Femtosecond electron microscopy of relativistic electron bunches

Abstract

The development of plasma-based accelerators has enabled the generation of very high brightness electron bunches of femtosecond duration, micrometer size and ultralow emittance, crucial for emerging applications including ultrafast detection in material science, laboratory-scale free-electron lasers and compact colliders for high-energy physics. The precise characterization of the initial bunch parameters is critical to the ability to manipulate the beam properties for downstream applications. Proper diagnostic of such ultra-short and high charge density laser-plasma accelerated bunches, however, remains very challenging. Here we address this challenge with a novel technique we name as femtosecond ultrarelativistic electron microscopy, which utilizes an electron bunch from another laser-plasma accelerator as a probe. In contrast to conventional microscopy of using very low-energy electrons, the femtosecond duration and high electron energy of such a probe beam enable it to capture the ultra-intense space-charge fields of the investigated bunch and to reconstruct the charge distribution with very high spatiotemporal resolution, all in a single shot. In the experiment presented here we have used this technique to study the shape of a laser-plasma accelerated electron beam, its asymmetry due to the drive laser polarization, and its beam evolution as it exits the plasma. We anticipate that this method will significantly advance the understanding of complex beam-plasma dynamics and will also provide a powerful new tool for real-time optimization of plasma accelerators.

Real-time visualization of the laser-plasma wakefield dynamics

“…On the basis of this understanding, it is shown that, as the bunch propagates further into the density downramp (Fig. 2, B9 and B10), its transverse sizes gradually increase due to the weakening of the plasma focusing force (31,46). Additionally, the transverse scale of the wakefield (represented by the trailing periodic bright-dark pattern) also increases along with the drive electron beam, behaving as a quasi-linear beam-driven plasma wave (47).…”

“…This was confirmed by slightly reducing the input laser energy to avoid ionization injection, which revealed that the strength of the wakefield was notably weaker compared to that observed with an injected electron beam. Furthermore, the elliptical hollow structure preceding the wakefield was recently discovered to resemble the transverse charge-density distribution of the electron bunch (31). On the basis of this understanding, it is shown that, as the bunch propagates further into the density downramp (Fig.…”

“…Let us now use the linear pertubation model given by (31,32) to estimate the evolution of the wakefield strength from the experimental data presented in Fig. 2B.…”

“…A newly developed technique called femtosecond relativistic electron microscopy (FREM) offers an alternative approach for diagnostics (26,27). By using electron beams generated from laser-plasma as a probe, FREM enables measurements of highly transient, microscopic field structures like plasma wakefields with high spatiotemporal resolution, owing to its few femtosecond pulse durations (22,28,29), micrometer-scale source sizes (30,31), and relativistic particle energies. Additionally, the electron probe encompasses several distinctive properties.…”

“…Last, the electron probe is highly sensitive to the space-charge fields of the charged particle bunch. Leveraging these unique properties, FREM has been successfully used to probe the linear structure of wakefields (26), the rear spikes of nonlinear wakefields (27), and electron bunches in plasma (31), and an algorithm for field reconstruction was also recently developed (32). Yet, a comprehensive experimental investigation of the entire evolution of the plasma wakefield from its excitation until the end remains elusive, of which the explanation still mostly relies on the support from computationally expensive postprocessing numerical simulations.…”