Experimental validation of a radio frequency photogun as external electron injector for a laser wakefield accelerator

Abstract

A purpose-built RF-photogun as external electron injector for a laser wakefield accelerator has been thoroughly tested. Different properties of the RF-photogun have been measured such as energy, energy spread and transverse emittance. The focus of this study is the investigation of the smallest possible focus spot and focus stability at the entrance of the plasma channel. For an electron bunch with 10 pC charge and 3.7 MeV kinetic energy, the energy spread was 0.5% with a shot-to-shot stability of 0.05%. After focusing the bunch by a pulsed solenoid lens at 140 mm from the middle of the lens, the focal spot was 40 μm with a shot-to-shot stability of 5 μm. Higher charge leads to higher energy spread and to a larger spot size, due to space charge effects. All properties were found to be close to design values. Given the limited energy of 3.7 MeV, the properties are sufficient for this gun to serve as injector for one particular version of laser wakefield acceleration, i.e., injection ahead of the laser pulse. These measured electron bunch properties were then used as input parameters for simulations of electron bunch injection in a laser wakefield accelerator. The arrival time jitter was deduced from measurements of the energy fluctuation, in combination with earlier measurements using THz coherent transition radiation, and is around 150 fs in the present setup. The bunch length in the focus, simulated using particle tracking, depends on the accelerated charge and goes from 100 fs at 0.1 pC to 1 ps at 50 pC. When simulating the injection of the 3.7 MeV electron bunch of 10 pC in front of a 25 TW laser pulse with a waist of 30 μm in a plasma with a density of 0.7 × 1024 m−3, the maximum accelerated charge was found to be 1.2 pC with a kinetic energy of ∼900 MeV and an energy spread of ∼5%. The experiments combined with the simulations show the feasibility of external injection and give a prediction of the output parameters that can be expected from a laser wakefield accelerator with external injection of electrons.

Collimated quasi-monochromatic beams of accelerated electrons in the interaction of a weak-contrast intense femtosecond laser pulse with a metal foil

“…However, this scheme has difficulties with synchronizing the electron beam and the laser pulse, and a relatively large length of the electron bunches. Currently, an injector based on a radio frequency (RF) accelerator, the photocathode of which is irradiated by a femtosecond pulse exactly synchronized with the strong pulse exciting a plasma wave [3] , is being actively developed. However, practical results have not been published yet.…”

“…The initial momentum of the electrons has to be sufficient for them to stay in an accelerating phase of the plasma wave traveling with speed close to the speed of light [1] . Many solutions have been proposed to solve the problem of electron trapping, such as external injection [3,4] , self-injection by a nonlinear plasma wave in the so-called bubble regime [5][6][7] , counterpropagating laser pulses [8] , using a mixture of gases, one of which has a high ionization threshold [9][10][11] , or plasma density gradients (downramp) [12] . But the problem still persists.…”

Physics of Phase Space Matching for Staging Plasma and Traditional Accelerator Components Using Longitudinally Tailored Plasma Profiles

“…The first configuration is the so-called injector-accelerator, where a ∼ 100 MeV class electron beam produced by a short, high-density injector stage is further accelerated to ∼ GeV level using a second low-density accelerator stage [11,24,25]. The second example is the external injection scheme where a highquality, relativistic electron bunch is first generated using an RF accelerator and then injected into a PBA [26][27][28][29][30]. The third example concerns the proposed PBA driven light source [31][32][33], where a high-quality electron beam needs to be coupled from the plasma wake to an undulator.…”

Summary report of Working Group 5: Beam and radiation generation, monitoring and control

“…Discussions in the working group pointed out that in all-optical injector often there is limited possibility of varying independently the various beam parameters, while an external injector will add freedom to control the different beam parameters. There are a number of groups in Europe working on the topic of external injection and we had two presentations from Brussaard [1] and Greyenbuck [2] on plans and simulations to use an RF photoinjector to generate a beam to be injected into a laser wakefield-based accelerating structure. The main issues that still require significant research and development to demonstrate the feasibility of this option are the transverse matching of the beam as well the timing and synchronization of the electrons with the plasma wave.…”