Combined Surface Heating by Laser Beam and Subsonic Nitrogen Plasma Jet

Abstract

The paper describes new combined heating capability of the IPMech RAS inductively coupled plasma facility VGU-4. A 200 W ytterbium laser was added to the facility as a source of radiative heating. The cylindrical specimen made of the Buran orbital vehicle’s heat-shielding tile material with a black low catalytic coating was exposed to subsonic pure nitrogen plasma jet and laser radiation. The specimen surface temperature reached 1325 ${}^{\circ }$C during combined radiative and convective heating. The maximum heat flux obtained in the combined mode for a laser incident power of 47 W and a VGU-4 HF-generator anode power of 22 kW was 32.1 W/cm${}^2$. The convective heat flux from the nitrogen plasma jet at the same anode power was 12.6 W/cm${}^2$. Adding a laser to an existing inductively coupled plasma facility gives the future opportunity to better simulate entry into the atmospheres of Mars, Venus, the outer planets and their moons.

1. Introduction

Arc-jet plasma torches and inductively coupled plasma (ICP) facilities (plasmatrons) are widely used in experiments on modeling thermochemical interaction of dissociated flow with material surface in Russia (TsNIIMash [1], TsAGI [2], IPMech RAS [3]), the USA [4,5] and Europe [6]. The technical solutions that made it possible to design powerful high-frequency induction plasmatrons are based on the works of G.I. Babat [7] and the idea of vortex stabilization proposed by T. Reed [8]. To date, several powerful electrodeless plasmatrons have been created to study the heat transfer processes in high-enthalpy gas flows and to test thermal protection materials. In Russia, there are a 1 MW plasmatron U-13VCHP in TsNIIMash [1], a 250 kW plasmatron VAT-104 in TsAGI [2] and 1 MW and 100 kW plasmatrons VGU-3 and VGU-4 in IPMech RAS [9]. In Western Europe, there are a 1200 kW plasmatron at the von Karman Institute for Fluid Dynamics [10] and at the PWK3 facility in Germany [11]. In the USA, the University of Texas at Austin and the University of Vermont have developed small 50 kW and 30 kW inductively coupled plasma torches [12,13]. Experiments on heat exchange in jets of high-frequency induction plasmatrons have also been carried out in Japan [14] and China [15].

It is possible to distinguish a number of main scientific tasks for modern high-power electrodeless plasmatrons:

• Thermochemical resistance of materials [16,17,18,19,20];
• Catalytic properties of materials in a dissociated gas with respect to a heterogeneous recombination of atoms [21,22,23,24];
• Compliance with the laboratory tests of the thermochemical effect on the material, realized during the movement of the body at high speed in the atmosphere [25,26].

Studies of the thermochemical resistance of the materials are also carried out in arc-jet plasma torches in Russia [1], the USA [27,28], Europe [29,30,31] and China [32]; however, the contamination of the flow with electrode destruction products makes it difficult to use such facilities in problems sensitive to the chemical purity of gas (for example, for the study of heterogeneous catalysis).

Along with modeling the entry into the Earth’s atmosphere, ground tests of thermal protection systems for missions to Mars, Venus, the outer planets and their moons are also of great interest [33,34,35,36,37,38,39,40,41,42,43,44].

Arc-jet and inductively coupled plasma facilities provide convective heating of a specimen surface. The addition of a laser radiation source can expand their abilities in modeling the actual atmospheric entry conditions. In this case, it is possible to combine radiation and convective heating environments. A combined heating mode by plasma flow and laser radiation has been implemented at the Interaction Heating Facility (IHF) in the USA (Ames Research Center). As part of the Orion program, the capabilities of the IHF facility with a power of 60 MW were significantly expanded due to the use of ytterbium laser radiation sources that provided additional radiation heating of the surface of material specimens. The complex of the Laser-Enhanced Arc-jet Facility (LEAF) was created [45,46,47,48]. The LEAF facility used a 50 kW fiber-optic laser system. A water-cooled semiconductor laser was used in [49] for the combined heating of a SiC specimen surface in the arc-jet plasma wind tunnel. It should be mentioned that laser facilities that provide only radiative heating are also used for ground tests [50,51].

