Thomson scattering diagnostics and computational modeling of a low pressure microwave excited microplasma source

Summary form only given. Low-pressure microwave-excited microplasmas¹ are promising sources of VUV photons for a variety of applications, including photoionization for mass spectrometry. A split-ring resonator microstrip architecture can be used to initiate and sustain these microplasmas using the extremely high electric field generated in the sub-millimeter gap between electrodes. The VUV flux, primarily the result of resonance radiation following electronic excitation of rare gas atoms, is a sensitive function of the distribution of electron energies. Direct measurement of the electron energy distribution (EED) could provide critical insight into the physics of the microplasma operation. We present Thomson scattering measurements of the EED in a split-ring resonator argon microplasma operating at 2.5 GHz and approximately 1 Torr. The diagnostic consists of a Q-switched Nd:YAG laser operating at 532 nm. Thomsonscattered light is collected with a high throughput (f/2) triple grating imaging spectrometer and an intensified CCD camera gated to the laser pulses. Stray and Rayleigh-scattered laser light, which can exceed the intensity of the Thomsonscattered light by factors of over 105, is filtered out using a mask placed between the first two gratings, which are operated in subtractive mode. Other available diagnostics include VUV flux measured using a vacuum UV monochromator. Plasma parameters are measured as a function of gas flow rate, absorbed microwave power, and spatial location both within the plasma cavity and in the downstream plume. Experimental results of the EED and VUV flux are compared with computational modeling using the Hybrid Plasma Equipment Model (HPEM).² In this model, the EED and radiation transport are computed using Monte Carlo simulations, and neutral gas and plasma transport are addressed using fluid techniques. These experimental and computational modeling results provide a means for optimizing the VUV flux produced by the source.