Resonance Absorption In A Microwave Plasma Interaction

Plasma Parameters

Resonance absorption in an unmagnetized, inhomogeneous plasma occurs when a plane EM wave is obliquely incident with the wave electric field polarization in the plane of incidence ( P polarization). The work reported herein is concerned primarily with the interaction of low intensity microwave radiation with an inhomogeneous plasma.

At higher power levels (500 W), the growth of localized large amplitude electric fields, E 2, has been observed, near the critical layer together with the production of hot electrons, I hot, and quasistatic magnetic fields, B y. Figure 1 shows the typical oscilloscope traces for an incident RF power level of 500 W.

Figure 1. Time histories of RF pulse ( P0=500 W ), electric field intensity E2, hot electron current ( > 20 eV) Ihot, time derivative of quasisteady magnetic field By and quasisteady magnetic strength By.

Observation of wave convection saturation

As can be seen in Figure 1, the electric field initially begins to grow, then exhibits one or two inflection points or shoulders before rising to its peak value and subsequently decreasing to a significantly smaller value. We have identified convective saturation followed by cold plasma wave breaking, respectively, as begin responsible for the above temporal behavior of the electric field. In order to study the expected transition from pure convection saturation to wave-breaking saturation, detail studies in region 10 mW < P 0 < 100W has been performed. Figure 2. shows the saturated electric field intensity is plotted as a function of the RF input power.

Figure 5. Saturated electric field intensity as a function of incident rf power. "A" corresponds to the wave convection case and "B" to the wavebreaking case. SOlid lines are the theoretical results with fixed density gradient and dashed lines are those with measured local modification included

Observation of Ion wave steamers

When the microwave input power level exceeds 600W, a sharp transition to significantly more complicated behavior has been observed. Accompanying the growth and collapse of the electric fields we observe density disturbances ( streamers) which propagate up and down the density gradient and originate in the region of maximum field intensity. These are apparently produces by the enhanced fields where the radiation presure [Ez2(z=0)/8pi] approaches and exceeds the background plasma pressure. The properties of the streamers were measured with Langmuir probes located remotely ( 2 cm) from the enhanced fields. Figure 3 shows the amplitude of the steamer as a function of rf pulse length.

Figure 3. Ion streamer amplitude as a function of rf pulse width. Time history of the electric field intensity is superimposed. Inset shows typical streamer waveform

Observation of higher harmonics

For P0 > 50 W, the electric fields appear to be initially saturated by plasma wave convection. However, during the subsequent phase of the wave-plasma interaction, the density profile modification slows the plasma wave propagation and the field amplitude grows again until the wave breaks. Spatially resolved measurements of the plasma waves were made interferometrically by mixing a fixed reference probe signal with that from a movable probe. We find that the wave characteristics are quite different for the lower power an higher power cases. Specifically, at higher over levels we observe shorter wavelength components indicative of wave steepening and breaking. as the amplitude of the electron oscillations increase during the approach to wave breaking, strong harmonic content in their velocity and associated electric field spectra is anticipated. Figure 4 shows the time histories of the total electric field intensity together with that of the second and third harmonics observed.

Figure 4. Time histories of the fundamental, second and third harmonics of the electric field intensity

Observation of Magnetic field

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