A five-channel microwave interferometer with high temporal resolution and high phase resolution has been developed for measuring electron density profiles and fluctuations on the Sino-UNIted Spherical Tokamak. A correction algorithm, based on the low signal amplitude detection, is proposed to eliminate the fringe jump errors. The correction algorithm achieves an accuracy of 92%. The density profile is reconstructed through the Park-matrix method, with the geometry of magnetic surfaces calculated by the equilibrium fitting. The reconstructed density profiles for discharges with supersonic molecular beam injection are in good agreement with the results of another 94 GHz single-channel horizontal interferometer and the Langmuir probes. The temporal resolution of the system is 0.5 µs and the line-integrated electron density resolution is 1 × 1015 m−2, which benefits from the single sideband modulation technique. Therefore, the multichannel interferometer system is capable of studying the details of the high-frequency (up to 200 kHz) density fluctuation such as that in the minor disruption event.

A quasi-coherent (QC) mode was observed in the core region of low-density ohmic plasmas in Sino-UNIted Spherical Tokamak (SUNIST). In experiments on the QC mode, two sets of moveable Langmuir probes (LPs) were used to measure the local parameters including floating potential, electron temperature, electron density, and so on, as well as their profiles. To monitor the magnetohydrodynamic (MHD) activities, a Mirnov probe (MP) was used to measure the poloidal magnetic fluctuation. The QC mode can be seen in the spectra of floating potential, but there is no similar peak in the spectra of magnetic fluctuation. Thus, the QC mode is probably electrostatic. By analyzing the electrostatic potential fluctuations from the LPs, the features of the QC mode including frequency, wavenumber, propagation direction, and dependence on collisionality are identified, which are consistent with the characteristics of dissipative trapped electron mode (DTEM).

Since the major/minor radius of the Sino-United Spherical Tokamak (SUNIST) is 0.3/0.23 m, respectively, the space left for magnetic diagnostics in the high field side (HFS) is quite limited. At the same time, a good signal-to-noise ratio and a high-frequency response (up to 1 MHz) are required for equilibrium reconstruction (ER) and Alfven eigenmode studies. Such a magnetic probe array must be extremely thin and tightly close to the central column, not exceeding the inner limiter and leaving the aspect ratio of the spherical tokamak unchanged. Therefore, the turn number and the shape of windings should be highly optimized to enable both a high-frequency response and an enough effective area. A 32-channel magnetic probe array fulfilling these requirements on the central column is designed, calibrated, and installed in the SUNIST. The array consists of 16 probes. Each of them consists of two perpendicular windings, which can measure toroidal and poloidal magnetic fields simultaneously. The effective area and frequency response of each probe are calibrated using a Helmholtz coil and an LCR bridge based on an equivalent probe-and-cable circuit model. After that, an expression of the magnetic diagnostic response to the field coil currents is used to calibrate the installation error. With the full coverage of magnetic probes in the poloidal direction, more reliable ER can be obtained, and the features of magnetohydrodynamic activities in the HFS can be studied.

Absolute x-ray ultraviolet diagnostics ensures 2D coverage of the radiation emission region that enables tomographic reconstruction. However, retrieving the local emissivity via tomography remains a challenge due to its ill-posed nature. Tikhonov regularization with smoothness operation generally performs well but tends to over-smooth regions with steep gradients and local structure in the radiation profile and may introduce artifacts. In this paper, a tomography method based on compressive sensing theory with Tikhonov regularization terms is developed. Experimental results on multiple phantom sets show that the proposed method improves the reconstruction accuracy and quality in regions with steep gradients compared with the Tikhonov regularization method and suppresses the unphysical negative emissivity. The analysis of reconstruction uncertainty shows that the dictionary learning process provides more accurate prior information about steep gradients to improve the quality of reconstructed images, and compressive sensing has the denoising capability to reduce the impact of noise. Finally, the method is validated by data from the Sino-UNIted Spherical Tokamak, showing fewer artifacts and more reliable reconstruction images than the earlier method.