In this work, Doppler-free two photon optogalvanic spectroscopy is used to measure the electric field strength in the cathode fall region of a hollow cathode discharge, operated in pure hydrogen, via the Stark splitting of the 2S level of atomic hydrogen. The cathode fall characteristics are analysed for various pressures and in a wide range of discharge currents. Tungsten is used as the cathode material, because it allows for reliable measurements in a fairly wide range of discharge conditions and because of its minimal sputtering. Two cathode diameters (10 mm and 15 mm) are used to study the dependence of the cathode fall on discharge geometry. The measurements reveal that the cathode fall characteristics are quite independent on the cathode diameter for equal cathode current density; hence the measurements can be used to test one dimensional modelling of the cathode fall region for low pressure hydrogen discharges using e.g. plane parallel electrodes.

In this work, we present Doppler free two photon optogalvanic measurements to determine the local electric field strength (E-field) in the cathode fall region of a hollow cathode discharge, operated in pure hydrogen, via the Stark splitting of the 2S level of hydrogen. The main aim of this article is the comparison of the measurements made with different cathode materials, stainless steel, and tungsten; both of them are widely used in the low-pressure discharges. Sputtering of stainless steel is the principle cause of the differences observed for the E-field variation, and the differences are analyzed in the frame of the sputtered material in a wide range of discharge conditions. Complementary images of the discharge luminosity allow for the conclusion; the cathode dark zone corresponds in good approximation to the cathode fall length.

Doppler-free two photon optogalvanic spectroscopy was employed in extensive studies to measure the electric field strength in the cathode fall region of a hollow cathode discharge (HCD), operated in pure hydrogen, via the Stark splitting of the 2S level of atomic hydrogen. The high quality measurements, based on an improved cathode design and laser spectroscopic set-up, reveal clear differences in the recorded spectra obtained for different cathode material (stainless steel and tungsten) at otherwise identical discharge conditions. It is well known, that the sputtering rate of tungsten is about four orders of magnitude less compared to stainless steel; hence the hydrogen plasma in front of the stainless steel cathode is much more contaminated by iron compared to tungsten. This study is focussed on analyzing the distortion of the spectra, i.e. the corresponding local electric field strength, depending on cathode material and laser power. We refer the more pronounced distortion of the spectra in case of a stainless steel cathode to the related large contamination of the hydrogen plasma due to atomic iron which is also expanding into the central discharge. Spectra recorded for different laser power, i.e. different spectral irradiance, allow verifying spectroscopic conditions, where the distortion of the spectra becomes quite negligible even for stainless steel cathode.

In this work, a non-intrusive, spatially resolved, spectro-tomographic optical diagnostic of the electronic density and temperature on the linear plasma column Mistral is presented. Coupling of spectroscopy and tomography technique gives access to the local plasma light emission at different wavelengths (visible and near IR) in an argon plasma. Taking advantage of the symmetry of the Mistral experiment, the diagnostic results are validated and the plasma is found to correspond to a corona equilibrium state. With the use of another spectrometer and a Langmuir probe, we propose a non-intrusive method to determine the electronic density and temperature of each pixel of the tomographic images of the plasma. The obtained results are in good agreement with the Langmuir probe ones.Spectro-tomography combines the advantage of these two approaches to simultaneously offer spatial and spectral resolution. This method provides 2D maps of light intensities at different wavelengths [12], giving access to the plasma electronic density and temperature.The experimental set-up and the tomographic method are presented in section 2 and 3, respectively. The experimental results are detailed in section 4 with 3 sub-sections: Experimental confirmation of the corona equilibrium in Mistral,  Validation of the spectro-tomographic results,

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