Sheath-wave-related resonances in the frequency response of acylindrical monopole in a plasma

Reference is made to a floating or negatively biased antenna immersed in a plasma that is surrounded by an ion sheath. Such an antenna-sheath-plasma system may support slow surface waves at driving frequencies below the electron plasma frequency. Resonances associated with these so-called sheath waves are observed at certain frequencies in the antenna's response to an applied sinusoidal signal. An experimental study of these resonances is presented for a short cylindrical monopole in a low-pressure isotropic argon plasma. The effect on the resonance frequencies of a DC bias applied to the antenna and of plasma density and antenna length was investigated. From the experimental data, the relationship between sheath thickness and antenna potential, and the frequency dependence of the antenna admittance could be derived     

Monopole probe fed by coupling gold rods

In this paper, we investigate the characteristic of near-field localization and enhancement in the coupling metallic structures. Then we combine a cylindrical monopole probe with the basic structures aforementioned. The excited strong fields on the resonant coupling rods inspire the resonance of the probe analogous to a traditional radio frequency monopole antenna. With that, the dimension of the localized optical hot spot is reduced to about a half. Simultaneously, the enhancement of near field increases to about twice, and the optical background is greatly suppressed. Detailed studies of the structure show that the E field along the probe (including the amplitude and the phase), excited by the basic structure, plays a key role in the enhancement of the structure. This gives a guidance to design relevant devices to further improve the performance of localization and enhancement. For comparison, the case of a single-rod feeding monopole probe is also studied.     

Current Distribution of a Cylindrical Antenna in a Warm Plasma

The current distribution of a center-fed dipole cylindrical antenna of arbitrary dimension immersed in unbounded lossless warm plasma is considered, taking into account the commonly neglected end-cap effect. The approximated solution of the integral equation for the current distribution is obtained with the aid of Fourier analysis, and the expression is derived for the current distribution I(z) as the function of position z along the cylindrical surface of the antenna. Sample calculations are made for a half-wavelength dipole antenna with different values of thickness parameter ?=(a/l)<0.25 and of the normalized plasma density parameter po=(?p/?)2 <1. Here a and l denote the radius and the half-length of the antenna, respectively. ?p and ? denote the plasma angular frequency and the radiation angular frequency. The variation of the amplitude and phase of the current with the position, and the variation of radiation resistance with po and ? are examined and discussed in detail.     

Theoretical analysis of ion cyclotron range of frequency antenna array for HT-7U

This paper considers the coupling analysis of phased antenna array designed to excite fast wave in the ion cyclotron range of frequency. The coupling of the antenna is calculated in slab geometry. The coupling code based on the variational principle gives the self-consistent current flowing in the antenna, this method has been extended so that it can be applied to a phased antenna array. As an example, this paper analyses the coupling prosperities of a 2 × 2 phased antenna array. It gives the optimum geometry of antenna array. The fields excited at plasma surface are found to more or less correspond to the antenna current phasing.     

On the transverse distribution of a resonance field excited by a Gaussian electromagnetic beam on the critical surface of a radially nonuniform plasma sphere

The transverse distribution of the resonance field excited by a Gaussian electromagnetic field on the critical surface of a radially nonuniform plasma sphere is studied. Analytical expressions are obtained for this distribution. It is shown that when a laser beam is focused in front of or behind a spherical target, identical values of the integrated resonance coefficient can correspond to substantially different (in width) distributions of the resonance field over the spherical critical surface. Zh. Tekh. Fiz. 67, 125–128 (October 1997)     

Excitation of the half-frequency of the input signal in theresponse of an antenna immersed in a plasma

The nonlinear response of a cylindrical monopole antenna immersed in a steady-state collisionless magnetic-field-free plasma and driven by a single-frequency RF source is investigated. The thickness of the ion sheath surrounding the antenna is controlled by a DC bias applied to the latter. It is found that at frequencies of the driving signal close to twice the frequency of a sheath wave resonance, the 1/2-subharmonic is excited, provided that the input power level is sufficiently high. The dependence of this process on the sheath thickness and on the electron plasma density is studied in the vicinity of several sheath wave resonances. This nonlinear effect is interpreted as a parametric excitation due to the periodic modulation of the sheath thickness, and hence its capacitance, by the applied RF signal. A similar effect can be obtained by replacing the antenna-sheath-plasma structure in the measuring circuit by a variable capacitance diode. A numerical analysis of the differential equation describing the current waveform in a microwave network modeling the antenna-sheath-plasma system is presented. It confirms the fact that a periodically modulated capacitance embedded in a linear circuit is responsible for the observed excitation of the half-frequency of the input signal     

