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Abstract Comparison between the Maxwell demon and a planar electrode has been revisited with an in-depth analysis of whether the angular momentum trap of the Maxwell demon indeed provides better energy selectivity than a small planar electrode that absorbs electrons indiscriminately. The evolutions of the EEDF under the influence of these heating techniques is directly analyzed, as well as the resultant plasma parameters. Experimental results show that the Maxwell demon indeed provides better energy selectivity as shown by its better retention of hot electrons than an indiscriminative absorption surface, which in turn results in smaller disturbance to the plasma potential a smaller reduction of the plasma density in the heating process. Experimental result also shows no electron heating when the demon is replaced by an ion-sheath forming large electrode, this is consistent with Mackenzie’s original results (MacKenzie et al 1971 App. Phys. Lett. 18 529). While it is possible to obtain the exact same plasma parameters replacing the Maxwell demon with a suitably sized planar plate and additional plasma parameters control, for experiments sensitive to the exact processes from which plasma parameters are formed, one should not overlook the physical differences of these heating methods. 

Abstract For unmagnetized low temperature Ar plasmas with plasma density ranging from 3 × 10 8 to 10 10 cm −3 and an electron temperature of ∼1 eV, the expansion of the ion collecting area of a double-sided planar Langmuir probe with respect to probe bias is experimentally investigated, through a systematic scan of plasma parameters. In accordance with many existing numerical studies, the ion collecting area is found to follow a power law for a sufficiently negative probe bias. Within our experimental conditions, the power law coefficient and exponent have been parameterized as a function of the normalized probe radius and compared with numerical results where qualitatively comparable features are identified. However, numerical results underestimate the power law coefficient while the exponent is overestimated. Our experimental measurements also confirm that ion–neutral collisions play a role in determining the expanded ion collecting area, thus changing values of the power law coefficient and exponent. This work suggests that a power law fit to the ion collecting area must be performed solely based on experimentally obtained data rather than using empirical formulae from simulation results since material and cleanness of the probe, type of working gas, and neutral pressure may also affect the expansion of the ion collecting area, factors which are difficult to model in a numerical simulation. A proper scheme of analyzing an I – V characteristic of a Langmuir probe based on a power law fit is also presented.