The Stark effect of the Hydrogen Balmer-lines as a versatile tool for plasma diagnostics

The Stark effect offers the possibility to use the shift and the broadening of spectral lines to determine the strength of the electric field surrounding the radiating atoms. Depending on the plasma investigated the electric field may arise from the moving charged particles of the plasma itself or from the applied voltage of a power supply used to drive the discharge creating the plasma. In either case the Stark effect will be useful to deduce the plasma parameters desired. In the talk the results of experiments carried out in our research group at Kiel University will be discussed. Various plasma sources have been used for the investigations that cover a wide range of electron densities. In a second part electric field strength results will be communicated that were obtained in the cathode fall of a low density gas discharge. Plasma sources: Electron densities spanning a range of more than ten orders of magnitude were obtained in essentially five different gas discharges. Starting at low densities the Stark effect of the fine structure of Hydrogen was investigated in low pressure gas discharges. Intermediate densities could be obtained in hollow cathode arcs at low pressure. With constricted arcs at low and high pressure we were able to span the the region between several times 1014 to 1017 cm-3. The highest electron densities were observed in laser produced plasmas and capillary discharges. Calibration: The shift and the broadening of the Balmer lines due to Stark effect can be used as diagnostic tool for determining the electron density of plasma with high accuracy. In order to do that you have either to rely on theoretical calculations of the Stark effect or you have to come up with a reliable and independent experimental way to calibrate your spectra. As experimentalists we were very much concerned with the second approach. However, it turns out that the different regions of the electron density require different experimental methods to measure the plasma density with high accuracy. For low temperature and low density plasmas Langmuir probe measurements are widely used in the community. However, noninvasive methods are desirable instead. For low densities the detuning of a microwave cavity by the plasma served as independent electron probe. For the intermediate range two-wavelength laser interferometry proved to give accurate results. At the very highest densities only relative measurements of the shift and the broadening of the respective lines have been performed so far. Electric fields: Investigations of the electric field strength in the cathode fall of a low pressure gas discharge were preformed using opto-galvanic spectroscopy. The sensitivity to the field is limited at the present by the Doppler-broadening of the lines. Doppler-free measurements would increase the sensitivity by almost two orders of magnitude. (author)