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Effect of laser intensity on microwave radiation generated in nanosecond laser-plasma interactions

Jiang Wei-Man Li Yu-Tong Zhang Zhe Zhu Bao-Jun Zhang Yi-Hang Yuan Da-Wei Wei Hui-Gang Liang Gui-Yun Han Bo Liu Chang Yuan Xiao-Xia Hua Neng Zhu Bao-Qiang Zhu Jian-Qiang Fang Zhi-Heng Wang Chen Huang Xiu-Guang Zhang Jie

Effect of laser intensity on microwave radiation generated in nanosecond laser-plasma interactions

Jiang Wei-Man, Li Yu-Tong, Zhang Zhe, Zhu Bao-Jun, Zhang Yi-Hang, Yuan Da-Wei, Wei Hui-Gang, Liang Gui-Yun, Han Bo, Liu Chang, Yuan Xiao-Xia, Hua Neng, Zhu Bao-Qiang, Zhu Jian-Qiang, Fang Zhi-Heng, Wang Chen, Huang Xiu-Guang, Zhang Jie
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  • 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.
      Corresponding author: Li Yu-Tong, ytli@iphy.ac.cn ; Zhang Zhe, zzhang@iphy.ac.cn
    [1]

    Campbell E M, Goncharov V N, Sangster T C, Regan S P, Radha P B, Betti R, Myatt J F, Froula D H, Rosenberg M J, Igumenshchev I V, Seka W, Solodov A A, Maximov A V, Marozas J A, Collins T J B, Turnbull D, Marshall F J, Shvydky A, Knauer J P, McCrory R L, Sefkow A B, Hohenberger M, Michel P A, Chapman T, Masse L, Goyon C, Ross S, Bates J W, Karasik M, Oh J, Weaver J, Schmitt A J, Obenschain K, Obenschain S P, Reyes S, Wonterghem V 2017 Matter Radiat. Extremes 2 37

    [2]

    Snavely R A, Key M H, Hatchett S P, Cowan T E, Roth M, Phillips T W, Stoyer M A, Henry E A, Sangster T C, Singh M S, Wilks S C, MacKinnon A, Offenberger A, Pennington D M, Yasuike K, Langdon A B, Lasinski B F, Johnson J, Perry M D, Campbell E M 2000 Phys. Rev. Lett. 85 2945

    [3]

    Mangles S P D, Murphy C D, Najmudin Z, Thomas A G R, Collier J L, Dangor A E, Divall E J, Foster P S, Gallacher J G, Hooker C J, Jaroszynski D A, Langley A J, Mori W B, Norreys P A, Tsung F S, Viskup R, Walton B R, Krushelnick K 2004 Nature 431 535

    [4]

    Phuoc K T, Corde S, Thaury C, Malka V, Tafzi A, Goddet J P, Shah R C, Sebban S, Rousse A 2012 Nat. Photon. 6 308

    [5]

    Rousse A, Phuoc K T, Shah R, Pukhov A, Lefebvre E, Malka V, Kiselev S, Burgy F, Rousseau J, Umstadter D, Hulin D 2004 Phys. Rev. Lett. 93 135005

    [6]

    Liao G, Li Y, Liu H, Scott G G, Neely D, Zhang Y, Zhu B, Zhang Z, Armstrong C, Zemaityte E, Bradford P, Huggard P G, Rusby, D R, McKenna P, Brenner C M, Woolsey N C, Wang W, Sheng Z, Zhang J 2019 Proc. Natl. Acad. Sci. USA 116 3994

    [7]

    Robinson T S, Consoli F, Giltrap S, Eardley S J, Hicks G S, Ditter E J, Ettlinger O, Stuart N H, Notley M, de Angelis R, Najmudin Z, Smith R A 2017 Sci. Rep. 7 983

    [8]

    Meng C, Xu Z Q, Jiang Y S, Zheng W G, Dang Z 2017 IEEE Trans. Nucl. Sci. 64 10

    [9]

    Pearlman J S, Dahlbacka G H 1978 J. Appl. Phys. 49 457

    [10]

