An ideal PWFA scenario consists of a driver followed by a trailing bunch that maintain its quality, i.d. its brightness B=varepsilon_x varepsilon_y over traveled distance (with I the beam r.m.s. current and varepsilon the normalized emittance along the x,y axes). It is also desirable to keep the beam energy spread as small as possible. To achieve these goals both the driver and the trailing bunch need to be finely manipulated, it is necessary to control with great accuracy their charge, shape

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... d dimensions; i.d. it has been shown that the trailing bunch need to have a trapezoidal shape to avoid beam loading. No geometrical conditions are given on the driver. In this paper we discuss the characteristics the driver have to meet in order to generate a stable bubble, that in turns allows for a stable trailing bunch propagation with no quality loss. We observe that the driver into a plasma channel naturally evolves toward a stable configuration; by taking advantage of this natural behavior we discuss how the driver can be injected at different dimensions without influencing the trailing bunch evolution. We can rephrase the previous sentence by saying that for PWFA the driver can be injected into the plasma also at un-matched conditions allowing for some beneficial flexibility. Such a flexibility is beneficial when considering a realistic case where a single photo-injector creates and transport both bunches up to the plasma accelerating section. The practical difficulties reside in tuning the photo-injector optics for two significantly different bunches at the same time. A single photo-injector can be tuned only around the requirements of one of the two: this is particularly true for plasma acceleration where the beam parameters result to be quite "extreme" in terms of dimensions and a slight initial mismatch can lead to disastrous results. The simplest strategy to overcome this technological limitation is to employ two different accelerator lines. This, however, poses a number of issues on the correct alignment and s [...]