1.Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China 2.School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China 3.National Astronomical Observatories, Chinese Academy of Science, Beijing 100012, China 4.Department of Astronomy, Beijing Normal University, Beijing 100875, China 5.Shanghai Institute of Optical and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China 6.Shanghai Institute of Laser Plasma, China Academy of Engineering Physics, Shanghai 201800, China 7.Songshan Lake Materials Laboratory, Dongguan 523808, China 8.Collaborative Innovation Center of IFSA (CICIFSA), Shanghai Jiao Tong University, Shanghai 200240, China
Fund Project:Project supported by the Science Challenge Project, China (Grant No. TZ2016005), the CAS-JSPS Joint Research Program (External Cooperation Program of the BIC, Chinese Academy of Sciences) (Grant No. 112111KYSB20160015), the National Natural Science Foundation of China (Grant Nos. 11520101003, 11827807, 11861121001), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB16010000).
Received Date:04 April 2019
Accepted Date:20 April 2019
Available Online:01 June 2019
Published Online:20 June 2019
Abstract:Microwave radiation in several gigahertz frequency band is a common phenomenon in laser-plasma interactions. It can last hundreds of nanoseconds and cause huge electromagnetic pulse disturbances to electrical devices in experiments. It has been found that the microwave radiation might originate from the oscillation of charged chambers, the return current on target holders, the dipole radiation, the quadrupole radiation, and the electron bunch emitted from the plasma to the vacuum. The microwave radiation waveform, frequency spectrum, and intensity depend on many factors such as laser pulse, target, and chamber parameter. To distinguish the microwave radiation mechanisms, the influence of the experimental parameters on the radiation characteristics should be investigated systematically. In this paper we investigate the microwave radiation influenced by the laser intensity in nanosecond laser-plasma interactions. It is found that the microwave radiation intensity varies nonmonotonically with the laser intensity. For the lower laser intensity, the radiation intensity first increases and then decreases with laser intensity increasing, the radiation field continuously oscillates in tens of nanoseconds, and the radiation spectrum contains two components below and above 0.3 GHz, respectively. For the higher laser intensity, the radiation intensity increases with the laser intensity increasing, the radiation field has a unipolar radiation lasting tens of nanoseconds, and the radiation spectrum mainly includes the component below 0.3 GHz. The waveform and spectrum analysis show that these phenomena are due to the difference of the radiation mechanisms at different laser intensities. The frequency component below and above 0.3 GHz are induced by the electron bunch emitted from the plasma to the vacuum and the dipole radiation respectively. At low laser intensity, both the dipole radiation and the electron bunch emitted from the plasma contribute to the microwave radiation. At high laser intensity, the microwave radiation is mainly produced by the electron beam emitted from the plasma to the vacuum. This work is significant for understanding the microwave radiation mechanisms in nanosecond laser-plasma interactions, and implies the potential to provide a reference to the diagnosing of the escape electrons and the sheath field on the target surface by the microwave radiation in laser-plasma interaction. Keywords:intense laser/ microwave radiation/ electromagnetic disturbance/ nanosecond laser plasma
图 4 入射激光强度分别为(a) 5.7 × 1014, (b) 7.4 × 1014, (c) 1.5 × 1015, (d) 2.0 × 1015, (e) 2.9 × 1015, (f) 6.2 × 1015 W/cm2时, 靶前靠近法线方向上电场的频谱分布 Figure4. Frequency spectra of the electric fields detected by the monopole antenna-3 at laser intensities of (a) 5.7 × 1014, (b) 7.4 × 1014, (c) 1.5 × 1015, (d) 2.0 × 1015, (e) 2.9 × 1015, and (f) 6.2 × 1015 W/cm2.
在较高和较低的激光强度下, 激光与靶相互作用产生的辐射场波形与频谱特征具有显著差别, 这表明不同强度的激光入射到金属靶上, 主导微波辐射的机制应当不同. 在纳秒激光装置中, 出射电子的时间尺度在纳秒量级, 由于电子逃逸会在靶面形成很强的电势, 反过来引起高能电子向靶面回流, 激发偶极辐射. 根据Felber[13]提出的偶极辐射的模型, 这一机制会产生以1/4τ为基频的辐射, 其中τ为激光与物质作用形成靶面电子回流的时间尺度. 本文实验中, 脉宽为1 ns的激光, 可以在1 ns之内产生高密的等离子体以及逃逸的高能电子, 对应的辐射基频约为0.25 GHz. 实验中观察到的大于0.3 GHz的辐射频谱与这一特征频率几乎一致, 这两个频率值不完全相同可能是由于高能电子首次被靶面电势拉回的时间尺度小于1 ns. 当激光强度增加到一定程度, 被加热的高能电子具有更高的能量, 可以克服靶面电势而不容易被拉回, 则偶极辐射不再起主导作用. 此时, 靶上因电子束向真空出射对辐射的驱动作用得以体现. 文献[24, 26, 27]利用纳秒激光与平面靶相互作用，在实验中发现激光作用结束后, 向真空出射的电子束流可持续数十纳秒. 这一过程中, 电子束向真空运动会产生辐射; 接地的金属靶杆会产生中和靶上电荷不平衡的电流回流, 并引起辐射. 其特征均表现为辐射场波形受电子束出射波形影响, 是一个持续数十纳秒的单极性脉冲. 这与图3(e)和图3(f)中频谱低于0.3 GHz的单极性辐射脉冲的特征一致. 本文实验中, 平面靶与接地金属腔室之间绝缘, 因此辐射主要由电子束向真空出射驱动. 为了更清楚地说明不同探测方向上辐射场波形和频谱特征, 我们对两个典型激光强度与靶相互作用的情况进行讨论. 当入射的激光强度为1.5 × 1015 W/cm2时, 不同方向上测量的电场波形和频谱分布如图5所示. 各个方向辐射场均表现为数十纳秒的振荡, 且其辐射频谱均包含低于0.3 GHz和高于0.3 GHz两个部分. 偶极辐射与电子束向真空出射产生的辐射共同作用产生了观测到的辐射场. 图 5 入射激光强度为1.5 × 1015 W/cm2时, 不同方向测量的电场波形及其频谱分布 (a)和(e)对应单极天线-1; (b)和(f)对应单极天线-2; (c)和(g)对应单极天线-3; (d)和(h)对应单极天线-4 Figure5. Electric field waveforms and their corresponding frequency spectra detected by the four monopole antennas. (a) and (e) correspond to the monopole antenna-1, (b) and (f) correspond to the monopole antenna-2, (c) and (g) correspond to the monopole antenna-3, (d) and (h) correspond to the monopole antenna-4. The laser intensity is 1.5 × 1015 W/cm2.