    Gerdin G, Tanis M J, Venneri F 1986 Plasma Phys. Control. Fusion 28 527

    [11]

    Mead M J, Neely D, Gauoin J, Heathcote R, Patel P 2004 Rev. Sci. Instrum. 75 4225

    [12]

    Raimbourg J 2004 Rev. Sci. Instrum. 75 4234

    [13]

    Felber F S 2005 Appl. Phys. Lett. 86 231501

    [14]

    Remo J L, Adams R G, Jones M C 2007 Appl. Opt. 46 6166

    [15]

    Miragliotta J, Brawley B, Sailor C, Spicer J B, Spicer J W M 2011 Proc. SPIE 8037 80370N-1

    [16]

    Chen Z Y, Li J F, Yu Y, Wang J X, Li X Y, Peng Q X, Zhu W J 2012 Phys. Plasmas 19 113116

    [17]

    戴宇佳, 宋晓伟, 高勋, 王兴生, 林景全 2017 物理学报 66 185201

    Dai Y J, Song X W, Gao X, Wang X S, Lin J Q 2017 Acta Phys. Sin. 66 185201

    [18]

    Englesbe A, Elle J, Reid R, Lucero A, Pohle H, Domonkos M, Kalmykov S, Krushelnick K, Schmitt-Sody A 2018 Opt. Lett. 43 4953

    [19]

    Brown C G, Bond E, Clancy T, Dangi S, Eder D C, Ferguson W, Kimbrough J, Throop A 2010 J. Phys.: Conf. Ser. 244 032001

    [20]

    Tao Y, Yang M, Wang C, Yang W, Li Y, Liu S, Jiang S, Ding Y, Xiao S 2016 Photon. Sens. 6 249

    [21]

    Bradford P, Woolsey N C, Scott G G, Liao G, Liu H, Zhang Y, Zhu B, Armstrong C, Astbury S, Brenner C, Brummitt P, Consoli F, East I, Gray R, Haddock D, Huggard P, Jones P J R, Montgomery E, Musgrave I, Oliveira P, Rusby D R, Spindloe C, Summers B, Zemaityte E, Zhang Z, Li Y, McKenna P, Neely D 2018 High Power Laser Sci. Eng. 6 e21

    [22]

    Brown C G, Ayers J, Felker B, Ferguson W, Holder J P, Nagel S R, Piston K W, Simanovskaia N, Throop A L, Chung M, Hilsabeck T 2012 Rev. Sci. Instrum. 83 10D729

    [23]

    Brown C G, Clancy T J, Eder D C, Ferguson W, Throop A L 2013 EPJ Web of Conferences 59 08012

    [24]

    Consoli F, de Angelis R, de Marco M, Krasa J, Cikhardt J, Pfeifer M, Margarone D, Klir D, Dudzak R 2018 Plasma Phys. Control. Fusion 60 105006

    [25]

    Chen Z Y, Li J F, Li J, Peng Q X 2011 Plasma Scr. 83 055503

    [26]

    Consoli F, de Angelis R, Duvillaret L, Andreoli P L, Cipriani M, Cristofari G, Di Giorgio G, Ingenito F, Verona C 2016 Sci. Rep. 6 27889

    [27]

    Krása J, de Marco M, Cikhardt J, Pfeifer M, Velyhan A, Klír D, Řezáč K, Limpouch J, Krouský E, Dostál J, Ullschmied J, Dudžák R 2017 Plasma Phys. Control. Fusion 59 065007

  • 图 1  实验布局图

    Figure 1.  Experimental setup.

    图 2  不同激光强度下, 四个方向上对应的电场峰幅值

    Figure 2.  Peak E-field magnitude versus laser intensity in the four different directions.

    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

    Figure 3.  Electric field waveforms 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.

    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

    Figure 4.  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.

    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

    Figure 5.  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.

    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

    Figure 6.  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.

    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

    Figure 7.  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.

