Diagnostics for plasma-based electron accelerators
Diagnostics for plasma-based electron accelerators
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DOI：
10.1103/revmodphys.90.035002
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发表时间：
2018-08-08
影响因子：
44.1
通讯作者：
Kaluza, M. C.
中科院分区：
文献类型：
--
作者：
Downer, M. C.;Zgadzaj, R.;Kaluza, M. C.
基于等离子体的加速器可以在几厘米内为电子或正电子提供高达几GeV的能量增益，这已经产生了一种新的诊断技术，与传统射频（rf）加速器所使用的诊断技术截然不同。等离子体加速器的微米尺度和瞬态动态结构与传统加速器的米尺度和静态结构形成了鲜明对比，因此需要新的诊断方法。由于这种微米级的源尺寸，等离子体加速的电子脉冲可以产生更小的归一化横向发射度（epsilon(n) < 0.1 mm mrad）和更短的持续时间（tau(b)类似于1 fs）。通过对电子发射的电磁辐射进行宽带宽光谱测量，我们回顾了单次诊断方法，可以无侵入性地确定如此小的epsilon(n)和tau(b)，并具有高分辨率：epsilon(n)来自电子与等离子体加速器横向内部场或外部光场或波动器相互作用时发射的x射线；τ (b)从太赫兹到穿越界面时发射的光相干跃迁辐射。类似于1fs束的持续时间也可以通过采样共传播光脉冲的单个周期或通过使用横向探针脉冲测量相关的磁场来测量。由于等离子体加速器的光速和微米尺寸，其结构的演变是加速器性能的关键决定因素，在实验室中可视化是非常具有挑战性的。本文回顾了新一代的实验室诊断，这些诊断产生了基于飞秒电磁或电子探针脉冲的相位调制或偏转的激光和粒子束产生的等离子体加速器结构的快照，甚至电影。讨论了这些成像技术的时空分辨率限制，以及通过分析图像并将其与模拟等离子体结构进行比较而产生的基于等离子体的加速物理的见解。
Plasma-based accelerators that impart energy gain as high as several GeV to electrons or positrons within a few centimeters have engendered a new class of diagnostic techniques very different from those used in connection with conventional radio-frequency (rf) accelerators. The need for new diagnostics stems from the micrometer scale and transient, dynamic structure of plasma accelerators, which contrasts with the meter scale and static structure of conventional accelerators. Because of this micrometer source size, plasma-accelerated electron hunches can emerge with smaller normalized transverse emittance (epsilon(n) < 0.1 mm mrad) and shorter duration (tau(b) similar to 1 fs) than bunches from rf linacs. Single-shot diagnostics are reviewed that determine such small epsilon(n) and tau(b) noninvasively and with high resolution from wide-bandwidth spectral measurement of electromagnetic radiation the electrons emit: epsilon(n) from x rays emitted as electrons interact with transverse internal fields of the plasma accelerator or with external optical fields or undulators; tau(b) from THz to optical coherent transition radiation emitted upon traversing interfaces. The duration of similar to 1 fs bunches can also be measured by sampling individual cycles of a copropagating optical pulse or by measuring the associated magnetic field using a transverse probe pulse. Because of their luminal velocity and micrometer size, the evolving structure of plasma accelerators, the key determinant of accelerator performance, is exceptionally challenging to visualize in the laboratory. Here a new generation of laboratory diagnostics is reviewed that yield snapshots, or even movies, of laser- and particle-beam-generated plasma accelerator structures based on their phase modulation or deflection of femtosecond electromagnetic or electron probe pulses. Spatiotemporal resolution limits of these imaging techniques are discussed, along with insight into plasma-based acceleration physics that has emerged from analyzing the images and comparing them to simulated plasma structures.