Thermal ionization instability of an air discharge plasma in a microwave field

Results are presented from experimental studies of the initial stage of an air discharge initiated in a linearly polarized quasi-optical microwave beam. The discharge was excited at an air pressure at which the electron-neutral collision frequency in the discharge plasma was considerably higher than the circular frequency of the electromagnetic field and at a microwave field amplitude close to the threshold field for air breakdown. The experiments revealed relatively bright plasma channels stretched along the microwave electric field. The development rate of these channels and their characteristic transverse dimensions are estimated. A comparison of the experimental data and theoretical estimates indicates that the channels observed arise due to the onset of thermal ionization instability in the microwave discharge plasma.     

Electric discharge in air in a deeply subcritical field of a quasi-optical microwave beam

We describe the results of experiments on initiation of an electric discharge in air in a quasi-optical microwave beam by an electromagnetic vibrator fixed above the screen. This method for initiating the electrical breakdown makes it possible to obtain a discharge at a level of the electric field component in the microwave, which two orders of magnitude lower than the minimal critical field of the electrodeless breakdown of air. In our experiments, the threshold value of the air pressure determining the low- and high-temperature forms of the microwave discharge are determined depending on the field level.     

Breakdown threshold in the microwave field at low and high pressures in electronegative gas mixtures

The dependence of the critical value of the microwave field amplitude in electronegative mixtures, which corresponds to the local equality of the ionization frequency and the dissociation attachment frequency, is investigated theoretically in a wide range of pressures with allowance for electron temperature pulsations. The resultant values are compared with the critical value for the static field. The range of pressures in which electron temperature pulsations become significant is determined, and the excess of the critical amplitude over the value in the static field is calculated in the high-pressure limit. Simple formulas are recommended for estimating the critical value of the microwave field amplitude.     

Deeply undercritical microwave discharge excited by the field of a quasi-optical electromagnetic beam in a supersonic air jet

Electric discharge in a supersonic air jet is studied. It is ignited in a linearly polarized quasi-optical microwave electromagnetic beam the initial field intensity of which is much lower than the breakdown level. Electric breakdown is initiated by a tubular electromagnetic vibrator, one end of which has spikes and is covered by a quartz tube. Atmospheric air enters into a low-pressure working chamber through the inner channel of the vibrator. As a result, an immersed supersonic air jet forms in the chamber at the outlet from the quartz tube. A microwave discharge ignited in this jet is “attached” to aft spikes of the vibrator. The energy deposit into the discharge plasma and the effective area of energy interaction between the discharge and excited microwave field are estimated from the temperature and stagnation pressure distributions in the wake of the discharge.     

Breakdown conditions and thermal instability in air

Summary Reduced electric fields (E/N) responsible for electrical breakdown in air have been calculated by solving a stationary Boltzmann equation including superelastic vibrational collisions. The results show a decrease ofE/N with increasing gas temperature. The possibility of air instability due to chemical processes producing electrons is then investigated by calculating the threshold of this instability as a function of a characteristic time for heat dissipation τ.     

宽气压范围空气中微波击穿电场的计算公式

为了简便快捷地计算微波击穿电场,依据电子扩散模型的基本理论,结合气体放电的基本参量,应用特征扩散长度的概念,给出了适合于规则结构微波部件的击穿电场的计算方法。为避免各种气体参数的不确定性对计算准确度的影响,对等效直流电场与特征扩散长度之间的实验关系进行了拟合,并根据等效直流电场的定义,得出了一个适用于较高气压范围的击穿电场计算表达式。为了将该计算表达式扩展到更低的气压范围,综合考虑了电子扩散模型和基于二次电子发射现象的真空微放电机理,引入了一个合理形式的等效扩散长度,进一步给出了适合于更广气压范围的微波击穿电场的计算表达式,计算结果更符合A.D.Macdonald的实验结果。     

Breakdown in the transverse magnetic field

The breakdown between coaxial cylindrical electrodes in the homogeneous axial magnetic field in the pressure range around the Paschen minimum is studied. On the right of this minimum the breakdown voltage is not practically influenced by a weak magnetic field. On the left of this minimum the breakdown U-B curves can be divided into two branches: the upper ones can be approximated by the magnetron cut-off parabola, the lower ones correspond qualitatively in some cases to the second solution of the equation for breakdown in the inhomogeneous electric field corrected with respect to the losses of electrons caused by recapturing on the cathode.     

Electromagnetic vibrator: An initiator of air breakdown in the subcritical field of a quasi-optical microwave beam

The subject of investigation is the resonance properties of cylindrical electromagnetic vibrators with different diameters and spherically rounded ends acting as initiators of air breakdown in the subcritical field of a quasi-optical microwave beam. The vibrator is placed both in the field of a traveling electromagnetic wave and at the antinode of the field above a screen inserted in the microwave beam. The shortening of the half-wave vibrator that governs its ability to initiate air breakdown, the field at the top-pole of this vibrator, and a tolerance on its resonance length are found from experimental data.     

Phase-dependent electron-ion recombination in a microwave field

Using picosecond laser photoionization of Li in a microwave field we have observed phase-dependent reccombination of the photoelectrons with their parent Li+ ions. Recombination occurs at phases of the microwave field such that energy is removed from the photoelectron in the first microwave cycle after excitation, and there are two maxima in the recombination in each microwave cycle. These observations are consistent with observations made using an attosecond pulse train phase locked to an infrared pulse and with the "simpleman's" model, modified to account for the fact that the photoelectrons are produced in a Coulomb potential.     