  • [1]

    Campbell E M, Goncharov V N, Sangster T C, Regan S P, Radha P B, Betti R, Myatt J F, Froula D H, Rosenberg M J, Igumenshchev I V, Seka W, Solodov A A, Maximov A V, Marozas J A, Collins T J B, Turnbull D, Marshall F J, Shvydky A, Knauer J P, McCrory R L, Sefkow A B, Hohenberger M, Michel P A, Chapman T, Masse L, Goyon C, Ross S, Bates J W, Karasik M, Oh J, Weaver J, Schmitt A J, Obenschain K, Obenschain S P, Reyes S, Wonterghem V 2017 Matter Radiat. Extremes 2 37

    [2]

    Snavely R A, Key M H, Hatchett S P, Cowan T E, Roth M, Phillips T W, Stoyer M A, Henry E A, Sangster T C, Singh M S, Wilks S C, MacKinnon A, Offenberger A, Pennington D M, Yasuike K, Langdon A B, Lasinski B F, Johnson J, Perry M D, Campbell E M 2000 Phys. Rev. Lett. 85 2945

    [3]

    Mangles S P D, Murphy C D, Najmudin Z, Thomas A G R, Collier J L, Dangor A E, Divall E J, Foster P S, Gallacher J G, Hooker C J, Jaroszynski D A, Langley A J, Mori W B, Norreys P A, Tsung F S, Viskup R, Walton B R, Krushelnick K 2004 Nature 431 535

    [4]

    Phuoc K T, Corde S, Thaury C, Malka V, Tafzi A, Goddet J P, Shah R C, Sebban S, Rousse A 2012 Nat. Photon. 6 308

    [5]

    Rousse A, Phuoc K T, Shah R, Pukhov A, Lefebvre E, Malka V, Kiselev S, Burgy F, Rousseau J, Umstadter D, Hulin D 2004 Phys. Rev. Lett. 93 135005

    [6]

    Liao G, Li Y, Liu H, Scott G G, Neely D, Zhang Y, Zhu B, Zhang Z, Armstrong C, Zemaityte E, Bradford P, Huggard P G, Rusby, D R, McKenna P, Brenner C M, Woolsey N C, Wang W, Sheng Z, Zhang J 2019 Proc. Natl. Acad. Sci. USA 116 3994

    [7]

    Robinson T S, Consoli F, Giltrap S, Eardley S J, Hicks G S, Ditter E J, Ettlinger O, Stuart N H, Notley M, de Angelis R, Najmudin Z, Smith R A 2017 Sci. Rep. 7 983

    [8]

    Meng C, Xu Z Q, Jiang Y S, Zheng W G, Dang Z 2017 IEEE Trans. Nucl. Sci. 64 10

    [9]

    Pearlman J S, Dahlbacka G H 1978 J. Appl. Phys. 49 457

    [10]

    Gerdin G, Tanis M J, Venneri F 1986 Plasma Phys. Control. Fusion 28 527

    [11]

    Mead M J, Neely D, Gauoin J, Heathcote R, Patel P 2004 Rev. Sci. Instrum. 75 4225

    [12]

    Raimbourg J 2004 Rev. Sci. Instrum. 75 4234

    [13]

    Felber F S 2005 Appl. Phys. Lett. 86 231501

    [14]

    Remo J L, Adams R G, Jones M C 2007 Appl. Opt. 46 6166

    [15]

    Miragliotta J, Brawley B, Sailor C, Spicer J B, Spicer J W M 2011 Proc. SPIE 8037 80370N-1

    [16]

    Chen Z Y, Li J F, Yu Y, Wang J X, Li X Y, Peng Q X, Zhu W J 2012 Phys. Plasmas 19 113116

    [17]

    戴宇佳, 宋晓伟, 高勋, 王兴生, 林景全 2017 物理学报 66 185201

    Dai Y J, Song X W, Gao X, Wang X S, Lin J Q 2017 Acta Phys. Sin. 66 185201

    [18]

    Englesbe A, Elle J, Reid R, Lucero A, Pohle H, Domonkos M, Kalmykov S, Krushelnick K, Schmitt-Sody A 2018 Opt. Lett. 43 4953

    [19]

    Brown C G, Bond E, Clancy T, Dangi S, Eder D C, Ferguson W, Kimbrough J, Throop A 2010 J. Phys.: Conf. Ser. 244 032001

