Session PR2: Capacitively Coupled Plasmas II

The role of the singlet metastables in capacitively coupled oxygen discharges

The roles of the singlet metastable molecules O$_2$(a$^1\Delta_{\rm g}$) and O$_2$(b$^1\Sigma_{\rm g}$) in a capacitively coupled rf driven oxygen discharge at 50 mTorr are explored using the one-dimensional object-oriented PIC/MCC code oopd1. Earlier we have demonstrated that the metastable molecule O$_2$(a$^1\Delta_{\rm g}$) has a significant influence on the discharge properties such as the electronegativity, the effective electron temperature and the electron heating processes [1]. A recent global model study indicates that the density of O$_2$(b$^1\Sigma_{\rm g}$) state can be higher than the density of the O$_2$(a$^1\Delta_{\rm g}$) state [2]. Thus the oxygen discharge model now includes the O$_2$(b$^1\Sigma_{\rm g}$) molecule and related reactions. The singlet metastable states of the oxygen molecule have significant influence on the discharge properties. Electron heating is only observed in the sheath region and the electron energy probability function becomes even more concave or bi-Maxwellian when the O$_2$(b$^1\Sigma_{\rm g}$) state is included in the simulation. The center electronegativity is in the range of 0.67 - 1.9. \\[4pt] [1] J T Gudmundsson and M A Lieberman, Plasma Sources Sci. Technol. 24 (2015) 035016\\[0pt] [2] D A Toneli et al., J. Phys. D. accepted 2015   

Experimental observation of multi-layer excitation structure in capacitively coupled SF6 plasmas

Electron excitation dynamics in capacitively coupled SF6 plasmas driven at 9 MHz $\sim$ 16 MHz are studied by using phase resolved optical emission spectroscopy (PROES) of trace rare gas. Multi-layer excitation structure inside the bulk plasma of capacitive discharges operating in SF6 is experimentally observed for the first time. Experimental results show that with the decrease of the rf power and/or the increase of the pressure, the multi-layer excitation structure becomes noticeable while the gap between two adjacent layers is almost kept constant. By increasing the driving frequency with a constant electrode gap, however, the number of layers increases while the layer gap decreases. The layer structure disappears at the driving frequency larger than 16 MHz. The electrode gap is found to have a negligible effect on the gap between two adjacent excitation layers, nevertheless only the number of excitation layers is increased when enlarging the electrode gap. The multi-layer formation may be due to a large modulation of the F- negative-ion density throughout the bulk plasma, and is more pronounced at intermediate and low frequencies, since F- negative ions do not respond to the time-varying electric field at high frequencies (\textgreater 16 MHz).   

Harmonic generation of microwave frequencies in plasmas

The ability of RF plasma discharges to generate harmonics of the source frequency was reported as early as 1950s by Margenau and Hartmann [1]. Experiments by Krenz and Kino [2] measured up to seventh harmonic in a spherical discharge with considerable efficiencies. Since the Ions and heavy species are usually slow in responding to a high-frequency signal it is the dynamics of electrons that determine the overall discharge characteristics. The equation of motion that governs the electron dynamics in a plasma discharge and the relation between the current and electron velocity is non-linear. These two factors lead to creation of harmonics of the input frequency. In this work we study the physics of harmonic generation in a microdischarge using a computational model. The model is based on fluid description of plasma. A discharge which is electrostatically excited by a microwave frequency source is simulated and its response is measured. The model is used to provide insights into the non-linear process in the plasma that leads to the creation of higher order harmonics. Microdischarges and large-size discharges are analyzed for their ability to produce harmonics. \\[4pt] [1] H. Margenau and L.M. Hartman, ``Theory of high frequency gas discharges. II - Harmonic components of the distribution function.'' Phys. Rev. 73, 309 (1948).\\[0pt] [2] J.H. Krenz and G.S. Kino, ``Harmonic Generation and Parametric Oscillations in Plasma,'' Journal of Applied Physics 36, 8 (1965)   

Generation of anomalously energetic suprathermal electrons by an electron beam interacting with a nonuniform plasma

Generation of anomalously energetic suprathermal electrons was observed in simulation of a high-voltage dc discharge with electron emission from the cathode. An electron beam produced by the emission interacts with the nonuniform plasma in the discharge via a two-stream instability. Efficient energy transfer from the beam to the plasma electrons is ensured by the plasma nonuniformity. The electron beam excites plasma waves whose wavelength and phase speed gradually decrease towards anode. The short waves near the anode accelerate plasma bulk electrons to suprathermal energies. The sheath near the anode reflects some of the accelerated electrons back into the plasma. These electrons travel through the plasma, reflect near the cathode, and enter the accelerating area again but with a higher energy than before. Such particles are accelerated to energies much higher than after the first acceleration. This mechanism plays a role in explaining earlier experimental observations of energetic suprathermal electrons in similar discharges.