An electron cyclotron resonant, microwave resonant cavity lithium plasma source.

Abstract

A unique lithium plasma source featuring a microwave resonant cavity with an electron cyclotron resonant region has been constructed for use in laser-enhanced isotope separation experiments. Lithium-argon discharges have been created in the plasma source and properties of those discharges have been extensively studied. The discharge is maintained within a quartz cup at the base of a microwave resonant cavity operating at 2.45 GHz. The electron cyclotron resonant region is created by eight SmCo rare-earth magnets arranged in a multicusp configuration. Lithium is introduced to the system in the form of lithium chloride, a solid placed directly in the discharge region, which is then heated by a background argon discharge allowing dissociation of lithium. The dissociation is evidenced by the observation of strong Li-I lines in the discharge using optical emission spectroscopy. This unique method of creating a lithium plasma has the advantage of greater control of contaminants and longer source lifetime because the device does not contain an electrode. Additionally, the discharge can be maintained at pressures as low as 1 mTorr and input microwave powers of 100 Watts. A number of diagnostics have been used to characterize the discharge. Double Langmuir probe studies indicate ion densities of 10$\sp{10}$-10$\sp{11}$ cm$\sp{-3}$ and electron temperatures of about 3 eV with operating pressures of 4-20 mTorr and input powers of 100-250 Watts. Optical emission spectroscopy indicates that the lithium concentration increases with decreasing argon flow and partial pressure. Single Langmuir probe measurements of the electron energy distribution function indicate that the electrons have a Druyvesteyn distribution at low operating pressures. Plasma density was found to decrease exponentially with downstream distance from the source. Peak saturation current densities of 1.2 mA/cm$\sp2$ were measured with a 7.5 cm diameter, negatively-biased, aluminum collector. A two-dimensional CCD array has been used to image the ECR region, providing a real-time diagnostic of the plasma emission within a wavelength region defined by low-bandpass filters. This newly-developed technique has been used for electromagnetic and uniformity of heating studies of the device.