Thomson scattering with laser intra-cavity multi-pass system to study fast changing structures in fusion plasma

Advanced Thomson Scattering (TS) diagnostics with a Laser Intra-cavity Multi-pass Probing (LIMP) system were developed and implemented in several tokamaks in recent decades. The LIMP system provides a significant gain of the laser probing energy along with generation of many pulses at a high repetition rate within a single pumping pulse. Probing lasers with a LIMP system have considerably extended the capability of the TS diagnostic and have opened new application fields in plasma physics.The paper presents the principles of the LIMP system which underlie its efficiency as well as some of its applications in physical studies performed in the TEXTOR tokamak. The TS diagnostic with LIMP has enabled detailed measurements of fluctuations of electron temperature and density along the whole plasma diameter. The structure of thesefluctuations, which are typically1-2% in amplitude, has been resolved. The diagnostic made it possible to measure the fine structure of rotating magnetic islands in the TEXTOR plasma. The observed structuressignificantly differ from the classical shape of helical magnetic perturbations. LIMP isapplied for measurementsofnot only electrons butalso of other plasma particles. Laser-heated dust micro-particles were detected in plasma via their thermal radiation. The diagnostic provides the location of particlesalong with its velocity, temperature and dimension.