Faraday rotation study of plasma bubbles in GeV wakefield accelerators
Faraday rotation study of plasma bubbles in GeV wakefield accelerators
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DOI：
10.1063/5.0072262
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发表时间：
2021-09
影响因子：
2.2
通讯作者：
Yen-Yu Chang;Xiantao Cheng;A. Hannasch;M. LaBerge;J. Shaw;K. Weichman;J. Welch;A. Bernstein;W. Henderson;R. Zgadzaj;M. Downer
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文献类型：
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作者：
Yen-Yu Chang;Xiantao Cheng;A. Hannasch;M. LaBerge;J. Shaw;K. Weichman;J. Welch;A. Bernstein;W. Henderson;R. Zgadzaj;M. Downer
在密度为Ne≈5×10 cm−3的等离子体中，我们通过成像λPr=1.0 5μm和持续时间τPr=2 ps或1 ps直角穿过气泡路径的线偏振探测脉冲上的法拉第旋转图案，可视化了由0.67pW激光脉冲驱动的等离子体气泡。当气泡捕获并加速几十到几百pC的电子电荷时，我们观察到两条平行的条纹，长度为cτPr，横跨驱动脉冲传输轴，被∼45μm隔开，其中探测器极化反向旋转0.3◦到大于5◦。伴随的模拟表明，它们是由于法拉第旋转在致密的气泡侧壁的一部分内产生的，这些气泡侧壁在探测器穿过气泡时被加速电子的方位向磁场所弥漫。对条纹宽度的分析表明，气泡内的准单能高能电子和尾随的低能电子贡献了观测信号的明显部分，而鞘电子的相对论流动抑制了气泡后方的法拉第旋转。结果表明，法拉第旋转诊断的良好标度比先前演示的等离子体密度低40倍。
We visualize plasma bubbles driven by 0.67 PW laser pulses in plasma of density ne ≈ 5 × 10 cm−3 by imaging Faraday rotation patterns imprinted on linearly-polarized probe pulses of wavelength λpr = 1.05μm and duration τpr = 2 ps or 1 ps that cross the bubble’s path at right angles. When the bubble captures and accelerates tens to hundreds of pC of electron charge, we observe two parallel streaks of length cτpr straddling the drive pulse propagation axis, separated by ∼ 45 μm, in which probe polarization rotates by 0.3◦ to more than 5◦ in opposite directions. Accompanying simulations show that they result from Faraday rotation within portions of dense bubble side walls that are pervaded by the azimuthal magnetic field of accelerating electrons during the probe transit across the bubble. Analysis of the width of the streaks shows that quasi-monoenergetic high-energy electrons and trailing lower energy electrons inside the bubble contribute distinguishable portions of the observed signals, and that relativistic flow of sheath electrons suppresses Faraday rotation from the rear of the bubble. The results demonstrate favorable scaling of Faraday rotation diagnostics to 40× lower plasma density than previously demonstrated.