利用三维模拟计算正流注现象的无线电发射
Calculating radio emissions of positive streamer phenomena using 3D simulations
AI总结：
本研究通过三维模拟结合两种光电离方法，探究正流注的无线电发射特性，明确不同流注过程的发射规律，给出电流近似式与电流矩拟合公式。
摘要：
我们利用三维模拟研究空气中正流注的无线电发射，通过求解Jefimenko方程计算辐射电场。模拟在0.5 bar气压下开展，采用两种光电离方法：用于光子密度的Helmholtz近似法，以及使用离散光子的Monte Carlo方法，后者更具真实性。我们研究了单一流注、流注分支、流注与预电离相互作用以及流注-流注相遇等多种情形。我们未在分支过程中或分支后观测到强甚高频（VHF）无线电信号，这一结果得到了实验室实验的证实。这表明流注内部的电流放电在分支过程中近似连续演化。另一方面，由Monte Carlo光电离导致的流注传播随机波动，会使100 MHz及以上频率的无线电发射量增加。另一种产生此类高频发射的过程是流注与弱预电离区域的相互作用，该区域可能由先前的放电产生。与既往研究一致，我们观测到流注相遇产生的发射最强、频率最高。总辐射能量似乎主要取决于背景电场，与流注的具体演化过程关联较小。最后，我们给出了流注通道最大电流的近似式，以及流注电流矩的拟合公式。
英文摘要：
We study radio emissions from positive streamers in air using 3D simulations, from which the radiated electric field is computed by solving Jefimenko's equations. The simulations are performed at 0.5 bar using two photoionization methods: the Helmholtz approximation for a photon density and a Monte Carlo method using discrete photons, with the latter being the most realistic. We consider cases with single streamers, streamer branching, streamers interacting with preionization and streamer-streamer encounters. We do not observe a strong VHF radio signal during or after branching, which is confirmed by lab experiments. This indicates that the current inside a streamer discharges evolves approximately continuously during branching. On the other hand, stochastic fluctuations in streamer propagation due to Monte Carlo photoionization lead to more radio emission being emitted at frequencies of 100 MHz and above. Another process that leads to such high-frequency emission is the interaction of a streamer with a weakly preionized region, which can be present due to a previous discharge. In agreement with previous work, we observe the strongest and highest-frequency emission from streamer encounters. The amount of total energy that is radiated seems to depend primarily on the background electric field, and less on the particular streamer evolution. Finally, we present approximations for the maximal current along a streamer channel and a fit formula for a streamer's current moment.
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