Electron acceleration during streamer collisions in air.
Electron acceleration during streamer collisions in air.
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DOI：
10.1002/2016gl072216
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发表时间：
2017-03-16
影响因子：
5.2
通讯作者：
Neubert T
中科院分区：
文献类型：
--
作者：
Köhn C;Chanrion O;Neubert T
高压实验室实验表明，在一米间隙内产生的最大电压为 1 MV 的空气放电可能会产生光子能量高达 1 MeV 的 X 射线。有人认为，光子是由正负流光碰撞期间的脉冲增强场加速的电子产生的轫致辐射。为了探索这个过程，我们对流光相遇进行了首次自洽粒子模拟。我们的仿真模型是一个二维、圆柱对称、细胞内粒子代码，用于追踪电子动力学并求解空间电荷场，并采用蒙特卡罗方案来解释碰撞和电离。我们将电子密度、电场和速度分布呈现为空间和时间的函数。假设背景电场是击穿场的 1.5 倍，我们发现电子密度达到 2·1021 m−3，相遇区域的大小为 ∼3·10−12 m3，并且电场在 ∼10−11 s 内增强到击穿场的 ∼9 倍。我们进一步发现，径向分量与平行分量相当，这与角散射一起导致电子几乎各向同性分布。这与 X 射线各向同性发射的实验室观察结果一致。然而，模拟中达到的电子最大能量为 ∼600 eV，远低于解释观测结果所需的能量。原因是遭遇区域的尺寸和持续时间都很小。对于观察到的光子能量，必须在更大的区域和/或更长的时间内增强场。流注碰撞结束后，新形成的双头流注尖端的场得到提升尽管在碰撞过程中观察到了大的增强场，但电子运动仍保持准各向同性在相遇期间，高于 150 eV 的指数能量分布的特征尺寸为 40 eV
High‐voltage laboratory experiments show that discharges in air, generated over a gap of one meter with maximal voltage of 1 MV, may produce X‐rays with photon energies up to 1 MeV. It has been suggested that the photons are bremsstrahlung from electrons accelerated by the impulsive, enhanced field during collisions of negative and a positive streamers. To explore this process, we have conducted the first self‐consistent particle simulations of streamer encounters. Our simulation model is a 2‐D, cylindrically symmetric, particle‐in‐cell code tracing the electron dynamics and solving the space charge fields, with a Monte Carlo scheme accounting for collisions and ionization. We present the electron density, the electric field, and the velocity distribution as functions of space and time. Assuming a background electric field 1.5 times the breakdown field, we find that the electron density reaches 2·1021 m−3, the size of the encounter region is ∼3·10−12 m3 and that the field enhances to ∼9 times the breakdown field during ∼10−11 s. We further find that the radial component becomes comparable to the parallel component, which together with angular scattering leads to an almost isotropic distribution of electrons. This is consistent with laboratory observations that X‐rays are emitted isotropically. However, the maximum energy of electrons reached in the simulation is ∼600 eV, which is well below the energies required to explain observations. The reason is that the encounter region is small in size and duration. For the photon energies observed, the field must be enhanced in a larger region and/or for a longer time. After the end of the streamer collision, the field at the tips of the newly formed double‐headed streamer gets elevated The electron motion remains quasi‐isotropic despite the large enhanced field observed during the collision During the encounter the exponential energy distribution above 150 eV has a characteristic size of 40 eV