Microwave Interferometry Techniques Research Articles

Doppler-free, Stark broadened profiles at low plasma densities in helium

This work utilizes Doppler-free saturation spectroscopy to measure Doppler-free, Stark broadened spectral profiles for the π-polarization of the HeI 21P → 61D transition in a low density (ne=7×1010−2×1012 cm−3) helium plasma. The measurements were performed in an electron-cyclotron resonance discharge at 23 mTorr with electron density being diagnosed using a combination of Langmuir probe and microwave interferometry techniques. The Doppler-free profiles were observed to be nearly symmetric at ne < 1011 cm−3 but markedly asymmetric above this transition point. Electron density is extracted from the spectral data via fitting to a spectral model based on quasi-static Stark broadening. The fit results are compared to ne measurements obtained using a combination of Langmuir probe and microwave interferometry techniques. The fit and measured ne are shown to agree within 20% on average. Finally, the quasi-static model is shown to be valid in the low-density regime for 21P → 61D helium transition.

Electro-optic modulation of a laser at microwave frequencies for interferometric purposes.

A multi-point microwave interferometer (MPMI) concept was previously proposed by the authors for spatially-resolved, non-invasive tracking of a shock, reaction, or detonation front in energetic media [P. Specht et al., AIP Conf. Proc. 1793, 160010 (2017).]. The advantage of the MPMI concept over current microwave interferometry techniques is its detection of Doppler shifted microwave signals through electro-optic (EO) modulation of a laser. Since EO modulation preserves spatial variations in the Doppler shift, collecting the EO modulated laser light into a fiber array for recording with an optical heterodyne interferometer yields spatially-resolved velocity information. This work demonstrates the underlying physical principle of the MPMI diagnostic: the monitoring of a microwave signal with nanosecond temporal resolution using an optical heterodyne interferometer. For this purpose, the MPMI concept was simplified to a single-point construction using two tunable 1550 nm lasers and a 35.2 GHz microwave source. A (110) ZnTe crystal imparted the microwave frequency onto a laser, which was combined with a reference laser for determination of the microwave frequency in an optical heterodyne interferometer. A single, characteristic frequency associated with the microwave source was identified in all experiments, providing a means to monitor a microwave signal on nanosecond time scales. Lastly, areas for improving the frequency resolution of this technique are discussed, focusing on increasing the phase-modulated signal strength.

Microwave Diagnostic Results from the Gaseous Electronics Conference RF Reference Cell.

Electron density measurements with even the simplest microwave interferometry techniques can range over three to four orders of magnitude, can be responsive on time scales as fast as 50 ns, and are simple to obtain and interpret. Three groups have published electron density data taken in the Gaseous Electronics Conference (GEC) reference reactor using microwave interferometry. The agreement in the data from these groups at higher pressures is excellent especially when one considers that the GEC reactors involved have some key differences. These may have been the cause of some differences between the results obtained at low pressures, although, the manner in which the measurements were interpreted may also have contributed. The electron densities compare favorably in argon, helium, and nitrogen above 33.3 Pa (250 mTorr); but, the measurements tend to diverge some at 13.3 Pa (100 mTorr) and in 133 Pa helium above approximately 200 mA. It is speculated that the latter difference occurs as the discharges change from a bulk ionization or α-mode to a secondary electron emission or γ-mode, and that this transition occurs at lower voltages and currents for reactors with aluminum electrodes than it does for those with stainless steel electrodes. In addition, time resolved electron densities are presented. There is agreement between time resolved measurements in the two reactors, in particular, the electron density in helium discharges is found to rise dramatically after the rf excitation is turned off while the electron densities in argon and nitrogen glows exhibit only slight increases.

Determination of pinch plasma engine exhaust profiles by microwave interferometry techniques

Determination of pinch plasma engine exhaust profiles by microwave interferometry techniques