Conceptual design of a tomographic system for helicon plasma device

An optical emission tomography system has been developed to obtain the 2D radial emission profile of a permanent magnet based helicon plasma source system. The plasma system is a RF driven cylindrical plasma source operating at 13.56 MHz. The technique is based on measurement of line integrated value of optical emission which is the intensity of line emission from a cross-sectional plane using an optical detection system. Inversion of the line integrated data gives the 2D distribution of emissivity over the plane. From this local emission profile, the distribution of the plasma parameters such as density, temperature can be calculated. The detection system having a fan beam geometry primarily consists of a pinhole, a lens and a CCD based linear optical array detector. The light from the plasma after passing through the pinhole is collimated using a precision a spheric lens and allowed to fall on the CCD based sensor. Optical band pass filters for some specific wavelengths are also used to reduce optical noise. The filter module is designed to quickly exchange filters for a particular wavelength within the experimental setup. The whole assembly of these components constitutes a camera. 6 such cameras are proposed to scan the entire plasma system and the cameras are placed symmetrically around the periphery of the cylindrical plasma system. The optical signals are acquired from the camera using software application built in LabVIEW through USB interface with configurable scan and exposure time settings. A tomography algorithm based on maximum entropy concept has been developed to get the local emission profile. (author)