Er Erik Kieft

Thomson scattering measurements on an atmospheric Ar dc discharge lamp

Thomson scattering (TS) experiments have been performed in the region near the electrodes of a dc powered model lamp filled with 1–2 bar argon gas. In order to suppress the false stray light and Rayleigh scattered photons, a triple grating spectrograph was used. In this way the electron density and electron temperature could be measured in the near-electrode region for different arc currents. In order to get the electron density and electron temperature out of the TS spectrum, both collective and non-collective TS fitting procedures were used for the data processing. It was found that the radial profile of the electron temperature is more or less flat in the anode region with a value of about 1 eV. The electron density increases with arc current and is of the order of 1021 m−3. From the result, we can see that the plasma regions investigated are not in local thermal equilibrium in the sense that, locally, the rate of ionization is much larger than that of recombination.

Subnanosecond Thomson scattering setup for space and time resolved measurements with reduced background signal

In order to reduce the level of recorded background radiation in Thomson scattering (TS) experiments, we have designed and built a new setup for time and space resolved subnanosecond TS. Compared to our old setup, the new one is based on a faster camera, a laser with shorter pulse duration, and an optical delay line in the laser path. It has been characterized both by ray-trace simulations of the spectrograph and test experiments using Rayleigh and rotational Raman scattering on atmospheric pressure nitrogen gas. Although it was designed in the first place for experiments on a vacuum arc discharge in tin vapor for production of extreme ultraviolet light, our setup can also be applied to other plasmas that emit high levels of radiation or in which fast phenomena play a role.

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.

Characterization of highly transient EUV emitting discharges

The method of disturbed Bilateral Relations (dBR) is used to characterize highly transient plasmas that are used for the generation of Extreme Ultra Violet (EUV), i.e. radiation with a wavelength around 13.5 nm. This dBR method relates equilibrium disturbing to equilibrium restoring processes and follows the degree of equilibrium departure from the global down to the elementary plasma-level. The study gives global values of the electron density and electron temperature. Moreover, it gives a method to construct the atomic state distribution function (ASDF). This ASDF, which is responsible for the spectrum generated by the discharge, is found to be far from equilibrium. There are two reasons for this: first, systems with high charge numbers radiate strongly, second the highly transient behaviour makes that the distribution over the various ionization stages lags behind the temperature evolution.