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Electron sheaths have previously only been measured near a positively biased small electrode, in which a potential dip was often observed. In this paper, we present an experimental study on the electron sheath near a stainless steel plate in the presence of a weak electron beam. It is shown that the electron beam, though its density is much lower than that of the background plasma, will substantially alter the sheath structure, i.e., it causes the disappearance of the potential dip when the beam energy just exceeds the ionization potential of the neutral gas but later enhances the dip for higher energies. It is also shown that proper biases on the plate and chamber wall are the key to the formation of the electron sheath and the dip. For a fixed plate bias but with different electron beam energy, the measured thickness of the ion-free Child–Langmuir sheath agrees well with that of the theoretical model.

A two-color homodyne Mach–Zehnder (M–Z) optical fiber interferometer with wavelengths of 1.55 and 1.31 µm was developed for long-time measurement of line-integrated plasma electron density. A novel phase difference demodulation algorithm based on a single 3 × 3 optical coupler was implemented in a two-color optical fiber interferometer scheme for the first time. Our laboratory tests showed that this new optical fiber interferometer could determine the phase shift due to the low-frequency ambient vibration and could maintain high phase resolution measurement. The resolution of the new interferometer was less than 0.04 rad in 1000 s, corresponding to a line-averaged electron density of less than 1.0 × 1019 m−2. Actual plasma discharge experiments performed on KTX-CTI, which is a new compact torus injector (CTI) constructed at the Keda Torus eXperiment (KTX), showed that this interferometer has excellent several-second stability.

A homodyne Mach–Zehnder (M-Z) optical fiber interferometer (OFI) based on the optical coupler demodulation with 1.55 μm wavelength is firstly developed for high plasma density measurement on the theta pinch (IMP-θ) plasmas in the Institute of Modern Physics for the research on magnetic-inertial fusion. The evolutions of center line-averaged electron density in the theta pinch discharges are measured by the interferometer. The OFI is full-fiber structure and the line-average electron number density resolution is 10 18 m -2 . The minimum time resolution of OFI is 1ns, depended on the bandwidth of terminal InGaAs detectors. Finally, the dynamics of theta pinch plasmas are reported, which are analyzed from the density waveforms under different gas pressures and charging voltages of capacitor banks. It shows the OFI is a highly reliable and flexible routine high plasma density diagnostic tool for theta pinch plasmas.

The flux of magnetic helicity characterizes how the topology of a magnetic field changes and helicity and its transport play essential roles in plasma equilibrium and instabilities in the reversed-field pinch (RFP) device configuration. Used to suppress magnetohydrodynamic instabilities effectively, the stabilizing conducting shell located at the boundary of an RFP device is always characterized by an eddy-current distribution that evolves rapidly with time. After measuring and analyzing the eddy currents on this stabilizing shell during tokamak discharges in the Keda Torus eXperiment device, the distributions of multiple physical quantities on the shell are obtained, including the magnetic fields, eddy currents, and electric fields. An experimental technique for measuring helicity flux density is demonstrated, combined with eddy-current diagnosis and these physical vectors on the boundary. How the magnetic and current helicity flux densities evolve is given by a feasible mathematical analysis, and data suggest that the plasma mediates both the generation and transport of magnetic helicity during the discharge. The experimental results of the distribution of the magnetic helicity flux densities (MHFDs) due to the instability on the boundary are given, and the average of these MHFDs over the whole boundary surface is far less than the maximum value of the MHFDs. In addition, the results of current helicity flux densities on the boundary are also presented.

An optical fiber Mach-Zehnder interferometer at a wavelength of 1.55 µm has been developed for measurements of high electron density on compact torus (CT) plasmas with a high time resolution of 0.1 µs and high phase resolution of 6.4 × 10-4 rad. To improve density measurement accuracy, the phase noise of the interferometer has been investigated in detail and optimized. In the bench test, the interferometer was calibrated using a piezoelectric ceramic actuator with known stroke. Initial results on CT plasma show that the optical fiber interferometer provides reliable density measurements at two spatial locations and the bulk velocity of plasma can be determined by the method of time of flight.

