de N Nienke Vries

Absolute measurements of the continuum radiation to determine the electron density in a microwave-induced argon plasma

A method for the determination of the electron density (ne) using the continuum radiation is presented. The radiation is calibrated with a standard tungsten ribbon lamp and thus expressed in absolute units. This method is applied to a microwave-induced argon plasma, created by a surfatron (2.45?GHz), for which the standard settings are: wavelength region at 648?nm, power of 60?W, pressure of 15?mbar, gas flow of 70?sccm and axial distance from the launcher of 3?cm. Due to the low degree of ionization, the influence of electron?ion interactions can be neglected; the radiation is predominantly generated by free?free interactions between electrons and atoms. The method provides the electron density values in the order of 1019?m?3 for different plasma settings. It is observed that the measured ne follows the well-known trends?it decreases in the direction of the propagating surface wave and increases with power.

Thomson scattering measurements on a low pressure surface wave sustained plasma in argon

Thomson scattering (TS) experiments have been made on a low pressure surfatron induced plasma. TS is an active diagnostic method and the experimental results are directly related to important plasma properties such as the electron density, ne, and the electron temperature, Te. Therefore, the TS results for ne and Te can be used to calibrate passive diagnostic methods which are often based on plasma models. However, to apply TS on a surfatron induced plasma inside a quartz tube is experimentally demanding because of the large amount of stray light and a low intensity of the TS signal. To achieve low detection limits and high stray light rejection, a triple grating spectrograph was used in the detection branch and an iCCD was used to record the TS spectrum. For a typical plasma condition with an argon pressure of 10 mbar and an absorbed power of 50 W, the measured electron density was found to be equal to ne ≈ 4 × 1019 m−3 and the electron temperature Te ≈ 1.2 eV. In addition, frame-averaged results for 6, 10, 15 and 20 mbar argon plasmas for absorbed microwave powers in between 25 ≤ Pab ≤ 60 W are presented. The trends found in the dependence of the pressure and power density are according to theory.

Polydiagnostic calibration performed on a low pressure surface wave sustained argon plasma

The electron density and electron temperature of a low pressure surface wave sustained argon plasma have been determined using passive and active (laser) spectroscopic methods simultaneously. In this way the validity of the various techniques is established while the plasma properties are determined more precisely. The electron density, ne, is determined with Thomson scattering (TS), absolute continuum measurements, Stark broadening and an extrapolation of the atomic state distribution function (ASDF). The electron temperature, Te, is obtained using TS and absolute line intensity (ALI) measurements combined with a collisional–radiative (CR) model for argon. At an argon pressure of 15 mbar, the ne values obtained with TS and Stark broadening agree with each other within the error bars and are equal to (4 ± 0.5) × 1019 m−3, whereas the ne value (2 ± 0.5) × 1019 m−3 obtained from the continuum is about 30% lower. This suggests that the used formula and cross-section values for the continuum method have to be reconsidered. The electron density determined by means of extrapolation of the ASDF to the continuum is too high (~1020 m−3). This is most probably related to the fact that the plasma is strongly ionizing so that the extrapolation method is not justified. At 15 mbar, the Te values obtained with TS are equal to 13 400 ± 1100 K while the ALI/CR-model yields an electron temperature that is about 10% lower. It can be concluded that the passive results are in good or fair agreement with the active results. Therefore, the calibrated passive methods can be applied to other plasmas in a similar regime for which active diagnostic techniques cannot be used.

Time resolved Thomson scattering measurements on a high pressure mercury lamp

Time resolved Thomson scattering (TS) measurements have been performed on an ac driven high pressure mercury lamp. For this high intensity discharge (HID) lamp, TS is coherent and a coherent fitting routine, including rotational Raman calibration, was used to determine ne and Te from the measured spectrum. The maximum electron density and electron temperature obtained in the centre of the discharge varied in a time period of 5 ms between 1 × 1021 m−3 < ne < 5 × 1021 m−3 and 6500 K < Te < 7100 K. In order to test the non-intrusive character of TS, we have derived a general expression for the heating of the electrons. By applying this to our mercury lamp and laser settings, we have confirmed the non-intrusiveness of our method. This is supported by the experimental findings. Furthermore, because the TS results were obtained directly, thus, without the local thermodynamic equilibrium (LTE) assumptions, they enabled us to follow the deviations from LTE as a function of time. Contrary to the generally made assumption that HID lamps are in LTE, we have found deviations from both the thermal and chemical equilibrium inside the high pressure mercury lamp at different phases of the applied current.

Investigation of Local Thermodynamic Equilibrium in HID lamps

Summary form only given. High intensity discharge (HID) lamps have a small volume, high pressure and high luminance. This makes them very useful for illumination of large surface areas such as streets and stadiums. A numerical model is being set up in order to fully understand the energy balance and transport processes in such lamps. The model can be greatly simplified if the discharge of the HID lamp is in Local Thermodynamic Equilibrium (LTE). LTE was experimentally investigated by determining the temperature of a model HID mercury lamp. This lamp has an arc length of 39 mm and an 18 mm diameter. It contains pure mercury and has no outer jacket. The validation of the LTE assumption requires the measurements of different temperatures. This was done using three different techniques. X-ray absorption (XRA) measures the heavy-particle temperature, Thomson scattering (TS) the electron temperature, absolute line intensity (ALI) measurements determine the excitation temperature. By comparing the different temperatures the LTE assumption can be validated. XRA uses an x-ray source, produced by a molybdenum anode, for the plane-wave illumination of the entire lamp. This is done for both lamp-on and off measurements. The Hg density distribution follows from the ratio of these, from which the heavy particle temperature can be determined using the ideal gas law. TS is the scattering of (laser) light on the electrons in the plasma. From the scattered photons the electron temperature and density can be determined. Emission spectroscopy (ALI) is used to determine the state density of mercury from the different optically thin lines. An atomic state distribution function (ASDF) then gives the excitation temperature. The TS temperature profile shows a relatively constant radial profile, whereas the XRA profile is parabolic. This is a clear indication of departure from LTE in the outer regions of the discharge.