Christoph Eichhorn

Oxygen plasma flow properties deduced from laser-induced fluorescence and probe measurements

Estimation of the local dissociation degree and the local mass-specific enthalpy of a pure oxygen plasma flow determined mainly from laser-induced fluorescence measurements are reported. Measurements have been conducted for several generator parameters in an inductively heated plasma wind tunnel. Additional probe measurements of total pressure together with the deduced translational temperature are used to estimate the local mass-specific enthalpy. For a reference condition, full dissociation has been measured. The measured translational temperature of atomic oxygen for this condition is T = 3500 K. Subsequently, the local mass-specific enthalpy has been derived using these local density and temperature measurements. For the reference condition the estimated value of h = 27 MJ/kg is in good agreement with the probe measurements and results from diode laser absorption spectroscopy.

Plasma Wind Tunnel Investigation of European Ablators in Air Using Emission Spectroscopy

Plasma wind tunnel tests using the European ablative materials AQ61 and MONA were conducted in a nitrogen/ oxygen atmosphere at three relevant heat fluxes. Results of the emission spectroscopic investigation of the plasma radiation are presented, for both the freestream (without probe body) and the boundary layer 5 mm in front of the test samples. Emission spectroscopy was conducted at several wavelength ranges (320 nm < λ < 580 nm) and spectral resolutions. Results are compared with respect to the temperatures of the most common plasma species. Here, comparisons are drawn between the plasma emission at the different heat flux regimes and the test specimen material.

Improved Abel Inversion Method for Analysis of Spectral and Photo-Optical Data of Magnetic Influenced Plasma Flows

This paper presents the derivation of an alternative method called spline method to perform inverse Abel transformation of measured data sets. At first the general approach of Abel transformation is shown. The following section shows the principle of operation of the existing methods. Then the new spline method is derived, which is a combination of the known Fourier method and f-Interpolation method with polynomials with an analytical solution of the forward Abel transform. All methods are tested on three test cases: an off centered Gaussian function, a cubic function and photo-optical data of a plasma flow in front of a spherical probe head. The results of the different methods are compared, limitations are shown and their advantages are pointed out. The new method is advantageous for noisy data and due to its spline based approach no assumptions about the local distribution are needed.

Plasma Wind Tunnel Investigation of European Ablators in Nitrogen/Methane Using Emission Spectroscopy

For atmospheric reentries at high enthalpies ablative heat shield materials are used, such as those for probes entering the atmosphere of Saturn’s moon Titan, such as Cassini-Huygens in December, 2004. The characterization of such materials in a nitrogen/methane atmosphere is of interest. A European ablative material, AQ60, has been investigated in plasma wind tunnel tests at the IRS plasma wind tunnel PWK1 using the magnetoplasma dynamic generator RD5 as plasma source in a nitrogen/methane atmosphere. The dimensions of the samples are 45 mm in length with a diameter of 39 mm. The actual ablator has a thickness of 40 mm. The ablator is mounted on an aluminium substructure. The experiments were conducted at two different heat flux regimes, 1.4 MW/m2 and 0.3 MW/m2. In this paper, results of emission spectroscopy at these plasma conditions in terms of plasma species’ temperatures will be presented, including the investigation of the free-stream species, N2 and , and the major erosion product C2, at a wavelength range around 500 nm–600 nm.

Theoretical and Experimental Approach of Multi-Photon Spectroscopy on Xenon for Application on Ion Thruster Plasma Parameter Investigations

Three excitation schemes for two-photon laser-induced fluorescence (TALIF) on neutral xenon are presented. An overview on possible transitions is given and two alternative candidates for neutral xenon ground state diagnostics are experimentally investigated in a xenon-filled cold gas cell. Comparisons to the well-known transition at ‚ = 2 ◊ 225.5 nm are given. It was found that alternative excitation schemes showed remarkable signal intensities and can be used for future TALIF measurements in neutral xenon additionally. With respect to singly ionized xenon ground state diagnostics, some theoretical aspects three-photon spectroscopy are discussed. Three-photon transition rates are expected to have transition rates » ten orders of magnitude lower than those of two-photon processes. A scheme to estimate multi-photon absorption cross sections based on lowest-order perturbation theory and Coulomb Approximation is given.

