1.Aerospace Information Research Institute, Chinese Academy of Sciences, Beijing 100094, China 2.Princeton Plasma Physics Laboratory, Princeton University, New Jersey 08543, USA 3.School of Electronic, Electrical and Communication Engineering, University of Chinese Academy of Sciences, Beijing 100049, China 4.Beijing Institute of Aerospace Micro-electromechanical Technology, Beijing 100094, China
Abstract:Based on the finite temperature plasma dielectric tensor model which contains the particle thermal effect, by numerically solving the eigenmode dispersion relation of electromagnetic waves propagating in radially uniform and magnetized warm plasma column which is surrounded by conducting boundary, the mode coupling characteristic and liner damping mechanism induced wave power deposition properties of helicon and Trivelpiece-Gould (TG) waves are parametrically analyzed. The detailed investigations show as follows. Under typical helicon plasma parameter conditions, i.e. wave frequency ω/(2π) = 13.56 MHz, ion temperature is much smaller than electron temperature, for the helicon wave, there exist a cut-off magnetic field B0,H,cutoff and a cut-off plasma density n0,H,cutoff, for which under the conditions of B0 > B0,H,cutoff or n0 < n0,H,cutoff, the helicon wave becomes an evanescent wave. When the magnetic field intensity changes from 48.4 to 484 G, i.e., ω/ωce ranges from 0.01 to 0.1, for the power deposition intensity, Landau damping of TG wave dominates for the m = 0 mode, meanwhile, for the m = 1 mode, which wave, i.e. helicon wave or TG wave, plays a major role in power deposition mainly depends on the magnitude of the magnetic field. On the other hand, for a given magnetic field B0 = 100 G, when ωpe/ωce changes from 3 to 100, for both the m = 0 mode and the m = 1 mode, the power deposition induced by Landau damping of TG wave plays a major role, further, one may notice that the power deposition of TG wave decreases while the power deposition of the helicon wave increases as plasma density increases. Finally, for both the m = 0 mode and the m = 1 mode, the power deposition due to the Landau damping plays a dominant role. All these conclusions provide us with some useful clues to better understanding the high ionization mechanism of helicon wave discharges. Keywords:helicon plasma/ mode coupling/ power deposition/ dispersion relation
图 4 螺旋波与TG波横向波数对等离子体密度的依赖关系 Figure4. The perpendicular wave number of helicon and TG waves given as functions of plasma density.
图5描述了在${T_{{\rm{eV, e}}}} = 3\;{\rm{eV}}$, ${p_{{\rm{Ar}}}} = 3\;{\rm{mTorr}}$参数条件下, 螺旋波轴向波数的实部与虚部随轴向静磁场/等离子体密度的变化关系. 图5(a)暗示, 在${n_0} = 1 \times {10^{12}}\;{\rm{c}}{{\rm{m}}^{{\rm{ - 3}}}}$等离子体密度条件下, 在$\omega /{\omega _{{\rm{ce}}}} \approx 0.2$(${B_0} = 30\;{\rm{G}}$)处螺旋波开始出现回旋阻尼, 且随着轴向静磁场的减小, 回旋阻尼强度显著增大; 当波频率$\omega /(2{\text{π}}) = 1\;{\rm{GHz}}$时, 在$\omega /{\omega _{{\rm{ce}}}} \approx $0.8—0.9范围内开始出现回旋阻尼[28]. 图5(b)暗示, 在${B_0} = 30\;{\rm{G}}$条件下, 螺旋波在${\omega _{{\rm{pe}}}}/{\omega _{{\rm{ce}}}} \approx 100$(${n_0} = 1 \times {10^{12}}\;{\rm{c}}{{\rm{m}}^{{\rm{ - 3}}}}$)处开始出现回旋阻尼, 且随着等离子体密度的增大, 回旋阻尼强度逐渐增大. 图 5 螺旋波轴向波数随参量变化情况 (a)轴向波数随轴向静磁场变化; (b)轴向静磁场随等离子体密度变化 Figure5. The axial wave number of the right hand polarized wave is given as a function of (a) axial static magnetic field and (b) plasma density.
图6描述了${T_{{\rm{eV, e}}}} = 3\;{\rm{eV}}$, ${n_0} = 1 \times {10^{12}}\;{\rm{c}}{{\rm{m}}^{{\rm{ - 3}}}}$, ${B_0} = 48.4\;{\rm{G}}$($\omega /{\omega _{{\rm{ce}}}} = 0.1$), ${p_{{\rm{Ar}}}} = 3\;{\rm{mTorr}}$参数条件下, 螺旋波与TG波的径向功率沉积分布. 图6(a)显示, 对于$m = 0$模, 螺旋波与TG波碰撞阻尼与Landau阻尼致使的功率沉积均在中心处取得峰值, 且TG波Landau阻尼致使的功率沉积占据主导地位; 图6(b)显示, 对于$m = 1$模, 回旋阻尼与异常多普勒阻尼致使的功率沉积在中心处取得峰值, 而碰撞阻尼与Landau阻尼致使的功率沉积在偏离中心处取得峰值且占据主导地位. 图 6 螺旋波与TG波径向功率沉积分布 (a) m = 0 角向对称模; (b) m = 1 角向对称模 Figure6. Radial power deposition profiles of the helicon and TG waves for: (a) m = 0 mode; (b) m = 1 mode.
