激光与近相对论临界密度薄层相互作用产生大电量高能电子束
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摘要
研究了激光与近相对论临界密度等离子体薄层相互作用时所产生的高能电子束的主要特征,包括平均有效温度以及截止能量等.实验结果表明,电子束的电量超过nC量级,平均有效温度可达8 MeV以上.PIC数值模拟证明,近相对论临界密度等离子体内,相对论自透明效应和激光钻孔效应共同形成一条磁化等离子体通道,电子与激光将在角向磁场的协助下发生Betatron共振.激光可将电子直接加速到很高能量,因此电子束平均有效温度("斜坡温度")远远超过Wilks定标率预计的平均温度.该研究为产生高亮度X射线源提供了一种新的可能途径.
In this paper, we report our results from interactions between sub-picosecond laser and relativistic near-critical density plasma layer. To create the near-critical density plasma layer, low density foam targets are utilized in our experiments. The foam is comprised of tri-cellulose acetate. Their average densities vary from 1 mg/cm~3 to 5 mg/cm~3,corresponding to full ionization densities ranging from 0.6nc to 3nc. When laser pulse is incident on the near-critical density plasma, some energetic bunches with a large quantity of charges are measured in most of the shots. The maximum charge quantity reaches to 6.1 nC/sr. Furthermore, the observed electron energy spectrum is Boltzmann-like with a wide plateau at the tail of the energy spectrum, rather than a Maxwell-like. The concept of average temperature is not available any more, and we define average effective temperature instead, namely the slope temperature. Fitting the Boltzmann-like spectrum exponentially, we find that the average effective temperature even exceeds 8 MeV at7.5 × 10~(19)W/cm~2, far beyond the ponderomotive limit. Aiming at analyzing the implication of physics, several twodimensional particle-in-cell(PIC) simulations are performed. The PIC simulations indicate that the hole-boring effect and relativistic self-transparency play an important role in the electrons heating process. At the earlier stage of heating process, a short plasma channel is created by the hole-boring effect and relativistic self-transparency. The length and the width of the plasma channel are about tens of micrometers and several micrometers respectively. Around the plasma channel, there is an intensive azimuthal magnetic field. The magnitude of the azimuthal magnetic field is 100 MGs.However, the radical electrostatic field is not seen. The possible reason is that the plasma channel would be cavitated by the hole-boring effect. As a result, the electrons will experience Betatron resonance in the magnetized plasma channel.The traverse momentum of the electron would be converted into forward momentum. Assisted by the Betatron resonance,the electrons gain energies from the laser directly and efficiently. Thus, the average effective temperatures of the electron bunches are much higher than predicted by the ponderomotive scaling law. Besides, we also conducte another simulation to instigate the differences by adopting different laser polarizations. Within our expectation, the electron spectrum of the P-polarization accords well with the experimental result, while the electron spectrum of the S-polarization obviously deviates from the experimental result. It also demonstrates that the Betatron resonance heating dominates the electron acceleration process. This research paves the way to generating the highly energetic bunches with a large quantity of charges, and wound also be helpful for producing the high-bright laser bremsstrahlung sources in future.
In this paper, we report our results from interactions between sub-picosecond laser and relativistic near-critical density plasma layer. To create the near-critical density plasma layer, low density foam targets are utilized in our experiments. The foam is comprised of tri-cellulose acetate. Their average densities vary from 1 mg/cm~3 to 5 mg/cm~3,corresponding to full ionization densities ranging from 0.6nc to 3nc. When laser pulse is incident on the near-critical density plasma, some energetic bunches with a large quantity of charges are measured in most of the shots. The maximum charge quantity reaches to 6.1 nC/sr. Furthermore, the observed electron energy spectrum is Boltzmann-like with a wide plateau at the tail of the energy spectrum, rather than a Maxwell-like. The concept of average temperature is not available any more, and we define average effective temperature instead, namely the slope temperature. Fitting the Boltzmann-like spectrum exponentially, we find that the average effective temperature even exceeds 8 MeV at7.5 × 10~(19)W/cm~2, far beyond the ponderomotive limit. Aiming at analyzing the implication of physics, several twodimensional particle-in-cell(PIC) simulations are performed. The PIC simulations indicate that the hole-boring effect and relativistic self-transparency play an important role in the electrons heating process. At the earlier stage of heating process, a short plasma channel is created by the hole-boring effect and relativistic self-transparency. The length and the width of the plasma channel are about tens of micrometers and several micrometers respectively. Around the plasma channel, there is an intensive azimuthal magnetic field. The magnitude of the azimuthal magnetic field is 100 MGs.However, the radical electrostatic field is not seen. The possible reason is that the plasma channel would be cavitated by the hole-boring effect. As a result, the electrons will experience Betatron resonance in the magnetized plasma channel.The traverse momentum of the electron would be converted into forward momentum. Assisted by the Betatron resonance,the electrons gain energies from the laser directly and efficiently. Thus, the average effective temperatures of the electron bunches are much higher than predicted by the ponderomotive scaling law. Besides, we also conducte another simulation to instigate the differences by adopting different laser polarizations. Within our expectation, the electron spectrum of the P-polarization accords well with the experimental result, while the electron spectrum of the S-polarization obviously deviates from the experimental result. It also demonstrates that the Betatron resonance heating dominates the electron acceleration process. This research paves the way to generating the highly energetic bunches with a large quantity of charges, and wound also be helpful for producing the high-bright laser bremsstrahlung sources in future.
