Characterization of Weakly Ionized Argon Flows for Radio Blackout Mitigation Experiments

For reproducing the so called ExB communication blackout mitigation scheme inside the L2K arc heated facility of the DLR in weakly ionized argon flows a flat plate model has been equipped with a superconducting magnet, electrodes and a setup comprising microwave plasma transmission spectroscopy (MPTS). A thorough characterization of the weakly ionized argon flow has been performed including the use of microwave interferometry (MWI), Langmuir probe measurements, Pitot probe profiles and spectroscopic methods like diode laser absorption spectroscopy (DLAS) and emission spectroscopy.


Introduction
A reliable technique for solving the radio communication blackout problem during reentry is important for improving mission safety.Communication blackout occurs during reentry due to the ionization process of the surrounding shock layer, which creates a plasma layer.This plasma layer is not transparent for electromagnetic waves of a frequency below the plasma frequency thus blocking radio communications [1,2,3].
For reproducing communication blackout in a ground based experiment a flat plate model has been equipped with a setup comprising microwave plasma transmission spectroscopy.This model setup has been tested inside the L2K arc heated facility of the DLR in weakly ionized argon flows.Argon flows have been used due to the reduced chemical complexity of the plasma to simplify comparison to numerical simulations.Suitable flow conditions for blackout mitigation tests have been identified by measuring the electron density by both MWI and MPTS.
The so called ExB communication blackout mitigation scheme is based on reducing the electron density of the plasma layer locally above the antenna position by combined electric and magnetic fields.The combination avoids the need of both strong magnetic and electric fields, therefore minimizing the required energy and improving Electromagnetic Compatibility (EMC) [1,2].
A thorough characterization of the argon flow conditions has been performed to ensure a good comparability between experiments and numerical simulations.In particular the plasma parameters electron density and electron temperature have been determined carefully by different techniques.The characterization is necessary for verification of numerical codes which are needed to transfer the blackout mitigation scheme to flight conditions.One main difference between flight application and the experiments in arc heated facility L2K is that in L2K the flow ionization is generated inside the arc heater whereas in flight case it is caused by the heating in the shock layer of the reentry vehicle.
In the following used flow conditions and methods for their characterization are presented and discussed.The model design for testing the ExB communication blackout mitigation scheme is described.Transmission spectra of the flow without applying the mitigation scheme are presented to verify that this experimental design is able to experimentally simulate the communication blackout.This is the first step of experimentally investigating the ExB blackout mitigation scheme.The principle design can be applied to other blackout mitigation schemes as well.

Arc heated facility L2K
For the tests the L2K facility has been used.L2K is one of the test legs of DLR's arc heated facility LBK.A sketch of LBK is given in Fig. 1.The principal component of L2K is a Huels-type arc heater that has a maximum electrical power of 1.4 MW allowing to achieve cold wall heat fluxes in air flow up to 4 MW/m 2 at stagnation pressures up to 250 hPa.Hypersonic free stream velocities are provided by a convergent-divergent nozzle.The nozzle's expansion part is conical with a half angle of 12°.Different throat diameters from 14 mm to 29 mm are available and can be combined with nozzle exit diameters of 50 mm, 100 mm, 200 mm, and 300 mm.So, the facility setup can effectively be adapted to particular necessities of a certain test campaign.A more detailed description of the L2K facility is given by Gülhan et al. [4][5][6].

Flow characterization methods
Coriolis gas mass flow meter and reservoir pressure transducer have been used to continuously monitor the mass flow rate and the reservoir pressure during the tests.A Pitot probe has been applied for determining the Pitot pressure profile across the flow.
Emission spectroscopy with a Fourier transform infrared spectrometer (FT-IR) has been used to study the species spectra and the electronic excitation state of both the free stream and within the reservoir chamber of the arc heater.For a summary of emission spectroscopy results see [7,8].Laser Induced Fluorescence Spectroscopy (LIF) on NO molecules seeded to the Argon has been used to determine the rotational temperature and concentration within the free stream and the bow shock layer [9,10].DLAS on CO molecules has been used to determine the heavy particle translational temperature as well the flow velocity of the free stream.MWI has been used for line-of-sight determination of the electron density as well as determining the flow velocity.Different types of electrostatic probes have been used to estimate the electron temperature and electron density.Transmission spectra of microwaves have been used to determine the plasma frequency and thus the maximum electron density of the free stream.These four characterization techniques are discussed in detail in the following sections.