This work demonstrates the possibilities of new combined heating modes of the VGU-4 ICP facility at the Ishlinsky Institute for Problems in Mechanics RAS. The heat-shielding material specimen was exposed to a subsonic jet of nitrogen plasma and additionally heated by an ytterbium laser radiation.

The motivation for this study was to show that it was possible to significantly expand the operating envelope of high-power plasmatrons without a global rebuild. This is important because facilities of this type have long creation and use times and often do not meet the new challenges of the aerospace industry.

2. Materials and Methods

This study was done in subsonic pure nitrogen plasma jets of the VGU-4 HF-plasmatron with a vortex gas stabilization manufactured by the Ishlinsky Institute for Problems in Mechanics RAS. Its general parameters are specified in

Table 1. General parameters of the VGU-4 HF-plasmatron.

Parameter	Value
Electric power supplied to plasmatron, kW	100
Generator anode power supply, kW	72
Frequency, MHz	1.76
Discharge channel diameter, mm	80
Stagnation pressure, hPa	$6÷1000$
Gas mass flow rate, g/s	$2÷6$
Working gases	Air, N${}_2$, CO${}_2$, Ar

. A detailed description of the VGU-4 HF-plasmatron is given in [9]. The VGU-4 HF-plasmatron has a vertical orientation of its discharge channel and the plasma jet moves vertically upwards. The important feature of the VGU-4 facility is the use of a quartz discharge channel without any forced cooling [52].

A conceptual sketch of the experiment is shown in Figure 1. The test cylindrical specimen of 30 mm in diameter with a rounded face was made of the heat-shielding tile material for the Buran orbital vehicle (TZMK-10 heat insulation) with a black low catalytic coating [53,54,55,56,57,58,59,60], its main properties are presented in

Table 2. Main properties of the TZMK-10 heat-shielding material with a black low catalytic coating.

Property	Value
Density, kg/m${}^3$	$140÷150$
Thermal conductivity, W/m·${}^{\circ }$C	0.05 (at 20 ${}^{\circ }$C and p = 1 atm.)
	0.22 (at 800 ${}^{\circ }$C and p = 1 atm.)
Heat capacity, kJ/kg·${}^{\circ }$C	0.8 (at 20 ${}^{\circ }$C)
Total emissivity	$0.8÷0.9$
Operating temperature, ${}^{\circ }$C	1100 ${}^{\circ }$C (long time)
	1250 ${}^{\circ }$C (short time)

. The detailed specimen geometry is shown in Figure 2, its photos before exposure are shown in Figure 3.

The experiment was carried out at a constant HF-generator anode power value of 22 kW. A low anode power was chosen to prevent overheating of the specimen. The mass flow rate of nitrogen through the discharge channel was 2.4 g/s and was controlled by a Bronkhorst MV-306 flow meter. The pressure in the test chamber was 100 hPa. The distance from the outlet section of the discharge channel to the specimen was 120 mm. A Tandem VS-415U thermal imager was used to record the temperature distribution over the specimen surface: thermal images were recorded at a set value of spectral emissivity ${\epsilon }_{\lambda }$ of 0.87 at a wavelength of 0.65 microns (a narrowband filter for this wavelength was used to eliminate the influence of the laser radiation) [60]. To obtain thermal images, a correction factor of 0.93 was used for the transmission of the pressure chamber quartz window. The thermal imager was calibrated according to the absolutely black-body model in the temperature range from 800 to 1500 ${}^{\circ }$C (for the chosen light filter).

An IPG Photonics YPLN-1-100-200-R pulsed fiber ytterbium laser with a wavelength of 1.064 microns was added to the VGU-4 ICP facility in this work to provide radiative heating. Details for the YPLN-1-100-200-R laser are presented in

Table 3. General parameters of the YPLN-1-100-200-R ytterbium laser.

