Experimental Study On Emissive Probe Technique And Plasma Sheath Structure Diagnostics

The primary reason of that the current electrostatic probe theory can't achieve quantitative accuracy measurements of plasma is that the structure of sheath is not completely clear.The limitation of describing the region near the boundary of plasma sheath with the existing theories of plasma and sheath shows that the electric field intensity at the boundary between plasma and sheath is inconsistent.In order to solve the discontinuity at the boundary between plasma and sheath,some scholars proposed to smoothly connect plasma with sheath by a transition region,while others believed that the boundary electric field between plasma and sheath should be defined as Te/e?D.The main reason why the plasma sheath boundary is still unresolved is the lack of accurate experimental data of sheath diagnostic.In order to resolve the problem of sheath structure and further achieve the quantitative accuracy measurements of plasma with the electrostatic probe theory,we consider starting the research with the emissive probe diagnostic of plasma sheath structure.The sheath structure diagnostic difficulties focus on the accurate measurement of the potential distribution near the plasma sheath boundary,because the existing emissive probe diagnostic techniques can't achieve an accurate enough measurement accuracy(-0.1 V)as the potential drop near the boundarty is usually very small(?1 V).Therefore,we firstly study the emissive probe diagnostic techniques for space potential measurements.According to the research results,we have proposed an improved emissive probe inflection point potential method with zero emission limit,suggesting that the potentials obtained by linearly extrapolating the curve between the inflection point potentials(Vip)of the emissive probe I-V characteristics and the probe heating currents(Iht)to the heating current where the electron emission current(Iemis)is zero are the most accurate results for plasma potential measurements.The measurement of the vacuum space potential distribution between two parallel plates shows the improved inflection point potential method has a measurement accuracy of 0.1 V in space potential measurements,providing a gurantee for accurate measurements of sheath potential distribution.Due to the tedious operation steps of the improved inflection point potential method,the manual execution of the method in the measurement of sheath potential distribution is hard to achieve because of the large workload and the long measuring time.Therefore,based on the improved inflection point potential method with zero emission limit,we have independently develpoed an automatic emissive probe diagnostic apparatus.The device can automatically apply and adjust the heating current and the probe bias to the emissive probe,automatically acquire and analyze the experimental data,quickly give measurement results,and greatly improve the efficiency of accurately measuring space potentials with emissive probe.The potential distribution of the negative cathodic sheath measured by the automatic emissive probe diagnostic device and the related theoretical analysises of the experimental data show that there is a transition region between the sheath and the presheath predicted by Riemann.The potential distribution in the sheath satisfies the Child's law,and the potential distribution in the presheath fits the corresponding expression of the presheath potential distribution.The experimental electric field intensity at the boundary between the transition region and the presheath coincides with the sheath boundary condition of the transition theory,verifying the correctness of sheath boundary electric field in the transition theory.In addition,we have also studied the space potential measurements at higher pressure.The measurements of vacuum space potential and microwave ECR argon plasma potential in higher pressure environments show that the improved inflection point potential method can reliably measure the space potentials with a pressure under 350 Pa.In the environment above 350 Pa,the effective space potential measurements can't be complished by the tungsten emissive probe because the tungsten wire is easily burnt out due to the high oxygen content in the vacuum chamber.The reliable measurements of space potentials in higher pressure environments with emissive probe provide a basis for the further research of collisional sheath structure.