Variation in electron emission time in weakly nonlinear laser wakefield acceleration

Abstract

As an advanced accelerator concept, laser wakefield acceleration, with its attractive acceleration gradient, has invoked great interest worldwide. The electron beams from laser wakefield acceleration were assumed to have the advantage of "jitterfree" or relatively small jitter, since the emergence of quasimonoenergetic bunch structures were considered to result from the trapping of electrons into the first bucket of the plasma wave. However, the emission times of the electron bunches from laser wakefield acceleration have not been real time monitored in experiments up to now. To examine this, we introduced the electro-optic spatial decoding method. The relative emission times of the electrons were observed to have variations with different plasma densities when using a laser with moderate intensity. The study shows that timing issues should not be ignored in some occasions of laser wakefield acceleration. The method used in this paper could be a candidate serving as a single-shot nondestructive electron bunch timing monitor for laser wakefield experiment and potential ultrafast pump-probe studies.

Electro-optic 3D snapshot of a laser wakefield accelerated kilo-ampere electron bunch

“…3a . The signal with background subtraction was 54 , 65 , where Γ is the phase retardation and δ ext indicates the extinction rate of the polarizer pair 53 , 54 . The λ /2 plate was rotated by θ 2 = 3° to perform a near-cross-polarization detection 50 .…”

Experimental demonstration of 7-femtosecond electron timing fluctuation in laser wakefield acceleration

“…Although exact bunch shape cannot be measured in such a set-up, the detection of center timing of the electron bunch was not affected. 23,30) Next, for the estimation of the upper limit of the electron bunch duration, the λ/2 plate was set to an angle of θ = 2°to perform a near-cross-polarization measurement. The λ/4 plate was tuned to diminish the residual birefringence caused by the EO crystal.…”

“…, 25) where Γ is the phase retardation, I probe is the probe laser intensity and δ ext = 1.25 × 10 −5 is the extinction ratio 30) of the polarizer pair. With laser but no gas (Γ = 0),…”

“…Relatively stable electron beams were generated with the mixture gas at a pressure of 0.65 MPa, corresponding to a peak neutral gas density of 2.16 × 10 18 cm −3 . At this condition, collimated electron beams were generated every shot and the pointing fluctuation was <2 In our previous work with a moderate drive laser intensity (4 × 10 17 W cm -2 , a 0 =0.57), 30) the relative emission times of electron bunches had quite large fluctuations due to uncontrolled self-injection. We had suggested using a controllable injection mechanism to achieve minimum jitter.…”

“…The polarization of a linear polarized probe laser becomes elliptical due to phase retardation, which can be used to reconstruct the original temporal distribution of the electron bunch. Usually, there are two ways of generating the signal electric field: one is to directly detect on the Coulomb field of the electron bunch; 27,[29][30][31] the other is to generate coherent transition radiation (CTR) pulses by inserting a thin metal foil in the electron path. 28) By setting a relative angle between the probe laser and the signal field, the longitudinal distribution of the electron bunches is encoded to the transverse intensity profile of the probe laser.…”