An intense, high-energy electron or positron beam can have focused intensities rivaling those of today's most powerful laser beams. For example, the 5 ps ͑full-width, half-maximum͒, 50 GeV beam at the Stanford Linear Accelerator Center ͑SLAC͒ at 1 kA and focused to a 3 micron rms spot size gives intensities of Ͼ10 20 W/cm Ϫ2 at a repetition rate of Ͼ10 Hz. Unlike a ps or fs laser pulse which interacts with the surface of a solid target, the particle beam can readily tunnel through tens of cm of steel. However, the same particle beam can be manipulated quite effectively by a plasma that is a million times less dense than air! This is because of the incredibly strong collective fields induced in the plasma by the Coulomb force of the beam. The collective fields in turn react back onto the beam leading to many clearly observable phenomena. The beam paraticles can be: ͑1͒ Deflected leading to focusing, defocusing, or even steering of the beam; ͑2͒ undulated causing the emission of spontaneous betatron x-ray radiation and; ͑3͒ accelerated or decelerated by the plasma fields. Using the 28.5 GeV electron beam from the SLAC linac a series of experiments have been carried out that demonstrate clearly many of the above mentioned effects. The results can be compared with theoretical predictions and with two-dimensional and three-dimensional, one-to-one, particle-in-cell code simulations. These phenomena may have practical applications in future technologies including optical elements in particle beam lines, synchrotron light sources, and ultrahigh gradient accelerators.
Plasma-wakefield excitation by positron beams is examined in a regime for which the plasma dynamics are highly nonlinear. Three dimensional particle-in-cell simulations and physical models are presented. In the nonlinear wake regime known as the blowout regime for electrons, positron wakes exhibit an analogous "suck-in" behavior. Although analogous, the two wakefield cases are quite different in terms of their amplitudes, wavelengths, waveforms, transverse profiles, and plasma density dependence. In a homogenous plasma, nonlinear positron wakes are smaller than those of the corresponding electron case. However, hollow channels are shown to enhance the amplitude of the positron wakes.
As Langmuir waves (LWs) are driven to large amplitude in plasma, they are affected by nonlinear mechanisms. A global understanding, based on simulations and experiments, has emerged that identifies various nonlinear regimes depending on the dimensionless parameter kλD, where k is the Langmuir wave number and λD is the electron Debye length. The nonlinear phenomena arise due to wave-wave and wave-particle coupling mechanisms, and this basic separation between fluid-like nonlinearities and kinetic nonlinearities depends on the degree to which electron and ion Landau damping, as well as electron trapping, play a role. Previous ionospheric heating experiments [Cheung et al. Phys. Plasmas 8, 802 (2001)] identified cavitation/collapse and Langmuir decay instability (LDI), predominantly wave-wave mechanisms, to be the principal nonlinear effects for driven LWs with kλD<0.1, in agreement with fluid simulations [DuBois et al. Phys. Plasmas 8, 791 (2001)]. In the present research, collective Thomson scattering measurements of LWs driven by stimulated Raman scattering in laser-plasma experiments are used to study both wave-wave and wave-particle nonlinearities [Kline et al. Phys. Rev. Lett. 94, 175003 (2005)]. For kλD<0.29, multiple LWs are detected and are attributed to LDI, a wave-wave nonlinear regime. For kλD>0.29, a single-wave, frequency-broadened spectrum is observed associated with electron trapping, a wave-particle nonlinear regime. The transition from wave-wave to wave-particle nonlinear behavior is qualitatively consistent with particle-in-cell simulations and with the crossing of the LDI threshold above that for LW self-focusing. The fact that LDI is observed in both ionospheric and laser-plasma experiments for similar values of kλD, though vastly differing in plasma conditions and scales, and that simulations predict the various observed nonlinear regimes over a large range of kλD, supports our global view of LW nonlinear behavior.
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