Power transmission of the neutral beam heating beams at TJ-II

A carbon fiber composite (CFC) target calorimeter has been installed at TJ-II to study in situ the power density distribution of the neutral beams. The thermographic print of the beam can be recorded and analysed in a reliable way due to the highly anisotropic thermal conductivity of the target material. Direct conversion from temperature to power density is possible due to the independent thermocouple measurements on two adiabatically mounted copper buttons. With the combined thermographic and calorimetric measurements it has been possible to determine the power density distribution of the beams. It has been found that a large beam halo is present, which can be explained by the extreme misalignment of the grids. This kind of halo has a deleterious effect on beam transport and must be minimized in order to improve the plasma heating capability of the beams.