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Asymmetrical capacitively coupled RF discharges in oxygen, argon and hydrogen have been experimentally investigated with the innovative technique of the phase resolved optical emission spectroscopy. This diagnostic tool allows to measure spatio-temporally resolved emission intensities of electronically excited species with a high resolution. The spatial (axial) resolution was better than 1 mm and a temporal resolution of about 1.5 ns has been achieved. Therefore the plasma induced optical emission within the RF cycle (TRF = 73.75 ns) from the RF sheath region with a typical mean sheath thickness of about 5mm has been studied. Spatio-temporally resolved optical emission patterns of the following optical transitions have been measured for a total gas pressure in the range of 20 to 100 Pa and self-bias voltages between -50 and -550 V: Oxygen plasma Emission at 777.4 nm and 844.6 nm (atomic oxygen) Argon plasma Emission at about 751 nm and 841 nm (argon) Hydrogen plasma Emission at 656.3nm (atomic hydrogen, H alpha-line) These transitions are the most prominent ones of the investigated excited species in these plasmas as could be shown from overview spectra of the plasma induced optical emission in the range from 350 to 850 nm. For the first time such extensive PROES measurements in oxygen CCRF plasmas are presented in this work. The additional investigations of argon and hydrogen plasmas serve as a reference and for a direct comparison with results from the literature. The temporal behavior of the emission intensity is influenced by the effective lifetime of the emitting states which is on the order of the nanosecond time scale of the RF cycle. Therefore, it does not represent the real temporal behavior of the excitation. A simple method has been applied to calculate relative excitation rates from the measured emission intensities to distinguish different excitation mechanisms and their correct relative temporal behavior. In a close collaboration within the framework of the Sonderforschungsbereich Transregio 24 'Fundamentals of Complex Plasmas' a newly 1d3v PIC-MCC code for simulations of capacitive RF discharges in oxygen has been developed by Matyash et al. The very close coupling of experiment and modeling allowed a really detailed and microscopic understanding of the processes and dynamics from the sheath to the bulk plasma in CCRF discharges. The spatio-temporally resolved excitation rate profiles show four different excitation structures (I-IV). Excitation processes due to the following mechanisms in CCPs could be identified and characterized: I Electrons expelled from growing sheath II Electrons detached from negative ions (collisions with neutrals) + secondary electrons from the electrode surface (ion bombardment) III Field-reversal effect, reduced mobility of electrons (electron-neutral collisions) IV Heavy-particle collisions These excitation mechanisms are characterized by different temporal and spatial behaviors of the excitation rate within the RF cycle. Additionally it has been shown that the excitation by electron impact in the investigated oxygen plasmas results mainly from dissociative electron impact excitation (O2 + e -> O + O* + e) and not from direct electron impact excitation (O + e -> O* + e). Actinometry measurements show that the results are not really credible. Thus actinometry is not applicable on the investigated oxygen RF plasma. A challenge in interpretation is the observed excitation pattern IV. Pattern IV has to be caused in connection with heavy particle collisions nearby the electrode surface and could be observed in all the three plasmas oxygen, argon and hydrogen. It is located directly in front of the powered electrode and appears during almost the whole RF cycle. The temporal modulation is nearly sinusoidal and weak in comparison to the first three patterns. This is due to the weak RF modulation of the ion flux towards the electrode surface which has been proven by a PIC simulation. It could be shown that the modulation degree of pattern IV depends on the transition time of the corresponding positive ions through the RF sheath which is influenced by the ion mass. In oxygen as well as in argon CCRF plasmas pattern IV is less modulated than in hydrogen CCRF plasmas due to the heavier ions in oxygen and argon. Additionally the modulation degree increases with increasing pressure due to the more confined plasma at higher pressures which is yielding in a stronger modulated ion current towards the powered electrode.
Beams of ions and electrons are a source of free energy which can be transferred to waves via an instability. Beams exist in almost all plasma environments, but their instabilities are particularly important for the dynamics of space plasmas. In the absence of collisions, the instability drives waves to large amplitudes and forms nonlinear structures such as solitary waves. The electric fields in these waves can scatter particles in the background plasma, or disrupt currents. Both of these effects are important for the overall dynamics of the plasma. In this thesis, both electron and ion beam plasma instabilities have been investigated in the linear plasma device VINETA and using a Particle-in-Cell simulation. The electron beam instability has been demonstrated by previous authors to be a useful diagnostic for the plasma density. The spatial resolution of previous results was confirmed at a few millimetres, and a temporal resolution of 1ms was shown for the first time. An ion beam was generated with a double plasma discharge. Compared to space, this environment and indeed most laboratory plasmas have considerably higher collisionality and a limited spatial extent which introduces gradients in the plasma. Gradients perpendicular to the beam propagation direction are linked to a decrease of both the wavelength and amplitude of the instability. It was observed in both experiment and simulation that gradients in sheaths at the boundaries of the plasma not only affect the time averaged plasma parameters, but also excite instabilities. Fluctuations within the sheath spread the beam in velocity space, effectively increasing its temperature. Warmer beams require a higher drift velocity to excite an instability. This was also confirmed by experimental and numerical results. Collisions are shown to be the dominant damping force for the electron beam instability. For ions, collisions play an important role in the simulation, but appear to be overshadowed by Landau damping from impurities in the experiment. When boundary conditions are removed from the simulation, wave amplitudes increase and nonlinear effects become important. Saturation by particle trapping and coalescence of phase space holes is observed, which could eventually lead to the solitary waves as they are observed in space plasmas.
Electromagnetic Drift Waves
(2010)
In the rf-plasma of the linear magnetized VINETA experiment, different types of low-frequency waves are observed. The emphasis in this work is on the interaction mechanism between drift waves on the one and kinetic Alfven waves on the other hand. In the peaked density profile of the plasma column drift waves occur as modulation of the plasma density. As gradient driven instability, they draw their energy from the radial density gradients. Alfven waves as magnetic field fluctuations are stable in the present configuration. They are launched by a magnetic excitation antenna. Parallel conduction currents in the plasma are common to both wave phenoma. A B-dot probe as standard diagnostic tool is used to detect the fluctuating magnetic fields of both wave types. The challenge are the small induced voltages due to the low wave frequency. The probe design with an integrated amplifier close to the probe head takes this into acount. The developed B-dot probe is mounted to different positioning systems to characterize both wave phenomena. For Alfven waves, the dispersion relation is recorded experimentally. It is found to be in good agreement with the prediction of the Hall-MHD theory with included resistive term, accounting for the cold collisional plasma. The fluctuating magnetic field pattern is recorded with azimuthal scans. The current density is obained by Amperes law. It is concentrated in helically twisted current filaments. For the unstable drift waves, similar investigations are done with simultaneously recorded density fluctuations. In the azimuthal plane, the locations of the parallel current filaments and the fluctuating density are found to be in phase, supporting the predicted drive of parallel currents by pressure gradients. A mutual influence of the two wave types is observed in an interaction experiment. Assuming parallel currents as coupling quantity, an interpretation of the experimental findings is given based on the linear theory of drift waves.