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 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 CT density profile during CT propagation is obtained. The number of injected particles, the CT velocity and density on the discharge parameters 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 exceed 120 km/s. It is firstly observed that multiple CTs can be produced and emitted during the very short period (<100 μs) in one discharge, which is significant for the future study of repetitive CT injection, even with an ultra-high frequency.

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.

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.

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A five-chord interferometer based on terahertz solid state sources has been successfully installed on the Keda Torus eXperiment (KTX), a reversed field pinch machine. The optical design has been carefully optimized for the uniform distribution of beam light to fully use the limited power source (∼2 mW). By setting the telescopic mirror unit, the beam waist is located in the center of the vacuum vessel and its diameter is in the range of the Rayleigh length. The beam width across the plasma area is improved to ∼20 mm to minimize crosstalk and beam energy loss. After careful beam alignment, the phase noise for each channel can reach 0.004π. The radial profiles of electron density on the KTX are inverted, and density fluctuation associated with instabilities is shown based on the forward-scattering signals.