Thermal Image Measurements of the Radio-Frequency Ion Thruster Accelerating Electrode Temperature

Аuthors

Abgarian V. K.^*, Abgaryan M. V.^**, Mogulkin A. .^***, Semenov A. A.^****

Moscow Aviation Institute (National Research University), 4, Volokolamskoe shosse, Moscow, А-80, GSP-3, 125993, Russia

*e-mail: vka.mai@mail.ru
**e-mail: abgmvk@gmail.com
***e-mail: riame@sokol.ru
****e-mail: heat204@mail.ru

Abstract

The electrojet engines running is being accompanied by the structural component heating up to the temperatures of about 300–400°C. Temperature measurements in the electrojet engines are necessary at both design stage and while the engine certification. Measured temperatures are being employed while the computational thermal models verification as well.
The article discusses both contact and non-contact techniques of temperature measurement in radio-frequency ion thruster (RIT), which is one of the ion engines schemes. Contact measurements by thermocouples, thermal sensors, etc. in the said thruster scheme are hampered in view of the fact that that under conditions of the RF plasma discharge, the wires connected to the thermal sensors represent the receiving antennas, in which high-frequency currents distorting the measurements results are being induced.
A relatively small number of non-contact temperature measurements have been previously performed in the world for several RITs of various sizes with pyrometers and thermal imagers. The temperature was being determined from the value of the heat flux measured by these instruments in the infrared range from the Stefan-Boltzmann law, with account for the degree of surface blackness. Measurements are associated with overcoming a number of problems, the main of which are:
- uncertainty in the values of the degree of frequency, including its dependence on the temperature of the object under study;
- low resolution of the optical system in terms of the large wavelength of the radiation received from the IR range;
- limitations on the surface details resolution stipulated by the digital form of еру thermograms (pixelation of images) in the case of noticeable temperature gradients on the surface
- the presence of reflected radiant heat fluxes from surrounding surfaces in cases of temperature measurements in the structures with complex geometry.
The temperatures on the accelerating electrode surface, being the external electrode of the ion-optical system of the RF ion thruster with an ion beam of the 8 cm diameter, was being measured in the presented work with the thermal imager. The electrode was perforated with a large number of orifices, through which the intense thermal radiation from the discharge chamber of the thruster entered, which hampered the measurements. While the measurements, the thermal imager was placed as close as possible to the measured surface at a distance of 0.5 m, which allowed obtaining  the accelerating electrode thermograms suitable for processing.
The impact of the reflected streams was significantly suppressed by the black mark method application. A soot mark, with a degree of blackness close to unity, was applied to the surface areas of the accelerating electrode, in which the temperature measurements were conducted. Thus, the impact of the reflected flows could be ignored.
Measurements were hampered due to the bombardment of soot with the ions of recharging with energies of the order of 100 eV. The ions sprayed the soot at the very beginning of the measurements by virtue of the low adhesion of the soot. The sections of the hexagonal structure, characteristic to the surface erosion from ions of recharging, were exposed herewith to the initial surface material. In this case, the electrode was manufactured from titanium, which emissivity factor was quite lower from the soot one and was 0.3 ...0.4, which might distort the measurements. Thus, the thermograms temperature readings were being registered at the points of the accelerating electrode surface equidistant from the orifices edges in the electrode and the electrode erosion areas.
As a conclusion of the work, it may be affirmed that the technique for the thermal imaging measurements with the black mark method application, which solved the problems arousing while contactless temperature measurements of in the RF ion thrusters, was worked out.

Keywords:

radio-frequency ion thruster, ion-optical system, accelerating electrode, thermal imager, thermogram, black mark method

