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.

This work presents a study on laser wakefield electron acceleration in the self-modulated regime (SM-LWFA) using 50-fs laser pulses with energy on the mJ scale, at λ = 0.8 µm, impinging on a thin H₂ gas jet. Particle-in-cell simulations were performed using laser peak powers ranging from sub-terawatt to a few terawatts and plasma densities varying from the relativistic self-focusing threshold up to values close to the critical density. The differences in the obtained acceleration processes are discussed. Results show that bunched electron beams with full charge on the nC scale and kinetic energy in the MeV range can be produced and configurations with peak density in the range 0.5–5 × 10²⁰ atoms/cm³ generate electrons with maximum energies. In this range, some simulations generated quasimonoenergetic bunches with ∼0.5% of the total accelerated charge and we show that the beam characteristics, process dynamics, and operational parameters are close to those expected for the blowout regime. The configurations that led to quasimonoenergetic bunches from the sub-TW SM-LWFA regime allow the use of laser systems with repetition rates in the kHz range, which can be beneficial for practical applications.

The ever-growing need for radiopharmaceuticals, i.e., compounds containing pharmaceutical drugs and radioisotopes used for medical diagnostic imaging (SPECT/PET scan) and treating neoplasms, is significantly leading to an increased demand for such substances in hospitals and clinics worldwide. Currently, most large-scale productions of radioisotopes required for radiopharmaceuticals are carried out in research reactors, via the fission of highly enriched uranium. However, because large amounts of radioactive waste are produced as byproducts in this process, new greener methods are needed for radioisotope production. This work presents an integrative literature review and summarizes enriched uranium-free methods for radioisotope production, accomplished through the adoption of new reaction routes, distinct acceleration technologies, or by using other physical processes. This review considered forty-eight studies published from 2010 to 2021 on three established virtual databases. Among these selected works, a cyclotron is the most adopted HEU-free method for radioisotope production, and ${}^{44}$Sc, ${}^{68}$Ga, and ${}^{99\mathrm{m}}$Tc are the medical radioisotopes most often reported as produced by using the investigated HEU-free production methods.

While laser wakefield acceleration (LWFA) in the bubble regime demands ultra-short, high-peak-power laser pulses, operation in the self-modulated regime (SM-LWFA) works with more relaxed pulse conditions, albeit at the cost of lower beam quality. Modern laser systems can deliver few-terawatt pulses with tens of femtoseconds at kilohertz repetition rates. These systems are well-suited for developing SM-LWFA applications where high average energy and charge are prioritized over beam quality. Such beams could be used to generate high-energy bremsstrahlung photons, capable of triggering photonuclear reactions to produce radioisotopes like molybdenum-99. This isotope decays into technetium-99m, the most widely used medical radioisotope, with over 30 million applications worldwide per year. This work explores the use of Bayesian optimization to maximize the energy and charge of electron beams accelerated via SM-LWFA. Particle-in-cell (PIC) simulations model a 5 TW, ∼60 fs-long (FWHM) Gaussian laser pulse, propagating through tailored hydrogen gas-density profiles. In these simulations, over multiple iterations, the algorithm optimizes a set of input parameters characterizing the gas-density profile and the laser focal position. Three distinct profiles, with lengths ranging from 200 to 400 μm and combining ramps and plateaus, were investigated. Optimal configurations produced electron beams with median energies ranging from 14 to 17 MeV and charges of 600 to 1300 pC, considering electrons with energies above 8 MeV. Preliminary estimates of molybdenum-99 yields for the optimal beams were obtained using their phase space data from PIC simulations as radiation sources in Monte Carlo simulations irradiating a tantalum-molybdenum target.

This work presents an energy criterion to define the halo of homogeneous and mismatched charged particle beams. In the simulations used in this work, the beam is considered to be azimuthally symmetric, initially cold and is confined by an external constant magnetic field inside a cylindrical conducting pipe. The energy criterion is established through the analysis of the beam energy distributions with time. The obtained results are in reasonable agreement with the past results that considered the beam phase-space topology, for many values of the beam initial envelope mismatch.