Time-Resolved Plasma Density Measurements in a Fluorescent Tube Using Microwave Interferometry by Coherent Microwave Scattering off Surfaces

We present the use of microwave interferometry by Coherent Microwave Scattering (CMS) off surfaces to measure path-averaged electron densities in a T12 fluorescent bulb. By monitoring the phase and attenuation of the microwaves via the CMS return signal, plasma parameters and their dynamics were sampled at a 1-kHz laser repetition rate. In sampling shot-to-shot at the laser repetition rate, plasma parameters and their oscillatory behavior were measured in real time. The time-averaged measured phase shift was 30.85◦, with 5◦ peak-to-peak oscillations at 120 Hz. The phase shift and attenuation yielded the real and imaginary parts of the refractive index of the plasma which were used to calculate the plasma and electron-neutral collisional frequencies. The measured time-averaged electron density was 3.8 × 10¹¹ cm⁻³ with peak-topeak oscillations of 0.45 × 10¹¹cm⁻³ at 120 Hz. The time-averaged electron-neutral collisional frequency was 9900 MHz with peak-to-peak oscillations of 2100 MHz. The electron densities measured correspond to plasma frequencies of 5500 ±300 MHz.