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The inductively-heated plasma generator IPG6-B at Baylor University has been established and characterized in previous years for use as a flexible experimental research facility across multiple applications. The system uses a similar plasma generator design to its twin-facilities at the University of Stuttgart (IPG6-S) and the University of Kentucky (IPG6-UKY). The similarity between these three devices offers the advantage to reproduce results and provides comparability to achieve cross-referencing and verification. Sub- and supersonic flow conditions for Mach numbers between M a = 0 . 3 − 1 . 4 have been characterized for air, argon, helium and nitrogen using a pitot probe. Overall power coupling efficiency as well as specific bulk enthalpy of the flow have been determined by calorimeter measurements to be between η = 0 . 05 − 0 . 45 and h s = 5 − 35 MJ kg−1 respectively depending on gas type and pressure. Electron temperatures of T e = 1 − 2 eV and densities n e = 1 0 1 8 − 1 0 2 0 m−3 have been measured using an electrostatic probe system. At Baylor University, laboratory experiments in the areas of astrophysics, geophysics as well as fundamental research on complex (dusty) plasmas are planned. The study of fundamental processes in low-temperature plasmas connects directly to electric propulsion systems. Most recent experiments include the study of dusty plasmas and astrophysical phenomena and the interaction of charged dust with electric and magnetic fields. In this case, dust can be used as a diagnostic for such fields and can reveal essential information of the magneto-hydrodynamics in low-temperature plasmas. Although some of these goals require further advancement of the facility, it is proposed that several phenomena relevant to electric propulsion as well as to other fields of plasma physics can be studied using the existing facility.

The objective of this work is to study the behavior of surface layers of high-temperature metals in their interaction with a powerful pulsed plasma flow produced by a high-power ablative pulsed plasma thruster. This plasma generator produces plasma flows with a directed velocity of (7–9) × 106 cm/s, an initial diameter of 1.5–2 cm, and a maximum number density of about 1018 cm–3, as well as a maximum power of 5 GW. The main measured values are the residual temperature of the tungsten specimens and the evaporated mass. Also, metallographic analysis of the specimens was performed. The basis of the research method is to analyze the experimental data with the help of a numerical model describing the heating and evaporation of the material upon absorption of pulsed energy fluxes taking into account the evaporation kinetics based on the Hertz–Knudsen expression. Based on the developed numerical model and the obtained experimental data, the kinetics of evaporation of tungsten at high power fluxes to the surface (up to 1 GW/cm2) is investigated.

The present work investigates the formation and stability of different oxides on PM1000, an Oxide dispersion-strengthened (ODS) Ni-Cr superalloy, and on SiC coated C/C-SiC after being exposed to highly dissociated nitrogen and oxygen flows in a Plasma Wind Tunnel (PWT). PM1000 and C/C-SiC are candidate materials for re-entry vehicles Thermal Protection System (TPS) and were used e.g. on the European eXPErimental Re-entry Testbed (EXPERT). The surface characterisation covers the identification of the oxide layers formed on the specimen's surface via X-ray Diffraction (XRD) and Energy-Dispersive X-ray spectroscopy (EDX). Grain sizes are assessed from XRD patterns and different topographical features are identified for Cr2O3 and NiO on PM1000 samples from Secondary Electron Microscopy (SEM). Surface spectral and total normal emissivities of the different oxides are assessed showing a significant dependency on the oxide, which is verified with previous investigations on ground and with flight data from the FOTON capsule.

