Experiments and analysis of wave propagation, absorption and backscattering in plasmas

The authors present measurements and analysis of the absorption, reflection and scattering of electromagnetic waves due to electron cyclotron absorption zones and the presence of scatterers in a plasma. The 12 cm diameter by 2 m laboratory plasma is produced by a 2.45 GHz microwave source. The plasma density that can be reached is in the range of 5 x 109 cm-3 15 x 1011 cm-3 with electron temperatures in the 3--6 eV range and 0.5--1.5 kG magnetic fields. Wideband wave absorption of 30--50 dB due to the electron cyclotron resonance zone in the 1.5--3.0 GHz range is measured. They also measure the strong wave damping rate, ki, near cyclotron resonance and compare it with WKB predictions. Measurements using a modulated homodyne detection system are made that are sensitive to the backscatter from a movable probe in the plasma. This system measures a reduction in backscatter signal produced by an intervening cyclotron resonance zone between the transmitter and the encapsulated modulated scatterer. The phase sensitive system is used to measure the local k-spectrum in the inhomogeneous plasma and is compared with WKB theory. Experiments which examine the frequency sensitivity of launched ion acoustic modes from the modulated PIN diode dipole scatterer will also be presented. The authors also present studies of TMAE (tetrakis-dimethylamino-ethylene) gas plasma formation by ultraviolet photoionization. Both ultraviolet flashtube and 193 nm, 10 ns, 10 mJ excimer laser pulses are used to create plasmas from TMAE gas. The density and temperature of these transient plasmas is measured using Langmuir probe techniques. These basic studies are carried out to determine the best methods for plasma creation for subsequent microwave propagation and attenuation measurements through the plasma thus formed