A two-dimensional ͑2-D͒, finite-difference computer code is developed to examine helicon antenna coupling, wave propagation, collisionless Landau, and collisional heating mechanisms. The code calculates the electromagnetic wave fields and power absorption in an inhomogeneous, cold, collisional plasma. The current distribution of the launching antenna, which provides the full antenna spectra, is included in the model. An iterative solution that incorporates warm plasma thermal effects has been added to the code to examine the contribution of collisionless ͑Landau͒ wave absorption by electrons. Detailed studies of the wave fields and electron heating profiles at low magnetic fields (B 0 Ͻ100 G), where both Trivelpiece-Gould ͑TG͒ and helicon ͑H͒ modes are present, are discussed. The effects of the applied uniform magnetic field (B 0 ϭ10-1000 G), 2-D (r,z) density profiles (n e0 ϭ10 11 -10 13 cm Ϫ3 ), neutral gas pressures of 1-10 mTorr and the antenna spectrum on collisional and collisionless wave field solutions and power absorption are investigated. Cases in which the primarily electrostatic ͑TG͒ surface wave dominates the heating and the power is absorbed near the edge region and cases in which the propagating helicon wave transports and deposits its energy in the core plasma region are examined.

Abstract-A computer code for modeling existing and new helicon sources for materials processing has been developed. The Nagoya type-111, helical, and Stix coil antennas have been modeled to study and examine plasma density and temperature profile effects on power absorption of a small fraction ( n f e / n e N 5%) of fast electrons (T,+,* N 40 eV) which provide ionization of the neutral gas in the experiment, and bulk (T,,1,, N 3 eV) electron distributions in an argon gas. The "ANTENA" computer code, originally written by B. McVey to study ion cyclotron waves, was modified and used to study and model helicon sources. A collisional model that includes radial density and temperature profiles was added to the code to study the effect of collisions on the heating mechanisms. The competing effects of collisional and Landau damping heating mechanisms have been investigated in detail, and results indicate that collisions play an important role in the plasma absorption profile at high densities (ne 2 1013 emp3).The radio frequency wave absorption profiles are sensitive to the plasma density and temperature profiles. The partial-turn helix antenna, that solely excites the m = +1 azimuthal mode, is found to be more efficient in coupling the power to an assumed plasma profile than the Nagoya type-111. The Stix coil is also found to be promising due to its on-axis peaking of the wave heating fields.

Wave, antenna impedance, plasma density, and temperature anisotropy measurements are carried out for a helicon plasma source in nonuniform and uniform static magnetic fields. Strong axial density gradients associated with the nonuniform magnetic fields are observed to affect wave fields, absorption, and source efficiency. The wave field and antenna input impedance measurements are compared with a new simulation code which also calculates Poynting power flow and wave absorption profiles. Wave amplitude measurements are shown to decay more rapidly and the phase velocity varies over a wide range for the nonuniform static magnetic field case.

Abstract-A two-dimensional (r; z) computer code is utilized to examine wave propagation, collisionless Landau, and collisional heating mechanisms in a helicon source operation. neutral pressure ranging from 1 to 10 mtorr which corresponds to -at an electron temperature of 3 eV. Fig. 1 illustrates that at a low magnetic field ( G) and an Ar pressure of 1 mtorr, the absorbed collisional power is localized near the antenna and the coupling is dominated by the TG mode which propagates near the outer plasma edge region. The arrows describe the direction and magnitude of the Poynting vector. The contour plots illustrate the spatial profiles of the absorbed power. Fig. 2 shows that at higher magnetic fields ( G) the coupling is dominated by the helicon mode which transports and deposits the coupled power well away from the antenna region. The TG mode is strongly damped and thus deposits some of the power near the edge of the plasma. The ANTENA2 [6] code is used to illustrate the existence of both modes since MAXEB solves for the total wave field solution and cannot separate the two modes. The contribution of the TG mode to the total absorbed collisional power is significant and dominates the heating process near the surface of the plasma column. In contrast, the H mode carries and deposits the power through collisional damping away from the antenna region. IndexStudies using the new, warm plasma, term of the dielectric tensor show that collisional damping (by electronneutral collisions) is the dominant heating mechanism for gas pressures greater than 2 mtorr and higher densities ( 10 cm ). Landau damping on the 3 eV Maxwellian background at a magnetic field G and gas pressure of 5 mtorr is found to be negligible and the power is absorbed entirely 0093-3813/99$10.00

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