Magnetic Control of Particle Injection in Plasma Based Accelerators

The use of an external transverse magnetic field to trigger and to control electron self-injection in laser- and particle-beam driven wakefield accelerators is examined analytically and through full-scale particle-in-cell simulations. A magnetic field can relax the injection threshold and can be used to control main output beam features such as charge, energy, and transverse dynamics in the ion channel associated with the plasma blowout. It is shown that this mechanism could be studied using state-of-the-art magnetic fields in next generation plasma accelerator experiments.

Figure 1

3D OSIRIS simulation of a magnetized LWFA. (a) Electron density isosurfaces in the uniform $B$ field region. (b)–(d) Density profile associated with the $y=0$, $\xi =0$, and $x=0$ planes of (a) revealing the off-axis injection. Self-injected electrons (darker dots) are closer to the bubble axis, while the electrons farther from the bubble axis escape from the trapping region (lighter dots). (e) Phase space of the plasma electrons and spectrum (solid line), showing a quasimonoenergetic ($\sim 6\%$ FWHM spread) electron bunch. (f) Trajectories of the self-injected beam particles, with the propagation axis shown by the dashed line. The $B$ field profile is also represented.

Figure 2

3D OSIRIS simulation of $B$ injection in PWFA (2D slices represented). (a) Electron density and (b) transverse density slice at the back of the bubble in the unmagnetized case. (c) Electron density after the $B$ field down-ramp and (d) transverse density slice at the back of the bubble, revealing the trapped particles.

Figure 3

Parameter scan for the self-injected charge in the first bucket retrieved from 2D simulations. (a) Down-ramp length scan, revealing that charge decreases with $L^{\mathrm{ramp}}$. (b) Peak $B$ field scan, revealing the charge increases with $B_y^{\mathrm{ext}}$.