Thermal Analysis and Testing of Different Designs of LaB₆ Hollow Cathodes to Be Used in Electric Propulsion Applications

• Main Authors: Ugur Kokal, Nazli Turan, Murat Celik
• Format: Article
• Language: English
• Published: MDPI AG 2021-08-01
• Series: Aerospace
• Subjects: electric propulsion; hollow cathode; LaB6 emitter; thermal analysis
• Online Access: https://www.mdpi.com/2226-4310/8/8/215

<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>L</mi><mi>a</mi><msub><mi>B</mi><mn>6</mn></msub></mrow></semantics></math></inline-formula> emitters are commonly used in hollow cathodes that are utilized in electric space propulsion systems. In order to obtain necessary surface current emission densities of 1–10 A/cm² for cathode operations, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>L</mi><mi>a</mi><msub><mi>B</mi><mn>6</mn></msub></mrow></semantics></math></inline-formula> emitters require temperatures above 1500 °C. Hence, the design for <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>L</mi><mi>a</mi><msub><mi>B</mi><mn>6</mn></msub></mrow></semantics></math></inline-formula> cathodes presents thermal and mechanical challenges. In this paper, several design iterations for <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>L</mi><mi>a</mi><msub><mi>B</mi><mn>6</mn></msub></mrow></semantics></math></inline-formula> hollow cathodes are presented and thermal analyses are conducted for each design. Temperature and heat flux distributions that are obtained from thermal analyses are investigated. The designs are evaluated according to the required heat input to the emitter, and the radiative and conductive heat loss mechanisms. In addition to the thermal analyses, experimental tests are conducted for different cathode designs and, based on these tests, various modes of failure are determined. Revising the cathode design and the material selection iteratively to eliminate the encountered failure mechanisms, a novel cathode design is achieved. Experimental tests of this novel cathode are conducted and current-voltage characteristics are presented for different mass flow rates and for discharge currents between 0.5 and 12 A. Tests and analysis results show that the selection of materials and design are crucial for a sturdy and long lifetime cathode.