Electron-ion collision frequency in very cold plasma*

We have generated very cold plasma in carbon monoxide gas in a large-diameter cooled copper microwave cavity supported within a larger vacuum chamber. The cylindrical cavity is 57 cm in diameter and 61 cm tall and has a microwave resonance at 872 MHz. The cavity wall and the interior CO gas are cooled to 90 K by liquid nitrogen. Earlier experiments showed that CO is an effective electron cooling agent as a consequence of its dipole moment [Phys. Plasmas 17, 033508 (2010)]. With hot-filament discharge currents in the range 1 - 16 mA, densities are in the range 10$^{8}$ - 10$^{9}$ cm$^{-3}$ and electron temperatures are as low as 0.019 eV ($\sim $200 K). At room temperature, the Q of the cavity is reduced from 396 without plasma to 320 with plasma, which provides a measure of the electron-ion collision frequency. Experiments are continuing with cooled plasma to make the first measurements of the electron-ion collision frequency at these low temperatures.