雷射尾場加速可藉由將數兆瓦雷射脈衝聚焦於高密度的薄氣靶來實現。在此情況下，於雷射脈衝內實現的自我聚焦效應(self-focusing effect)可進一步縮小脈衝的橫向尺寸，而自我調變機製(self-modulation effect)可於縱方向壓縮脈衝時寬，使等效提高的雷射脈衝強度有效激發非線性電漿波並加速注入電漿波內的電子。所實現的數兆瓦雷尾流場加速可望由新興的高重複頻率二極體激發雷射所產生之兆瓦級雷射脈衝所驅動，以提高電子束的平均電流，增加其在電子束治療或產生X光輻射的應用價值。本研究之重點即為發展次毫米尺度，可提供> 5×10^19 cm^-3的電子密度的氮氣氣腔與噴嘴，並研析引入僅二至一兆瓦尖峰功率的雷射脈衝驅動下所產生的電子束特性。實驗結果顯示當40 飛秒、1 兆瓦、波長為810奈米的雷射脈衝聚焦至以20 psi背壓充入氮氣的450微米長氣腔時，氣靶內電漿電子密度達5.3×10^19 cm^-3，並在重現性 > 90 %的情況下產生能量峰值約 9.1 MeV、電荷量約 25 pC 的電子束。這些電子束具有約11 MeV 的能量擴散度、約40 mrad 的電子束擴散角和約14 mrad 的指向穩定性。而在使用直徑為178微米孔徑之噴嘴時，可在400 psi氮氣背壓下使具高斯密度分佈之氣靶的峰值電漿密度達2.8×10^19 cm^-3，並在重現性> 90 %的情況下產生能量峰值約 7 MeV、電荷量約 14 pC 的電子束。這些電子束具有約6.4 MeV 的能量擴散度、約35 mrad 電子束擴散角和約24mrad的指向穩定性。此實驗結果顯示了以小於50 mJ的雷射能量驅動的雷射尾流場可產生穩定的類單能MeV 電子束，同時給予我們發展高重複率LWFA甚至是X光微影等應用的可能性。

Laser wakefield acceleration (LWFA) can be implemented by focusing a few-terawatt (TW) laser pulse onto a thin, dense gas target. In this way, the self-focusing effect reduces the transverse size of the pulse and the self-modulation instability causes the longitudinal pulse compression, resulting in a greatly increased pulse intensity capable of exiting a nonlinear plasma wave for accelerating injected electrons. It is expected that few-TW LWFA can be implemented with the high-repetition-rate, diode pumped laser system for generating electron bunches at kHz-level frequencies and raising the beam current suitable for downstream applications such as electron therapy or X-ray images. Therefore, the major goal of this study is to realize sub-mm, dense gas target that can provide plasmas electron density up to 5×10^19 cm^-3 with the commensurate reduction of laser peak power and demonstrate the LWFA driving by a low laser peak power at 1 TW. By focusing a 40-fs, 1-TW, 810 nm pulse on a 450 μm long nitrogen gas cell, quasi monoenergetic electron bunches can be generated with peak energy ~ 9.1 MeV, charge ~ 25 pC and reproducibility >90% at a gas backing pressure of 20 psi. These output electrons exhibit an energy spread ~ 11 MeV, a beam divergence ~ 40 mrad, and a pointing fluctuation ~ 14 mrad. On the other hand, by focusing a 40 fs, 1 TW, 810 nm pulse on a 178 μm size nitrogen orifice, quasi monoenergetic electron bunches can be generated with peak energy ~ 7 MeV, charge ~ 14 pC and reproducibility >90% at a gas backing pressure of 400 psi. These output electrons exhibit an energy spread ~ 6.4 MeV, a beam divergence ~ 35 mrad, and a pointing fluctuation ~ 24 mrad. Our scheme advances the frontier of few-TW LWFA for generating few-MeV electrons with on-target laser energy less than 50 mJ, while maintaining a beam quality sufficient for downstream applications.

[1] T. Toshiki and J. M. Dawson, "Laser electron accelerator," Phy.Rev.Lett. 43, 267,(1979).
[2] E. Esarey, C. B. Schroeder, and W. P. Leemans,” Physics of laser-driven plasma-based electron accelerators” Rev. Mod. Phys. 81, 1229 (2009).
[3] C. Joshi, Plasma Phys. Controlled Fusion 61, 104001 (2019).
[4] D. Strickland and G. Mourou, "Compression of amplified chirped optical pulses," Opt. Commun. 56, 219-221,(1985).
[5] E. Kaksis, G. Almási, J. A. Fülöp, A. Pugžlys, A. Baltuška, and G. Andriukaitis, "110-mJ 225-fs cryogenically cooled Yb: CaF2 multipass amplifier," Optics express. 24, 28915-28922,(2016).
[6] T. Nubbemeyer, M. Kaumanns, M. Ueffing, M. Gorjan, A. Alismail, H. Fattahi, J. Brons, O. Pronin, H. G. Barros, and Z. Major, "1 kW, 200 mJ picosecond thin-disk laser system," Opt. Lett. 42, 1381-1384,(2017).