Silicon Carbide (SiC) is a low Z material discussed as an alternative to graphite for fusion devices. The retention of hydrogenous species is an important plasma-surface interaction property. Deuterium was implanted into SiC, SiCf/SiC, Cf/C-SiC and SiC coated graphite under various particle energy and substrate temperature conditions. A TDS process was used to characterise the deuterium retention of the implanted specimens. While all SiC materials show elevated retention levels compared to graphite, the differences are limited to about a factor of two over the range of parameters investigated. Keywords: Deuterium Retention, Silicon Carbide, Ceramic Composite Materials, Low Z Materials

To investigate the dissociation and recombination processes in a plasma jet, the characterization of its composition is important. For this purpose, an automated fitting algorithm for emission spectroscopic measurements, based on the radiation simulation tool PARADE, is developed. The algorithm is demonstrated by qualitatively analyzing single carbon dioxide spectra of the inductive plasma generator IPG4, operated at 125 kW anode power with a CO2 mass flow rate of 2.2 g/s. The species O, C, CO, CO+, CN and C2 are fitted with regards to number density, as well as vibrational and rotational temperature. A radial analysis of species composition is performed at two axial positions in the plasma jet. The results show different regions inside the plasma, suggesting increasing CO dissociation towards the centerline, as well as recombination of atomic carbon with molecular nitrogen. Moreover, the species composition distribution is found to be an indicator for the flow conditions in underexpanded free turbulent plasma jets.

At the Institute of Space Systems (IRS), experiments have been performed using the high enthalpy, inductively heated plasma generator (IPG) in plasma wind tunnel 3 (PWK3), in order to assess the catalytic behaviour of different materials. Utilising the Upwind Relaxation Algorithm for Non-equilibrium Flows of the University of Stuttgart (URANUS), a methodology for determining catalytic efficiencies by obtaining atomic recombination probabilities γ for high temperature materials, has been developed. This method eliminates the inherent uncertainties produced when using catalytic properties of previously tested materials to infer those of new materials. In this work, eight different candidates for the Thermal Protection System (TPS) of an entry vehicle have been studied, of which six are ceramic materials and the other two are metallic alloys. Thermochemical properties of these specimens are given for surface temperatures between 1000 and 2000 K in pure oxygen and pure nitrogen plasmas. The high enthalpies and relatively low pressure conditions in which these material samples have been tested in PWK3 are relevant for entry applications from Low Earth Orbit (LEO).

Research of magnetoplasmadynamic Pulsed Plasma Thrusters (PPTs), or iMPDs, at the Stuttgart Institute of Space Systems (IRS), led to the thruster design ADD SIMP-LEX. For optimization of the thruster's discharge behavior and of the plasma acceleration, the magnetic self-field is measured. Inductive miniaturized probes are introduced and used to acquire data at points of the volume inside of the thruster's discharge channel. The results are used for analysis to identify the extend of the discharge zone along the discharge channel of the PPT. Measured magnetic field signals along the centerline of the discharge channel are presented. The underlying distribution of the dynamic discharge current is deduced from Amperes circuital law and interpreted in a two-dimensional plot against camera images for the current-plasma-interaction, current motion and validity of today's discharge models. The evaluation of the calculated integral discharge current in a plane of the discharge channel against the recorded thruster discharge current is presented and discussed.

The newly qualified IRS facility for the determination of total and spectral emissivities and its recent numerical optimization is described. Values of measured total emissivities of the ceramics HfO2, Al2O3, Yt2O3 and the metals/alloys tungsten, TZM and PM1000 in the temperature regime of 750–1800 K are given. The drastic influence of the oxidation state of PM1000 on the emissivity is discussed. Additionally, results of an investigation of the influence of surface roughness and surface topology on emissivity are presented. Therefore, three SSiC samples with surface roughness from R q = 0.05 to 0.66 have been prepared using common finishing operations. The tests showed that the emissivity increased about 10% with an increase of the surface roughness even in the regime where R q values are in the same magnitude or much smaller than the maximum emitting wavelength. Recombination coefficients for the abovementioned materials have been determined in pure oxygen plasma. The methodology for the determination of recombination coefficients of ceramic and metallic thermal protection system (TPS) materials in single species gases used at IRS and its latest improvements is presented. Test results for the recombination coefficients in oxygen plasma are shown between 1469 and 2072 K.

