Development of planar langmuir probes for supersonic plasma flows

摘要

Langmuir probes are a long established tool for the investigation and char-udacterization of plasmas. Diagnostic use of planar Langmuir probes in sta-udtionary low temperature plasmas is a well understood and long establishedudtechnique. When the plasma possesses a drift velocity greater than the ionudsound speed the flow is considered to be supersonic. Under such conditionsudthe theory for Langmuir probes is less than satisfactory. Where the flow isudsupersonic the Mach probe theory of Hudis and Lidsky [1] can be applied foruda magnetized plasma. However in the case of an unmagnetized plasma thereudis no satisfactory theory. It has been observed that in orientating a planarudLangmuir probe parallel to the direction of flow, the ion current due to theudflow is eliminated. Under such conditions the behaviour of the plasma’s in-udteraction with the probe bears close resemblance to the conditions seen inudplasma immersion ion implantation (PIII).ud This thesis describes the adaptation’s made to PIII analytical model’s toudtake advantage of these similarities and its use to then describe the ion currentudof planar Langmuir probes in unmagnetized plasmas possessing a supersonicudflow. In adapting a suitable analytical model for planar Langmuir probesudunder such conditions, extensive use of both 1D and 2D hybrid particle inudcell (PIC) simulations have been made. Additionally the work required theuddevelopment of a 2D hybrid PIC code where the probe is embedded withinudthe grid. This allowed the investigation of the impact of edge effects on theudanalytical model’s performance. The theory for and structure of the 2D codeudis also presented as part of this work.ud Three different probe orientations are considered, firstly the parallel case,udthe other two concerns the near parallel cases of the probe surface orientatedudboth into and away from the direction of flow. The model’s performanceud udunder these conditions is evaluated and discussed. Finally the use of thisudmodel in allowing a planar Langmuir probe to act as a Mach probe is alsoudconsidered.ud In testing the success of the analytical model against experimental data,udcomparisons are made between the models results and those of the 2D hybridudPIC. The experimental results used for this work were for xenon plasma withuda range of moderately supersonic velocities and a highly supersonic silver laserudablated plasma plume.ud