Pengcheng Yu

Design and construction of Keda Space Plasma Experiment (KSPEX) for the investigation of the boundary layer processes of ionospheric depletions

In this work, the design and construction of the Keda Space Plasma EXperiment (KSPEX), which aims to study the boundary layer processes of ionospheric depletions, are described in detail. The device is composed of three stainless-steel sections: two source chambers at both ends and an experimental chamber in the center. KSPEX is a steady state experimental device, in which hot filament arrays are used to produce plasmas in the two sources. A Macor-mesh design is adopted to adjust the plasma density and potential difference between the two plasmas, which creates a boundary layer with a controllable electron density gradient and inhomogeneous radial electric field. In addition, attachment chemicals can be released into the plasmas through a tailor-made needle valve which leads to the generation of negative ions plasmas. Ionospheric depletions can be modeled and simulated using KSPEX, and many micro-physical processes of the formation and evolution of an ionospheric depletion can be experimentally studied.

Laboratory generation of broadband ELF waves by inhomogeneous plasma flow

Transversely accelerated ions and the associated heating of the high-latitude ionosphere have been attributed to broadband extremely low-frequency (BBELF) turbulence. Controlled laboratory tests of the hypotheses on the formation mechanism of BBELF waves have involved only a few examples, e.g., current-driven and shear-driven instabilities. In this work, electrostatic fluctuations in the ion-cyclotron frequency range have been excited by inhomogeneous energy-density driven instability (IEDDI). This was achieved using the interpenetrating plasma method with a much larger electric field scale size LE comparable to the ion gyroradius ρi, which was challenging earlier because of plasma conditions. The peak frequency of the IEDDI spectrum falls as low as ω≈0.3ωci, where ωci is ion cyclotron frequency. This is an interesting result because the previous attempts could not produce such low frequency IEDDI, although it was known theoretically to be possible. The observations made by FAST, Freja, and THEMIS satellites might be explainable in terms of the reported experimental results.

Laboratory experiments in the argon plasma perturbed by injections of the electronegative gases

In this study, laboratory observations of the perturbations of the magnetic field are reported due to the injection of attachment chemicals (CF4, SF6, and CO2) into argon plasmas. Besides the well-known electron density reduction, we also observed magnetic field perturbation in the experiment. The measured induced voltage B, which is taken as a proxy of the time-changing electromagnetic field, fluctuates in the boundary layer between the ambient plasmas and negative ions plasmas. Perturbations of the magnetic field were investigated by changing the ambient pressure and ratio of attachment chemicals. The measured B keeps increasing in these lower pressures; but it no longer increases as the ambient pressure higher than a threshold, e.g., for CF4, SF6, and CO2, the transition pressure is 6Pa, 5Pa and 4Pa, respectively. The magnitude of the B increase with the change of the ratio of release flow until at higher ratios, e.g., 40%. We transformed these time-sampled data into the frequency domain and found c...

The transition mechanisms of the E to H mode and the H to E mode in an inductively coupled argon-mercury mixture discharge

In our experiment, the transition points between the two operational modes of capacitive coupling (E mode) and inductive coupling (H mode) were investigated at a wide range of mercury vapor pressures in an inductively coupled plasma, varying with the input radio-frequency powers and the total filling pressures (10 Pa–30 Pa). The electron temperatures were calculated versus with the mercury vapor pressures for different values of the total filling pressures. The transition power points and electron density also were measured in this study. It is shown that the transition powers, whether the E to H mode transition or the H to E mode transition, are lower than that of the argon discharge, and these powers almost increase with the mercury vapor pressure rising. However, the transition electron density follows an inverse relationship with the mercury vapor pressures compared with the transition powers. In addition, at the lower pressures and higher mercury vapor pressures, an inverse hysteresis was observed clearly, which did not appear in the argon gas plasma. We suggest that all these results are attributed to the electron-neutral collision frequency changed with the additional mercury vapor pressures.