The interaction between a magnetic field and plasma close to a nonconductive surface is of interest for both science and technology. In space, crustal magnetic fields on celestial bodies without atmosphere can interact with the solar wind. In advanced technologies such as those used in fusion or spaceflight, magnetic fields can be used to either control a plasma or protect surfaces exposed to the high heat loads produced by plasma. In this paper, a method will be discussed for investigating magnetic field plasma interactions close to a nonconductive surface inside a Gaseous Electronics Conference reference cell employing dust particles as probes. To accomplish this, a magnet covered by a glass plate was exposed to a low power argon plasma. The magnetic field was strong enough to magnetize the electrons, while not directly impacting the dynamics of the ions or the dust particles used for diagnostics. In order to investigate the interaction of the plasma with the magnetic field and the nonconductive surface, micron-sized dust particles were introduced into the plasma and their trajectories were recorded with a high-speed camera. Based on the resulting particle trajectories, the accelerations of the dust particles were determined and acceleration maps over the field of view were generated which are representative of the forces acting on the particles. The results show that the magnetic field is responsible for the development of strong electric fields in the plasma, in both horizontal and vertical directions, leading to complex motion of the dust particles.

MHD effects arising in plasma sources, such as short-pulsed magnetoplasmadynamic generators and inductively heated plasma generators, are analyzed with both algebraic models and measured data. Functional principles of the sources based on their MHD behavior are explained. Moreover, Stewart numbers in the order of magnitude of at least 10 −1 are calculated for the systems and qualified as an identifier for the magnetic influence on plasmas. Here, the considered plasma systems that are a priori known as MHD systems were used to determine typical values of the Stewart number. Based on this experience the concept of a plasma probe to magnetically influence or control weakly ionized free stream plasma flows is presented.

The method for determining the recombination coefficients for ceramic materials in air plasma and pure oxygen plasma is described. Additionally, a new method lor the in situ determination of the spectral emissivity of high-temperature ceramic materials is presented. Values of the recombination coefficients for considered materials, resulting from the described methodology, are given at temperatures between 1483 and 1851 K and pressures between 356 and 950 Pa and compared with literature. Values of the spectral emissivities of the investigated materials are presented in a temperature range from about 1200 to 1820 K and compared with values obtained by other authors. Obtained results will be used for the development of catalytic sensors in the frame of the European reentry program EXPERT.

Laser absorption spectroscopy was applied to pure oxygen flows generated by an inductively heated plasma generator IPG3. Temporal variation of translational temperature was deduced from measured absorption line broadening of OI 777.19nm because the flow properties fluctuated at 300Hz in synchronization with the induction current. The specific total enthalpy and mole fraction of oxygen were estimated from the deduced temperature assuming thermo-chemical equilibrium. Consequently, the averaged degree of dissociation of oxygen is 92%. The specific total enthalpy on the centerline was estimated at 33.7±2.9MJ/kg; 43% of it was in the form of chemical potential. Results show good agreement with intrusive measurements.

Apart from magnetoplasmadynamic plasma generators (MPG) and thermal arcjet devices (TPG) for re-entry simulation, inductively heated plasma generators (IPG ) have been developed for basic TPS material tests at the IRS. These inductively heated plasma generators consist mainly of a water-cooled coil surrounding a tube in which the plasma is located Due to this setup no electrode erosion appears; impurities in the plasma are minimized. Hence, both the behaviour of the gas components can be examined singularly and basic material tests such as the determination of the catalytic behaviour can be performed. With IPG3 an rf-source is provided where the coil is closer to the plasma than it was with previous designs. Therefore, the electromagnetic field loss is reduced. The water cooling system surrounds both the induction coil and the plasma container. IPG3 was qualified up to an anode power of 180 kW (argon), which is an essential improvement regarding the intended re-entry simulations in combination with the IRS MPG-simulations in the plasma wind tunnels 1 and 2 (PWK). The structure of the rf-plasma source IPG3 and the facility PWK3 are presented in this paper. This paper describes the design of IPG3 and the first experimental results for the plasma source. Power characteristics of IPG3, which were measured under variation of gas, are shown. Within this variation of gas frequency measurements were made, which will be a help for later numerical simulations of IPG3. A CID camera was used to measure the radial intensity of the plasma through an axial optical window of IPG3. The results led to a rough determination of the skin depth for the operation with air.

