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.

Feasibility study for a new high resolution Thomson scattering system for the ASDEX Upgrade pedestal

A new Thomson scattering diagnostic is proposed for the study of fast plasma dynamics in the pedestal of ASDEX Upgrade. The diagnostic will measure electron temperature and density profiles over a similar to 3 cm wide area in the edge transport barrier region, with similar to 1-2 mm spatial resolution and similar to 10 kHz sampling rate. A challenging goal of the project is the study of the bootstrap current in the plasma pedestal by measuring the distortion and shift of the electron distribution along the toroidal direction. Expected spatial and time resolutions of the current density measurements are similar to 3 mm and similar to 1 ms correspondingly. The new diagnostic will be used to study the fast dynamic behaviour of the pedestal bootstrap current, where models indicate that it plays a key role in regulating edge stability, e.g. during ELMs. The diagnostic design is based on the intra-cavity multi-pass system currently in operation in TEXTOR, which uses a probing ruby laser, a grating spectrometer and two fast CMOS cameras for scattered light detection, and has achieved measuring accuracies of the order of similar to 1% for n(e) and similar to 2% for T-e. Parts of that system will be reused in ASDEX Upgrade (some with significant modifications), but the laser multi-pass and light collection systems are entirely redesigned. Restrictions in space and line-of-sight availability have led to the adoption of a design which uses in-vessel multi-pass mirrors and light collection optics, requiring a number of innovative technical solutions to permit remote laser alignment and light collection. We give an overview of the project, discuss the underlying physics basis and present a number of technical solutions employed

Tokamak plasma self-organization-synergetics of magnetic trap plasmas

Analysis of a wide range of experimental results in plasma magnetic confinement investigations shows that in most cases, plasmas are self-organized. In the tokamak case, it is realized in the self-consistent pressure profile, which permits the tokamak plasma to be macroscopically MHD stable. Existing experimental data permit suggesting a hypothesis about the mechanism of pressure profile regulation and to give an explanation of such unusual phenomena as a nonlocal character of transport coefficients, enhanced speed of heat/cold pulse propagation and many modes of tokamak operation

Comparison of measured and simulated fast ion velocity distributions in the TEXTOR tokamak

Here we demonstrate a comprehensive comparison of collective Thomson scattering (CTS) measurements with steady-state Monte Carlo simulations performed with the ASCOT and VENUS codes. The measurements were taken at a location on the magnetic axis as well as at an off-axis location, using two projection directions at each location. The simulations agree with the measurements on-axis, but for the off-axis geometries discrepancies are observed for both projection directions. For the near perpendicular projection direction with respect to the magnetic field, the discrepancies between measurement and simulations can be explained by uncertainty in plasma parameters. However, the discrepancies between measurement and simulations for the more parallel projection direction cannot be explained solely by uncertainties in plasma parameters. Here anomalous fast ion transport is a possible explanation for the discrepancy