Parameter	Description
Configuration	Laser head with control unit (separate)
Technology	Ytterbium fiber laser
Operation mode	Repetitively pulsed laser
Wavelength	1.064 $\mathsf{\mu }\mathrm{m}$ (infrared)
Maximum power (time average)	200 W
Maximum pulse energy	1 mJ
Pulse width	100 ns
Repetition rate	1000 kHz

.

A laser radiation source is an infrared fiber laser with a high beam directivity and emission stability. A short-wavelength infrared laser (1.064 μm) was chosen due to its high radiation absorption coefficient for a wide range of materials.

For experiments on heating materials, the operating mode with a high pulse repetition rate was chosen. At a pulse repetition rate of 1 MHz, the pulse power was just several times higher than the average power. For surface heating, a high-repetition-rate laser mode practically does not differ from a CW one.

An external optical system was designed to transport radiation from the laser head to the specimen surface along the induction plasmatron jet axis forming the irradiation spot with a known intensity distribution. The elements of the laser beam transport system are shown in Figure 1 integrated in the HF-plasmatron system. The radiation transport system included a folding mirror designed to align the axes of the laser beam and the plasma jet. The position of the mirror and its tilt angle were adjusted using micrometer screws to fine-tune the position of the irradiation spot on the specimen surface. The size of the irradiation spot was governed by the laser beam spatial characteristics and the parameters of the lens located on the axis of the radiation transport system. The specimen was placed in the divergent part of the laser beam behind the lens focus $\mathrm{F}$, so that the spot size depended on the ratio of the lens focal distance to the distance from the lens to the specimen along the optical axis.

In the experiments, the specimen surface irradiance distribution in the spot was close to a Gaussian distribution with a radius of 10.25 mm. The laser beam was injected into the vacuum volume of the test chamber through a transparent quartz window.

Radiation losses on the elements of the optical system due to absorption, scattering and backreflection from transparent surfaces resulted in a $20\%$ lower incident laser power on the specimen than the laser output power. The dependence of the incident laser power on the set output laser power was controlled by means of precalibrated laser power meter OPHIR Starlite with thermal sensor head 50(150)A-BB-26, where 26 means the receiving pad diameter in mm. The power meter head was positioned in the divergent beam so that the ratio of the radiation spot and the receiving pad diameters was the same as on the tested specimen. When calculating the specimen surface irradiation distribution, the axial irradiation value was determined from the condition that the irradiation integral over the surface should be equal to the measured incident laser power.

3. Results

The experiment was divided in two parts.

The first part (test 22-181-1) consisted of heating the specimen only with a laser beam without exposure to a plasma jet. The specimen was mounted in the facility in the same way as in a future combined heating mode (Figure 1). The specimen was heated by laser radiation in two stages. At the first stage, the incident power of the pulsed ytterbium laser was ${\mathrm{N}}_{\mathrm{laser}}^{\mathrm{incident}}=31\mathrm{W}$ and at the second stage, the incident power was increased to 47 W. Figure 4 shows the changes in maximum temperature in the central area of the specimen surface that occurred with the increase in the laser incident power (the temperature field in the heating area at the last second of exposure is also given). The maximum surface temperature of the specimen measured by the thermal imager reached 1197 ${}^{\circ }$C (${\mathrm{N}}_{\mathrm{laser}}^{\mathrm{incident}}=47\mathrm{W}$).

The second part of the experiment (test 22-181-2) consisted of a combined heating of the specimen with a subsonic nitrogen plasma jet generated by the VGU-4 ICP facility and laser radiation (Figure 5). The specimen was exposed to a plasma jet at a constant anode power (${\mathrm{N}}_{\mathrm{a}.\mathrm{p}.}=22\mathrm{kW}$), and the incident power of the laser changed stepwise (from 0 to 47 W).

Figure 6 shows the changes in the maximum temperature in the central area of the specimen surface that occurred with an increase in the laser incident power. The maximum surface temperature of the specimen measured by the thermal imager reached 1325 ${}^{\circ }$C (${\mathrm{N}}_{\mathrm{laser}}^{\mathrm{incident}}=47\mathrm{W}$, ${\mathrm{N}}_{\mathrm{a}.\mathrm{p}.}=22\mathrm{kW}$). The temperature in the central area of the specimen surface heated only by subsonic nitrogen plasma jet was ∼1010 ${}^{\circ }$C (laser off, ${\mathrm{N}}_{\mathrm{a}.\mathrm{p}.}=22\mathrm{kW}$).

