Category: Publications

Development of a small-scale helical surface dielectric barrier discharge for characterizing plasma-surface interfaces

Nazli Turan

Understanding plasma-surface interactions is important in a variety of emerging research areas, including sustainable energy, environmental remediation, medicine, and high-value
manufacturing. Plasma-based technologies in these applications utilize surface chemistry driven by species created in the plasma or at a plasma-surface interface. Here, we develop a helical dielectric barrier discharge (DBD) configuration to produce a small-scale plasma that can be implemented in a diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) cell and integrated with a commercial Fourier transform infrared (FTIR) spectrometer instrument to study plasma interactions with inert or catalytic solid media. The design utilizes the entire surface of a cylinder as its dielectric, enhancing the plasma contact area with a packed bed. In this study, we characterize the electrical and visual properties of the helical DBD design in an empty reaction cell and with added potassium bromide (KBr) powder packing material in both air and argon gas environments at ambient conditions. The new surface DBD configuration was integrated into a DRIFTS cell and the time evolution of water desorbing from the KBr packed bed was investigated. Measurements show that this configuration can be operated in filamentary or glow-like mode depending on the gas composition and the water content absorbed on KBr solid media. These results not only set the basis for the study of plasma-surface interactions using a commercial FTIR, but also show that controlling the gas environment and water content in a packed bed might be useful for studying different plasma regimes that are typically not possible at atmospheric pressure.

You can reach the paper via this link: Nazli Turan et al 2020 J. Phys. D: Appl. Phys

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The impact of transition metal catalysts on macroscopic dielectric barrier discharge (DBD) characteristics in an ammonia synthesis plasma catalysis reactor

Nazli Turan

When non-equilibrium, low-temperature plasmas and catalysts interact, they can exhibit
synergistic behavior that enhances the chemical activity above what is possible with either process alone. Unlike thermal catalysis, in plasma-assisted catalysis the non-equilibrium state of the plasma produces reactive intermediates, such as excited species, that may play an important role in the catalytic process. There are two primary plasma-surface mechanisms that could produce this synergy: the effect of the plasma on the catalyst (e.g., enhanced adsorption / reaction of plasma activated species, change of surface structure / morphology, hot spots, etc.) and the effect of the catalyst on the plasma state. This work focuses on the latter. We use a laboratory-scale, packed bed, dielectric barrier discharge (DBD) reactor to observe the influence of multiple alumina (Al2O3) supported, transition metal ammonia (NH3) synthesis catalysts on the plasma electrical and optical properties. We find that while the rates of ammonia synthesis over the materials considered, including Fe/Al2O3, Ni/Al2O3, and Co/Al2O3, are different, the macroscopic properties of the DBD are statistically indistinguishable. These results support the argument that the observed synergy in our catalysis experiments is not due to the catalyst modifying the characteristics of the plasma itself, but rather arises from differences in how the plasma environment and plasma-generated species modify chemistry at the catalyst surface, although the specific mechanism is still an outstanding question.

You can reach the paper via this link: Journal of Physics D: Applied Physics, Volume 52, Number 22

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Experimental Study of the Effects of the Cathode Position and the Electrical Circuit Configuration on the Operation of HK40 Hall Thruster and BUSTLab Hollow Cathode

Nazli Turan

HK40 Hall thruster, designed and developed at the Bogazici University Space Technologies Laboratory (BUSTLab), is an SPT type Hall thruster with a 40 mm discharge channel. HK40 was initially designed to operate with SmCo permanent magnets. To optimize the magnetic field topology, the permanent magnets were replaced with iron-core electromagnets. The thruster is operated with different magnetic coil currents to observe the changes in discharge characteristics. Magnetic field topology of the thruster is examined to determine the proper location of a LaB6 hollow cathode, which is also designed and built at BUSTLab. External magnetic field topology of a Hall thruster has an important characteristic called magnetic field separatrix defining the boundary between closed magnetic surfaces and open magnetic field lines. To investigate the effects of the separatrix surfaces, the location of the cathode is changed in-situ with respect to the Hall thruster with a 2-D translational stage in two different grounding configurations, one connecting the vacuum chamber to the same ground with the power supplies, and the other with the power supplies connected to a common floating ground. We show that the influence of the external magnetic field strength on the thruster efficiency can be predicted from the electron current coming from the cathode emitter surface. We also show that the cathode to ground voltage provides a way to estimate the efficiency with respect to the cathode placement. The mechanisms and the efficiency values of two setups are compared to explain the ground and the space operations.

