Design and commissioning of a new diagnostic for routine determination of Zeff from visible bremsstrahlung measurements on TEXTOR

On TEXTOR a new diagnostic was commissioned for the determination of the profile of the ion effective charge Zeff from bremsstrahlung emissivity measurements in the visible. The line-integrated bremsstrahlung emissivity is measured with high spatial resolution along 28 fiber optic channels, in one poloidal cross section of the tokamak. Twenty-four channels are coupled to a cooled fast charge-coupled device (CCD) camera and four to photomultiplier tubes for higher sampling rate. The design of the diagnostic is described, including a technique for coping with CCD image smearing, without the need for the calculation of the point spread function of the system. An in-vessel calibration was performed by means of an integrating sphere. A convenient alternative way is outlined for determining the relative calibration from plasma emissivity measurements only. Bremsstrahlung emissivity profiles can be reconstructed by Abel inversion of the line integrals measured with the camera. Overestimation of edge Zeff values due to nonbremsstrahlung contributions to the continuum radiation can be minimized via Tikhonov or maximum entropy regularization. The propagation of such edge continuum contributions towards the center of the reconstructed profile is illustrated in the case of ergodization of the edge plasma.