is the RFQ (see [1]). This cavity is fed by 8 couplers that must handle up to 200 kW of RF power. During commissioning, this power handling capability has proven to be challenging, and studies to understand and push the limits are on-going. One hypothesis is locally excessive heating due to RF losses. A numerical model was built to investigate this possibility. The model is based on a multiphysics simulation done with version 6.2 of COMSOL ([2]). A two steps simulation was performed: first a RF simulation produces the surface currents that are then used as heat sources in a thermal simulation, through ohmic losses. The roughness of the surface needs to be considered to correctly estimate these losses. Moreover, in the thermal simulation, it was found that the natural convection has a significant role in the heat transfer from the inner to the outer conductor of a coaxial structure. This was checked with a third step simulation using a laminar flow solver. However, this step required too much computation power, so it is approximated with an effective thermal conductivity for the inner air, 4 times higher than its natural value. Once the model was built and validated, it is applied to the couplers of the RFQ. The results shown that thermal heating due to RF losses cannot explain alone the observed degradation, even where the inner conductor is narrowest, meaning that multipacting effects need to be considered. |
