Electron Acceleration
In this chapter we discuss the generation of high-energy electrons in laser-plasma interactions, in two very different regimes. First, we consider electron acceleration in wake waves generated in underdense plasmas, which is the concept behind the develop
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Electron Acceleration
Abstract In this chapter we discuss the generation of high-energy electrons in laser-plasma interactions, in two very different regimes. First, we consider electron acceleration in wake waves generated in underdense plasmas, which is the concept behind the development of laser-plasma electron accelerators for high energy physics. Second, we consider the case of an overdense plasma, where electrons are accelerated at the interface where the laser impinges. Such problem is strongly connected to the general issue of collisionless absorption in an overdense plasma, possibly the most complex and less understood topic of laser-plasma interactions.
4.1 Underdense Plasmas: Laser Wakefield Accelerators 4.1.1 Wakefield Generation How does a surfer gain velocity from a sea wave? It is necessary for the surfer to have some initial velocity along the direction of propagation of the wave. If possible, the initial velocity should be equal to the phase velocity of the wave, because in such a case in its rest frame the surfer would see a constant field which can provide a net acceleration, in contrast to an oscillating field for which the acceleration averages to zero over a cycle. Of course, the process of acceleration has a negative feedback because a change in the surfer’s velocity leads to dephasing from the wave. The above example brings us to two necessary conditions for an electromagnetic wave to accelerate efficiently a charged particle: there must be an electric field component along the propagation direction, and the phase velocity must be such that an optimal phasing between the wave and the particle can occur. Electron plasma waves thus appear as a serious candidate for particle acceleration. In fact, those are longitudinal waves, and the phase velocity υ p is not determined by the plasma frequency, leaving hope to “construct” a wave with the optimal value of υ p . The other attractive point with plasma waves is that electric fields much higher than in conventional A. Macchi, A Superintense Laser-Plasma Interaction Theory Primer, SpringerBriefs in Physics, DOI: 10.1007/978-94-007-6125-4_4, © The Author(s) 2013
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4 Electron Acceleration
accelerators may be attained, for the trivial reason that there is no risk of electrical breakdown in an already ionized medium! Since the final goal is to accelerate relativistic particles, the issue is to generate a plasma wave with phase velocity close to (but not exceeding) that of light, υ p c, so that relativistic particles (electrons, for definiteness) may remain in phase with the wave. Notice that for highly relativistic energies the acceleration process may be more efficient because a large change in energy corresponds to a small change in velocity, thus the particle may get out of phase only after a long time. The original work on the “laser electron accelerator” by Tajima and Dawson (1979) proposed to use a wakefield (Sect. 3.6.1) generated by an intense laser pulse1 propagating at the group velocity υg = c(1 − ω2p /ω2 )1/2 c in an underdens
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