The Design Of A Collimated Faraday Probe And Its Application In Plasma Beam Diagonostics Of SPT

The reason why a nude Faraday probe can' t truly measure the ion current density in Hall thruster is discussed and the design of a collimated Faraday probe is reported in this paper.The resonant charge exchange(CEX) collisions of directed plume ions with the random background population of neutrals will bring low energy ion,which will be recorded as ion current by Faraday probe.A collimated probe design is attractive because it offers the possibility of obtaining the true ion current density profile regardless of the facility pumping speed.For improving the measuring precision of Faraday probe,firstly the paper analyses the basic principle and the physics process of plasma plume diagnose with it.Then it provides some assumptions to simplify the physics processes.Based on these assumptions,the expressions for the scale factor are derived.The structure and the size of the Faraday probe is decided by these theoretical expressions.Also the materials of the collector inside the collimated probe and the collimated tube are chosen.After all, the processing and fixing for the accessories of the collimated probe were be finished in 801 Institute.Experiments were conducted using a traditional nude Faraday probe and a collimated probe.The probes are evaluated using a 660W Hall thruster(SPT-70,which is made in 801 Institute.) in a facility capable of maintaining background pressures of 1.7-2.3×10^-2 Pa for cathode flow rates of 2sccm and anode flow rates of 16-20sccm. After preparing testing circuit,measurements were made for each position with the two probes either floating or biased to the collectors to evaluate them.Detailed examination of the results has shown that the collimator can act primarily as a filter for low energy ions introduced in the plume by charge exchange collisions and the collimated probe is interacting strongly with the plasma inside the collimator. The pressure and electric fields inside the collimator are considered in this paper.Finally other investigations may interrogate the probe interior itself by measuring the internal pressure and plasma properties with ionization gauges and electrostatic probes.Numerical modeling of the probe is also being considered,as are alternative designs to the probe geometry and materials.Further study is required to bring the collimated Faraday probe to a maturity level sufficient for use in evaluating Hall thruster plumes.