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 N2+, and the major erosion product C2, at a wavelength range around 500 nm–600 nm.

In order to develop inductive electric propulsion as a flexible, throttleable technology for future space operations, a greater understanding of discharge transitions within the inductive plasma generator discharge chamber is required. This paper presents a non-intrusive method to determine the conditions under which transitions between the capacitive, low inductive, and high inductive regimes occur with greater accuracy, as well as determining the proportion of a single discharge cycle the plasma spends in either capacitive or inductive regime. Such a method allows a more robust method of classification of inductive discharges than previously available and can be applied to numerous gases. This approach presents an advantage over previous methods which relied on strongly radiating or thermally reactive gases to exhibit certain behaviour (due to the restriction of classical diagnostics on such high power sources) before a transition could be confirmed. This paper presents results from the proposed method applied to a pure oxygen plasma as well as two combinations of argon and oxygen (at 1:1 and 3:2 Ar:O2 volumetric ratios) in order to assess the tunability of electromagnetic regime transitions through modifications of gas composition rather than mechanical alterations. Transitions to the higher inductive mode were observed for much lower input powers for the argon–oxygen blends, as was expected, allowing final discharge conditions to occupy the inductive regime for 94% and 85% of a single discharge cycle for the 3:2 and 1:1 Ar:O2 mixtures, respectively. Pure oxygen achieved a maximum inductive proportion of 71% by comparison.

A molecular relaxation model, a dissociation model, and a recombination model for gas phase particles have been developed making use of the Conservative Weighting Scheme. The models are discussed in detail and are verified on basis of single cell simulations. Verification results show a strong increase of computational efficiency for the resolution of trace species' macroscopic properties. Additionally, several catalysis models are discussed with respect to expected pros and cons. Analysis results allow for identification of the most promising candidate. Implementation of catalysis is currently ongoing.

The development status of a code allowing for a variable thermo-chemical modeling of nonequilibrium flows is presented. A high enthalpy flow around a cylinder has been calculated using different modeling of the vibrational nonequilibrium. The effects due to different models on the temperature and the mol fraction of the species are shown. The results of the new code are compared to already verified codes.

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.

For the numerical investigation of re-entry phenomena the modeling of gas-surface interactions is crucial. Mission relevant phenomena are catalysis i.e. the surface material dependent recombination of atomic species and the oxidation processes of ceramic thermal protection systems. The latter is closely related to the following questions: What is the size of available surface area for catalysis? What is the dominating process at peak heating during the atmospheric entry maneuver – active or passive oxidation? In this work we will discuss the two main catalysis mechanisms, the Eley-Rideal and the Langmuir-Hinshelwood process. We give a brief introduction to this topic and discuss relevant implementations in catalysis related numerical tools. Also, we present some details on oxidation modeling and its implementation in the URANUS 2D code.

An experimental investigation was carried out in order to evaluate the possible use of a newly adapted calorimetric heat flux probe in combination with a set of 8 strong permanent ring magnets to influence a high enthalpy argon plasma flow. The plasma wind tunnel facility 1 (PWK1) of Institut fur Raumfahrtsysteme (IRS) was used together with the high enthalpy magneto-plasmadynamic plasma source RD5. The probe was first qualified at well known plasma conditions. Thereafter, it was equipped with permanent magnets and tested at a newly defined argon plasma condition in order to evaluate a possible influence of the magnetic field on the stagnation point heat flux. For each case, with and without magnet, two radial profiles have been recorded as well as several measurements at the center position inside the plasma flow. A definite influence on the stagnation point heat flux has been found, though the results show an increase of 17.78 % at the center position (18.15 % mean value) instead of a decrease. A possible explanation for this result is, that it might originate from the magnetic field configuration of the permanent magnets.

