In GyM, a linear magnetized plasma device, low-frequency electrostatic fluctuations are measured by means of Langmuir probes. To complement electrostatic probe measurements of plasma turbulence and the study of plasma structures, a nonperturbative direct imaging system has been used on GyM, including a fast-framing Photron APX-RS camera and an image intensifier unit. From the line-integrated camera images, we compute time-resolved emissivity profiles of the plasma by applying a tomographic reconstruction technique using a pixel method and solving an overdetermined set of equations by singular value decomposition. The validity and robustness of the tomographic reconstruction technique are examined with respect to noise and wall reflection effects.

The tomographic reconstruction is applied to fast camera movies acquired with a frame rate of 75 kHz and 4 s of exposure time to obtain the temporal evolution of the emissivity fluctuations. Plasma structures can be detected and tracked in the reconstructed emissivity movies with a finest spatial resolution of 2 cm. Conditional average sampling is used to determine the size and speed of turbulent structures.