Observation of the low electron density and electron temperature in an unmagnetized cascaded arc helium plasma by laser Thomson scattering approach

In this study, the electron density (n_e ) and temperature (T_e ) in an unmagnetized cascaded arc helium (He) plasma are precisely determined using cutting-edge laser Thomson scattering. In our experimental scope, n_e is only 10¹⁸ m⁻³ and T_e is less than 0.2 eV, both of which are substantially lower than in linear plasma devices (LPDs). The comparison of n_e and T_e values in He plasma with those in cascaded arc Ar plasma reveals that these two parameters are likewise significantly lower in He plasma than they are in Ar plasma on average. In comparison to Ar gas, the degree of ionization of He is low due to its high ionization potential, and diffusive loss dominates due to its light weight, both of which result in a lower n_e . Meanwhile, these two characteristics render the three-body recombination interaction between electrons and He⁺ ions in He plasma insignificant, thus the electrons cannot be heated effectively, explaining why T_e is lower. This study will provide foundational data and build the groundwork for a thorough knowledge of cascaded arc He plasma in LPDs and plasma windows.