Laser and Radiofrequency Air Plasma Sources

Abstract

Research on 193 nm laser initiation and rf sustainment of seed gases and air plasmas has been carried out. We have obtained 10 14 /cm 3 , T e = 0.3 eV laser plasmas of 500 cm 3 volumes using an organic gas which has been seeded (8-60 mTorr) in nitrogen at pressures up to atmospheric conditions. Fast Langmuir probe measurements of density and temperature have been carried out utilizing a special shielded probe and boxcar sampling techniques. Photomultiplier measurements of optical emission have shown increased emission with nitrogen pressure which indicates strong coupling and longer lifetimes (r = lOfis) of the absorbed laser flux to the dominant nitrogen species. We have observed and analyzed a new delayed ionization process of a superexcited seed gas state which interacts with the nitrogen background resulting in longer plasma lifetimes. We have also carried out seed argon measurements of radiofrequency power absorption from 1-100 mTorr yielding densities of 2 X 10 13 /cm 3 and temperatures of 3 eV with 8500 cm 3 volumes in a magnetic field of 1.4 kG. The antenna and plasma loading characteristics have been compared with our MAXEB 2-D simulation model and are in very good agreement. We have since carried out experiments in a nitrogen/argon mix at 1-2 Torr pressures and have achieved plasma densities of 5 x 10 12 /cm 3 at 1 eV with 7000 cm 3 volumes at low 0.03 W/cm 3 radiofrequency power levels. The experimental antenna and matching circuit measurements have been compared to our ANTENA-II simulation code and are in good agreement. We have carried out optical emission measurements on these rf-produced plasmas to determine the lifetime under different gas mix conditions to minimize radiofrequency power requirements. We have also developed an analysis and carried out simulations of highly collisional plasmas created by a radiofrequency source and obtained plasma loading resistances identical to that observed by Eckert and Kelly in atmospheric air at 760 Torr. These models have been used to design and scale our antenna and matching system for our laser initiated seed gas atmospheric air radiofrequency plasma sustainment experiment which has been carried out. We have carried out laser initiation and radiofrequency sustainment in a 5 cm diameter chamber with seed gases and nitrogen at up to 10 Torr pressures. Antenna and matching optimization have provided sustained densities of 10 12 /cm 3 which could not be produced without the laser initiation. 010 DTIC QUALITY INSPECTED 3REPORT DOCUMENTATION. PAGE Public reporting burden for this collection of information is estimated to average 1 hour per responsi gathering and maintaining the data needed, and completing and reviewing tta collection of inform« collection of information, including suggestions for reducing this burden, to Washington Headquarte Davis Highway. Suite 1204. Arlington, VA 22202-4302. and to the Office of Management and Budj