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Register a new userRadiation of a Bunch Crossing a Boundary between a Vacuum and Cold Magnetized Plasma in a Waveguide
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We analyze the electromagnetic field of a small bunch that uniformly moves in a circular waveguide and transverses a boundary between an area filled up with cold magnetized electron plasma and a vacuum area. The magnetic field is supposed to be strong but finite so the perturbation technique can be applied. Two cases are studied in detail: the bunch is flying out of the plasma into a vacuum, and, inversely, the bunch is flying into the plasma out of the vacuum area of waveguide. The investigation of the waveguide mode components is performed analytically with methods of the complex variable function theory. The main peculiarities of the bunch radiation in such situations are revealed.
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Here we develop a general theory of mode transformation (diffraction) at the flat transverse boundary between cold magnetized electron plasma and isotropic vacuum-like medium inside a circular waveguide. The obtained results can be also directly applied to the narrow-band Cherenkov radiation generated in plasma (or in isotropic medium) by a moving charged particle bunch.
If a charged particle bunch propagates near a plasma-vacuum boundary, it excites a surface wave and experiences a force caused by the boundary. For the linearly responding plasma and ultra-relativistic bunch, the spatial distribution of excited fields is calculated, and the force exerted on a short and narrow bunch is approximated by elementary functions. The force attracts the bunch to the boundary, if the bunch is in the vacuum, and repels otherwise. There are also additional focusing and defocusing components of the force.
We use a relativistic ionization front to provide various initial transverse wakefield amplitudes for the self-modulation of a long proton bunch in plasma. We show experimentally that, with sufficient initial amplitude ($ge(4.1pm0.4)$ MV/m), the phase of the modulation along the bunch is reproducible from event to event, with 3 to 7% (of 2$pi$) rms variations all along the bunch. The phase is not reproducible for lower initial amplitudes. We observe the transition between these two regimes. Phase reproducibility is essential for deterministic external injection of particles to be accelerated.
Plasma wakefield dynamics over timescales up to 800 ps, approximately 100 plasma periods, are studied experimentally at the Advanced Wakefield Experiment (AWAKE). The development of the longitudinal wakefield amplitude driven by a self-modulated proton bunch is measured using the external injection of witness electrons that sample the fields. In simulation, resonant excitation of the wakefield causes plasma electron trajectory crossing, resulting in the development of a potential outside the plasma boundary as electrons are transversely ejected. Trends consistent with the presence of this potential are experimentally measured and their dependence on wakefield amplitude are studied via seed laser timing scans and electron injection delay scans.
We propose a new method for self-injection of high-quality electron bunches in the plasma wakefield structure in the blowout regime utilizing a flying focus produced by a drive-beam with an energy-chirp. In a flying focus the speed of the density centroid of the drive bunch can be superluminal or subluminal by utilizing the chromatic dependence of the focusing optics. We first derive the focal velocity and the characteristic length of the focal spot in terms of the focal length and an energy chirp. We then demonstrate using multi-dimensional particle-in-cell simulations that a wake driven by a superluminally propagating flying focus of an electron beam can generate GeV-level electron bunches with ultra-low normalized slice emittance ($sim$30 nm rad), high current ($sim$ 17 kA), low slice energy-spread ($sim$0.1%) and therefore high normalized brightness ($>10^{19}$ A/rad$^2$/m$^2$) in a plasma of density $sim10^{19}$ cm$^{-3}$. The injection process is highly controllable and tunable by changing the focal velocity and shaping the drive beam current. Near-term experiments using the new FACET II beam could potentially produce beams with brightness exceeding $10^{20}$ A/rad$^2$/m$^2$.
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