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激光尾场加速是一种利用超强飞秒激光与气体靶作用加速电子的新型加速技术, 经过40多年的发展已经日益成熟, 但是重复频率相比传统加速器还有很大的差距. 高重复频率加速是未来激光尾场加速的一个重要发展方向, 届时气体靶给真空系统带来的负载将不可忽视, 这可能会成为限制重复频率的重要因素. 本文设计了一种应用于中小规模激光器的微气室喷嘴, 并通过三维流体模拟对比了这种喷嘴和常用的超音速喷嘴的喷气量差异, 证明它不仅能够产生和超音速喷嘴类似的密度分布, 还能够大幅降低喷气量, 从而减小真空系统的负载, 提高重频的上限. 此外, 把这种微气室喷嘴应用于激光尾场加速实验中, 在多条件下产生了稳定性很好的电子束. 这个工作将为高重频、高稳定性的尾场加速做出贡献.After forty-year tremendous advances, laser wakefield acceleration (LWFA), in which an ultra-intense femtosecond laser interacts with a gas target to produce energetic electrons, is becoming more and more mature. Acceleration with a high repetition rate will be an important topic in the near future. When operating at a high repetition rate, the influence of the gas load on the vacuum system cannot be neglected. Among the widely used gas targets, gas cells have a lower flow rate than supersonic gas nozzles. However, most of gas cells are several centimeters long, unsuitable for a moderate-size laser facility. In this work, we design a kind of micro gas cell with a sub-centimeter length. The flow rate of the micro gas cell and the supersonic nozzle are compared by hydromechanics simulations. Comparing with the supersonic nozzle, the flow rate of the micro gas cell is reduced by 97%. Moreover, the gas cell sustains a longer flattop region. The reduced flow rate is attributed to two reasons. The first reason is that the area of the nozzle exit decreases significantly. In the case of the supersonic nozzle, the laser interacts with the gas jet outside the nozzle exit. Therefore, the exit size is determined by the interaction length. In the case of the micro gas cell, the laser interacts with the gas inside the gas cell. The exit only needs to be larger than the laser focal, which is much smaller than the interaction length. The second reason is that the velocity of the gas jet decreases. When using a supersonic nozzle, the velocity at the nozzle exit must be high enough to generate a flattop density distribution, which is required by LWFA. As a comparison, in the micro gas cell, the gas is confined by the cell wall. As a consequence, the gas velocity has little influence on the density distribution inside the cell. By changing the inner radius of the cell, 1–4 mm-long flattop regions can be generated while keeping a low flow rate. Experiments using the micro gas cell are conducted on a 45 TW femtosecond laser facility at the Laser Fusion Research Center. The stable electron beams with maximum energy of 250 MeV are generated. This study will contribute to the investigation of stable and high-frequency laser wakefield acceleration.
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Keywords:
- laser wakefield acceleration /
- high repetition rate /
- micro gas cell
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图 1 超音速喷嘴(a)与微气室喷嘴(b)截面示意图
Fig. 1. Cross sections of the supersonic nozzle (a) and the micro gas cell (b).
图 2 超音速喷嘴(a)与微气室(b)气流密度分布的模拟结果; (c)比较了两个喷嘴在激光传播方向上((a)和(b)中黑色虚线位置)的分子密度分布
Fig. 2. Density distribution of the supersonic nozzle (a) and the micro gas cell (b); (c) the molecular number density along the laser propagation direction (dash lines) are compared in (c).
图 3 超音速喷嘴出口(a), (b)和微气室喷嘴内壁锥孔(c), (d)的气体密度(a), (c)和流速(b), (d)分布
Fig. 3. Density (a), (c) and velocity (b), (d) distribution at the exit of the supersonic nozzle (a), (b) and the internal interface of the micro gas cell exit (c), (d).
图 4 不同内径的微气室喷嘴中N2的分子密度分布
Fig. 4. Molecular number density in micro gas cells of different inside diameters.
图 5 采用内径1 mm的微气室喷嘴在分别在5 kPa (a)和8 kPa (b)下连续10发获得的电子能谱
Fig. 5. Electron sepctra in 10 consecutive shots using a 1 mm micro gas cell backing at 5 kPa (a) and 8 kPa (b).
表 1 使用4 mm微气室不同工作频率下的真空度
Table 1. Vaccum at different repetition rates using 4 mm micro gas cell.