Breakdown characteristics of air in the lower atmosphere

This study is devoted to an analysis of the ionization and loss of electrons at early stages of discharge development in air at an altitude of 0–12 km. Ionization in an external electric field and background ionization, electron attachment and detachment from atomic and molecular oxygen, charge exchange, and negative ion conversion are considered. The dependence of the process rate constants on pressure is taken into account. It is demonstrated that the electric field at which effective air ionization begins decreases with increasing altitude.     

Electrical discharge excited by the deeply undercritical field of a microwave beam in a high-speed jet of air and air-propane mixture

An electrical gas discharge initiated by a tubular linear electromagnetic vibrator is studied. The discharge is excited by the deeply undercritical linearly polarized field of a quasi-optical microwave beam. Experiments are conducted at a flow velocity of several hundreds of meters per second. The discharge region is photographed, and the stagnation temperature of the flow in the wake of the discharge is measured. It is shown that a deeply undercritical microwave discharge may arise in air even at such high flow velocities. Moreover, it is found that the discharge ignites and stabilizes the discharge region in a “lean” air-propane mixture. In such an experimental design, propane completely burns out when the flow velocity is smaller than some threshold value. When the flow velocity is high and the Mach number of the flow approaches unity, throttling effects and “thermal blocking” of the jet are observed.     

Breakdown magnetic field in an inductively coupled plasma

The Paschen curve shows that the breakdown voltage over a gap is a non-linear function of the product of the gas pressure and the gap distance for plasmas which are generated inside an electric field driven source. The Paschen breakdown relationship does not depend on the applied frequency and covers secondary emission at the electrodes of the discharge. Adapting the Paschen curve, a breakdown curve for a time-dependent magnetic field, typical for inductively coupled plasmas which sources are without electrodes, is examined here. It is shown that in this case the magnetic field breakdown curve does depend on the applied frequency.     

Spin-polarized current in a Rashba ring pumped by a microwave field

We report a theoretical study on generation of a spin polarized charge current with arbitrary spin polarization, including the fully-spin-polarized current. In a two-terminal mesoscopic ring device, the Rashba spin-orbit coupling (RSOC) is considered as well as a microwave field applied on one of arms of the ring. It is shown that at zero external bias a spin current can be produced in addition to the usual charge current pumped by the microwave field, which is attributed to the the quantum interference effect of the RSOC induced spin precession phase. By varying the system parameters such as the microwave frequency and the RSOC strength, not only the magnitude but also the direction of the spin current can be efficiently controlled, moreover, the spin-polarization degree of the charge current can readily be tuned by these system parameters in the range [-1,1]. Since all the parameters can be controlled electrically in our study, the proposed device may shed light on the possibility of an all-electrical generation and tuning of a spin-polarized current in the field of the spintronics.     

Quantum Poincare recurrences for a hydrogen atom in a microwave field

We study the time dependence of the ionization probability of Rydberg atoms driven by a microwave field, both in classical and in quantum mechanics. The quantum survival probability follows the classical one up to the Heisenberg time and then decays algebraically as P(t) approximately 1/t. This decay law derives from the exponentially long times required to escape from some region of the phase space, due to tunneling and localization effects. We also provide parameter values which should allow one to observe such decay in laboratory experiments.     

Resonant scattering of three-level Rydberg atoms in a microwave field

We examine the theory of potential scattering of Rydberg atoms in a microwave field. The model of a three-level atom is employed to calculate the radiative force emerging in the resonant coherent interaction with the microwave field for the case of a two-photon resonance and high intensities, using the method of quasienergies of the system consisting of the atom and the field. We determine the probabilities of Landau-Zener transitions in the spatial regions where under two-photon resonance conditions the quasienergies of the atoms approach one another by a small quantity. We also study the dynamics of the variation of the spatial profile of a beam of Rydberg atoms caused by resonant scattering. Finally, we give the results of the first experimental observation of the variation of the transverse beam profile when Rydberg atoms pass through a nonuniform microwave field formed in a rectangular waveguide and in resonance with the two-photon 36P–37P transition. Zh. éksp. Teor. Fiz. 111, 796–815 (March 1997)     

Collisions between linear polar molecules trapped in a microwave field

The collisions between linear polar molecules, trapped in a microwave field with circular polarization, are theoretically analyzed. We demonstrate that the collisional dynamics is mostly controlled by two ratios ν/B and x=μ₀E₀/ħ B (ν is the microwave frequency, B is the molecular rotational constant, μ₀ is the dipole moment, and E₀ is the electric field amplitude). We discuss the dependence of collision cross sections on these ratios in order to find an advantageous condition for evaporative cooling.     

Parametric excitation of a magnetic nanocontact by a microwave field

We demonstrate that magnetic oscillations of a current-biased magnetic nanocontact can be parametrically excited by a microwave field applied at twice the resonant frequency of the oscillation. The threshold microwave amplitude for the onset of the oscillation decreases with increasing bias current, and vanishes at the transition to the auto-oscillation regime. Theoretical analysis shows that measurements of parametric excitation provide quantitative information about the relaxation rate, the spin transfer efficiency, and the nonlinearity of the nanomagnetic system.