VHF current distribution on a cylindrical VLF antenna in a magnetoplasma

We obtain, solution for the current distribution on a linear antenna excited by a given e.m.f. The antenna is an infinitely, long, perfectly conducting cylinder immersed in a plasma and aligned with an external magnetic field. The obtained current distribution in the VLF band is contributed by the eigenmode guided by the cylinder and waves of the continuous spatial spectrum. We show that the current distribution of a sufficiently thin antenna is determined mainly by the eigenmode. The antenna current is calculated both analytically and numerically under ionospheric conditions. The validity criteria for using the transmission-line theory to obtain the current distribution on a linear antenna in a magnetoplasma are discussed. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 8, pp. 750–764, August 1999.     

Toward the theory of a loop antenna in an anisotropic plasma

We consider the problem of determining the current distribution along a loop antenna consisting of an infinitely thin perfectly conducting strip coiled into a ring. The antenna is immersed in an anisotropic plasma medium perpendicular to the external magnetic field and is excited by a given EMF. Primary attention is focused on the frequency band in which the excitation of electrostatic waves is possible. The problem is reduced to solving a system of integral equations with logarithmic and singular kernels. Using the solution of these equations, we found expressions for the antenna current distribution and input impedance. The analysis of these expressions is presented. Radiophysical Research Institute, Lobachevsky State University, Nizhny Novgorod, Russia. Translated from izvestiya Vysshikh Uchbnykh Zavedenii, Radiofizika, Vol. 41, No. 3, pp. 358–373, March, 1998.     

Optimization of Antenna Current Feeding for the Alfvén Eigenmodes Active Diagnostic System of JET

The possibility of exploring proper phasing of the feeding currents in the existing antenna of the Alfvén Eigenmodes Active Diagnostic system of JET, to excite pure toroidal spectra of Toroidal Alfvén Eigenmodes, is numerically investigated. Special attention is given to the actual perturbed fields excited in the plasma, which are calculated self-consistently using the antenna version of the CASTOR code. It is found that due to the close spacing of the JET antenna modules and quasi degeneracy of modes with medium to high values of the toroidal mode number n, although a proper choice of the phasing of the feeding currents of the antenna modules indeed leads to an increase of the perturbed fields of the selected mode, modes with nearby values of n are also excited with large amplitudes, so that a scheme to proper select the detected modes remains necessary. A scheme using different antenna position distribution is proposed to achieve successful optimization.     

Influence of Negative Ions on Drift Waves in a Low‐Density Ar/O2‐Plasma

The effect of negative ions on the drift wave instability has been studied in detail in a linear device by means of Langmuir probes and cross‐correlation analysis. Drift waves are excited in low‐density (5 × 10¹⁴ m^–3) and strongly magnetized (0.5 T) pure argon plasmas and in the presence of an oxygen admixture. The radial density profile of negative ions is hollow. For increasing concentration of negative ions the wave frequency decreases by about 25%. Despite of an axial density gradient, a global wave frequency is established for the entire column length. While for the noble gas case the drift wave frequency is given by the equilibrium plasma parameters in the mid‐plane, there is no such relationship for the argon plasma with oxygen admixture. This different finding is attributed to the inhomogeneous distribution of the negative ions (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optimization of Operating Parameters of Plasma Antenna

In this work, the operating parameters of the plasma antenna are optimized using a kinetic model based on Particle in Cell‐Monte Carlo Collisions (PIC‐MCC) method. This optimization study is performed via the investigation of variations in the operating parameters of the plasma antenna, i. e., its dimensions, background gas pressure, and the applied voltage frequency and their consequent effects on the plasma frequency, kinetic energy of electrons and plasma current density of plasma antenna. While the antenna performance is improved at higher tube lengths and applied frequencies, it is optimized at a particular tube radius. Moreover, higher background pressures have increasing effects on the plasma antenna operation. Based on this parametric study, the optimum operating parameters of the plasma antenna are established. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Numerical Simulation of Plasma Antenna with FDTD Method

We adopt cylindrical-coordinate FDTD algorithm to simulate and analyse a 0.4-m-long column configuration plasma antenna. FDTD method is useful for solving electromagnetic problems, especially when wave characteristics and plasma properties are self-consistently related to each other. Focus on the frequency from 75 MHz to 400 MHz, the input impedance and radiation efficiency of plasma antennas are computed. Numerical results show that, different from copper antenna, the chaxacteristics of plasma antenna vary simultaneously with plasma frequency and collision frequency. The property can be used to construct dynamically reconfigurable antenna. The investigation is meaningful and instructional for the optimization of plasma antenna design.     

