Abstract
Electrons emitted from a solid surface can noticeably affect characteristics of plasma sheath surrounding that surface by modifying current balance at wall, charge separation in sheath region and Bohm criterion at sheath edge. We establish a static sheath model with kinetic electrons and cold ions to emphasize the effect of different total emitted electron velocity distribution functions (EEVDFs) on classic sheath solution and its structure transition. Four total EEVDFs with same average energy are considered separately. It is found that total EEVDFs influence the sheath solution and the threshold of total electron emission coefficient (EEC) for classic sheath dramatically, and can cause no solution for critical space-charge limited (SCL) sheath. These results indicate that, as EEC increases from zero gradually, the sheath will not transit from classic sheath to SCL sheath structure for some special total EEVDFs.
Highlights
Plasma sheaths usually described as space charge layers always appear near boundaries of finite plasmas
With above kinetic sheath model and solution method, we found that different total electron velocity distribution functions (EEVDFs) can noticeably influence sheath solutions (i.e. E, Φw, and Γc,space-charge limited (SCL))
We found that different total EEVDFs significantly affect sheath boundary parameters Φw and E, and the threshold of Γ for classic sheath
Summary
Plasma sheaths usually described as space charge layers always appear near boundaries of finite plasmas. Sodha and Mishra found that sheath potential and dust levitation near lunar surface changes noticeably when using well-established half Fermi Dirac distribution of photoelectrons to replace intuitive half Maxwellian distribution.[18] it seems that different total EEVDFs lead to different sheath structure transitions, i.e. from classic sheath to SCL sheath[6,20,21,22,23] or inverse sheath.[25,26] it naturally makes a question that how different total EEVDFs affect sheath potential and its structure transition For this purpose, four total EEVDFs will be considered in this work, i.e. single-velocity distribution (SD), uniform-velocity distribution (UD), half-Maxwellian distribution (hMD) and parabola-like Maxwellian distribution (pMD). Maxwellian speed distribution, which has one peak) to approximately characterize the total EEVDF induced by a Maxwellian-distribution incident electron group
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