Radiation safety challenges in plasma accelerators

Abstract

Plasma accelerators are rapidly evolving toward user-relevant machines with increasing repetition rates, particle energies and average beam powers. Despite their compact size, the operational characteristics of plasma accelerators are comparable to those of radio-frequency linacs, involving the continuous generation and dumping of electron bunches. However, beam properties and loss patterns can differ substantially from those of conventional accelerators, leading to radiation safety considerations dominated by high peak charges and distributed beam losses relevant for both personnel protection and machine integrity. Using established scaling laws, we show that significant dose rates already occur at electron energies of only a few tens of MeV, underscoring the relevance of radiation protection even for comparatively low-energy plasma accelerators. Based on a combination of Monte Carlo and particle-in-cell simulations, supported by radiation measurements from plasma accelerator experiments at DESY, we analyze typical radiation fields with a particular focus on radiation generated close to the plasma source. These findings highlight the need for dedicated shielding and beam-dump concepts tailored to plasma accelerators, especially in view of increasing average beam powers and future application-oriented operation.

Figures (6)

• Figure 1: Radiation generation mechanisms in electron accelerators as a function of electron energy. Both subfigures are redrawn as a vector graphic based on Fig. 6 and Fig. 17 of IAEA TRS 188 Swanson1979RadiologicalAccelerators.
• Figure 2: Simplified schematic overview of the KALDERA setup and the position of radiation monitors in- and outside the accelerator tunnel (not to scale).
• Figure 3: Top-view FLUKA simulation of radiation dose fields (electrons, photons, and neutrons) in the horizontal plane at beamline level for simplified KALDERA beam parameters. The tunnel geometry and a beam-dump setup are included and radiation monitor positions are indicated.
• Figure 4: Time-resolved scintillator signal measured with the LB6419 detector during KALDERA operation, showing a double-peak structure attributed to radiation originating from different locations along the beamline.
• Figure 5: FBPIC simulation of electrons escaping the plasma. (a) Simulation setup and virtual detectors; (b) Energy-angle distribution of electrons in the case with injection; (c) Spectra of emitted electrons; (d) cumulative kinetic energy spectrum. The value at e.g. 100 MeV represents the total kinetic energy of all electrons with an energy lower or equal to 100 MeV. (e) Same as (d), with the abscissa representing the electron propagation angle with respect to propagation direction.