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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.

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.

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.