Injection in plasma-based electron accelerators
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Plasma-based accelerators aim to efficiently generate relativistic electrons by exciting plasma waves using a laser or particle beam driver, and "surfing" electrons on the resulting wakefields. In the blowout regime of such wakefield acceleration techniques, the intense laser radiation pressure or beam fields expel all of the plasma electrons transversely, forming a region completely devoid of electrons ("bubble") that co-propagates behind the driver. Injection, where initially quiescent background plasma electrons become trapped inside of the plasma bubble, can be caused by a variety of mechanisms such as bubble expansion, field ionization or collision between pump and injector pulses. This work will present a study of the injection phenomenon through analytic modeling and particle-in-cell (PIC) simulations. First, an idealized model of a slowly expanding spherical bubble propagating at relativistic speeds is used to demonstrate the importance of the bubble's structural dynamics in self-injection. This physical picture of injection is verified though a reduced PIC approach which makes possible the modeling of problem sizes intractable to first-principles codes. A more realistic analytic model which takes into account the effects of the detailed structure of the fields surrounding the bubble in the injection process is also derived. Bubble expansion rates sufficient to cause injection are characterized. A new mechanism for generation of quasi-monoenergetic electron beams through field ionization induced injection is presented, and simulation results are compared to recent experimental results. Finally, a technique for frequency-domain holographic imaging of the evolving bubble is analyzed using PIC as well as a novel simulation method for laser probe beam propagation.