Development and application of Thomson scattering diagnostic on Multiple Plasma Simulation Linear Device (MPS-LD)

To achieve high-precision non-invasive measurements of electron density (ne) and electron temperature (Te) on the Multiple Plasma Simulation Linear Device (MPS-LD), this paper presents the design and experimental demonstrate of the Thomson scattering (TS) diagnostic system, and applies the developed system to investigate helicon discharge physics. The system uses a Nd: YAG laser with a wavelength of 532 nm, combined with a collection system of a grating spectrometer and an ICCD, and the TS spectrum is obtained through optical-path alignment with a translational resolution of 0.01 mm and a rotational resolution of 0.01°, a MASK with a suppression ratio exceeding 99%, spectrometer calibration, and rotational Raman scattering characteristic-line calibration. At typical discharge conditions (ne ~ 4×10¹⁹ m⁻³), the relative uncertainties are δ_(n_e )⁄n_e ~ 3.47% and δ_(T_e )⁄T_e ~ 4.83%. The reliable diagnostic range of this system is determined to be approximately 4×1017–1×1020 m-3. The TS diagnostic is benchmarked against Langmuir probe measurements at the nearest axial position, showing reasonable agreement. Both diagnostics are applied to argon discharge. Experimental results show that, at fixed RF power, the electron density exhibit peak values with increasing gas injection. At a constant gas flow rate, an optimal magnetic field exists at fixed RF power, beyond which the on-axis electron density decreases. Under the conditions of the optimal magnetic field and saturated gas injection, transitions of the helicon mode are observed at RF powers of 1000 W and 3000 W, characterized not only by a stepwise increase in electron density but also by a simultaneous stepwise increase in electron temperature, indicating that a high-density plasma state can be sustained solely through helicon-wave-dominated power coupling. To further investigate the radial deposition of helicon wave power, the development of radially resolved TS diagnostics is required in future work.