Description
STEP is a UK-based project to design, construct and operate a demonstration fusion reactor delivering net electricity to the grid and achieving tritium self-sufficiency. Before reaching the ultimate goal, STEP will undergo commissioning and physics model test phase in a non-fusion hydrogen plasma. For that phase, and possibly for the initial DT phase, a more comprehensive set of plasma diagnostics is being considered.
LIDAR Thomson Scattering (TS) is one of the possible candidates for measuring electron temperature and density profiles. This type of diagnostic was operated at JET for almost 40 years but so far has never been implemented on any other tokamak. The concept combines a Thomson backscatter diagnostic with the time-of-flight or LIDAR principle implemented to resolve the profiles via measuring the time delay between short laser pulse entering the plasma and the scattering signal reaching the detector.
LIDAR TS is an attractive option for reactor-grade fusion machines compared with conventional TS, due to less demanding front-end (near the plasma) design, smaller footprint on the first wall, larger tolerance to the laser beam alignment and the absence of glass optical components, especially optical fibres, in areas with high neutron fluences.
The proposed design is based on the dual colour fundamental + 2nd harmonic Nd:YAG laser (1064nm+532nm) with laser pulse energies of 5J + 3J and pulse length of 300ps. The diagnostic will implement GaAs/GaAsP detectors identical to those previously used in the JET system. The light detection wavelength range is limited by the Quantum Efficiency (QE) of these detectors to 350-800nm, which is nonetheless sufficient for a good quality measurement of a wide Te range. Spectral self-calibration based on dual-wavelength Thomson Scattering will be used to compensate changes in the wavelength sensitivity of collection optics. The proposed system is therefore not relying on R&D of new technologies and can be built using components currently available on the market.