Implementation of low temperature spectrometers for the JET high resolution Thomson scattering diagnostic for disruption plasma measurements.

This work presents a system upgrade of the High Resolution Thomson Scattering (HRTS) diagnostic [Pasqualotto et al., Rev. Sci. Instrum. 75, 3891-3893 (2004)] on JET that allows it to measure low temperature (1-500 eV) plasma pre- and post-Thermal quench (TQ), which would help us further understand the Shattered Pellet Injection (SPI) physics. The upgrade was done by connecting optic fibers from the original HRTS system to four spectrometers specialized in measuring low temperature plasmas. The upgraded system allows for the measurement of low temperature plasma at up to 12 spatial points, which can be flexibly distributed throughout the JET outer midplane profile during the dedicated SPI experiments. In other JET experiments, four previously unused groups of fibers were used to measure the central plasma to provide disruption data without changing the standard HRTS system. The low temperature Thomson scattering system was installed, commissioned, and cross-calibrated against the standard HRTS diagnostic in a calibration pulse. The system worked reliably during the JET experiments and provided electron density and temperature profiles for pre- and post-TQ low temperature plasmas in the SPI campaign.

Radiometry for the vertical electron cyclotron emission from the runaway electrons at the COMPASS tokamak.

Due to an increased interest in runaway electron (RE) phenomena in tokamak research, the need for diagnostics of runaway electron population in plasma has emerged. A novel diagnostic of the nonthermal electron cyclotron emission from runaway electrons can be utilized for this purpose. It was designed and installed at the COMPASS tokamak based on the available heterodyne radiometer. The vertical ECE (V-ECE) system uses a 16-channel heterodyne radiometer with a vertically placed E2-band horn antenna with a 76.5-90 GHz frequency range front-end. Simulations with the ray-tracing SPECE code have shown a measurement feasibility of the runaway electrons with energies up to 1 MeV. Due to a low optical depth of the plasma in COMPASS during RE discharges, reflected waves from the tokamak wall can be detected as well. First results show strong connection with other RE diagnostics at COMPASS. The V-ECE can obtain important information about RE population evolution and primary generation mechanism.

Use of soft x-ray diagnostic on the COMPASS tokamak for investigations of sawteeth crash neighborhood and of plasma position using fast inversion methods.

The soft x-ray diagnostic is suitable for monitoring plasma activity in the tokamak core, e.g., sawtooth instability. Moreover, spatially resolved measurements can provide information about plasma position and shape, which can supplement magnetic measurements. In this contribution, fast algorithms with the potential for a real-time use are tested on the data from the COMPASS tokamak. In addition, the soft x-ray data are compared with data from other diagnostics in order to discuss possible connection between sawtooth instability on one side and the transition to higher confinement mode, edge localized modes and productions of runaway electrons on the other side.