Diode Laser Absorption Spectroscopy
DLAS was applied on carbon monoxide (CO) molecules for measuring the flow velocity and the translational temperature using a diode laser emitting in the wavelength range from 2330 to 2335 nm at room temperature.DLAS is a line-of-sight method and the interpretation of the measurements has to take into account the homogeneity of the flow field.The velocity of the gas molecules is proportional to the spectral shift of the absorption signal and the translational temperature is determined from the half width of the absorption profile.In order to provide the required carbon monoxide for DLAS measurements, the argon flow was seeded with 20% of carbon dioxide which partially dissociates to carbon monoxide at high temperatures.The measured velocity did not change significantly when the amount of seeding was changed from 5% to 40%.For the measurement conditions in L2K, the divergence of the flow direction tends to increase the extracted velocity value while shear layer effects in the outer region of the free stream tend to decrease it.Concerning the temperature measurement the broadening of the signal due to the shear layer leads to increased temperature values.As no quantitative information about the divergence of the flow vectors and the thickness of the shear layer is available, the presented data should be used for a qualitative interpretation of the flow properties [10].The determined flow velocity and the translational temperature of all studied flow conditions are listed in Tab. 1 and discussed in section 4.

Microwave Interferometry
MWI is used for measuring the electron density and the flow velocity of the free stream.A MWI 2650 microwave interferometer working at 26.5 GHz measures the phase shift of microwaves passing the plasma.Electrons in the measurement volume reduce the permittivity and thus the optical path length on the way through the plasma.Thus the phase shift is directly linked to the integral line of sight electron density in the measurement volume between emitter and detector.The effective geometrical path length is determined by Pitot probe profiles.The flow velocity is measured using two coupled microwave interferometer setups.An additional MWI 2650-V interferometer allows to determine the time fluctuations in the electron density need to travel with the flow from one interferometer arm to the other.By knowing the distance between the two measurement volumes the flow velocity can be evaluated.The MWI 2650 is placed in 230 mm distance from the nozzle exit, the MWI 2650-V in 430 mm distance.In contrast to the above described DLAS technique no seeding of the flow is required.
The experimental setup of the MWI 2650 is sketched in Fig. 2. The measured electron densities and flow velocities for all studied flow conditions are listed in Tab. 1 of section 4. A comparison with other methods for determining electron densities is given in section 4.2.

Electrostatic probes
Langmuir probes in single [11], double [12] and triple configuration [13] were used to determine the electron density as well as the electron temperature of the flow.To exclude shock effects between the wires the probe is placed behind the bow shock in the stagnation point region of a stagnation point model of 70 mm diameter.A photograph of the Langmuir triple probe setup in Argon flow is given in Fig. 3.In this picture the bow shock in front of the triple wire arrangement is clearly visible.The electron temperature is supposed to be effected only by a small amount by the shock due to the higher speed of sound of the electron gas, so that the free stream electron temperature can be approximated by the measured value.In contrast the electron density is supposed to be effected by the bow shock, which has to be taken into account when comparing it to the results of other methods.During the single and double probe measurements the probes was placed on the flow axis.With the triple probe also density profiles across the flow have been recorded.

Microwave Plasma Transmission Spectroscopy
Radio transmission is the essential test parameter for blackout mitigation.To examine the radio blackout a compact sender antenna is integrated within a flat plate model.The microwaves are transmitted through the quartz surface of the model and half the beam width of the L2K weakly ionized argon flow.They are received by an antenna placed within the L2K chamber outside the flow.A spectrum analyser with a tracking generator measures spectra of the received signal strength.The transmission of the argon flow is determined by subtracting received spectra recorded without flow from those with flow.The setup is sketched in Fig. 4.
The sender antenna is a cavity backed spiral antenna with a diameter of 55 mm and a height of 50 mm.A conical log spiral antenna is used as receiver antenna.The spectrum analyser is a Rohde & Schwarz FSL 18.The output signal of the spectrum analyser has been amplified external amplifiers by an additional 20 dB.This setup is able to record spectra in a range from 2 GHz to 10 GHz.The noise level of the setup is at -80 dB.
The antenna polarization is circular polarized, as the L2K chamber has a metallic surface which is highly reflective for radio waves.Reflection of the waves changes the handedness of the circular polarization.So the receiver antenna is much less sensitive to single time reflected waves, suppressing signal interferences.A set of both LHCP and RHCP antennas is available.n e is the electron density, e the elementary charge, m e the electron mass, and ε 0 the electric constant.
In this paper also the (ordinary) plasma frequency f p defined by Eq. ( 2) is used.

