Two-dimensional density profile measurement with a sheet thermal Li beam on CPD

A sheet thermal Li beam to study the density contour near X-point and divertor strike points has been developed for the spherical tokamak CPD. The CCD camera with a time resolution of 1 ms and spatial resolution of ∼1 mm can record two-dimensional images of LiI resonance line (300 mm × 500 mm). The performance of the sheet beam is absolutely calibrated by a quartz crystal monitor. The Li flux of 10¹⁸-10¹⁹ m^-2 s^-1 is achieved near the X-point. The sheet characteristics of beam width and uniformity are also confirmed. This technique is applied during plasma initiation and electron cyclotron heating phase of plasma in a simple torus with a small vertical field. The formation of vertically stretched plasma at the position of the cyclotron resonance is clearly found. The expansion process towards the low field side is observed and an over dense plasma (>1 × 10¹⁸ m^-3) is obtained. A model calculation of LiI emission with density range of 1-5 × 10¹⁸ m^-3 agrees fairly well with the observations. Over dense plasma formation is also discussed.