High-Quality Electron Beams from Beam-Driven Plasma Accelerators by Wakefield-Induced Ionization Injection

Abstract

We propose a new and simple strategy for controlled ionization-induced trapping of electrons in a beam-driven plasma accelerator. The presented method directly exploits electric wakefields to ionize electrons from a dopant gas and capture them into a well-defined volume of the accelerating and focusing wake phase, leading to high-quality witness bunches. This injection principle is explained by example of three-dimensional particle-in-cell calculations using the code OSIRIS. In these simulations a high-current-density electron-beam driver excites plasma waves in the blowout regime inside a fully ionized hydrogen plasma of density 5×10(17)cm-3. Within an embedded 100 μm long plasma column contaminated with neutral helium gas, the wakefields trigger ionization, trapping of a defined fraction of the released electrons, and subsequent acceleration. The hereby generated electron beam features a 1.5 kA peak current, 1.5 μm transverse normalized emittance, an uncorrelated energy spread of 0.3% on a GeV-energy scale, and few femtosecond bunch length.

Efficient numerical modelling of the emittance evolution of beams with finite energy spread in plasma wakefield accelerators

“…Besides the demands for high beam energies, these applications entail stringent requirements with regard to the beam quality. While a number of numerical studies [6][7][8][9][10][11] indicated the potential of PWFAs to generate high quality beams, one of the major challenges constitutes the preservation of these qualities during the witness beam extraction and transport to the interaction region. In addition, the beams may have to be transported in between successive plasma sections and re-injected without significant quality deterioration in staged acceleration concepts, such as proposed e.g.…”

Optical plasma torch electron bunch generation in plasma wakefield accelerators

“…These applications are in particular relevant to PWFA, where low ionization thresholds and multiple gas types can be used. Here the gas density of different gas species can be uniform over the whole acceleration stage-whereas the production of multicomponent mixtures with (independently) differing, localized gas density profiles [29,38] is experimentally challenging.…”

“…2(b), the situation is substantially different due to the fact, that preionized hydrogen is used. Therefore the wakefield is much stronger and would at the same hydrogen density ionize and even trap helium electrons [24,29,35], which is unwanted here. Because of that, the hydrogen/helium densities are chosen an order of magnitude lower, increasing λ p;H ≈ 334 μm, and the driver beam charge Q was reduced from 3 to 1 nC.…”

“…At similar densities and drive beam currents as in (i) and (ii), continuous injection due to the strong wakefields would occur [24,29,35]. Therefore, more than an order of magnitude lower gas densities n He ¼ n H ¼ 1 × 10 16 cm −3 are used, and in addition the drive beam charge is reduced to 1 nC.…”

“…In both cases, injection of electron beams into the proper phase of the plasma wave is of paramount importance to obtain high-quality witness bunches from the plasma. A multitude of injection methods has been conceived, among those hydrodynamics-based plasma density transition [14][15][16][17][18][19][20], injection by additional ionization [21][22][23][24][25][26][27][28][29][30] and Trojan Horse-type methods [31][32][33][34][35][36][37][38].…”