当入射激光强度为6.2 × 1015 W/cm2时, 不同方向上测量的电场波形和频谱分布如图6所示. 各个方向辐射场均表现为单极性脉冲, 且其辐射频谱均低于0.3 GHz, 辐射由电子束向真空出射主导. 图 6 入射激光强度为6.2 × 1015 W/cm2时, 不同方向测量的电场波形及其频谱分布 (a)和(e)对应单极天线-1; (b)和(f)对应单极天线-2; (c)和(g)对应单极天线-3; (d)和(h)对应单极天线-4 Figure6. Electric field waveforms and their corresponding frequency spectra detected by the four monopole antennas. (a) and (e) correspond to the monopole antenna-1, (b) and (f) correspond to the monopole antenna-2, (c) and (g) correspond to the monopole antenna-3, (d) and (h) correspond to the monopole antenna-4. The laser intensity is 6.2 × 1015 W/cm2.
从辐射场的频谱分析可以看到, 不同强度的激光入射到靶上, 都会因电子束向真空出射产生低于0.3 GHz的分量. 为了研究不同辐射机制产生微波辐射的效率, 对比计算了不同方向上单位立体角内的总辐射能、电子束向真空出射的辐射能, 以及偶极辐射产生的辐射能. 图7(a)给出了四个探测方向上, 不同强度的激光作用于平面靶时在单位立体角内产生的辐射能. 图7(b)和图7(c)分别展示了在激光强度增加的过程中, 低于和高于0.3 GHz的频率分量相应的微波辐射能的变化规律. 可以看到, 不同方向上的辐射能随激光强度的变化规律与图2中辐射峰幅值随激光强度的变化规律相同: 辐射能先增加再减小, 最后缓慢增加. 由靶上电子束向真空出射引起的低于0.3 GHz的辐射能随激光强度的增加而增加. 这是由于高的激光强度能产生更多的逃逸电子数从而产生更强的辐射. 图 7 不同方向测量的微波辐射能量随激光强度的变化(a)单位立体角内产生的总辐射能; (b)单位立体角内产生的0.3 GHz以下的辐射能; (c)单位立体角内产生的0.3 GHz以上的辐射能 Figure7. Radiation energy versus laser intensity at different directions: (a) Total radiation energy detected by the antennas; (b) radiation energy at frequencies lower than 0.3 GHz; (c) radiation energy at frequencies upper than 0.3 GHz.
图 1 实验布局图
图 2 不同激光强度下, 四个方向上对应的电场峰幅值
图 3 入射激光强度分别为(a) 5.7 × 1014, (b) 7.4 × 1014, (c) 1.5 × 1015, (d) 2.0 × 1015, (e) 2.9 × 1015, (f) 6.2 × 1015 W/cm2时, 靶前靠近法线方向上的电场时间波形
图 4 入射激光强度分别为(a) 5.7 × 1014, (b) 7.4 × 1014, (c) 1.5 × 1015, (d) 2.0 × 1015, (e) 2.9 × 1015, (f) 6.2 × 1015 W/cm2时, 靶前靠近法线方向上电场的频谱分布
图 5 入射激光强度为1.5 × 1015 W/cm2时, 不同方向测量的电场波形及其频谱分布 (a)和(e)对应单极天线-1; (b)和(f)对应单极天线-2; (c)和(g)对应单极天线-3; (d)和(h)对应单极天线-4
图 6 入射激光强度为6.2 × 1015 W/cm2时, 不同方向测量的电场波形及其频谱分布 (a)和(e)对应单极天线-1; (b)和(f)对应单极天线-2; (c)和(g)对应单极天线-3; (d)和(h)对应单极天线-4
图 7 不同方向测量的微波辐射能量随激光强度的变化(a)单位立体角内产生的总辐射能; (b)单位立体角内产生的0.3 GHz以下的辐射能; (c)单位立体角内产生的0.3 GHz以上的辐射能
