Jinxiang Cao

Total absorption of electromagnetic radiation in overdense plasma

The energy transformation from electromagnetic wave to plasmas polaritons in overdense plasma is investigated by using the theory of hydrodynamics in the thin cylinder limit and surface wave resonator. The grating experiment certifies the excitation of the surface wave. Through studying the role of the magnetic field in excitation of the surface wave and analyzing the frequency domain spectrum of the reflected wave, the time series of reflection, transmission and plasma density are diagnosed when the electromagnetic wave transforms into the surface wave. The experimental scheme of Bliokh [Phys. Rev. Lett. 95, 165003 (2005)] is improved. A steady overdense plasma in a cylindrical cavity is obtained by dc high voltage discharging and measurement is taken in series. The diffraction grating is fixed in optimum position after the distance from it to the chamber is adjusted. The reflection ratios of plasma and a piece of tinfoil are compared to avoid the effect of the standing wave. The effect of incident polar...

Coherent structure generated in the boundary layer of a laboratory‐created ionospheric depletion

Laboratory experiments have been conducted to simulate the boundary processes of ionospheric depletion. The ionospheric depletion was modeled through releasing depletion chemical (SF6) into the ambient plasmas. These plasmas were segregated into two regions by a boundary layer of width electric scale length. In the localized boundary layer, the electron density decreased sharply that yielded steep density gradients. Meanwhile, the floating potential increased in the time scales of the lower hybrid (LH) period, which produced strong sheared electron flows. The shear frequency ωs=VE/LE, which characterizes the sheared flow, is much larger than the LH frequency ωLH. A coherent structure was observed when the floating potential fluctuations were analyzed using digital spectral analysis techniques. Comparison with the theory indicated that the structure is driven by the electron-ion hybrid instability which is generated owing to the nonlinear coupling between the electron density gradient and the sheared electron flow. Our results are important to study the early phase nonlinear evolution of the ionospheric depletion, especially in the development of plasma irregularities and turbulence in the boundary layer.

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.

E→H mode transition density and power in two types of inductively coupled plasma configuration

E → H transition power and density were investigated at various argon pressures in inductively coupled plasma (ICP) in a cylindrical interlaid chamber. The transition power versus the pressure shows a minimum transition power at 4 Pa (ν/ω=1) for argon. Then the transition density hardly changes at low pressures (ν/ω≪1), but it increases clearly when argon pressure exceeds an appropriate value. In addition, both the transition power and transition density are lower in the re-entrant configuration of ICP compared with that in the cylindrical configuration of ICP. The result may be caused from the decrease of stochastic heating in the re-entrant configuration of ICP. This work is useful to understand E → H mode transition and control the transition points in real plasma processes.

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.

Experimental study of surface-wave-assisted microwave transmission through a single subwavelength slit

In this study, we made a systematic investigation in the enhanced transmission of X-band microwaves through a single subwavelength slit aperture surrounded by periodic grooves in metallic aluminum plates. The influence of the grooves’ number, width, and depth on the transmission spectra was examined. By tuning these geometrical parameters, the transmission enhancement could be maximally optimized. The transmission spectra at s-polarization was also measured and compared to those at p-polarization. It is shown that the extraordinarily enhanced transmission at p-polarization is associated with the surface waves of Brewster–Zenneck modes on the metal-air interface. Our experimental results will be helpful to the control of the propagation of electromagnetic waves in the microwave regime.

Laboratory investigation of the boundary layer processes of artificially created ionospheric depletion

In the work, we have experimentally investigated the boundary layer processes of artificially created ionospheric depletions. Those ionospheric depletions were modeled via releasing attachment chemicals, such as SF6,CF4, and CO2, into the ambient plasmas. Boundary layer of width of electric scale length emerged and separated those plasmas into two regions, the ambient plasmas and the negative ions plasmas. In the localized boundary layer, those fluctuations of the electron density and the floating potential were investigated varying with the plasma pressure and the partial pressure of released chemicals. The electron density decreased sharply that yielded steep density gradients ∇ne, and the floating potential increased which generated sheared electron flows. It is found that the magnitude of fluctuating floating potential is proportional to that of the ∇ne. Those fluctuations were analyzed in detail using digital spectra analysis techniques. Vortex-like coherent structures were observed in the fluctuations of electrostatic potentials. These coherent frequencies are sensitive to the mass of the negative ions, and all lie in the lower hybrid range. By comparing the experimental results with theoretical predictions, the modes have been identified as the coherent structures resulting from the electron-ion hybrid instability. Our results are important to study the early phase nonlinear evolution of the ionospheric depletion and also may be applied to the plasma sheet boundary layer in where often encounters the narrow electron density gradients and sheared electron flows.

Electromagnetic fluctuations generated in the boundary layer of laboratory-created ionospheric depletions

Ionospheric depletions, produced by release of attachment chemicals into the ionosphere, were widely investigated and taken as a potential technique for the artificial modification of space weather. In this work, we reported the experimental evidence of spontaneously generated electromagnetic fluctuations in the boundary layer of laboratory-created ionospheric depletions. These depletions were produced by releasing attachment chemicals into the ambient plasmas. Electron density gradients and sheared flows arose in the boundary layer between the ambient and the negative ions plasmas. These generated electromagnetic fluctuations with fundamental frequency f0 = 70 kHz lie in the lower hybrid frequency range, and the mode propagates with angles smaller than 90° (0.3π–0.4π) relative to the magnetic field. Our results revealed that these observed structures were most likely due to electromagnetic components of the electron-ion hybrid instability. This research demonstrates that electromagnetic fluctuations also...

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.