Thermal images of the specimen recorded for different moments of exposure are shown in Figure 7. Figure 7a shows the moment of the experiment when the surface was heated only by a subsonic nitrogen plasma jet. In Figure 7b–d, the area of the surface additionally heated by the laser beam is clearly visible.

Due to the low thermal conductivity of the specimen material (the thermal conductivity of the TZMK-10 material is $0.22\mathrm{W}/(\mathrm{m}{·}^{\circ }$C) at a temperature of 800 ${}^{\circ }$C), the heat flux can be determined from the Stefan–Boltzmann equation excluding heat transfer through the wall:

$$q=\epsilon \sigma T^4,$$

(1)

$\epsilon$—total emissivity, $\sigma$—Stefan–Boltzmann constant, T (K)—surface temperature.

The total heat fluxes in the central part of the specimen surface obtained from thermal images for various moments of the experiment are shown in

Table 4. Total heat flux.

Test	Time, s	Laser Incident Power, W	HF-Generator Anode Power, kW	Total Heat Flux in the Central Area of the Specimen, W/cm${}^2$
22-181-1	68	31	0	13.4
	129	47		22.5
22-181-2	60	0	22	12.6
	171	15		13.4
	297	31		24.1
	416	39		28.0
	503	47		32.1

. The maximum heat flux obtained in the combined heating for a laser incident power of 47 W and a VGU-4 HF-generator anode power of 22 kW was 32.1 W/cm${}^2$. The convective heat flux from the nitrogen plasma jet at the same anode power was 12.6 W/cm${}^2$.

The temperature distribution along the specimen’s diameter (indicated by horizontal lines in Figure 7) at various stages of exposure is shown in Figure 8. The x coordinate in Figure 8 is given in pixel numbers along the horizontal axis of the full frames recorded by the thermal imager from which the temperature distributions were obtained for the same y coordinate. The temperature distribution near the central region of the specimen surface under combined and laser radiative heating conditions (for laser incident powers from 31 to 47 W) was Gaussian-like. The temperature distribution near the central region of the specimen heated only by the subsonic nitrogen plasma jet was almost uniform. The low laser incident power (15 W) caused a uniform temperature increase along the specimen’s diameter.

4. Discussion

From the temperature graph in Figure 4, we can determine that the maximum heating rate of the specimen surface when exposed to the laser beam was 50 ${}^{\circ }$C/s. For a more massive and thermally conductive specimen, the heating rate under the same exposure conditions would be lower. The high heating rate is important when studying the thermal shock resistance of materials. In the considered modes, it can be flexibly controlled by changing the laser incident power.

Figure 6 clearly demonstrates the effect of the combined heating, which consisted of a stepwise increase in surface temperature as the incident laser power increases. It should be noted that after switching off the laser radiation source, the surface temperature of the specimen returned to its initial value. This fact confirmed the stability of the operation mode of the HF-plasmatron and also showed that that the emissivity of the specimen surface did not change after exposure.

As can be seen from Figure 7, the maximum heat flux during combined heating was slightly displaced relative to the geometric center of the heated surface, which can be explained by the aiming errors of the laser beam. The clear maximum in the temperature and heat flux distribution in Figure 7 and Figure 8 is due to the extremely low thermal conductivity of the specimen material. For a material with a higher thermal conductivity value (metal or carbon-based material), a more uniform distribution can be expected in the combined heating mode. For further experiments with low thermal conductivity materials, it will also be important to achieve a uniform temperature distribution, since this is critical for the studies of catalytic and radiative surface properties.

