Theory of ionization-induced trapping in laser-plasma accelerators

Abstract

Ionization injection in a laser-plasma accelerator is studied analytically and by multi-dimensional particle-in-cell (PIC) simulations. To enable the production of low energy spread beams, we consider a short region containing a high atomic number gas (e.g., nitrogen) for ionization-induced trapping, followed by a longer region using a low atomic number gas (e.g., hydrogen), that is, free of additional trapping, for post acceleration. For a broad laser pulse, ionization injection requires a minimum normalized laser field of a0≃1.7, assuming a resonant Gaussian laser pulse. Effects of gas mix parameters, including species, concentration, and length of the mixture region, on the final electron injection number and beam quality are studied. The minimum energy spread is determined by the spread in initial ionized phases of the electrons in the wakefield due to the tunneling ionization process within the laser pulse. Laser polarization and intensity effects on injection number and final electron emittance are examined. Two-dimensional PIC simulations are used to study the ionization injection process and the transverse beam structure. With proper laser-plasma parameters, mono-energetic electron beams with 10 pC charge, a central energy at GeV level, and energy spread less than 1% can be produced in a mixed gas with ionized electron density of 1018cm-3. Lower density can give a higher final accelerated beam energy and reduce the final relative energy spread even further.

Two-Color Laser-Ionization Injection

“…A three-dimensional (3D) separatrix may be defined by trapped and focused orbits and has H s,3D = 1/γ p [6]. For an electron ionized in the wake by the injection pulse, the orbit is described by H(u z , ψ) = H i = 1 − φ(ψ i ) [14], assuming that the electron is ionized at rest and a 2 1 1, where ψ i is the wake phase at ionization. The trapping condition for the ionized electron is H s − H i > 0, and the optimal injection phase occurs where H s − H i is maximum, as shown in Fig.…”

“…Using ionization injection the emittance grows with increasing ionization laser intensity n ∝ a 0 . Since a 0 2 is required for electron trapping [14], reducing the injected beam emittance using single-pulse ionization injection is limited. One variant on ionization injection is to use a beam-driven plasma wake in the blow-out regime, followed by a laser pulse to trap electrons via ionization injection, and simulations of this method indicate that ultra-low emittance beams can be generated [16,17].…”

“…In order to improve the quality and stability of laserplasma-accelerated electron beams, controlled injection methods are actively being pursued, including colliding pulse injection [6,7], plasma density transitions [8,9], and ionization injection [10][11][12][13][14][15]. Ionization injection, compared to self-trapping, allows electron beam generation at lower plasma densities and, hence, higher beam energies can be achieved.…”

“…Ionization injection, compared to self-trapping, allows electron beam generation at lower plasma densities and, hence, higher beam energies can be achieved. In conventional ionization injection into laser-plasma accelerators [12][13][14], a single laser pulse is used both for wake generation and high-Z gas ionization. Typically a 0 2, with w 0 ∼ 10 µm, is needed to excite a sufficiently large wake so that an electron ionized near the peak intensity of the laser pulse can be on a trapped orbit, where w 0 is the laser spot size and a 0 is the peak normalized amplitude of the laser vector potential [14].…”

High quality electron beam acceleration by ionization injection in laser wakefields with mid-infrared dual-color lasers

“…One is improving the beam phase space distribution during the acceleration processes 8 , and the other is improving the injection processes at the very beginning of the acceleration [9][10][11] . Among the variety of injection schemes, the ionization-induced injection is found to be simple and effective [12][13][14][15][16][17][18][19][20] . By using different variations of this mechanism, electron beams with low emittances down to the nano-meter level [21][22][23] , or low energy spreads down to a few percent [24][25][26] were produced.…”