This paper describes the methodology of determining the recombination coefficients for candidate materials of the catalytic based sensor system PHLUX. The methodology was broadened in terms of evaluation of the specific heat flux on catalytic surfaces. This leads to the possibility of calculating species concentration and atom net fluxes on and at the surface directly. Recombination coefficients with an indication of temperature and pressure level at which they were determined is given. Total emissivities for these materials for temperatures up to 2300K are provided.

The interaction between a probe body and argon plasma flow is investigated to examine to what extent the thermal loads on a vehicle during the re-entry into the Earth’s atmosphere can be reduced by applying a magnetic field. The experiments are performed using a strong permanent magnet installed inside a probe body of the so called European standard. An insert of high magnetic permeability was used to increase the magnetic field strength in front of the probe, which then was measured to be 0.12 T. Former investigations showed only weak influence on the bow shock geometry but definite impact on the temperature as well as on the boundary layer. In order to obtain a stronger interaction, several aspects of the setup have been revised. In contrast to earlier experiments, a spherical probe head is used in this investigation. Also, the plasma generator has been changed in order to achieve higher enthalpies und thus stronger magnetic interaction. The argon plasma condition had to be defined and first characterization measurements have been made. All experiments performed were steady state and the test duration was about 5 minutes. Pictures of the experiments were analyzed and showed a definite impact of the magnetic field on the bow shock geometry. Due to the magnet, the bow shock distance increased by roughly 13% whereas the recorded light intensity of the bow shock region increased by a factor of 2.7. The analysis of the center line intensity of each test case indicates that the magnetic field is affecting the argon plasma flow upstream, beyond the bow shock.

Previous investigation of the propellant feed system for the thermal arcjet thruster of the lunar mission Bw1 concluded in the necessity that the pressure regulator of the propellant feed system needs to be modified to reach a higher pressure level inside the propellant feed system. The mass flow rate showed fluctuations of 15%, which are not acceptable for stable arcjet operation. To sustain a high temperature behind the vaporizer the propellant feed system is insulated by use of space blankets wrapped around the pipings. A new vaporizer concept is developed increasing the heat transfer into the ammonia by use of stainless steel srub inside the vaporizer. The necessary heating power for complete vaporization is calculated to be 50W. The results of the tests with the modificated propellant feed system show that first the fluctuations of the mass flow rate disappear by use of the modified pressure regulator. Secondly, the higher pressure level is reached but the mass flow is to high. Consequently, the diameter of the flow aperture has to be decreased.

Laser absorption spectroscopy was applied for diagnostics of inductively heated plasma generator IPG3 air and nitrogen flows. Although no absorption signal could be detected in the nitrogen flow, temporal variation of absorption profiles at OI 777.19 nm were measured in the air flow. As a result, the flow properties were found to fluctuate at 300 Hz with the plasma emitted duration of 46%. The specific total enthalpy and degree of dissociation in oxygen and nitrogen were estimated from the deduced total temperature assuming thermo- chemical equilibrium in the discharge tube. Consequently, the averaged total enthalpy, degree of dissociation in oxygen and nitrogen in the emitted duration time were estimated as 23.5±10.1 MJ/kg, 98% and 44 %, respectively.

The IRS sensor system PHLUX is based on catalysis effects. Two different material samples with different but known catalysis properties are placed close to each other, thus being exposed to the same plasma condition. In the course of choosing between suitable materials experimental investigations and numerical analysis has been performed. Catalysis models for SiO2, PM1000 and tungsten are implemented in the flow field solver URANUS in order to perform a numerical analysis of the material samples during the re-entry of the vehicle EXPERT. The recombination coefficients of oxygen and nitrogen on materials investigated here are based on recent measurements at the IRS and on data given in the literature.

