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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.

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.

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.

An equilibrium field (EQ) power supply (PS) circuit using a feedback H-bridge pulse-width-modulation controller has been developed to control the plasma horizontal position and to satisfy the requi...

The inward particle flux associated with the global long-lived mode (LLM) during the L–I–H transition on HL-2A tokamak has been measured. The inward particle flux arises from the phase change between density and radial velocity fluctuations, where density and velocity fluctuations are strongly correlated with magnetic fluctuations of LLM. Moreover, the radial velocity and its gradient rather than poloidal flow shear play an important role in particle transport associated with the large-scale mode. The strong nonlinear coupling between LLM and ambient turbulence has been confirmed, and this may contribute to most of the inward particle flux in the LLM during the I-phase state.

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.

The inward transport flux accompanied by the long-lived mode (LLM) has been observed by the edge Langmuir probe array during the L-H transition in the HL-2A tokamak. The cross phase between the poloidal electric field fluctuation and the electron density fluctuation in the LLM frequency changed greatly during the L-H transition, which led to the radial transport flux in the LLM frequency to change its transport direction in a given radial region near the pedestal barrier. By using the long distance correlation method, the phase shift of poloidal electric field fluctuation proved to be the key reason, which led to the cross-phase term to change in the LLM frequency. Furthermore, the shear effect has a mode-selecting characteristic on the LLM and turbulence because the direction of turbulent transport still remains radially outward during the H-mode. In addition, when the inward conducted heat flux accompanied by LLM occurs, the poloidal electric field fluctuation and electron temperature fluctuation both have a positive gradient in the LLM frequency; this may indicate that the positive gradient of radial velocity and electron temperature in the LLM frequency play an important role during the entire transport direction change process.

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.

A poloidal and toroidal magnetic field coupling circuit model is developed to simulate discharge waveforms of the Keda Torus eXperiment (KTX) reversed field pinch (RFP) to ensure that the design of the power supply systems is able to achieve the typical discharge parameters. The radial profiles of plasma current density and magnetic field used in the coupling model are characterized by modified Bessel function model (MBFM) derived from the well-known Taylor's theory. The power balance in the energy conservation of poloidal and toroidal fields is used to establish the connection between these two fields. The numerical solutions of the coupling show that the KTX power supply system is capable of achieving the designed discharge parameters, such as field reversal parameter ( F ), pinch parameter (Θ), magnetic field, plasma current and loop voltage. Furthermore, the delay of magnetic field penetration due to the effect of vacuum vessel is also considered in the model.

The Keda Torus eXperiment (KTX) is a new reversed field pinch (RFP) device at the University of Science and Technology of China. The construction and assembly of KTX, including the vacuum chamber, conducting shell, magnetic field windings, power supply system, active control coils, vacuum pump and data acquisition system, have been completed on August 1, 2015. Immediately following that, the first plasma was obtained on August 15, 2015. Intensive conditioning of the machine is underway to ramp up the plasma current toward its full operation. An active feedback mode control system has been built and has been implemented to control the error field around the vertical gaps of the conducting shell. The pulsed power supply systems of ohmic heating field and toroidal field (TF), using thyristor and energy storage capacitors, have been tested and commissioned. The TF power supply has flexibility in being able to operate with a reversed TF configuration and stable TF configuration. The fundamental diagnostic tools are developed for early KTX operation. Currently, the plasma current is up to 205 kA and the maximum discharge length is 21 ms, approaching to the conducting shell penetration time. Furthermore, typical RFP discharges are being produced with RFP state lasting 2 ms. These initial operation results for KTX are described in detail.

A 10-channel H α diagnostic system has been designed with the rapid response rate of 300 kHz, spatial resolution of about 40 mm, and overlap between adjacent channels of about 3%, and it has been implemented successfully on Keda Torus eXperiment (KTX), a newly constructed, reversed field pinch (RFP) experimental device at the University of Science and Technology of China (USTC). This diagnostic system is a very important tool for the initial KTX operations. It is compact, with an aperture slit replacing the traditional optical lens system. A flexural interference filter is designed to prevent the center wavelength from shifting too much as the increase of angle from vertical incidence. To eliminate the stray light, the interior of the system is covered with the black aluminum foil having a very high absorptivity. Using the H α emission data, together with the profiles of electron temperature and density obtained from the Langmuir probe, the neutral density profiles have been calculated for KTX plasmas. The rapid response rate and good spatial resolution of this H α diagnostic system will be beneficial for many studies in RFP plasma physics.

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.