Filament size of floating-emissive probe for low density plasmas with large space potential

Space potential φ s of non-neutral plasmas with a low density of n e ∼10 12 m −3 are measured by two floating-emissive probes. Nothing is different between them except the area S of filaments. Despite the fact that the thermionic current is sufficiently emitted, floating potential φ f outputted from the smaller filament is much larger than the realistic φ s at some measurement points, which is contrary to the widely known relation of φ f ⩽φ s in probe measurements. The result is attributed to the insufficient probe current I p collected in low- n e plasmas with a large φ s . This is because, in such a plasma, I p does not always satisfy the necessary condition of I p >φ s /R HI , where R HI is a high impedance resistor, although the value of I p required for the floating emissive method is very small. In order to correctly determine the φ s of the plasmas, S must be larger than φ s /en e 〈v e 〉R HI , where e is the electron charge and 〈v e 〉 is the mean speed of electrons collected to the probe.