In this paper, the elementary reaction steps adsorption/desorption of atoms as well as Eley-Rideal and Langmuir-Hinshelwood reaction mechanisms will be explained. Analytical solutions for recombination probabilities will be given for adsorption, desorption and Eley-Rideal chemistry as well as for adsorption, desorption and Langmuir-Hinshelwood chemistry. The main focus of the paper will be on the calibration of the catalysis model in comparison to models and experimental results of other authors. Finally, CFD results will be presented for the Shuttle equivalent hyperboloid. The results will be compared with published CFD results and experimental data.

The influence of chemical accommodation coefficients on the forebody heating of the ballistic MIRKA re-entry capsule was investigated numerically with the nonequilibrium Navier-Stokes code URANUS. Chemical accommodation coefficients have been modeled physically consistent for all possible catalytic recombination mechanisms at the surface, i.e. direct recombination, the Eley-Rideal reaction type as well as the Langmuir-Hinshelwood reaction type. The difference in surface temperature was below 6 K when comparing the thermal nonequilibrium simulations, taking into account chemical accommodation coefficients below unity, with the thermal equilibrium simulations. In a second step, typical plasma wind tunnel conditions with stagnation pressures of 800 Pa and a total specific enthalpy of 20 MJ kg used to determine recombination coefficients in the plasma wind tunnel at IRS have been addressed. Given the chemical accommodation coefficient, only little uncertainty in determining recombination coefficients has been found.

The laser-induced fluorescence measurement setup has been installed at the plasma wind tunnel PWK3 of the Institut fur Raumfahrtsysteme (IRS). Number densities and translational temperatures of atomic oxygen have been measured in pure oxygen plasma. The examined data has been used to calculate local specific enthalpies using measured tem- peratures and examined dissociation degree from the number density measurements. For a well investigated plasma condition, the results are compared to previous measurements using probe technique and absorption spectroscopy. The results agree well with previous experiments using probe techniques.

In the inductive stage of the DC-HF hybrid thruster Attila, power characteristics are measured and interpreted. This is done with two different induction coils of 2.5 and 4.5 turnsunder variation of ambient pressure. Test gases used are Nitrogen and Oxygen. At the present operation frequencies, generator power is coupled into the plasma with higher efficiency for Nitrogen than for Oxygen. Efficiencies determined for the inductive plasma generator are 44% and 43% in total.

The paper describes the development of the inductively driven plasma wind tunnel PWK3 enabling the electrodeless generation of high enthalpy plasmas for the development, investigation and qualification of heat shield materials required for space vehicles performing entry manouevres in the atmospheres of Venus, Earth and Mars. The facility, equipped with modular inductive plasma generators enabling the operation with gases such as carbon dioxide, air, oxygen and nitrogen has been qualified for thermal plasma powers up to 60 kW. The measurement of these powers, coil currents, operational frequencies, electromagnetic fields, heat fluxes on probes, Pitot pressures and Mach numbers allows the extensive characterization of both the plasma generators’ operational behavior and the plasma flow conditions. Newly developed models and numerical simulations, that correspond pretty well with the experimental data, enable a maximum plasma power in combination with long operational periods using different operational gases and gas mixtures by optimizing the plasma container wall thickness, the number of coil turns and the number of capacitors in the resonant circuit.

Inductive Plasma Generator developed at the Institut fur Raumfahrtsysteme (IRS) was used to produce high enthalpy pure oxygen plumes. Translational temperature was obtained by laser absorption spectroscopy using an absorption line from meta -stable atomic oxygen at 777.19nm. As a result, emission and absorption signals were found to fluctuate at 300Hz. Maximum translational temperature w as 6,700K on the centerline and 4000K at the edge of the plume. Then, the spatially averaged static enthalpy and kinetic enthalpy estimated by measured temperature and corresponding frozen loss calculated under thermo -chemical equilibrium were 6.5MJ/kg 5.5 MJ/kg and 15.5MJ/kg, respectively . Then, the total enthalpy was estimated to be 27.5MJ/kg. This value agrees with input enthalpy of 30MJ/kg.