Argon flow conditions for blackout mitigation tests at L2K
To fit the scale of the used flat plate models, the 200 mm nozzle exit diameter has been used for the blackout mitigation test.The previously studied argon flow condition FC 3 defined by a mass flow rate of 20 g/s and a reservoir pressure of 375 mbar is used [9].Two additional conditions FC 1 and FC 2 were defined using a mass flow rate at 20 g/s and reducing the reservoir pressure to 325 mbar and 350 mbar respectively resulting in a reduction in the enthalpy and thus the electron density.These tree states feature electron densities in the measuring range of all used electron density measurement techniques and also give the possibility to determine the effectiveness of blackout mitigation techniques for different electron densities.

Definition of flow conditions
The characterization results for each of the three conditions are listed in Tab. 1. From mass flow and reservoir pressure the specific enthalpy and total temperature were computed using the NATA code [14], which is a quasi-one dimensional flow solver including non-equilibrium chemistry.All other values are free stream measurement results taken at 330 mm distance from nozzle exit, measured by the given method.The MWI measures results of the electron density are taken at 230 mm distance from nozzle exit.The pitot pressure profiles across the flow are given in Fig. 6.The pressure peaks at each side are caused by side shocks.The Pitot pressure value listed in Tab. 1 is the on-axis value.Densities are determined from velocity and Pitot pressure, plasma frequencies are derived using Eq.
(1).The Pitot pressure profiles are used to derive density and the beam width, which is needed for getting the average electron density n e from the line-of-sight electron density determined by the MWI.No DLAS and LiF measurements have been done for FC 1 and FC 2 so far.Within the range of uncertainties the electron temperature values gained by one method do not change with the flow condition.The values of single and triple probe are both in the range of 2500 K to 3500 K whereas the values for the double probe are about a factor of 2 lower in the range of 1500 K.This is a hint that the electron gas is not in thermal equilibrium as the used standard evaluation method for Langmuir double probes assumes Maxwell-distributed electron energies.This also can contribute to the higher electron density value measured by the double probe compared to the other techniques.
Electron density profiles across the flow measured by Langmuir triple probe are shown in Fig. 6.The steps in the profile are due to the slow current data acquisition compared to the voltage data acquisition.The values obtained by the triple probe measurements in the range of the values obtained by MWI and MPTS.They have larger uncertainties than the other values due to the spread in the corresponding electron temperatures.
Further studies have to be done on the reliability of the values and the validity of using the standard analysis routines for determining the electron temperatures and densities from the obtained data.The flat plate model is shown in detail in Fig. 8.The magnet is placed beneath a 1mm quartz glass plate in the centre of the model.Two electrodes for generating the electric field of the ExB scheme are integrated into the model.The anode is placed between the model nose and the quartz plate.The cathode is placed at the rear end of the model.The compact sender antenna can be placed at two positions A1 and A2 just behind the magnet.In the following only the upper antenna position A1 is used.The axis of the receiver antenna is aligned to the axis of the sender antenna.Its distance to the model is 30 cm.The nose of the model is made from copper and cooled by water.
This setup in contrast to usual L2K models cannot be moved within the L2K chamber and it is directly mounted on the flow axis with a 0° angle of attack.As the HTS magnet is sensitive to high heat loads the test runs are performed only for a few second after arc heater ignition.From these data sets an averaged transmission spectrum for the respective flow condition is determined.First all recorded spectra have been filtered with a moving average of a specific frequency width.This moving average filter reduces noise and remaining interference patterns.Afterwards all filtered reference spectra and all filtered flow spectra are being averaged frequency-wise, resulting in a time averaged reference spectrum and a time averaged flow spectrum.The reference spectrum is subtracted from the flow spectrum to get the time-averaged transmission spectrum.For plotting the spectra a moving average of a width of 800 MHz is used.
The determination of a transmission spectrum is depicted in Fig.The different flow conditions show clearly distinct attenuation curves.The spectra for one condition are comparable within standard deviation.There is no significant difference between the antenna setups.The attenuation has a level between -20 dB (FC 1) and -28 dB (FC 3) at the low end of 2 GHz.At a specific edge frequency the transmission rises to a level between almost 1 dB (FC 1) and 3 dB (FC 3) at the high end of 10 GHz.
For each spectrum the edge frequency is evaluated by determining the intersection of the 0 dB axis with a linear extrapolation at the maximum slope.This edge frequency can be considered to be linked to the maximum electron density n e of the flow and its corresponding plasma frequency f p .The average edge frequency for each condition is given in Tab. 2. Corresponding electron densities n e are calculated using Eq. ( 1).These are also listed in Tab. 2. The resulting values are in good agreement to the values determined by the MWI free stream.As an angle of attack of 0° is used, the flow above the flat plate surface outside the boundary layer is expected to be similar to the free stream flow.
As expected, the plasma flow is transparent for microwaves above a specific frequency and microwaves are blocked beneath this frequency.Thus the principle of radio communication blackout can be experimentally reproduced in L2K facility for the selected argon flow conditions.As additionally expected the edge frequency is growing with the specific enthalpy of the flow.In conclusion this experimental design is a promising basis for testing blackout mitigation schemes as the ExB scheme, for which the previously described HTS model is designed.