    [20]

    Tao Y, Yang M, Wang C, Yang W, Li Y, Liu S, Jiang S, Ding Y, Xiao S 2016 Photon. Sens. 6 249

    [21]

    Bradford P, Woolsey N C, Scott G G, Liao G, Liu H, Zhang Y, Zhu B, Armstrong C, Astbury S, Brenner C, Brummitt P, Consoli F, East I, Gray R, Haddock D, Huggard P, Jones P J R, Montgomery E, Musgrave I, Oliveira P, Rusby D R, Spindloe C, Summers B, Zemaityte E, Zhang Z, Li Y, McKenna P, Neely D 2018 High Power Laser Sci. Eng. 6 e21

    [22]

    Brown C G, Ayers J, Felker B, Ferguson W, Holder J P, Nagel S R, Piston K W, Simanovskaia N, Throop A L, Chung M, Hilsabeck T 2012 Rev. Sci. Instrum. 83 10D729

    [23]

    Brown C G, Clancy T J, Eder D C, Ferguson W, Throop A L 2013 EPJ Web of Conferences 59 08012

    [24]

    Consoli F, de Angelis R, de Marco M, Krasa J, Cikhardt J, Pfeifer M, Margarone D, Klir D, Dudzak R 2018 Plasma Phys. Control. Fusion 60 105006

    [25]

    Chen Z Y, Li J F, Li J, Peng Q X 2011 Plasma Scr. 83 055503

    [26]

    Consoli F, de Angelis R, Duvillaret L, Andreoli P L, Cipriani M, Cristofari G, Di Giorgio G, Ingenito F, Verona C 2016 Sci. Rep. 6 27889

    [27]

    Krása J, de Marco M, Cikhardt J, Pfeifer M, Velyhan A, Klír D, Řezáč K, Limpouch J, Krouský E, Dostál J, Ullschmied J, Dudžák R 2017 Plasma Phys. Control. Fusion 59 065007

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  • Received Date:  04 April 2019
  • Accepted Date:  20 April 2019
  • Available Online:  16 August 2019
  • Published Online:  01 June 2019

Effect of laser intensity on microwave radiation generated in nanosecond laser-plasma interactions

    Corresponding author: Li Yu-Tong, ytli@iphy.ac.cn
    Corresponding author: Zhang Zhe, zzhang@iphy.ac.cn
  • 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

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.

    • 强激光与等离子体相互作用过程中, 可以在单位时间、单位空间内实现极高的能量密度, 从而在实验室中产生一系列原本只存在于核爆或者天体中的极端物理条件, 这使得人们可以在实验室中进行激光核爆模拟[1]和实验室天体物理等相关研究. 在激光等离子体相互作用过程中, 可以产生大量的高能粒子[2,3]、高亮的γ射线和X射线[4,5]、超强的太赫兹辐射[6]、微波辐射[7,8]等. 其中强激光等离子体作用中产生的微波辐射, 是强场物理研究伊始就被观测到的物理现象[918]. 频段在几个GHz、能够持续百纳秒量级的微波辐射在实验中通常被作为电磁干扰[1921]. 然而, 微波辐射的产生机制的研究还有待加深.

      在美国国家点火装置上(NIF), 研究人员连续多年测量了纳秒激光与等离子体相互作用过程中微波辐射的辐射特征[22,23]. 发现微波辐射场可以持续振荡百纳秒量级, 其频谱主要集中在2 GHz以下, 其强度与激光能量之间正相关. 最近, 又有研究者在捷克的Asterix激光装置上测量了脉宽0.35 ns、能量600 J、光强为1016 W·cm–2的激光入射到靶上以后产生的辐射[24], 其时间波形表现为小于50 ns的单极性辐射, 频谱集中在几百MHz以下. 在不同的纳秒激光装置下, 微波辐射的时域和频域特征都有很大不同, 这可能是由于不同激光以及靶参数条件下辐射机制的差异引起的. 通常认为在纳秒激光实验中, 微波辐射可能源于偶极辐射、电四极辐射、腔室充电后的振荡、接地金属靶杆上的电流回流产生的辐射等过程[11,13,24,25]. 随着实验条件, 如激光、靶、腔室等参数条件的变化, 不同装置、甚至同一装置上不同激光发次上测量的微波辐射场都呈现很大差异, 这给人们认识微波辐射的产生机制带来很大困扰. 在大能量纳秒激光与等离子体相互作用的过程中, 专门针对微波辐射特征和其机制进行系统研究的实验目前还相对较少. 为了更清楚地认识微波辐射的产生机制, 需要系统地研究实验参数对辐射特征的影响.

