Physics of Microwave Discharges

Artificially Ionized Regions in the Atmosphere

A Gurevich, N Borisov, G Milikh

• Publisher: CRC Press
• Year: 2018
• Print ISBN:
  9789056990084
• eBook ISBN:
  9781351424363
• Topic: Physical Sciences
• Subtopic: Environmental Science

A comprehensive and unique account of the creation of artificially ionized layers in the middle and upper atmosphere, using powerful radio waves. Major physical mechanisms associated with the formation of the ionized region are studied in detail. The main part of the author's research is devoted to problems associated with the breakdown mechanisms for radio frequency discharges in air. A special chapter deals with breakdown in intersecting pulsed beams and the effects of recombination, diffusion and atmospheric winds on the stability of the structure. The kinetics of the plasma produced are also described. The authors examine possibilities of inducing changes in the chemical composition of the upper atmosphere by means of radio frequence heating, with promising effects on the concentration of constituents such as ozone. The feasibility of using this phenomenon for; ozone healing - in connection with the ozone holes in the polar regions is investigated. The text is a timely treatment of key topics in the field of ionospheric modification.

CHAPTER I
OVERVIEW OF THE PROBLEM

1.1 Behavior of an Artificially Ionized Region (AIR) in the Atmosphere

The upper part of the Earth’s atmosphere, beginning at an altitude z ∼ 60 km, is ionized. The ionization results from the action of the Sun’s ultraviolet radiation. This creates a plasma layer surrounding the Earth, called the ionosphere. The electron and ion densities N in the ionosphere increase up to altitudes z ∼ 300 km, where they reach values N_max ∼ 10⁵ − 10⁶ cm⁻³; at higher altitudes N falls off slowly.

The ionosphere can reflect radio waves. This important property is widely used in radio communications at long, intermediate, and short wavelengths. The maximum radio frequency that can be reflected by the ionosphere is f_max ∼ 20–30 MHz; it is determined according to fmax∼N by the electron density at its maximum value in the ionosphere (Ginzburg, 1967). Radio waves with frequency f > f_max pass freely through the ionosphere without reflecting. It is therefore natural to ask about the possibility of raising the frequency f_max, i.e., whether the maximum electron density N_max in the ionosphere can be increased by means of additional ionization.

Bailey (1937) suggested that additional ionization of the ionosphere could be produced by using radio waves. For this it was proposed to use the gyroresonance, i.e., the resonance between the wave frequency and the natural gyration frequency of ionospheric electrons in the Earth’s magnetic field. The gyroresonance corresponds to the intermediate-wavelength range λ ∼ 200 m. Subsequent theoretical calculations (Ginzburg and Gurevich, 1960) and experiments (Bailey et al, 1952; Gurevich and Shlyuger, 1975) revealed that this technique requires an excessively high energy expenditure and cannot significantly increase the reflectivity of the ionosphere. Calculations of the effect on the lower ionosphere of radio waves of frequency f ∼ 50 MHz yielded similar results (Lombardini, 1965). The possibility of significantly increasing the maximum electron density N_max in the ionosphere by means of radio waves turns out to be unrealistic on account of the rapid increase in nonlinear absorption of the radio waves and the relatively rapid diffusion of electrons out of the disturbed region.

A different approach, pursued by A. V. Gurevich in the period 1972–1977, was to study the possibility of creating an artificially ionized layer in the atmosphere below the ionosphere, at an altitude between 30 and 60 km (Gurevich, 1980). It was proposed to create an ionized region via low-frequency breakdown of the air. Focused beams of radio waves would be used to initiate breakdown and to subsequently maintain the ionization. The ionization occurs in the region where the beams intersect. The breakdown is produced by a short radio pulse, and the ionization is maintained using either continuous or pulsed radiation. When it is maintained by pulsing the intersecting beams, a very thin plasma layer can be created, capable of reflecting radio waves with frequency f of up to 1–2 GHz, almost two orders of magnitude greater than the maximum frequency reflected by the natural ionosphere.

The purpose of the present treatise is to study the possibility of creating an artificially ionized region in the atmosphere at an altitude z ∼ 30–60 km and consider its possible applications. In this chapter we present a qualitative survey of the fundamental physical processes associated with the production of an ionized region.

1.1.1 Pulsed breakdown of the atmosphere

Under the action of a strong variable rf electric field, electrons acquire considerable energy and become able to ionize neutral air molecules through collisions. When enough ionization has taken place, breakdown occurs. The electron density N grows exponentially as a function of time, N ∼ exp(ν_ii). Here ν_i is the ionization rate, i.e., the number of ionization events produced on the average by a single electron per unit time. Attachment, recombination, and diffusion, which reduce the number of electrons, are unimportant for sufficiently large ν_i.

The ionization rate ν_i increases rapidly as a function of the ratio E₀/E_thr. Here E₀ is the amplitude of the variable electric field and E_thr is the critical field for breakdown in air. The latter depends on the wave frequency ω and the density N_m of air molecules:

Ethr=28.2C(νc/ω)(Nm2.7×1019cm−3)(1+ω2ν2)1/2kVcm−1,νc=1.7(Nm/107cm3)s−1.⁢(1.1)

Here ν_c is the characteristic electron collision frequency. In Eq. (1.1) the field Ethr is in kV/cm, N_m is in cm⁻¹ (at a pressure p = 760 torr and temperature T ≃ 300 K we have N_m = 2.7 × 10¹⁹ cm⁻³). The coefficient C(ν/ω) is of order unity; it will be discussed below in Sec. 2.1. For E₀ = E_thr the ionization rate is comparable with the electron attachment rate, and breakdown develops in a steady oscillatory field. Pulsed breakdown, however, must persist for a finite time, the pulse length τ_p. Consequently, it occurs only in fields E₀ > E_thr, where the magnitude of the threshold field E_p for pulse breakdown depends on the pulse length τ_p: the shorter the pulse, the larger E_p.

Fig. 1.1 Amplitude of the critical breakdown field as a function of altitude: 1) ω = 10¹⁰ s; 2) ω = 10⁹ s.

Fig. 1.2 Frequency f of ionizing radiation and the maximum frequency f_m reflected from an AIR as functions of altitude.

We can determine optimum conditions for ionization in connection with pulsed breakdown, when the largest fraction of the energy of the high-frequency pulse goes into the formation of electron–ion pairs. These conditions take the form

E0=Eop≈4−6Ethr,ω≈νc.⁢(1.2)

Hence it is clear that the frequency and power of the variable electric field at which ionization of the air is optimum depend only on the density N_m of the neutral molecules, and therefore (in the Earth’s atmosphere) on the height z above the Earth’s surface, since N_m = N_m(z). Figure 1.1, which shows the critical breakdown field as a function of altitude, serves to illustrate this, while Fig. 1.2 shows the frequency f ≃ ν_c/2π of the ionizing wave as a function of altitude. The atmospheric model of Blank et al. (1974) was used in the calculations.

1.1.2 Maintaining the ionization

The ionization can be maintained either by periodically repeating the breakdown using short pulses or by continuous heating of the plasma.

In response to a strong pulse, electrons acquire considerable energy, and their velocity distribution function departs greatly from its equilibrium (Maxwellian) form. The electron distribution rapidl...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Contents
• Preface
• List of Principal Symbols
• 1. Overview of the Problem
• 2. Pulsed Rf Breakdown in Air
• 3. Maintaining the Ionization
• 4. Structure of an Ionized Layer
• 5. Air Applications
• 6. Artificial Emission of Ionized Regions
• 7. Rf Effects on Air Chemistry
• Appendix: Ionization and Instability in illtra-Strong Fields
• Bibliography
• Index