The influence of gas pressure on E↔H mode transition in argon inductively coupled plasmas

Considering the gas pressure and radio frequency power change, the mode transition of E↔H were investigated in inductively coupled plasmas. It can be found that the transition power has almost the same trend decreasing with gas pressure, whether it is in H mode or E mode. However, the transition density increases slowly with gas pressure from E to H mode. The transition points of E to H mode can be understood by the propagation of electromagnetic wave in the plasma, while the H to E should be illustrated by the electric field strength. Moreover, the electron density, increasing with the pressure and power, can be attributed to the multiple ionization, which changes the energy loss per electron-ion pair created. In addition, the optical emission characteristics in E and H mode is also shown. The line ratio of I750.4 and I811.5, taken as a proxy of the density of metastable state atoms, was used to illustrate the hysteresis. The 750.4 nm line intensity, which has almost the same trend with the 811.5 nm line intensity in H mode, both of them increases with power but decreases with gas pressure. The line ratio of 811.5/750.4 has a different change rule in E mode and H mode, and at the transition point of H to E, it can be one significant factor that results in the hysteresis as the gas pressure change. And compared with the 811.5 nm intensity, it seems like a similar change rule with RF power in E mode. Moreover, some emitted lines with lower rate constants don’t turn up in E mode, while can be seen in H mode because the excited state atom density increasing with the electron density.Considering the gas pressure and radio frequency power change, the mode transition of E↔H were investigated in inductively coupled plasmas. It can be found that the transition power has almost the same trend decreasing with gas pressure, whether it is in H mode or E mode. However, the transition density increases slowly with gas pressure from E to H mode. The transition points of E to H mode can be understood by the propagation of electromagnetic wave in the plasma, while the H to E should be illustrated by the electric field strength. Moreover, the electron density, increasing with the pressure and power, can be attributed to the multiple ionization, which changes the energy loss per electron-ion pair created. In addition, the optical emission characteristics in E and H mode is also shown. The line ratio of I750.4 and I811.5, taken as a proxy of the density of metastable state atoms, was used to illustrate the hysteresis. The 750.4 nm line intensity, which has almost the same trend with the 811.5 nm line...

Double flush-mounted probe diagnostics and data analysis technique for argon glow discharge plasma

In this work, a double flush-mounted probe for measuring plasma parameters was designed and fabricated. The method to determine the plasma density and electron temperature using a floating double flush-mounted probe was characterized. To validate this method, the measurement results in an argon glow discharge plasma, including the electron density and temperature measurements, were compared with those obtained using a single probe and a double probe. Results indicate that the electron density measured using the double flush-mounted probe agrees well with those measured using other probes; the effective electron temperature values are also consistent within the admissible error range. These results suggest that the double flush-mounted probe can be used for accurate measurements at low pressure DC plasma discharges and also can be applied to other complex plasmas such as tokamaks, in the boundary-layer region without a reference electrode.

Comparisons of the Characteristic on the Mode Transition in an Inductively Coupled Discharge by Exciting Coil Change

The effects on the characteristics of mode transition were investigated in a spiral type inductively coupled discharge with different size antennas, such as different turns, intervals, and heights. In the case of the relatively small intervals, the transition powers, whether the E to H mode or the H to E mode, decrease with the intervals in the two-turn antenna discharge, and the transition powers of three-turn antenna discharge are much smaller than that of two-turn antenna as the heights stay constant. All these results can be attributed to the nonlinear enhancement mechanism of electron heating in the nonlocal kinetic overlapping region. Moreover, the transition powers decrease slowly with the intervals and finally can increase under the circumstance that the dissipated power in the plasma sheath is relatively obvious. Meanwhile, the transition powers seem like keeping at a minimum values in the three-turn antenna discharge with the same intervals. These experimental results can be caused by the increased power loss across the relatively large sheath and Joule heating in the antenna coil, which lead to a decrease in the radio frequency coupling efficiency with the increasing intervals. Therefore, the combined effect of the nonlinear enhancement effect and the thicken sheath is a key factor on the change laws of the mode transition powers in different type antenna discharges. This paper can be significant to industrial production applications and the foundation theory researches on the mode transition in inductively coupled discharges.

Laboratory simulation of the formation of an ionospheric depletion using Keda Space Plasma EXperiment (KSPEX)

In the work, the formation of an ionospheric depletion was simulated in a controlled laboratory plasma. The experiment was performed by releasing chemical substance sulfur hexafluoride (SF6) into the pure argon discharge plasma. Results indicate that the plasma parameters change significantly after release of chemicals. The electron density is nearly depleted due to the sulfur hexafluoride-electron attachment reaction; and the electron temperature and space potential experience an increase due to the decrease of the electron density. Compared to the traditional active release experiments, the laboratory scheme can be more efficient, high repetition rate and simpler measurement of the varying plasma parameter after chemical releasing. Therefore, it can effective building the bridge between the theoretical work and real space observation.