While laser-plasma accelerators have demonstrated a strong potential in theacceleration of electrons up to giga-electronvolt energies, few experimentaltools for studying the acceleration physics have been developed. In this paper,we demonstrate a method for probing the acceleration process. A second laserbeam, propagating perpendicular to the main beam is focused in the gas jet fewnanosecond before the main beam creates the accelerating plasma wave. Thissecond beam is intense enough to ionize the gas and form a density depletionwhich will locally inhibit the acceleration. The position of the densitydepletion is scanned along the interaction length to probe the electroninjection and acceleration, and the betatron X-ray emission. To illustrate thepotential of the method, the variation of the injection position with theplasma density is studied.
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