      本文通过改变激光能量调节到靶面的激光强度, 系统地研究了微波辐射强度的变化, 表征了不同激光强度下辐射场的时域波形和频谱特征的变化规律. 实验发现: 在较低激光强度下, 辐射强度随激光强度的增加先增加后减小, 辐射场时间波形呈现连续振荡的特征, 辐射频谱包含低于和高于0.3 GHz两部分分量; 在较高的激光强度下, 辐射强度随激光强度的增加而增加, 辐射场时间波形表现为数十纳秒的单极性辐射, 辐射频谱主要包括0.3 GHz以下的分量. 我们探讨了微波辐射的主要辐射机制随激光强度的变化, 认为较低激光强度下偶极辐射起主导作用, 而较高激光强度下靶上电子束向真空出射更重要.

    2.   实 验
    • 实验在神光Ⅱ高功率激光装置上运行, 实验布局图如图1所示. 与激光作用的靶为一个直径10 mm、厚1 mm的铜平面靶. 靶与靶架之间用5 cm长的绝缘材料相连, 避免了接地的靶室与靶之间形成过强的电流振荡, 干扰对微波辐射机制的研究. 铜盘南面中心位于球形真空靶室中心. 神光Ⅱ装置中单路激光最高可输出约250 J能量, 激光波长351 nm, 脉宽1 ns, 焦斑直径(半高全宽) 150 μm. 实验中南侧4路激光可作用于铜盘南面中心, 到达靶面的总能量在100—1100 J之间. 在真空靶室内部安放了四个相同的单极探测天线, 其位置均在靶室中心以下13 cm的平面内. 四个探测天线中心的坐标如图1所示. 探测天线的信号通过转接法兰上的线缆转接口连接到真空靶室外, 经过30 m长的同轴线缆以及功率衰减器后, 用同一台3 GHz LeCroy示波器进行记录. 为了避免激光与靶相互作用产生的微波辐射直接耦合到示波器, 示波器置于法拉第屏蔽箱中.

      Figure 1.  Experimental setup.

      由激光与等离子体作用区传递到天线探测面的瞬时辐射功率为P1 = A·S, 其中A (5 cm2)为天线的有效探测面积; S为瞬时电磁场能流密度的幅值, 它与辐射电场E之间的关系为S = ${ E}^2 $/377. 示波器接收的辐射功率为$P_2 = |V|^2/R $, 其中V为示波器的测量值, R (50 Ω)为示波器的阻抗. 考虑到天线到示波器之间的功率衰减, 传递到天线探测面的瞬时辐射功率与示波器测量的辐射功率之间满足P1 = αP2, 其中α为功率衰减因子, 包含天线增益、线缆衰减、功率衰减器等衰减, 实验中总衰减为58 dB. 通过示波器测量值与辐射电场之间的关系, 单极天线可用来表征辐射电磁场的电场信息. 单极天线的有效探测频段在2.2 GHz以下, 示波器的有效探测频段在3 GHz以下, 因而测量的电场主要对应频段在2.2 GHz以下的辐射场.

