Amplitude Of Rf Electric Field Research Articles

Parametric studies of stream instability-induced higher harmonics in plasma ionization breakdown near an emissive dielectric surface

In this work, we comprehensively investigate the generation of higher harmonic (HH) electric fields normal to the applied rf electric field in multipactor-coexisting plasma breakdown by fully kinetic particle-in-cell (PIC) simulations and a theoretical model. Firstly, a base case at driving frequency 1 GHz, transverse rf electric field amplitude 3 MV m−1, and background gas pressure 0.2 Torr, is studied in detail. The enhanced harmonic frequency observed is around ten times the fundamental rf frequency, significantly lower than the Langmuir frequency. A theoretical model reveals that the fundamental mechanism of HHs generation is stream–plasma instability, which originates from stream-like secondary electron emission interacting with plasma. The resulting HH frequency and the growth rate of its oscillating amplitude from the theoretical model, agree well with the PIC simulations. With increasing pressure, the HH oscillations are found to be significantly reduced. This is because at higher pressure the gas ionization rate is higher, which causes a more rapidly increasing plasma density, leaving less time for the growth of instability. Furthermore, the parameter space in terms of background gas pressure and rf field amplitude within which the HHs appear is revealed. Finally, the effect of the driving rf frequency on HHs is also investigated, and it is found that the instability-induced oscillating HHs field remains when the driving frequency is increased to 2 GHz, however, it is highly reduced at higher driving frequency of 5 GHz, as oscillations at the fundamental frequency start playing a more important role.

Effects of microwave frequency on growth rate of upstream dielectric multipactor

Effects of microwave frequency on the growth rate of an upstream dielectric multipactor are studied. Our previous research (2018 Plasma Sources Sci. Technol. 27 125006) shows that the growth rate is independent of microwave frequency in a strong microwave. However, this note shows that the growth rate would decrease quickly to a negative value if the microwave frequency increases to be above a critical value (far away from the first crossover point of unit secondary yield). This phenomenon is observed using electromagnetic particle-in-cell simulation on a dielectric exposed to microwaves of TEM modes TE01 mode, and TE11 mode, respectively. Consistent simulation results and theoretical results show that the suppression of the multipactor is induced from the returning difficulty of emitted electrons, and the critical microwave frequency corresponding to zero growth rate is in direct proportion to the rf electric field amplitude and is in positive correlation with the secondary electron yield.

Tracking multiple generation and suppression of secondary electrons on periodic triangular surface

To research the dynamic course of multipactor suppression on the periodically patterned surface, tens of electron collision processes are tracked by numerical calculation. The influences of microwave frequency, amplitude of RF electric field, slope angle, the local field enhancement, and the tilted incident electric field on the multipactor suppression are studied by tracking multi-generation electrons' trajectories, hopping and flight time, collision energy, and secondary emission yield. Meanwhile, the dynamic processes of secondary electrons on the periodic surface are analyzed by particle-in-cell (PIC) simulation. The PIC results are consistent with the analytical results in which the electrons fly reciprocatingly between the slopes and impact on the slopes; the methods of increasing the slope angle, enlarging the RF field, and lowering the frequency in a certain range are helpful to enhance the multipactor suppression steadily and persistently.

Dynamic Stark Spectroscopic Measurements of Microwave Electric Fields Inside the Plasma Near a High-Power Antenna

Fully dynamic Stark effect visible spectroscopy was used for the first time to directly measure the local rf electric field in the boundary plasma near a high-power antenna in high-performance, magnetically confined, fusion energy experiment. The measurement was performed in the superconducting tokamak Tore Supra, in the near field of a 1–3 MW, lower-hybrid, 3.7 GHz wave-launch antenna, and combined with modeling of neutral atom transport to estimate the local rf electric field amplitude (as low as 1–2 kV/cm) and direction in this region. The measurement was then shown to be consistent with the predicted values from a 2D full-wave propagation model. Notably the measurement confirmed that the electric field direction deviates substantially from the direction in which it is launched by the waveguides as it penetrates only a few cm radially inward into the plasma from the waveguides, consistent with the model.