The temperatures in Figure 7 were obtained by correcting the thermal data for the spectral emissivity of the surface, which may not be known reliably for another class of material. To date, our group has not solved the technical problem of measuring temperatures above 1500 ${}^{\circ }$C since there are no optical pyrometry devices for high temperatures among our measuring instruments that are not affected by interference from laser radiation. In the future, it is planned to use the spectral pyrometry method [61]. The main advantage of the spectral pyrometry method is the possibility to exclude scattered laser radiation from the obtained thermal spectrum. In addition, it is not necessary to know the emissivity of the surface.

The fact that the maximum heat fluxes for the test case without exposure to laser radiation (curve “laser off” in Figure 8) were observed along the edge of the specimen can be explained by an error due to the influence of the plasma radiation reflected from the rounded edge of the specimen (the thermal imager was placed at an angle, as shown in Figure 1).

Figure 8 confirmed that the nitrogen plasma jet had no noticeable effect on the shape of the laser beam’s heat flux density distribution. This result was expected because the experiments were performed at a low pressure in the test chamber (100 hPa), and therefore at a low plasma density in the discharge region. The question of laser beam scattering with increasing pressure (up to 1 atm.) needs to be studied.

Convective heat flux to the specimen surface depends on the subsonic flow of nitrogen plasma. A deeper understanding of the investigated mode requires a numerical simulation of the subsonic jet flowing around the specimen. The Laboratory of Interaction of Plasma and Radiation with Materials of IPMech RAS developed a program package for a complete numerical simulation of a subsonic plasma flow (air, nitrogen, oxygen, carbon dioxide, argon) in the induction plasmatron [62,63]. Some modes of subsonic nitrogen plasma jets for the same discharge channel geometry were numerically simulated in [64]. For the pressure in the test chamber of the VGU-4 facility of 50 hPa and the HF-generator anode power value of $35÷65$ kW, the enthalpy of the subsonic nitrogen plasma flow varied in the range from 26.7 to 46.4 MJ/kg. To date, preliminary calculations of the jet parameters in the outlet section of the discharge channel for the investigated combined heating mode have been performed using the above programs. For a pressure in the test chamber of 100 hPa and an HF-generator anode power value of 22 kW, the enthalpy of the subsonic nitrogen plasma flow was 20.4 MJ/kg, the velocity at the jet axis was 76 m/s and the dynamic pressure at the jet axis was 12 Pa. A more comprehensive study of plasma flow, including flow and heat exchange in the boundary layer, will be the next step of our study, as it is a separate and challenging task by itself.

5. Conclusions

The new combined heating capability of the IPMech RAS ICP facility VGU-4 was successfully demonstrated. A 200 W ytterbium laser YPLN-1-100-200-R was added to the facility as a source of radiative heating. This addition to the VGU-4 facility allows the material specimens to be simultaneously exposed to radiative and convective heating conditions and gives the future opportunity to better simulate entry into the atmospheres of Mars, Venus, the outer planets and their moons.

The VGU-4 induction plasmatron has several advantages in comparison with a similar LEAF complex created at the Ames Research Center as a result of the 60 MW arc-jet facility modernization [45]: a high chemical purity of gases and almost unlimited exposure time (up to one hour).

This work was aimed at showing the very possibility of the combined heating of the material surface by an induction plasma jet and laser radiation. The specimen made of the Buran orbital vehicle heat-shielding tile material was exposed to subsonic nitrogen plasma jet and laser radiation. The maximum heat flux obtained in the combined mode for a laser incident power of 47 W and a VGU-4 HF-generator anode power of 22 kW was 32.1 W/cm${}^2$. The convective heat flux from the nitrogen plasma jet at the same anode power was 12.6 W/cm${}^2$. Thus, it can be concluded that the effect of the combined radiative and convective heating was clearly demonstrated.

For the design of a spacecraft, it is necessary to know the total (convective and radiative) heating from the reentry environment, because the radiation heat transfer contributes a substantial amount of energy transfer in addition to the conductive and convective processes [65]. At the peak heating condition of the reentry vehicle, the radiation heat transfer rate can be comparable to the convective–conductive heat transfer rate and even greater in a shock-heated region [66,67,68]. That is why we find the obtained combined heating modes promising for testing heat-shield materials.