引文
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[3]Kulcsár G,Al Mawlawi D,Budnik F W 2000 Phys.Rev.Lett.84 5149
[4]Nodera Y,Kawata S,Onμma N 2008 Phys.Rev.E 78046401
[5]Hu G Y,Lei A L,et al.2010 Phys.Plasmas 17 083102
[6]Huang K,Li D Z,Yan W C,Li M H,Tao M Z,Chen ZY,Ge X L,Liu F,Ma Y,Zhao J R,Hafz N M,Zhang J,Chen L M 2014 Appl.Phys.Lett.105 204101
[7]Cao L H,Gu Y Q,Zhao Z Q 2010 Phys.Plasmas 17043103
[8]Haines M G,Wei M S,Beg F N,Stephens R B 2009Phys.Rev.Lett.102 045008
[9]Wilks S C,Kruer W L,Tabak M 1992 Phys.Rev.Lett.69 1383
[10]Glinec Y,Faure J,Le Dain L,Darbon S,Hosokai T,Santos J J,Lefebvre E,Rousseau J P,Burgy F 2005Phys.Rev.Lett.94 025003
[11]Cipiccia S,Islam M,Ersfeld B,Shanks R P,Brunetti E,Vieux G,Yang X,Issac R C,Wiggins S,Welsh GH,Anania M P,Maneuski D,Montgomery R,Smith G,Hoek M,Hamilton D J,Lemos N R C,Symes D,Rajeev P P,Shea V O,Dias J M,Jaroszynski D A 2011 Nature Phys.7 867
[12]Courtois C,Edwards R,Compant La Fontaine A,Aedy C,Barbotin M,Bazzoli S,Biddle L,Brebion D,Bourgade J L,Drew D,Fox M,Gardner M,Gazave J M,Lagrange J,Landoas O,Le Dain L,Lefebvre E,Mastrosimone D,Pichoff N,Pien G,Ramsay M,Simons A,Sircombe N,Stoeck C,Thorp K 2011 Phys.Plsamas 18023101
[13]Kalmykov S Y,Gorburov L M,Mora P 2005 Phys.Plasmas 12 033101
[14]Pukhov A,Gordienko S 2006 Phil.Trans.R.Soc.A 364623
[15]Lu W,Huang C,Zhou M 2006 Phys.Plasmas 13 056709
[16]Esaresy E,Schroeder C B,Leemans W P 2009 Rev.Mod.Phys.81 1229
[17]Atzeni S,Meyer-ter-Vehn J 2008 The Physics of Inertial Fusion(in Chinese)(Beijing:Science Press)[Atzeni S,Meyer-ter-Vehn J 2008惯性聚变物理(北京:科学出版社)]
[18]Kruer W L,Estabrook K 1985 Phys.Fluids 28 430
[19]Scott R H H,Perez F,Santos J J,Ridgers C P,Davies J R,Lancaster K L,Baton S D,Nicolai Ph,Trines R MG M,Bell A R,Hulin S,Tzoufras M,Rose S J,Norreys P A 2012 Phys.Plasmas 19 053104
[20]Pukhov A,Sheng Z M,Meyer-Ter-Vehn J 1999 Phys.Plasmas 6 2847
[21]Willingale L,Nagel S R,Thomas A G R,Bellei C,Clarke R J,Dangor A E,Heathcote R,Kaluza M C,Kamperidis C,Kneip S,Krushelnick K,Lopes N,Mangles S P D,Nazarov W,Nilson P M,Najmudin Z 2009 Phys.Rev.Lett.102 105002
[22]Kemp A,Sentoku Y,Tabak M 2008 Phys.Rev.Lett.101 075004
[23]Kemp A,Sentoku Y,Tabak M 2009 Phy.Rev.E 79066406
[24]Pukhov A,Sheng Z M,Meyer-ter-Vehn J 1999 Phys.Plasmas 6 2847