重复频率/Hz 1 2 5 10 真空度最大值/(10–3 Pa) 4.7 4.6 6.7 10 
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[1] Tajima T, Dawson J 1979 Phys. Rev. Lett. 43 267
Google Scholar
[2] Faure J, Glinec Y, Pukhov A, Kiselev S, Gordienko S, Lefebvre E, Rousseau J P, Burgy F, Malka V 2004 Nature 431 541
Google Scholar
[3] Geddes C, Toth C, Van Tilborg J, Esarey E, Schroeder C, Bruhwiler D, Nieter C, Cary J, Leemans W 2004 Nature 431 538
Google Scholar
[4] Mangles S, Murphy C, Najmudin Z, Thomas A, Collier J, Dangor A, Divall E, Foster P, Gallacher J, Hooker C 2004 Nature 431 535
Google Scholar
[5] Kim H T, Pae K H, Cha H J, Kim I J, Yu T J, Sung J H, Lee S K, Jeong T M, Lee J 2013 Phys. Rev. Lett. 111 165002
Google Scholar
[6] Wang X, Zgadzaj R, Fazel N, Li Z, Yi S A, Zhang X, Henderson W, Chang Y Y, Korzekwa R, Tsai H E, Pai C H, Quevedo H, Dyer G, Gaul E, Martinez M, Bernstein A C, Borger T, Spinks M, Donovan M, Khudik V, Shvets G, Ditmire T, Downer M C 2013 Nat. Commun. 4 1988
Google Scholar
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Google Scholar
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[9] Gonsalves A J, Nakamura K, Daniels J, Benedetti C, Pieronek C, de Raadt T C H, Steinke S, Bin J H, Bulanov S S, van Tilborg J, Geddes C G R, Schroeder C B, Tóth C, Esarey E, Swanson K, Fan-Chiang L, Bagdasarov G, Bobrova N, Gasilov V, Korn G, Sasorov P, Leemans W P 2019 Phys. Rev. Lett. 122 084801
Google Scholar
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Google Scholar
[11] Wiggins S M, Issac R C, Welsh G H, Brunetti E, Shanks R P, Anania M P, Cipiccia S, Manahan G G, Aniculaesei C, Ersfeld B, Islam M R, Burgess R T L, Vieux G, Gillespie W A, MacLeod A M, van der Geer S B, de Loos M J, Jaroszynski D A 2010 Plasma Phys. Contr. F 52 124032
Google Scholar
[12] Pollock B B, Clayton C E, Ralph J E, Albert F, Davidson A, Divol L, Filip C, Glenzer S H, Herpoldt K, Lu W, Marsh K A, Meinecke J, Mori W B, Pak A, Rensink T C, Ross J S, Shaw J, Tynan G R, Joshi C, Froula D H 2011 Phys. Rev. Lett. 107 045001
Google Scholar
[13] Buck A, Wenz J, Xu J, Khrennikov K, Schmid K, Heigoldt M, Mikhailova J M, Geissler M, Shen B, Krausz F, Karsch S, Veisz L 2013 Phys. Rev. Lett. 110 185006
Google Scholar
[14] Maier A R, Delbos N M, Eichner T, Hübner L, Jalas S, Jeppe L, Jolly S W, Kirchen M, Leroux V, Messner P, Schnepp M, Trunk M, Walker P A, Werle C, Winkler P 2020 Phys. Rev. X 10 031039
Google Scholar
[15] Schmid K, Veisz L, Tavella F, Benavides S, Tautz R, Herrmann D, Buck A, Hidding B, Marcinkevicius A, Schramm U 2009 Phys. Rev. Lett. 102 124801
Google Scholar
[16] Cole J M, Symes D R, Lopes N C, Wood J C, Poder K, Alatabi S, Botchway S W, Foster P S, Gratton S, Johnson S, Kamperidis C, Kononenko O, De Lazzari M, Palmer C A J, Rusby D, Sanderson J, Sandholzer M, Sarri G, Szoke-Kovacs Z, Teboul L, Thompson J M, Warwick J R, Westerberg H, Hill M A, Norris D P, Mangles S P D, Najmudin Z 2018 Proc. Natl. Acad. Sci. USA 115 6335
Google Scholar
[17] Wenz J, Schleede S, Khrennikov K, Bech M, Thibault P, Heigoldt M, Pfeiffer F, Karsch S 2015 Nat. commun. 6 7568
Google Scholar
[18] Wu Y, Zhu B, Li G, Zhang X, Yu M, Dong K, Zhang T, Yang Y, Bi B, Yang J 2018 Sci. Rep. 8 15888
Google Scholar
[19] Schmid K, Veisz L 2012 Rev. Sci. Instrum. 83 053304
Google Scholar
[20] Döpp A, Guillaume E, Thaury C, Gautier J, Ta Phuoc K, Malka V 2016 Rev. Sci. Instrum. 87 073505
Google Scholar
[21] Osterhoff J, Popp A, Major Z, Marx B, Rowlands-Rees T, Fuchs M, Geissler M, Hörlein R, Hidding B, Becker S 2008 Phys. Rev. Lett. 101 085002
Google Scholar
[22] Clayton C E, Ralph J, Albert F, Fonseca R, Glenzer S, Joshi C, Lu W, Marsh K, Martins S F, Mori W B 2010 Phys. Rev. Lett. 105 105003
Google Scholar
[23] Liu J, Xia C, Wang W, Lu H, Wang C, Deng A, Li W, Zhang H, Liang X, Leng Y 2011 Phys. Rev. Lett. 107 035001
Google Scholar
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