Effects of rf magnetic field and wave reflection on multipactor discharge on a dielectric

This paper analyzes the effects of the rf magnetic field and partial reflection of the circularly polarized electromagnetic wave on multipactor discharge on a dielectric. A statistical theory (taking into account the velocity spread of injected electrons) is constructed to evaluate the multipactor induced breakdown and saturation level. It is concluded that the spread of initial velocities considerably changes the condition for multipactor initiation in comparison with the dynamic approach. This effect is especially strong for the case of relatively low rf electric field amplitude (when the transit time essentially exceeds the rf period) and leads to the oscillation suppression of effective electron yield and to an increase in the threshold of multipactor growth. It is established that the rf magnetic field eliminates the upper boundary of the susceptibility diagram, while the low boundary almost remains unchanged. It is also found that the presence of partial reflection causes degradation of the saturation level (in comparison with the nonreflecting case) and results in decreasing of the characteristic time required to achieve the steady state.

Modelling of D majority ICRH at JET: impact of absorption at the Doppler-shifted resonance

Recent experiments in JET have provided new information on majority ICRF heating schemes in large tokamak plasmas. Adopting a wide range of available diagnostics, the plasma behaviour in a fundamental D majority ICRF heating scenario was investigated. The main results of the experiments are that, due to the modest RF power available at the frequencies necessary for central D absorption in JET associated with the reduced RF electric field amplitude near the cold ion-cyclotron resonance layer of the D majority ions, ICRH alone is barely capable of heating the plasma. On the other hand, when preheating the plasma using neutral beam injection, the wave–plasma coupling is improved and considerable plasma heating followed by increased neutron yield was observed in several discharges. The enhancement of the ICRH efficiency in the presence of NBI is attributed not only to the lower collisionality of the pre-heated plasma but also to the Doppler-shifted ion-cyclotron resonance absorption of the fast beam ions. This effect has been observed with several diagnostics and was confirmed by numerical simulations. The numerical modelling was done adopting a coupled full wave/quasi-linear Fokker–Planck (QLFP) code that takes into account the non-Maxwellian distributions of the injected beam ions in the wave equation, and the actual local RF electric field structure obtained by the full-wave computations in the Fokker–Planck description. By studying the response of the plasma to sudden changes in the ICRF power level, the experimental RF power deposition profiles were estimated and compared with the theoretical predictions. Finally, a qualitative link between the RF-heated beam distributions obtained in the QLFP simulations and some of the experimental results is also presented.

Excitation of the geodesic acoustic mode during ion cyclotron resonance heating

It is shown that poloidal polarization of the plasma during ion cyclotron resonance heating (ICRH) can provide the source of free energy for excitation of the geodesic acoustic mode (GAM). A rough estimate for the threshold rf electric field amplitude necessary for GAM instability is given by νQL∕νi>q−3, where νQL is the rate of ICRH-induced quasilinear diffusion in velocity space, νi is the ion collision rate, and q is the safety factor.

Experimental and theoretical investigation of waves and resonances of a bounded hot electron, cold ion magnetized plasma

A plasma column is produced by diffusion along a steady magnetic field by a helical microwave discharge source located at one end of a linear machine. Waves in the plasma are electrostatically excited by a split-cylindrical, slow-wave structure. Movable well-balanced RF double probes are used to measure RF electric field amplitude as a function of frequency. This frequency domain includes the Trivelpiece-Gould (T-G) mode (fLH<f<fpe), surface wave mode, electrostatic ion cyclotron mode (fci<f<fLH) and ion-acoustic mode (f<or approximately=fci). Experimentally observed resonances and dispersion curves of the bounded plasma system are in good agreement with theory for ion-acoustic waves, surface waves and T-G mode.