Demonstration tests were conducted using a diagnostic thermionic converter (N597) designed and manufactured at NII NPO LUCH (JV INERTEK). The converter at NII NPO LUCH was a subject of comparative research on mass loss rate of two ceramic materials (aluminium oxide and scandium oxide) at constant sample temperature in various modes of the converter operation. These investigations were continued in NMERI on the modified TISA Rig. Substantial corrosion of Al2O3was revealed after sample exposure in cesium plasma. It was also determined that Sc2O3does not interact with cesium plasma. Sc2O3mass loss rate in all thermionic converter operation modes does not exceed values corresponding to diffusion mode and is by several orders lower than Al2O3mass loss rate. Based on these results the lifetime of TFE electrode spacers was evaluated. © 1995American Institute of Physics