图7和图8描述了${n_0} = 1 \times {10^{12}}\;{\rm{c}}{{\rm{m}}^{{\rm{ - 3}}}}$, B0 = 48.4 G ($\omega /{\omega _{{\rm{ce}}}} = 0.1$), ${p_{{\rm{Ar}}}} = 0\;{\rm{mTorr}}$参数条件下, 螺旋波与TG波的功率沉积随电子温度/离子温度的变化关系. 在螺旋波等离子体典型电子温度范围内, 对于$m = 0$模, 图7(a)显示在${T_{{\rm{eV, e}}}} < 0.5\;{\rm{eV}}$范围内, 螺旋波和TG波碰撞阻尼致使的功率沉积占据主要地位, 而在${T_{{\rm{eV, e}}}} > 0.5\;{\rm{eV}}$范围内, 螺旋波和TG波Landau阻尼致使的功率沉积在整个功率沉积机制中占据主导地位; 对于$m = 1$模, 图7(b)显示在${T_{{\rm{eV, e}}}} < 0.5\;{\rm{eV}}$范围内, TG波碰撞阻尼致使的功率沉积占据主要地位, 而在${T_{{\rm{eV, e}}}} > 0.5\;{\rm{eV}}$范围内, TG波Landau阻尼致使的功率沉积在整个功率沉积机制中占据主导地位. 另一方面, 在给定电子温度(${T_{{\rm{eV, e}}}} = 1\;{\rm{eV}}$)条件下, 图8描述了螺旋波与TG波的功率沉积随离子温度的变化关系, 由图可知, 对于$m = 0$模与$m = 1$模, 离子温度的变化对螺旋波与TG波各类阻尼致使的功率沉积的影响完全可以忽略不计, 与功率沉积随电子温度变化不同的是, 在${T_{{\rm{eV, i}}}}/{T_{{\rm{eV, e}}}} \in (0.1\;, \;10)$范围内, TG波Landau阻尼致使的功率沉积始终在整个功率沉积机制中占据主导地位. 此外, 一个重要的结论是粒子热效应的引入显著地改变了波功率沉积特性: 与仅包含碰撞效应的冷等离子体模型计算结果不同的是, 热效应的计入导致的朗道阻尼、回旋阻尼及异常多普勒阻尼为我们提供了更加清晰的波能量沉积细节特性, 即, 对于$m = 0$模, 图7(a)揭示了螺旋波和TG波Landau阻尼在功率沉积中的主导地位; 而对于$m = 1$模, 图7(b)则揭示了TG波Landau阻尼在功率沉积中的主导地位; 回旋阻尼与异常多普勒阻尼亦对功率沉积有所贡献, 但在当前参量条件下其占比很小. 图 7 螺旋波与TG波功率沉积随电子温度的变化关系 (a) m = 0 模; (b) m = 1 模 Figure7. Power deposition profiles of helicon and TG waves are given as functions of electron temperature for: (a) m = 0 mode; (b) m = 1 mode.
图 8 螺旋波与TG波功率沉积随离子温度的变化关系 (a) m = 0 模; (b) m = 1 模 Figure8. Power deposition profiles of helicon and TG waves are given as functions of ion temperature for: (a) m = 0 mode; (b) m = 1 mode.
图9描述了${T_{{\rm{eV, e}}}} = 3\;{\rm{eV}}$, ${n_0} = 1 \times {10^{12}}\;{\rm{c}}{{\rm{m}}^{{\rm{ - 3}}}}$, ${p_{{\rm{Ar}}}} = 3\;{\rm{mTorr}}$参数条件下, 螺旋波与TG波的功率沉积在$\omega /{\omega _{{\rm{ce}}}} \in (0.01\;, \;0.1)$范围内的变化情况. 由图9(a)可知, 对于$m = 0$模, TG波Landau阻尼致使的功率沉积在整个功率沉积中占据主导地位, 且随着$\omega /{\omega _{{\rm{ce}}}}$的增大这种主导特性逐渐增强; 对于$m = 1$模, 图9(b)表明, 在$\omega /{\omega _{{\rm{ce}}}} \in (0.01\;, \;0.05)$范围内, 螺旋波的Landau阻尼致使功率沉积占据主导地位, 而在$\omega /{\omega _{{\rm{ce}}}} \in (0.05\;, \;0.1)$范围内, TG波的Landau阻尼致使功率沉积占据主导地位. 这些结论表明: 对于不同角向模数, 轴向静磁场对波能量沉积影响不同; 此外我们应注意到, 相比碰撞阻尼和Landau阻尼, 回旋阻尼与异常多普勒阻尼致使的功率沉积始终很小. 图 9 螺旋波与TG波功率沉积随轴向静磁场的变化关系 (a) m = 0 模; (b) m = 1 模 Figure9. Power deposition profiles of helicon and TG waves are given as functions of axial static magnetic field for: (a) m = 0 mode; (b) m = 1 mode.
图10描述了${T_{{\rm{eV, e}}}} = 3\;{\rm{eV}}$, ${B_0} = 100\;{\rm{G}}$, ${p_{{\rm{Ar}}}} = $3 mTorr参数条件下, 螺旋波与TG波的功率沉积在${\omega _{{\rm{pe}}}}/{\omega _{{\rm{ce}}}} \in (3\;, \;100)$范围内的变化情况. 对于螺旋波, 其功率沉积随等离子体密度的增大总体呈现上升趋势; 而对于TG波, 其功率沉积随等离子体密度的增大总体呈现下降趋势. 对于$m = 0$模和$m = 1$模, 图10(a)和图10(b)表明TG波的功率沉积在整个功率沉积机制中占据主导地位, 更精确地说, 是TG波Landau阻尼致使的能量沉积占据主导作用; 在两个角向模式中, $m = 1$模在TG波Landau阻尼致使的能量沉积过程占据主导地位. 图 10 螺旋波与TG波功率沉积随等离子体密度的变化关系 (a) m = 0 模; (b) m = 1 模 Figure10. Power deposition profiles of helicon and TG waves are given as functions of plasma density for: (a) m = 0 mode; (b) m = 1 mode.