[7] M. Kaumanns, V. Pervak, D. Kormin, V. Leshchenko, A. Kessel, M. Ueffing, Y. Chen, and T. Nubbemeyer, "Multipass spectral broadening of 18 mJ pulses compressible from 1.3 ps to 41 fs," Optics letters. 43, 5877-5880,(2018).
[8] F. Albert and A. G. R. Thomas,” Applications of laser wakefield accelerator-based light sources” ,Plasma Phys. Controlled Fusion 58, 103001 (2016).
[9] Darren Batey, “Ptychographic imaging of mixed states”, Department of Electronic and Electrical Engineering University of Sheffield, PhD, 2014.
[10] Cipiccia, S., Islam, M., Ersfeld, B. et al. "Gamma-rays from harmonically resonant betatron oscillations in a plasma wake," Nature Phys 7, 867–871 (2011).
[11] A. Hannasch, A. Laso Garcia, M. LaBerge, R. Zgadzaj, A. Köhler, J. P. Couperus Cabadağ, O. Zarini, T. Kurz, A. Ferrari, M. Molodtsova, L. Naumann, T. E. Cowan, U.
46
Schramm, A. Irman & M. C. Downer,” Compact spectroscopy of keV to MeV X‑rays from a laser wakefield accelerator”, Sci Rep 11, 14368 (2021).
[12] H. Stark, J. Buldt, M. Muller, A. Klenke, A. Tunnermann, and J. Limpert,“23 mJ high-power fiber CPA system using electro-optically controlled divided-pulse amplification,”Opt. Lett. 44 5529 (2019).
[13] T. Nubbemeyer, M. Kaumanns, M. Ueffing, M. Gorjan, A. Alismail, H. Fattahi, J. Brons, O. Pronin, H. G. Barros, Z. Major, T. Metzger, D. Sutter, and F. Krausz, “1 kW, 200 mJ picosecond thin-disk laser system,” Opt. Lett. 42 1381 (2017).
[14] C.-H. Lu, Y.-J. Tsou, H.-Y. Chen, B.-H. Chen, Y.-C, Cheng, S.-D. Yang, M.-C. Chen, C.-C. Hsu, and A. H. Kung, “Generation of intense supercontinuum in condensed media,” Optica 1, 400 (2014).
[15] M. Hanna, X. Delen, L. Lavenu, F. Guichard, Y. Zaouter, F. Druon, and P. Georges, “Nonlinear temporal compression in multipass cells: theory,” J. Opt. Soc. Am. B 34 1340 (2017).
[16] M. Kaumanns, V. Pervak, D. Kormin, V. Leshchenko, A. Kessel, M. Ueffing, Y. Chen, and T. Nubbemeyer, “Multipass spectral broadening of 18 mJ pulses compressible from 1.3 ps to 41 fs,”Opt. Lett. 43 5877 (2018).
[17] R. L. Wagner, "Laser-plasma electron accelerators and nonlinear, relativistic optics," Applied physics, University of Michigan, (1999).
[18] H. H. Chu, “Construction of a 10-TW Laser of High Coherence and Stability and Its Application in Laser-Cluster Interaction and X-Ray Lasers”, Department of Physics, National Taiwan University, PhD, (2005).
[19] W. Kruer, The physics of laser plasma interactions. CRC Press,(2018).
[20] W. Lu, M. Tzoufras, C. Joshi, F. S. Tsung, W. B. Mori, J. Vieira, R. A. Fonseca, and L. O. Silva, "Generating multi-GeV electron bunches using single stage laser wakefield
47
acceleration in a 3D nonlinear regime," Physical Review Special Topics-Accelerators and Beams. 10, 061301,(2007).
[21] H. Suk, N. Barov, J. B. Rosenzweig, and E. Esarey, "Plasma electron trapping and acceleration in a plasma wake field using a density transition," Phys. Rev. Lett. 86, 1011-1014,(2001).
[22] M. Chen, E. Esarey, C. Schroeder, C. Geddes, and W. Leemans, "Theory of ionization-induced trapping in laser-plasma accelerators," Phys. Plasmas. 19, 033101,(2012).
[23] G. Z. Sun, E. Ott, Y. Lee, and P. Guzdar, "Self‐focusing of short intense pulses in plasmas," Phys. Fluids. 30, 526-532,(1987).
[24] R. W. Boyd, S. G. Lukishova, and Y. R. Shen, Self-focusing: Past and Present: Fundamentals and Prospects. Springer, New York, NY,(2008).
[25] A. Buck, K. Zeil, A. Popp, K. Schmid, A. Jochmann, S. Kraft, B. Hidding, T. Kudyakov, C. Sears, and L. Veisz, "Absolute charge calibration of scintillating screens for relativistic electron detection," Rev. Sci. Instrum. 81, 033301,(2010).
[26] M.-W. Lin, T.-Y. Chu, Y.-Z. Chen, D. K. Tran, H.-H. Chu, S.-H. Chen, and J. Wang,” Laser wakefield acceleration driven by a fewterawatt laser pulse in a sub-mm nitrogen gas jet” Phys. Plasmas. 27, 113102,(2020).
[27] M.-W. Lin, C.-Y. Hsieh, D. Tran, and S.-H. Chen, "Simulation study of ionization-induced injection in sub-terawatt laser wakefield acceleration," Phys. Plasmas. 27, 013102,(2020).