Conclusions
Different Argon flow conditions have been defined for experimental blackout mitigation simulation tests.A thorough characterization of those conditions has been done with multiple methods including spectroscopic methods and different methods to determine the electron density as well as the electron temperature.
For studying the ExB blackout mitigation scheme, a flat plate model setup comprising a superconducting magnet has been constructed.With this setup the radio communication blackout can be experimentally reproduced in L2K facility for the selected argon flow conditions.From the resulting transmission spectra electron densities could be estimated which are in good agreement with the electron densities measured by MWI.Thus this principle experimental design is a promising basis for testing the ExB blackout mitigation scheme in particular and other blackout mitigation schemes in general.

Figure 2 :
Figure 2: Scheme of microwave interferometer setup

Figure 3 :
Figure 3: Langmuir triple probe in Argon flow

Figure 5 :4. 2
Figure 5: Pitot pressure profiles for FC 1, FC 2 and FC 34.2 Electron density and electron temperature measurementsThe values for the electron density n e obtained by MWI, MPTS and Langmuir double probe (2L) are compared in Tab. 2. The determination of the MPTS values is described and discussed in detail in section 5.2.The Langmuir probe values have been determined by taking average values over several test runs.The results of the MWI and MPTS are in good agreement for all states.The values for the Langmuir double probe (2L) are about a factor of 2 higher than those for the other methods.A higher value has to be expected according to the placement of the probe wires behind the bow shock.The results for the electron temperature T e of Langmuir single probe (1L), double probe (2L) and triple probe (3L) are compared in Tab. 3. The given values have been determined by taking average values over several test runs.The data points marked with (-) indicate those conditions for which not enough data was obtained to get a reliable value.

a 5 . Communication blackout experiments 5 . 1
Too few data points for this condition were taken to give a reliable value 6: Electron density profiles measured by Langmuir triple probe Flat plate model for testing ExB communication blackout mitigation scheme For testing the ExB communication blackout mitigation scheme a flat plate model has been constructed comprising a high-temperature superconducting (HTS) magnet.The magnet and its cryogenic system have been constructed by the Institute for Technical Physics (ITEP) of the Karlsruhe Institute of Technology (KIT).The cryogenic system is able to cool the magnet down to temperatures of about 8 K.The accessible magnetic field strength at the model surface is about 2 T. The setup is shown in Fig. 7.The cryostat is mounted in the L2K chamber together with the flat plat model.The magnet gets into the model by an opening in the model ground plate.The cryostat is protected from the flow and thus from high heat fluxes by a protective plate.

9 .
The green (lowermost) curves show the spectra with argon flow.The red (middle) curves show the reference spectra without argon flow.The blue (uppermost) curves show the resulting transmission spectra.For each of these spectral sets different moving averages with widths of 4 MHz, 200 MHz, 400 MHz, 600 MHz and 800 MHz are shown (colour saturation increasing with the width).The error bars show the respective standard deviation of the curves filtered with a width of 800 MHz.

Figure 9 :
Figure 9: Determination of a transmission spectrum

Table 1 :
Flow Conditions a No DLAS and LiF measurements have been done for these conditions.

Table 2 :
Comparison of n e measurement results

Table 3 :
Comparison of the T e measurement results of different Langmuir probes