    3.   实验结果与讨论
    • 在实验中利用南侧四束光路分别获得100, 130, 260, 360, 520和1100 J的激光能量输出, 对应到靶面的激光强度分别为5.7 × 1014, 7.4 × 1014, 1.5 × 1015, 2.0 × 1015, 2.9 × 1015和6.2 × 1015 W/cm2. 辐射电场峰的幅值随激光强度的变化如图2所示. 在激光强度从5.7 × 1014 W/cm2增加到1.5 × 1015 W/cm2的过程中辐射场强度逐渐增加; 继续增加激光强度到2.9 × 1015 W/cm2, 辐射场强度逐渐减小; 在2.9 × 1015 W/cm2以上, 增加激光强度又会引起辐射场强度的缓慢增加. 在连续增加激光强度的过程中, 辐射场强度并非单调增加, 这不同于以往实验中人们发现的激光强度越高, 辐射越强的现象[23]. 整体上靶前3个方向的辐射场强度高于靶后的辐射场, 这说明辐射场主要集中在靶前. 由于实验采用了1 mm厚度的铜靶与激光相互作用, 激光不会穿过如此厚的靶, 并在靶后有效地激发等离子体, 因此微波辐射主要与靶前的等离子体行为相关.

      Figure 2.  Peak E-field magnitude versus laser intensity in the four different directions.

      图3展示了不同激光强度下, 靶前靠近法线方向的单极天线-3测量的电场时间波形. 在激光强度低于2.0 × 1015 W/cm2时, 电场持续振荡数十纳秒; 激光强度高于2.0 × 1015 W/cm2时, 则不会产生振荡的电场, 仅有一个单极性的辐射峰. 进一步对辐射电场做傅里叶变换, 得到辐射场的频谱如图4所示. 激光强度低于2.0 × 1015 W/cm2时, 辐射频谱主要包含低于0.3 GHz和高于0.3 GHz的两部分连续谱. 激光强度高于2.0 × 1015 W/cm2时, 辐射频谱主要集中在0.3 GHz以下.

      Figure 3.  Electric field waveforms 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.

      Figure 4.  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)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.

      Figure 5.  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.

      Figure 6.  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)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.

      Figure 7.  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.

      由偶极辐射产生的频率高于0.3 GHz的辐射能, 随激光强度的增加却不是单调变化的. 在激光强度由5.7 × 1014 W/cm2增加到2.9 × 1015 W/cm2的过程中辐射能先增加后减小; 激光强度继续增加, 由于逃逸电子再被拉回靶面的效率很低, 因此产生偶极辐射的效率很低. 根据靶面电子回流产生偶极辐射的模型, 辐射总功率为[13]:

      式中, a为电子回流区半径; Te为临界密度面附近的电子温度; σ为斯必泽电导率, 它与${T_{\rm{e}}}^{3/2}$成正比. 在本文实验条件下, (1)式括号内第二项远远大于1, 括号内第一项可以省略. 因此可以近似认为, 偶极辐射的强度与${T_{\rm{e}}}^5$成正比, 与靶面电子回流的时间尺度τ2成反比. 在激光强度的增加过程中, 产生电子回流所需的特征时间与电子温度同时增加, 使得辐射场强度以及能量受二者的共同作用, 产生了图7(c)所示的非单调变化关系.

    4.   结 论
    • 本文在神光Ⅱ高功率激光装置上研究了大能量纳秒激光与平面靶相互作用过程中, GHz频段内的微波辐射随激光强度的变化规律. 在激光强度增加的过程中, 发现微波辐射强度非单调增加, 辐射波形由持续振荡转变为单极性辐射, 辐射频谱在高激光强度下向低频移动. 辐射场的时间波形和频谱分析表明, 这一现象是由于不同的激光强度作用下, 产生微波辐射的机制不同. 在较低的激光强度下, 微波辐射由偶极辐射和靶上电子束向真空出射共同作用产生, 而偶极辐射占主导作用; 在较高的激光强度下, 偶极辐射不再起作用, 微波辐射主要由靶上电子束向真空出射产生. 由于几个GHz以下的频段恰好对应纳秒激光与等离子体相互作用的时间尺度, 对微波辐射机制的理解有助于加深激光与等离子体相互作用过程中逃逸电子、靶面鞘层场等物理问题的理解. 另一方面, 明确影响微波辐射强度的因素, 也有助于人们在超强激光装置中合理选择实验条件, 以避免强电磁干扰的影响.

      感谢在上海神光Ⅱ高功率激光实验装置相关部门工作的所有员工对本文实验所做的贡献.

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