Non-invasive determination of electron energy and velocity distributions in lowtemperature, non-thermal plasmas has always been a great challenge for diagnostics. Thomson scattering has proved to be a very versatile technique and application has been made to lowpressure discharges. In inductively coupled (ICP) radio frequency (RF) discharges the electron velocity distribution function is harmonically modulated in time and this modulation is equivalent to the oscillating current density generated in the plasma by the induced electric field of the antenna. For the first time this oscillation is measured temporally resolved by Thomson scattering [1]. Further, we will introduce a novel phase resolved emission spectroscopic technique that allows absolute measurement of the same quantity by analyzing the modulation of the atomic excitation by electron collisions [1-3]. The experiment is carried out in an ICP (f = 13,56 MHz) with a planar antenna of 10 cm radius in argon at low pressures in the Pa regime. The induced electric field is directed mainly azimuthally and this is also the direction of the oscillation of the anisotropic part of the electron velocity distribution. This part of the velocity distribution function is measured by Thomson scattering with a frequency doubled Nd:YAG laser with a pulse length of 8 ns which determines the temporal resolution. Under the conditions of our experiment, the scattering is incoherent with the scattering parameter α << 1 and a Maxwellian electron velocity distribution results in a Gaussian spectral distribution. The slope on a logarithmic scale gives directly the electron temperature and the integral the electron density. A drift in the direction of the scattering vector leads to a certain displacement of the distribution. Phase resolved measurement of this displacement allows a direct determination of the drift oscillations.

Measurements of electron density and temperature of helium plasmas in a cw running magnetic multipole plasma source by repetitively laser-pulsed 90° Thomson scattering are reported. This is the first experiment in which this technique has been applied to such plasmas. Measurements are performed at a helium gas pressure of pg = 5 Pa, the discharge voltage was Ud = 100 V, the discharge current was 5 A ≤ Id ≤ 30 A, and the cathode heating current was 80 A ≤ Ih ≤ 140 A. Electron energy distribution functions obtained from the Thomson scattering spectra are studied. The obtained plasma parameters are electron temperature 1.5 eV ≤ kTe ≤ 5 eV and density 1012 cm−3 ≤ ne ≤ 4 × 1012 cm−3, respectively. The sensitivity of detection of the experiment is in the range of 109 electrons and the accuracy of the electron temperature and electron density are specified to 15% and 20%, respectively. In addition, the neutral density and helium gas temperature are obtained from the Rayleigh component of the scattered spectra. Langmuir probe measurements are performed under the same plasma conditions and a comparison of the results with Thomson scattering shows good agreement between the two diagnostics.

Sterilization of bio-medical materials using radio frequency (RF) excited inductively coupled plasmas (ICPs) has been investigated. A double ICP has been developed and studied for homogenous treatment of three-dimensional objects. Sterilization is achieved through a combination of ultraviolet light, ion bombardment and radical treatment. For temperature sensitive materials, the process temperature is a crucial parameter. Pulsing of the plasma reduces the time average heat strain and also provides additional control of the various sterilization mechanisms. Certain aspects of pulsed plasmas are, however, not yet fully understood. Phase resolved optical emission spectroscopy and time resolved ion energy analysis illustrate that a pulsed ICP ignites capacitively before reaching a stable inductive mode. Time resolved investigations of the post-discharge, after switching off the RF power, show that the plasma boundary sheath in front of a substrate does not fully collapse for the case of hydrogen discharges. This is explained by electron heating through super-elastic collisions with vibrationally excited hydrogen molecules.

Laser spectroscopic electric field measurements have the potential to become a versatile tool for the diagnostics of low-temperature plasmas. From the spatially and temporally resolved field distribution in the sheath close to electrodes or surfaces in general, a broad range of important plasma parameters can be inferred directly: electron temperature; ion density distribution; displacement-, ion-, electron-diffusion current density; and the sheath potential. Indirectly, the electron and ion energy distribution functions and information on the ion dynamics in the sheath can also be obtained. Finally, measurements in the quasi-neutral bulk can also reveal even the plasma density distribution with high spatial and temporal resolution. The basic concepts for analysis of the field data are introduced and demonstrated by examples in hydrogen discharges.

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