Simon Bohlen

The interaction of ultrafast lasers with plasmas has been studied for many years, primarily with respect to fundamental emission mechanisms. Only in recent years has ionizing radiation emerged as a safety concern in ultrafast laser-based material processing, where high pulse energies, repetition rates, and average powers, combined with continuous material supply, can lead to sustained X-ray emission. These processing-specific findings have informed German radiation protection legislation, which mandates notification or approval for laser systems exceeding irradiances of 1×10 ¹³ W/cm ² . However, this threshold does not distinguish between material processing and other ultrafast laser applications. In this work, we show that the conditions required for X-ray generation are highly specific and are typically only met during material processing. We assess the applicability of existing radiation studies to non-processing environments and present experimental results demonstrating negligible or no dose production under representative laboratory conditions, such as ultrafast laser interactions with underdense gas or stationary solid targets. We conclude that current legislation generalizes a processing-specific hazard to all ultrafast laser applications and does not adequately reflect the relevant physical conditions.

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 several 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.

The charge contained in an electron bunch is one of the most important parameters in accelerator physics. Several techniques to measure the electron bunch charge exist. However, many conventional charge diagnostics face serious drawbacks when applied to plasma accelerators. For example, integrating current transformers (ICTs or toroids) have been shown to be sensitive to the electromagnetic pulses (EMP) originating from the plasma, whereas scintillating screens are sensitive to background radiation such as betatron radiation or bremsstrahlung and only allow for a destructive measurement of the bunch charge. We show measurements with a noninvasive, cavity-based charge diagnostic (the DaMon), which demonstrate its high sensitivity, high dynamic range and resistance towards EMP. The measurements are compared to both an ICT and an absolutely calibrated scintillating screen.

The charge contained in an electron bunch is one of the most important parameters in accelerator physics. Several techniques to measure the electron bunch charge exist. However, many conventional charge diagnostics face serious drawbacks when applied to plasma accelerators. For example, integrating current transformers (ICTs or toroids) have been shown to be sensitive to the electromagnetic pulses (EMP) originating from the plasma, whereas scintillating screens are sensitive to background radiation such as betatron radiation or bremsstrahlung and only allow for a destructive measurement of the bunch charge. We show measurements with a noninvasive, cavity-based charge diagnostic (the DaMon), which demonstrate its high sensitivity, high dynamic range and resistance towards EMP. The measurements are compared to both an ICT and an absolutely calibrated scintillating screen.

Employing colliding-pulse injection has been shown to enable the generation of high-quality electron beams from laser–plasma accelerators. Here, by using test particle simulations, Hamiltonian analysis, and multidimensional particle-in-cell simulations, we lay the theoretical framework for spin-polarized electron beam generation in the colliding-pulse injection scheme. Furthermore, we show that this scheme enables the production of quasi-monoenergetic electron beams in excess of 80% polarization and tens of pC charge with commercial 10-TW-class laser systems.

Laser-plasma acceleration (LPA) is a compact technique to accelerate electron bunches to highly relativistic energies, making it a promising candidate to power radiation sources for industrial or medical applications. We report on the generation of electron beams from an 80 MeV-level LPA setup based on ionization injection (II) over a duration of 8 hours at a repetition rate of 2.5 Hz, resulting in 72,000 consecutive shots with charge injection and acceleration. Over the full operation time the moving averages of the total beam charge of 14.5 pC and the charge between 70–80 MeV did not drift on a detectable level. The largest source of shot-to-shot jitter was in the beam charge (26% standard deviation), which was most strongly correlated with fluctuations in the plasma density (3.6% standard deviation). Particle-in-cell simulations demonstrate that this was chiefly caused by stronger laser self-focusing in higher density plasmas, which significantly increased the ionized charge along with the emittance of the beam. The nonlinearity of this process imposes tight constraints on the reproducibility of the laser-plasma conditions required for a low jitter II-LPA output if self-focusing plays a role in the laser evolution.

A Correction to this paper has been published: https://doi.org/10.1038/s41467-020-20676-1