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Tailored plasma-density profiles for enhanced energy extraction in passive plasma beam dumps

Plasma-based deceleration could greatly improve the overall compactness of accelerator facilities. In the so-called passive plasma beam dump, collective oscillations of plasma electrons are used to absorb kinetic energy of spent high-energy electron beams. Moreover, due to the high-amplitude decelerating gradient and the low-density plasma medium, deceleration is achieved in a compact and safer way, if compared to conventional beam dumps. Adoption of this novel decelerating scheme might be critical for facilities aiming for high-energy, high-repetition-rates, as well as for transportable applications built upon plasma-based accelerator technology. However, key issues such as, for example, particle re-acceleration, need to be addressed. In this work, tailored plasma-density profiles are used to shift the defocusing phase of the beam self-driven transverse wakefield towards re-accelerated beam particles, and transversely eject them. PIC simulations are used to evaluate the total beam energy, energy deposited in the plasma, and energy transversely ejected, for each of the investigated plasma density profiles. Moreover, the average energy per ejected particle is estimated for each case. In particular, it is shown that the energy of ejected particles is affected by the rate at which the plasma wavelength decreases, since it determines the time that particles experience re-acceleration before being defocused.