The effects of experimental variations in the synchronization, laser power, and plasma density on the final beam parameters of externally injected electrons accelerated in a plasma wave are studied using a hybrid model. This model combines a relativistic fluid description of the plasma wave generated by the laser pulse with particle tracking of the accelerated electrons. For cases in which the effects of beam loading and laser depletion can be neglected, the two parts can be separated, allowing a significant reduction in computational power needed compared to particle in cell codes. Two different approaches to externally injecting electrons into plasma waves are studied: In the first case, the electrons are injected behind a laser pulse with a 0 ¼ 0:32. In the second case, electrons are injected in front of the laser pulse in three different laser regimes a 0 ¼ 0:32, a 0 ¼ 0:56, and a 0 ¼ 1:02, ranging from linear to nonlinear. For these four cases, the effects of expected experimental variations in synchronization (AE 500 fs), laser power (AE 10%), and plasma density (AE 30%) are studied. From these simulations, it becomes clear that in some cases, even a small variation in one of these parameters can create a large change in the final energy, energy spread, and trapped charge. For lower laser intensities, the method of injecting behind the laser pulse is the least sensitive to fluctuations while injection in front of the laser pulse becomes less sensitive at higher intensities.

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