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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.
In this work, studies with respect to the exhaust problem were performed
in the stellarator experiment Wendelstein 7-X with different target concepts and different magnetic field geometries. Different infrared cameras were used to study the heat flux from the plasma onto the PFC. In the first publication, the limiter set-up was used with a simpler magnetic topology in the plasma edge. The radial fall-off of the parallel heat flux for inboard limiters in W7-X shows, similar to inboard limiters in tokamaks, two different radial fall-off lengths, a short (narrow) one, characterizing the near-SOL, and a long (broad) characterizing the far-SOL. For the far-SOL, the heating power and connection length have been identified as the main scaling parameters, while for the near-SOL, the electron temperature close to the LCFS has been identified as the main scaling parameter. The two fall-off lengths differ by a factor 10, and the found scalings for both regimes differ from known models and experimental scalings in tokamaks. A turbulent-driven feature was discussed in the publication as a possible explanation for the behavior of the fall-off length in W7-X.
The gained information and data have been further used to support many
other publications, covering the symmetry of the heat loads, the
energy balance of the machine, and seeding experiments.
The heat exhaust in W7-X with an island divertor was studied in the second
and third publication. Definitions of parameters such as peaking factor and
wetted area were applied for the heterogeneous heat flux pattern on the
W7-X divertor. It was shown that the island divertor concept is capable
of spreading out the heat efficiently, resulting in large wetted areas of up to 1.5 m2. The reached values for the wetted area are comparable to the ones of the larger tokamak JET but with a much smaller ratio of wetted
area to the area of the last closed flux surface. Furthermore, a positive
scaling of the wetted area with the power in the SOL was observed. This
scaling is beneficial for future reactors but needs further investigation of the involved transport processes. The peaking factor (discussed in the second publication) describes how concentrated the heat load is within the region of the strike line. It was shown that this factor is decreasing for increasing densities without affecting the wetted area. The present work paves the way for further analysis of the transport processes of the heat flux towards the island divertor of Wendelstein 7-X.