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Experimental Investigation of the Effects of Cathode Current on HK40 Hall Thruster Operation

Nazli Turan

Hall effect thrusters utilize electric and magnetic fields to extract ions from a plasma discharge. The cathode is responsible for the ionization of the propellant and the neutralization of the ion beam by emitting an equal number of electrons to prevent spacecraft charging. Hollow cathode electrons are extracted from LaB6 insert surface by thermionic emission. The electrons leaving the surface generate a negative cathode voltage around LaB6 emitter. As the emitter surface expels electrons, the same amount of electrons are attracted from the ground. Those electrons are measured as the ground current. For Hall effect thrusters, the electron movements are determined by the external magnetic lines of the thruster. If electrons could not pass the magnetic field lines, they could not reach the anode and the magnitude of cathode to ground voltage increases. As a result, plume plasma potential increases. This study shows that by measuring the electron current coming from the emitter surface, influence of the external magnetic field strength on the efficiency of the thruster could be predicted.

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Investigation of the Effect of Hollow Cathode Neutralizer Location on Hall Effect Thruster Efficiency

Nazli Turan

Neutralization of ions is important for all electric thruster types when considering thruster efficiency and life. Hollow cathode is responsible for both creating plasma discharge and neutralization of the beam ions for Hall Effect Thruster (HET). In this study, appropriate placement of the cathode is investigated by taking into account that the decrease in cathode coupling voltage increases thruster efficiency. Regarding this, the effects of mass flow rate of the cathode and keeper current on the coupling voltage are investigated, according to available experimental results from the literature.

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Note: Coaxial-heater hollow cathode

Nazli Turan

The design and tests of a LaB6 hollow cathode with a novel heater are presented. In the new design, the heater wire is completely encapsulated around the cathode tube and a coaxial return electrode, thereby eliminating hot spots on the heater wire due to the free hanging regions. Since the new heater confines the Joule heating to the region of interest, where the LaB6 emitter is placed, the heater terminals are further secured from overheating. The cathode with the presented heater design has been successfully tested and is able to deliver currents in the 0.5-15 A range.

You can reach this paper with the following link:

Kurt, H.; Turan, N.; Kokal, U.; Celik, M. “Note: Coaxial-heater Hollow Cathode”, Review of Scientific Instruments, Vol. 88, No. 6, pg. 066103, 2017.

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Thermal Analysis and Testing of Different Designs of Lanthanum Hexaboride Hollow Cathodes

Nazli Turan

Electric propulsion systems provide a higher delta-V for the same mass of propellant when compared to chemical propulsion systems due to their higher Isp levels. Many electric propulsion systems utilize cathodes as electron source. Hollow cathodes generate electron current through thermionic emission mechanism. In this study conventional hollow cathode designs are investigated numerically and experimentally. Considering the problems that are encountered with the conventional designs, a new hollow cathode design is developed, which is called coaxial hollow cathode. Operational parameters of the coaxial cathode are investigated experimentally.

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Development of the New BUSTLab Hall Thruster with Internal Coaxial Hollow Cathode

Nazli Turan

HK40 Hall thruster is a low power SPT-type Hall thruster with a boron nitride (BN)
discharge channel of 40 mm in outer diameter. Based on the experience gained during the design and manufacturing of the HK40 Hall thruster, and the experiments that were conducted with this Hall thruster, an advanced larger diameter Hall thruster with an LaB6 internal cathode is developed. In the design, protection of thruster wall material is aimed by the appropriate magnetic field inside the thruster channel. Design optimization of this new Hall thruster is made by investigating the effects of different design parameters, such as magnetic circuit and discharge channel geometry. A thermal model of this thruster is developed in order to investigate the heat distribution for the thruster-cathode system.

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Design Improvements and Experimental Measurements of BURFIT-80 RF Ion Thruster

Nazli Turan

BURFIT-80, a prototype radio-frequency ion thruster, is designed, built and tested
at the Bogazici University Space Technologies Laboratory. This paper presents the design parameters and numerous design improvements of this thruster. Three different versions of the thruster, with the same discharge chamber inner diameter of 80 mm, have been built and tested. The latest version of this prototype thruster presents significant improvements of the DC electrical connections to the grids and RF electrical connections to the RF antenna. For the second version of the thruster, plume ion energy distribution measurements are conducted using an indigenously developed retarding potential analyzer, and some of the measurement results are presented.

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Study of Magnetic Field Configuration Effects on Coupling between Hall Effect Thruster and Hollow Cathode

Nazli Turan

Hall effect thrusters utilize electric and magnetic fields to extract ions from a plasma discharge. In Hall effect thrusters, the ionization of the propellant gas is achieved by collisions of the neutral propellant gas atoms with the emitted electrons from a cathode, typically a hollow cathode. Besides, the cathode is responsible for the neutralization of the ion beam by emitting an equal number of electrons to prevent spacecraft charging. Proper placement of the cathode strongly affects the neutralization of ions in addition to creating well coupled discharge plasma. Studies from the literature show that the cathode coupling voltage is a function of cathode placement and thruster efficiency. Cathode coupling voltage is related to the external magnetic field lines of the thruster. This study shows that depending on the external magnetic field topology of the thruster, there could be an optimum position for the cathode considering the separatrix region.

You can reach this paper with the following link:

8th Ankara International Aerospace Conference, Ankara, Turkey, September 2015, also AIAC-2015-119.

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