The objective of the presented work was the investigation of heat flux on a cooled copper surface in two different plasma flows, N2/O2 and N2/CO2. Both plasmas were generated by the thermal arcjet generator RB3. The mass fraction of CO2 was the same as the mass fraction of O2 in the air plasma. The formation of NO was investigated with laser-induced fluorescence measurements (LIF). The heat flux on copper is about two times higher in an N2/CO2–plasma than in an air plasma but NO recombination is much lower. The rotational temperature in front of the probe was estimated by comparison of excitation scans with simulated spectras.

The operating parameters and resulting plasma properties of a blind hollow cathode (BHC) discharge have been investigated. The hollow cathode was driven capacitively with a pulsed dc signal of 200 kHz in a power range between 50 and 100 W at an ambient pressure of about 10 Pa. The working gas was argon, which was introduced with a ceramic capillary at different positions of the longitudinal axis of the hollow cathode with flow rates of between 30 and 1000 sccm. The current–voltage characteristics were recorded.The pressure at the end of the BHC was measured with a miniaturized pressure transducer with varying volumetric flow rate and axial position of the capillary in the hollow cathode. To characterize the ignition behaviour of the system, the measured breakdown voltages were compared with phenomenological Paschen curves calculated from the pressure data.Optical emission spectroscopy was used to examine the origins of the light emission, comparing the glow mode and hollow cathode mode in particular.A high-speed camera recorded some plasma processes. A mounting with an indium tin oxide coated glass was used to observe the inner volume of the BHC along the longitudinal axis, while the plasma was operated with different parameters. The optical observations revealed an inhomogeneous plasma condition along the axis.

The inductively heated plasma source at the Institute of Space Systems was investigated with respect to the wall thickness of the plasma tube using an air plasma. For this, the wall thickness of the quartz tube was reduced in steps from 2.5 to 1.25 mm. The significance of reducing the wall thickness was analyzed with respect to both the maximum allowable tube cooling power and the coupling efficiency. While the former results from thermal stresses in the tube's wall, the latter results from a minimization of magnetic field losses near the coil turns of the inductively coupled plasma (ICP) source. Analysis of the thermal stress could be validated by experimental data, i.e. the measurement of the tube cooling power when the respective tube structure failed. The coupling efficiency could be assessed qualitatively by simplified models, and the experimental data recorded show that coupling was improved far more than predicted.

A facility for the determination of total and spectral emission coefficients is described. Values of measured total emission coefficients of the different high temperature ceramics such as HfO2, Al2O3, Yt2O3 and the metals/alloys Tungsten, TZM and PM1000 in the temperature regime of 750 - 1800 K are given. The drastic influence of the oxidation state of PM1000 on the emissivity is discussed. Additionally, results of an investigation of the influence of surface roughness and surface topology on emissivity are presented. Therefore, three SSiC samples with root-mean-squared roughness surface roughness from Rq = 0.05 to Rq = 0.66 have been prepared using common finishing operations. The tests showed that the emissivity increased about 10 % with an increase of the surface roughness even in the regime where Rq values are in the same magnitude or much smaller than the maximum emitting wavelength. Knowledge on emissivity is a basis for the analysis of thermo-chemical behavior and related properties (i.e. recombination coefficients) as the two properties influence each other. In a second step a high enthalpy inductively heated plasma wind tunnel (PWK3) is described and a methodology to derive recombination coefficients is explained. Recombination coefficients for the above mentioned materials have been determined in pure oxygen plasma. The methodology for the determination of recombination coefficients of ceramic and metallic Thermal Protection System (TPS) materials in single species gases used at IRS and its latest improvements are presented. Test results for the recombination coefficients in oxygen plasma are shown between 1469 and 2072 K.

