7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Investigation of Ionization Instability in a Linear Plasma Device
Subir Biswas1, 2 and Dibyajyoti Bora1, 2
1Institute of Advanced Study in Science and Technology2
Academy of Scientific and Innovative Researche-mail: subirb@iasst.gov.in

Hall thrusters are electric propulsion devices with crossfield (𝑬 × 𝑩) configuration, presently used for near earth
applications and emerging as a technology of choice inupcoming deep space missions like trips to Mars [1]. InHall thruster electrons are magnetized and trappedwhereas the unmagnetized heavier ion species areaccelerated by the electric field and extracted from theplasma producing thrust. In spite of the advancement of
the Hall thruster technology, there are several limitationsof its operation because of the various instabilities thatarises from the various free energy sources (e.g., density,temperature and velocity gradient etc) present in it.
Among the various instabilities, ionization instability also
known as breathing mode oscillations, which is related tothe ionization process is supposed to be one of the mostprominent instability which deteriorates the performanceof Hall thrusters [2]. This low frequency oscillation modein the longitudinal direction is accompanied by a strong
periodic depletion of neutrals in the ionization area. Theseoscillations can cause fluctuations in thrust and efficiency,
which can be detrimental to the thruster's performance.
Although the ionization nature of the breathing mode isgenerally accepted, exact mechanism remains poorly
understood and is an important area of research in basic
plasma physics.
Unavailability of the ports for the insertion of thediagnostics, limitation of space and simultaneousoccurrence of various instabilities in Hall thruster make it
difficult to study the ionization instability in it in control
manner. Therefore, linear plasma device can be helpful tostudy the instability in a controlled environment [3, 4]. A
linear plasma device producing uniform and quiescentplasma has been fabricated indigenously at IASST for
studying various waves and instabilities in a precise
manner [5].
High degree of quiescence was measured in the neutralpressure range of 1×10−4−1×10−3 mbar. However, theonset of certain instabilities results in a reduced value ofplasma quiescence beyond this neutral pressure range. To
investigate these phenomena, time series of the floating
potential and discharge current were recorded fromvarious spatial regions with an array of Langmuir probes
at different filling gas pressures. It was observed that selfexcited oscillations with distinctive frequency
components were present at both higher and lower filling
gas pressures as shown in Figure 1 (a-f). Frequency of theoscillation was found to be depend on filling gas pressure.
Interestingly, the fluctuations detected at lower filling gas
pressure appear only for a narrow pressure range justbefore the extinction of the plasma. Dependence of theseself-excited oscillations on filling gas pressure indicatesan association with the ionization instability. These
floating potential fluctuations showed no phase difference
when observed at different axial positions simultaneously,indicating formation of a standing wave. The dominantfrequency component of these self-excited oscillations
was found to be varied with different filling gases. Density
fluctuations were also investigated by monitoring thefluctuations in the ion saturation region. In the present
study, detailed experimental results will be presented.
References[1] D. Y. Oh, S. Collins et al, “Development of the psyche
mission for NASA’s discovery program,” in 35th
International Electric Propulsion Conference (ERPS,Atlanta, GA, 2017).
[2] J-P Boeff and L. Garrigues, J. Appl. Phys., 84, 3541
(1998)[3] P. W. Lee, S. W. Lee and H. Y. Chang, Appl. Phys. Lett.
69, 2024 (1996)[4] S. Biswas et. al., Phys. Rev. Lett.111, 115004 (2013)[5] D. Bora, A. Tarafder, S. Biswas, et. al., Phys. Scr. 98,045618 (2023)

Figure 1(a-f): Time series signal of (a-c) floating potential
fluctuations and (d-f) discharge current fluctuations atdifferent filling gas pressures.