Laser Induced Fluorescence Spectroscopy on Neutral Xenon: Two-Photon Cross Sections and Measurements in an Ion Thruster Plume

Two-photon absorption laser induced fluorescence (TALIF) measurements in neutral xenon are presented both in a cold gas cell and in the plume of the radiofrequency ion thruster RIT-10. Measurements in the cold gas cell concentrate on the characterization of TALIF schemes involving levels of the 6p and 6p multiplets with respect to natural lifetimes, collisional deactivation coefficients and two-photon absorption cross sections. The feasibility of two TALIF schemes including the 6p[1/2]0 and 6p [3/2]2 levels has been demonstrated in the plume of the RIT-10. Relative particle densities have been measured at a distance of 12 cm downstream the acceleration grid in the center of the plume axis.

Photon Laser-Induced Fluorescence of Neutral Xenon in a Thin Xenon Plasma

Two-photon absorption laser-induced fluorescence (TALIF) measurements of relative ground state densities of neutral xenon on the plume axis of the radio-frequency ion thruster RIT-10 are reported. ATALIF scheme involving excitation of the 6p0 1=2 0 was applied. TALIF signals have been analyzed both in the cold gas flow and during thruster operation. Results show the principle feasibility of the investigated TALIF scheme, which seems to be a useful extension to formerly applied TALIF transitions for diagnostics in a thin xenon plasma.

Flow Characterization Using Laser-induced Fluorescence Measurements of Atomic Nitrogen In Inductively Heated Nitrogen Plasma

First results of two-photon absorption laser-induced fluorescence (TALIF) measurements of atomic nitrogen in an inductively heated nitrogen plasma flow are presented. Although particle densities can not be derived yet, promising results have been obtained. For calibration of these measurements, several levels in neutral xenon have been investigated.

Two-Photon Excitation of Some np(n¸8) and nf(n¸5) Levels in Neutral Xenon

Results from two-photon excitation of a variety of levels including 8p, 9p, 5f and 6f configurations in neutral xenon are reported. Measurements have been taken in pressure regimes between 4 and 200 Pa, depending on the characteristics of the respective excitation scheme. Effects of collisional quenching are found to have significant influence on effective lifetimes of excited levels even at low pressures. For both 5f [3/2] J=2 and 5f [5/2] J=2 levels, Stern-Volmer plots yield consistent results for collisional deactivation constants and extrapolation of the spontaneous lifetimes. Results from excitation of the 8p [1/2] J=0 configuration indicate the applicability of the Stern-Volmer interpretation. Two-photon excitation of both 6f J=2 states have been verified, but show weaker signal intensities at the considered fluorescence transitions and are more difficult to interpret. Within the 9p manifold, comparably intense response was observed following excitation of the 9p [1/2] J=0 state. Besides for xenon diagnostics in general, results may be interesting to characterize two-photon reference transitions in xenon for signal intensity calibration of two-photon ground state diagnostics in atomic nitrogen.

Measurement of Two-Photon Absorption Cross Sections in Neutral Xenon: First Results

First results within an experimental approach to determine relative two-photon absorption cross sections in neutral xenon are presented. Two-photon absorption profiles of 7p, 8p, 9p (J=0,2) levels and 5f, 6f (J=2) have been measured. Two-photon absorption laserinduced fluorescence (TALIF) schemes including 9p and 6f levels have been found to be manifestly weaker compared to schemes including 7p, 8p and 5f levels. Investigations have been carried out in pressure regimes between 4 and 200 Pa. Evaluation of two-photon absorption cross sections still require measurements below the TALIF signal saturation limit, which has been estimated for the 5f [5/2]-level.