A compact torus injection system, KTX-CTI, has been developed for the planned injection experiments on the Keda Torus eXperiment (KTX) reversed field pinch (RFP) device to investigate the physics and engineering issues associated with interaction between a compact torus (CT) and RFP. The key interests include fueling directly into the reactor center, confinement improvement, and the injection of momentum and helicity into the RFP discharges. The CT velocity and mass have been measured using a multichannel optical fiber interferometer, and for the first time the time evolution of the CT density profile during CT propagation is obtained. The effects of discharge parameters on the number of injected particles, CT velocity and CT density have been characterized: the maximum hydrogen CT plasma mass, m CT , is 50 μg, corresponding to 30% of the mass in a typical KTX plasma; the CT velocity exceeds 120 km s−1. It is observed for the first time that multiple CTs can be produced and emitted during a very short period (μs) in one discharge, which is significant for the future study of repetitive CT injection, even with an ultra-high frequency.

Dust accumulation significantly affects the solar PV(Photovoltaic) performance, resulting in a considerable decrease in output power, which can be reduced by 40% with the dust of 4 g/m2. Understanding the dust deposition characteristics of PV modules can provide theoretical support for selecting dust cleaning methods and formulating cleaning strategies. This paper introduced the factors affecting dust accumulation and presented the research status of dust deposition mechanisms. Moreover, the dust cleaning methods were discussed and compared. In future research, a comprehensive particle deposition model including surface energy and other factors could be established to explore the dust accumulation mechanism. Additionally, investigating the particle deposition condition to determine the deposition of particles is beneficial to understanding the dust accumulation characteristics and predicting the dust accumulation amount. Compared with mechanical cleaning methods, the cleaning effect of electrostatic (cleaning efficiency can reach 90%), coating, and acoustic wave methods is superior. If the three methods can be combined, using an electrostatic or coating method to remove small particles and acoustic waves to remove large particles is a promising approach.

We report about the soft X-ray (SXR) tomography system which is being developed on Keda Torus eXperiment (KTX). The tomographic system aims at studying three dimensional effects in reversed field pinch with high plasma current, particularly in quasi-single-helicity state (QSH). The view distribution of the probes has been carefully designed to reflect the experimental constraints. Bayesian experimental design (BED) is used to optimize the signal camera location and camera opening angle, aiming at maximizing the gain information in future QSH states. The BED approach allows us to quantitatively evaluate the performance of the design, and to estimate the design robustness. The physical image of the plasma is tomographically reconstructed using Cormack approach and the reconstructed emissivity image may be compared to the emissivity model using the residual error χ. The camera location, camera opening angle, and sightline density are finally decided by quantitatively comparing diverse designs in order to achieve the best reconstruction.

We have developed a parametric method for eliminating the background component of the plasma current, which is measured by a Rogowski coil and polluted by the toroidal magnetic field in the vacuum vessel of the Keda Torus eXperiment (KTX) reversed field pinch (RFP) device. The method considers the toroidal magnetic field windings, the KTX vacuum chamber, and the Rogowski coil as a linear time-invariant system; in this case, a constant frequency response function characterizes the system. Using this response function, the current component caused by pollution from the toroidal magnetic field can be predicted exactly for an arbitrary input current to the toroidal magnetic field windings. Compared with the traditional proportional compensation method, the proposed method has great flexibility and universality and it is potentially applicable to cases in which the toroidal field current signal changes over time with plasma feedback signals. Furthermore, the method can be applied to other similarly affected signals, such as magnetic field signals. As an example, we have corrected the poloidal and toroidal magnetic field signals better to reveal the true physical processes for the RFP state.

The Keda Torus eXperiment (KTX) is still operated in the commissioning phase, and preparation for the operation capability of the KTX phase II upgrade is underway. The diagnostics in the KTX have been greatly developed: (1) the terahertz interferometer has been upgraded to seven chords for electron density profile inversion; (2) a Thomson scattering system with a 5 Joule laser has been installed and commissioning is in progress; (3) a 3D movable probe system has been developed for the electromagnetic turbulence measurement; (4) double-foil soft x-ray imaging diagnostics have been set up and a bench test has been completed; (5) an edge capacitive probe has been installed for the radial electrical field measurement; (6) a multi-channel spectrograph system has been built for detecting impurities of carbon and oxygen. In addition, the design of a new compact torus injection system has been completed for feeding and momentum driving. Pilot research, such as the 3D reversed field pinch physics and electromagnetic turbulence, etc, have been conducted in the discharge status of the KTX. The 3D spectra characters of electromagnetic turbulence are firstly measured using a classical two-point technique by Langmuir probe arrays set on the 3D movable probe system and edge magnetic sensors. The forward scattering is collected by the interferometer system, which shows the potential for turbulence research. The electromagnetic turbulence is tentatively investigated in the KTX. The formation of a quasi-single-helicity state in the KTX regime is also preliminarily explored in simulation.