First results of the application of an improved sensor design for local mass–specific enthalpy estimation are presented. The sensor is based on the fact that the reduction of heat flux to a probe body by mass addition into the boundary layer depends on the free stream enthalpy. In reverse, measurement of this heat flux reduction boundary layer mass addition can be used for enthalpy determination. However, the classical, theoretical approach to solve the boundary layer equations is based on several assumptions, the most important of which is the self similarity of the boundary layer. The new design of the probe meets the requirements of these theoretical assumptions. The coolant is now fed through one bore hole in the stagnation point of the spherical probe head. The cooled area corresponds to the area used for the calorimetric heat flux measurement. The heat flux reduction for different coolant mass flow rates behaves similar to the former probe design. For a pure high enthalpy argon plasma flow the determined enthalpy value of h = 25.9 MJ/kg compares well to the semi–empiric approach of Marvin and Pope.

The production of hi gh enthalpy flows in a stationary manner requires plasma wind tunnels using plasma ge nerators. Magneto plasmadynamic generators (MPG), thermal arcjet devices (TPG) and inductively hea ted plasma generators (IPG) have been de ve loped at the IRS for differ ent atmospheres, different atmos pheric entry regimes and/or different basic in vesti gations. The absence of electrodes enables the use of IPGs for basic TPS material tests (e.g. catalycity) and the simulation of re -active atmospheres such as the Martia n or Venerian atmosphere. With IPG3 an rf -source is provided where the coil is closer to the plasma than it was with previous de signs. Therefore, the electromagnetic field loss is re duced. The water cooling system surrounds both the induction coil and th e plasma container. This paper describes the design of IPG3 and the experimental results using O 2. Three test cases with pure O 2 plasma were investigated using the absorption spectroscopy system of Department of Advanced Energy , the University of Tokyo, in combination with a Pitot pressure probe and a material sample support system of IRS, Universitat Stuttgart. Number density and translational temperature distributions of atomic oxygen were obtained from the measured absorption line profiles including abso rption lines of meta -stable atomic oxygen at 777.19 and excited atomic oxygen at 844. 64 nm. Mach number was deduced by the ratio between pitot pressure and ambient pressure. Surface temperature of TPS materials was measured by thermocouplers . 1. Introduc tion A variety of stringent requirements upon technology are imposed by both space travel to a rather close Earth orbit and to more distant celestial bodies such as the planets of our solar system. Among those requirements there are the high temperature ef fects occurring during atmospheric entries. Here, spacecrafts encounter gases at velocities of more than ten km/s, thereby being subjected to great heat loads. This makes it necessary to use high temperature thermal protection systems (TPS) to prevent the destruction of the space vehicle. Both TPS and environment (plasma) during the entry have to be investigated by means of computational and ground facility based simulations. Such ground facilities are the five IRS plasma wind tunnels PWK1 -5 that reproduce the thermal, aerodynamic and chemical load on the surface of a space vehicle entering a celestial body’s atmosphere. MPGs enable gas flows on high enthalpy levels. They are mainly used to investigate the erosion of radiation cooled heat shield materials li ke C -C or C -SiC as well as the behavior of ablative ma terials under thermal and chemical loads. Two IRS plasma wind tunnels are operated with MPGs. Furthermore, a TPG producing moderate enthalpies and higher stagnation pressures to simulate the follow on flight path is in use [1, 2]. As an IPG is contactlessly heated by induction currents, exceptionally pure plasma flows of any gases can be produced. Therefore, the application of an IPG delivers two major advantages. First, researchers are aware that the catalytic behavior of TPS materials is one of the main topics to be investigated in future. A main part of the heat flux seen by a TPS material derives from the recombination of atomic plasma species. Using the IPG the related heat flux measurements are not falsified by impurities as with the electrode plasma generators. Furthermore, the catalytic mechanisms of single gases such as of O 2 and N 2 [3, 4] in the case of air of an atmosphere can be investigated. Fig. 1 shows a typical pure oxygen plasma plume of IPG3 at 500Pa ambient pressure, an oxygen mass flow rate 3g/s and at an anode voltage U A=6250V. The resonant circuit was adjusted to nominal 650kHz using a 5 -turn coil with 7 capacitors each of 6nF (see description below).