柱形等离子体辐射场和阻抗的数值计算

由柱形非磁化等离子体色散关系，得到信号沿柱形等离子体传输波矢空间分布，再根据天线方程推导得到柱形等离子体在轴向密度均匀、不均匀情况下辐射方向图、辐射阻抗与增益等参数.理论计算结果与实验都证明了柱形等离子体辐射方向图随等离子体参数变化而改变，同时实验结果表明，重新得到的数值计算结果更加精确，更加符合实际测量结果，这对等离子体作为天线传输具有重要的参考价值. 关键词： 柱形等离子体 辐射阻抗 辐射方向图     

Zur mikrophysikalischen Beschreibung des schwachionisierten Säulenplasmas von Glimmentladungen in Stickstoff-Neon-Gemischen III. Bilanzierung des Säulenplasmas und Berechnung seiner inneren Parameter

For the plasma of the low current positive column in the N,-Ne-mixture the equations for the concentration of different kinds of ions and excited particles are set up. The axial electric field strength, the degree of ionization and the concentration of excited Ne-atoms and of the different kinds of ions are calculated by simultaneous solution of these balances together with the electron Boltzmann equation for this mixture and the balances for the electric current and quasineutrality of the column plasma. All these macroscopic properties of the discharge column are investigated for electric currents of 0.5 mA to 13,5 mA, for total pressures of 0,37 torr to 3,7 torr and for any mixture ratio. A comparision between measured and calculated electric field strengths shows a good agree-ment. The ionization of the N,-molecules by collisions with the metastable Ne-atoms (Penning-ionization) is of great significance for the generation of new charge carriers in such a mixture. In these investigations the collision rate zp of the Penning-ionization was obtained to 3 · 10^?11 cm³ s^?1 by the aid of a comparision between measured and calculated electric axialgradients of the column.     

Simulation of a satellite antenna tuned by a plasma load

We have investigated experimentally and theoretically the effect of a RF discharge excited in a rarefied gas by the antenna field on the input and radiation characteristics of the antenna. Under the conditions of our experiments the discharge plasma is localized near the end of a vibrator antenna, and the parameters of this plasma inhomogeneity can be changed by varying the pressure of the gas, the frequency of the RF signal, and the power fed to the antenna. The excitation of the RF discharge led to the modification of the antenna near field and, as a result, to changes of its electrodynamic characteristics.Nizhnii Novgorod State University. Tranlated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 37, No. 6, pp. 707–714, June, 1994.     

Low-power rf plasma sources for technological applications: I. Plasma sources without a magnetic field

A general analytical theory is developed and numerical simulations are carried out of cylindrical plasma sources operating at an industrial frequency of f=13.56 MHz ω=8.52×10⁷ s⁻¹). Purely inductive surface exciters of electromagnetic fields (exciting antennas) are considered; the exciters are positioned either at the side surface of the cylinder or at one of its end surfaces. In the latter case, the plasma flows out of the source through the opposite end of the cylinder. A study is made of both elongated systems in which the length L of the cylinder exceeds its diameter 2R and planar disk-shaped systems with L<2R. The electromagnetic fields excited by the antenna in the plasma of the source are determined, and the equivalent plasma resistance, as well as the equivalent rf power deposited in the plasma, is calculated.     

An Algorithm for the Analysis of Inductive Antennas of Arbitrary Cross Section for Heating in the Ion Cyclotron Range of Frequencies

The application of ion cyclotron range of frequency (ICRF) heating to near-ignited plasmas will require launching structures that will be capable of withstanding the harsh plasma environment. The recessed antenna configuration is expected to provide sufficient protection for the structure, but to date no analysis has been done to determine if adequate coupling can be achieved in such a configuration. In this work we present a method for determining the current distribution for the antenna in the direction transverse to current flow and predict antenna loading in the presence of plasma. Antennas of arbitrary cross section are analyzed above ground planes of arbitrary shape. Results from the antenna design code (ANDES) are presented and compared to experimental results.     

Influence of nonlinear effects on whistler emission in magnetoactive plasma

The influence of thermal and strictional nonlinear effects on the whistler emission in magnetoactive plasma is studied experimentally. It is established that a nonlocal thermal nonlinearity determines the directional pattern of the antenna, while a strictional nonlinearity, which is strongest near the antenna surface, is responsible for the matching of the emitter with the surrounding plasma. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 8, 548–551 (25 April 1998)     

长金属椭球体顶点处线形单极子天线的辐射问题

本文分析了竖立在任意长的和任意偏心率的长金属椭球体的顶点上一个与椭球体同轴的并且底馈电的细线形单极子天线的辐射问题,获得了辐射场的普遍表示式。 关键词：