Thermodynamics of a magnetically expanding plasma with isothermally behaving confined electrons
Thermodynamics of a magnetically expanding plasma with isothermally behaving confined electrons
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DOI：
10.1088/1367-2630/aac877
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发表时间：
2018-06-20
影响因子：
3.3
通讯作者：
Hwang, Y. S.
中科院分区：
文献类型：
--
作者：
Kim, June Young;Chung, K. S.;Hwang, Y. S.
研究了磁膨胀等离子体（磁喷嘴（MN））的热力学问题，考虑了在外加磁场和自生双极静电势共同作用下形成的势阱中，受限电子在势阱中来回跳跃的情况.用双面平面朗缪尔探针分别测量了束缚电子和γ（e）约为5/3的膨胀电子的性质。电子能量概率函数（eepfs）上的平均电子密度与电子压力之间的关系清楚地表明MN中的受限电子具有近等温特性。等温约束电子和膨胀电子的存在将MN系统分成两个具有不同热力学性质的区域;一个是位于喷嘴喉部附近的近绝热区域，另一个是位于远离喷嘴的近等温区域。电子热力学性质沿着远离喷管喉部的转变可用双极静电势反弹回来的电子磁矩守恒来解释。麦克斯韦eepf的近绝热电子和高能量耗尽eepf的近等温电子的共存，使整个eepf形状低能量填充eepf，表明需要仔细分析的喷管喉部附近的测量eepf。尽管受限电子对eepf和整体电子热力学有显著贡献，但发现受限电子等温行为对MN中离子加速重要的双极静电势的产生没有贡献。本研究表明，离子加速不应该直接从多变指数γ（e）的值推断，因为MN的热力学性质受到等温行为的约束电子以及膨胀电子的影响。
Thermodynamics of a magnetically expanding plasma (magnetic nozzle (MN)) has been investigated considering the existence of confined electrons bouncing back and forth inside a potential well formed by a combination of external magnetic field and self-generating ambipolar electrostatic potential. The properties of confined electrons are distinguished from that of the adiabatically expanding electrons with gamma(e) approximate to 5/3 by the separate measurement of each species using a double-sided planar Langmuir probe. Relationship between the electron pressure versus electron density averaged over electron energy probability functions (eepfs) clearly reveals that the confined electrons in MN have a nearly isothermal characteristic. Existence of isothermally behaving confined electrons together with adiabatically expanding electrons separates the MN system into two regions with different thermodynamic properties; one is a nearly adiabatic region located near the nozzle throat and the other is nearly isothermal region located far from the nozzle. A transition of electron thermodynamic property along a distance from the nozzle throat can be explained with conservation of magnetic moment of electrons bounced back by ambipolar electrostatic potential. Coexistence of the nearly adiabatic electrons with Maxwellian eepf and the nearly isothermal electrons with high energy-depleted eepf makes the overall eepf shape low energy-populated eepf, indicating a need for careful analysis on the measured eepfs near the nozzle throat. In spite of significant contribution of confined electrons to eepf and overall electron thermodynamics, it is found that the confined electrons behaving isothermally do not contribute to the generation of ambipolar electrostatic potential which is important for ion acceleration in MN. The present study suggests that ion acceleration should not be directly inferred from the value of polytropic exponent gamma(e) because thermodynamic property of a MN is influenced by isothermally behaving confined electrons as well as adiabatically expanding electrons.