非熱平衡大氣壓電漿由於其低溫高壓態的特性與在應用上的潛力, 在過去的二十年內備受矚目。一般而言, 可以產生這類電漿的設備在尺寸上通常很小(通常是數個釐米), 並且由短脈衝類型的電源所驅動(奈秒脈衝是很常見的一個方法)。因此, 可以用來研究這類電漿的實驗方法是相當有限的。此外, 因為這類電漿的基本特性仍然不是很清楚, 因而減緩了相關應用的發展。這本論文當中, 作者發展了一個空間上一維速度上二維(1d2v) 的粒子模擬程式
(Particle-in-Cell), 合併使用蒙地卡羅碰撞(Monte Carlo Collisions) 演算法來研究這類的非熱均勻大氣壓電漿。此模擬程式模擬的系統是在近大氣壓(0.3 atm) 下, 由奈秒脈衝電壓驅動, 在平行電板間產生的氫電漿。
傳統的氣體放電機制可以由Townsend 理論[Rai91]解釋。該理論由兩個循環的步驟所組成。電子在往陽極前進的同時, 會與中性粒子碰撞並游離中性粒子, 且游離所產生的陽離子會往陰極移動。當這些陽離子與陰極產生碰撞時, 二次電子(secondary electron) 可能會在陰極表面產生, 因此造成有更多的電子往陽極移動。但這種情形並不會出現在非熱平衡大氣壓電漿當中。由模擬結果可以發現, 在非熱平衡大氣壓電漿中, 大多數因電子碰撞所游離出的陽離子在所考慮的時間之內幾乎是固定的, 僅有非常少數距離陰極很近的陽離子可以到達陰極表面。所以只有很少量的二次電子會在陰極表面發生。這些有限的二次電子在產生後會因為很強的陰極鞘層電場而加速, 進而導致了電子倍增(electron multiplication) 的發生。換句話說, 此氣體放電的過程在大部分游離出的陽離子到達陰極之前就已經產生。
另外, 模擬結果也發現此電子倍增會伴隨發生電子密度的快速增加。此電子密度的增加過程可以離子化波前的傳遞來描述。在模擬當中可以得到此離子化波前的傳遞速率及其在時空上的特徵, 且此模擬結果與實驗結果[IKCH10]一致。在模擬中, 針對傳遞速率與氣體壓力、施加電壓峰值和二次電子放射係數(對應不同的電極材料) 的相關性作了檢驗。結果指出, 傳遞速率大約對電壓峰值成正比, 對壓力成反比, 但和二次電子放射係數的相關性卻不明顯。但此離子化波前在不考慮二次電子放射的情況下將會消失。模擬結果也顯示, 電子可以短暫得形成非馬克士威分佈(non-Maxwellian distribution)。該分佈是由於兩種電子與中性粒子間強烈的非彈性碰撞所導致: v=0到v=1的振動態激發和由基態到b3S+u激發態的電子態激發。

Nonthermal plasmas at low gas temperature and high pressure have attracted much attention since last decade because of its potential for various novel applications in many fields. Such plasmas are generally generated in a device which is small in size, typically a few millimeters in length, and powered by a short period of pulses, for instance, a nanosecond pulse. As a result, only handful of experimental techniques can be used to investigate plasmas under these conditions. Furthermore, fundamental mechanisms of such plasmas still remain unclear and thus slow the development for possible applications. In this dissertation, a one dimensional in space and two dimensional in velocity (1d2v) particle-in-cell (PIC) simulation program with Monte Carlo collisions (MCC) scheme has been developed from scratch to study such nonthermal plasmas under high pressure. The simulation program is used to model hydrogen plasmas generated in between parallel electrodes driven by a nanosecond pulsed high voltage at near-atmospheric pressure (0.3 atm).
Traditionally, the mechanism for gas breakdown is well explained by Townsend theory [Rai91]. A repeated cycle consisted of two steps sustains the discharge. Firstly electron avalanche induced by electron-impact ionization while electrons travel toward the anode and the resultant ions move toward the cathode. The resultant ions reach the cathode and cause secondary electron emission. Therefore, more electrons move toward the anode and induce more ionization processes. But this appears not to be the case for nonthermal atmospheric-pressure plasmas. It is found that, in the simulations, ions generated by electron-impact ionization hardly reach the cathode in the time scale of interest except for those very close to the cathode. Consequently, there are very few secondary electrons caused by the ion-wall interaction. These finite (but very few) secondary electrons are later subjected to the cathode field built by the significantly accumulated space charge and the externally applied voltage. An electron multiplication process therefore takes place and the breakdown occurs. This process happens before most ions generated by electron-impact ionizations reach the cathode.
In addition, it has been found that the electron multiplication process is accompanied by a rapid increase of plasma density. This increased density characterizes a propagation of the ionization front. The propagation velocity obtained in simulations is in good agreement with earlier experimental observations [IKCH10]. Dependence of the propagation velocity on gas pressure p, peak applied voltage Vapp and coefficient of secondary electron emission (corresponding to different electrode materials) are also examined. The simulation results indicate that the velocity can be roughly scaled with Vapp/p ratio and has no clear dependence on the coefficient of secondary electron emission. However, such propagation of ionization front will disappear once the coefficient of secondary electron emission is set to be zero.
The simulations have also revealed that electrons can transiently form a non-Maxwellian distribution. The non-Maxwellian is mainly caused by two electron-neutral inelastic collisions: vibrational excitation of v=0 to v=1 and electronic excitation from ground state to b3S+u state.

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