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Register a new userPlasma ramps caused by outflow in gas-filled capillaries
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Plasma confinement inside capillaries has been developed in the past years for plasma-based acceleration to ensure a stable and repeatable plasma density distribution during the interaction with either particles or laser beams. In particular, gas-filled capillaries allow a stable and almost predictable plasma distribution along the interaction with the particles. However, the plasma ejected through the ends of the capillary interacts with the beam before the inner plasma, affecting the quality of the beam. In this article we report the measurements on the evolution of the plasma flow at the two ends of a 1 cm long, 1 mm diameter capillary filled with hydrogen. In particular, we measured the longitudinal density distribution and the expansion velocity of the plasma outside the capillary. This study will allow a better understanding of the beam-plasma interaction for future plasma-based experiments.
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A plasma flow behind a relativistic electron bunch propagating through a cold plasma is found assuming that the transverse and longitudinal dimensions of the bunch are small and the bunch can be treated as a point charge. In addition, the bunch charge is assumed small. A simplified system of equations for the plasma electrons is derived and it is shown that, through a simple rescaling of variables, the bunch charge can be eliminated from the equations. These equations have a unique solution, with an ion cavity formed behind the driver. The equations are solved numerically and the scaling of the cavity dimensions with the driver charge is obtained. A numerical solution for the case of a positively charged driver is also found.
The AWAKE experiment relies on the self-modulation instability of a long proton bunch to effectively drive wakefields and accelerate an electron bunch to GeV-level energies. During the first experimental run (2016-2018) the instability was made phase reproducible by means of a seeding process: a short laser pulse co-propagates within the proton bunch in a rubidium vapor. Thus, the fast creation of plasma and the onset of beam-plasma interaction within the bunch drives seed wakefields. However, this seeding method leaves the front of the bunch not modulated. The bunch front could self-modulate in a second, preformed plasma and drive wakefields that would interfere with those driven by the (already self-modulated) back of the bunch and with the acceleration process. We present studies of the seeded the self-modulation (SSM) of a long proton bunch using a short electron bunch. The short seed bunch is placed ahead of the proton bunch leading to self-modulation of the entire bunch. Numerical simulations show that this method have other advantages when compared to the ionization front method. We discuss the requirements for the electron bunch parameters (charge, emittance, transverse size at the focal point, length), to effectively seed the self-modulation process. We also present preliminary experimental studies on the electron bunch seed wakefields generation.
We present a pump-probe technique for monitoring ultrafast polarizability changes. In particular, we use it to measure the plasma density created at the temporal focus of a self-compressing higher-order pump soliton in gas-filled hollow-core photonic crystal fiber. This is done by monitoring the wavelength of the dispersive wave emission from a counter-propagating probe soliton. By varying the relative delay between pump and probe, the plasma density distribution along the fiber can be mapped out. Compared to the recently introduced interferometric side-probing for monitoring the plasma density, our new technique is relatively immune to instabilities caused by air turbulence and mechanical vibration. The results of two experiments on argon- and krypton-filled fiber are presented, and compared to numerical simulations. The technique provides an important new tool for probing photoionization in many different gases and gas mixtures as well as ultrafast changes in dispersion in many other contexts.
A linear theory of a wakefield excitation in a plasma-dielectric accelerating structure by a drive electron bunch in the case of an off-axis bunch injection has been constructed. The structure under investigation is a round dielectric-loaded metal waveguide with a channel for the charged particles, filled with homogeneous cold plasma. Derived theory was used to investigate numerically the spatial distribution of the bunch-excited wakefield components, which act on both the drive and witness bunches.
Plasma injection schemes are crucial for producing high-quality electron beams in laser-plasma accelerators. This article introduces the general concepts of plasma injection. First, a Hamiltonian model for particle trapping and acceleration in plasma waves is introduced; ionization injection and colliding-pulse injection are described in the framework of this Hamiltonian model. We then proceed to consider injection in plasma density gradients.
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