References

1. Goebel D.M., Katz I. Fundamentals of electric propulsion: ion and Hall thrusters. John Wiley & Sons, 2008. Vol. 1, 526 p. DOI:10.1002/9780470436448
2. Holste K., Dietz P., Scharmann S. et al. Ion thrusters for electric propulsion: Scientific issues developing a niche technology into a game changer. Review of Scientific Instruments, 2020, vol. 91, no. 6: 061101. DOI: 10.1063/5.0010134
3. Popov G.A. Electrical rocket engines-their development in Russia and a contribution of the Moscow Aviation Institute. Aerospace MAI Journal, 2005, vol. 12, no. 2, pp. 112–122.
4. Sinitsin A.P., Parakhin G.A., Rumyantsev A.V. Thermal design of cathode with barium thermo-emitter. Aerospace MAI Journal, 2020, vol. 27, no 2, pp. 71–80. DOI: 10.34759/vst-2020-2-71-80
5. Kaplin M.A., Mitrofanova O.A., Bernikova M.Yu. Development of very low-power PlaS-type plasma thrusters. Aerospace MAI Journal, 2021, vol. 28, no. 1, pp. 74–85. DOI: 10.34759/vst-2021-1-74-85
6. Kazbanov A.A., Dronov P.A., Spivak O.O., Derkachev A.B. Vestnik Samarskogo universiteta. Aerokosmicheskaya tekhnika, tekhnologii i mashinostroenie, 2021, vol. 20, no. 3, pp. 49–56. DOI: 10.18287/2541-7533-2021-20-3-49-56
7. Svotina V.V., Mogulkin A.I.; Kupreeva A.Y. Ion Source— Thermal and Thermomechanical Simulation. Aerospace, 2021, vol. 8, no. 7: 189. DOI: 10.3390/aerospace8070189
8. Fedorov V.A., Obukhov V.A., Mogulkin A.I. Trudy MAI, 2014, no. 77. URL: https://trudymai.ru/eng/published.php?ID=52975
9. Van Noord J.L. NEXT Ion Thruster Thermal Model. 43rd AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit (08-11 July 2007; Cincinnati, OH). DOI: 10.2514/6.2007-5218
10. Anderson J.R., Snyder J.S., Van Noord J.L., Soulas G.C. Thermal Development Test of the NEXT PM1 Ion Engine. 43rd AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit (08–11 July 2007; Cincinnati, OH). DOI: 10.2514/6.2007-5217
11. Waldemar G., Lotz B., Meyer B.K. 3D Thermal Simulation of a µN-RIT. 33rd International Electric Propulsion Conference (06–10 October 2013; The George Washington University, Washington, D.C., USA). IEPC-2013-298.
12. Dobkevicius M., Feili D. A coupled performance and thermal model for radio-frequency gridded ion thrusters. The European Physical Journal D, 2016, vol. 70, no. 11: 227. DOI: 10.1140/epjd/e2016-70273-7
13. McGee T.D. Principles and Methods of Temperature Measurement. John Wiley & Sons, Inc., New York, 1988, 608 p.
14. Domonkos M.T., Gallimore A.D., Patterson M.J. Thermographic investigation of 3.2 mm diameter orificed hollow cathodes. 34th AIAA/ASME/SAE/ASEE Joint Propulsion Conference (13–15 July 1998; Cleveland, OH). AIAA Paper No. 98-3793.
15. Proshkin S.S. Vestnik МАХ, 2014, no. 1, pp. 51–54.
16. Nikitin S.A., Stepanenko P.V. A technique for assessment of blackness level for heat resistant coatibgs. Aerospace MAI Journal, 2008, vol. 15, no. 5, pp. 58–61.
17. Bundesmann C., Tartz M., Scholze F. et al. In-situ temperature, grid curvature, erosion, beam and plasma characterization of a gridded ion thruster RIT-22. 31st International Electric Propulsion Conference (20–24 September 2009; University of Michigan, Ann Arbor, Michigan, USA). IEPC-2009-160, pp. 20–24.
18. Bundesmann C., Tartz M., Scholze F. et al. In Situ Thermal Characterization of the Accelerator Grid of an Ion Thruster. Journal of Propulsion and Power, 2011, vol. 27, no. 3, pp. 532–537. DOI: 10.2514/1.50049
19. Bundesmann C., Eichhorn C., Scholze F. et al. Electric Propulsion Thruster Diagnostic Activities at IOM. 34th International Electric Propulsion Conference (2015; Hyogo-Kobe). Paper IEPC-2015-392.
20. Bundesmann C., Tartz M., Scholze F. et al. Note: An advanced in situ diagnostic system for characterization of electric propulsion thrusters and ion beam sources. Review of Scientific Instruments, 2010, vol. 81, no. 4: 046106. DOI: 10.1063/1.3386585
21. Kruglov K.I. Modelirovanie teplofizicheskikh protsessov v vysokochastotnom ionnom dvigatele (Modeling of thermophysical processes in a high-frequency ion engine) PhD. thesis. Moscow, MAI, 2017, 143 p.
22. Abgaryan V., Balashov V., Kupreeva A., Nigmatzyanov V. Verification of numerical model of temperature calculations in radio-frequency ion thrusters. IOP Conference Series: Materials Science and Engineering. Vol. 927. XIII International Conference on Applied Mathematics and Mechanics in the Aerospace Industry (06–13 September 2020; Alushta, Russia). No. 1: 012010. DOI: 10.1088/1757-899X/927/1/012010