Radio-frequency (rf) photo-guns are the most common electron sources used in LINACs for free-electron laser and Compton facilities, as they provide very low-emittance, high-brightness electron bunches. The innovative C-band system (operating at 5.712 GHz) that we have developed and tested pushes the frontiers of high-gradient rf photo-guns, combining cathode peak fields above 160 MV/m with repetition rates of up to 1 kHz. Its realization relies on a novel brazing-free technology developed at the National Institute for Nuclear Physics (INFN, Italy), enabling significant improvements in the mechanical and electrical performance of high-gradient rf devices.

Highly overmoded iris-line waveguides offer an attractive solution for the efficient transportation of THz pulses over long distances. This paper presents the general (full-scatter) field analysis for the iris-line and its discontinuities under a paraxial excitation by an arbitrary source. By developing an advanced technique that combines Lorentz’s reciprocity theorem (with a generalized guided-field configuration), uniqueness theorem, and an equivalent current-source theorem by Schelkunoff, we derive the general scattered-field coefficients analytically and examine the effects of screen thickness, transient regime, diffraction loss, and ohmic loss on the propagation properties.

As large-scale superconducting-cavity accelerator facilities transition from the construction phase to the operational phase, the attention shifts from producing high-performance cryomodules to maintaining cryomodule performance. $In$ $situ$ plasma processing has helped to reduce field emission in superconducting linacs for protons and electrons. This paper reports the first results of plasma processing on a quarter-wave cryomodule for heavy ions at the Facility for Rare Isotope Beams.

Near-infrared optical transition radiation is used to measure density noise in relativistic electron bunches under conditions relevant to coherent electron cooling. We show that uncompressed beams exhibit noise consistent with the shot-noise limit, implying no reduction of the cooling rate, while compression leads to strong microbunching and coherent emission.

Laser wakefield acceleration can produce accelerating fields of hundreds of GV/m, enablng a compact alternative to conventional accelerators. However, reaching the potential of both electron charge and radiation brightness requires precise tuning of laser-plasma interactions. We identify the optimal laser pulse duration and focal spot size needed to maximize performance in subcritical density plasmas. Using 3D-PIC simulations, we show that a 550 TW laser pulse can generate massive electron beams with charges reaching tens of nanocoulombs and energies up to 500 MeV. Electron oscillations emit high-flux X-rays. Optimization is a significant step toward compact, high-brightness X-ray sources.

Superconducting RF photoinjectors with thread-mounted cathodes, which allow cavity cleaning with the photocathode installed, offer record accelerating fields for CW electron sources driving FELs, UEDs, and THz facilities. Recently, DESY’s SRF photoinjector demonstrated a 50 MV/m peak field on-axis with a copper photocathode. A unique feature of this layout is the photocathode’s direct interface to superfluid helium at 2 K. The critical question is whether UV drive-laser heating can impose operational limitations. To address this, we developed and studied transient and steady-state coupled multiphysics models.

A primary goal of next-generation Compton gamma sources (CGSs) is to combine mature accelerator technology with high-power Fabry-Perot cavities to achieve a two-to-three orders of magnitude increase in total scattered flux over state-of-the-art CGSs. In this work, we re-examine the performance of nearly-concentric two-mirror resonators as laser drivers for next-generation storage ring-based CGSs by introducing a new figure of merit, $S$, that quantifies their proximity to instability. We then demonstrate how to improve the design choices for the two-mirror resonator to enhance the achievable gamma-ray beam flux.

This paper describes the beam dynamics of an ultrafast electron diffraction facility now under construction at the National Centre for Nuclear Research (NCBJ). The first stage is due for commissioning in June 2026. The second stage, in which the room-temperature injector will be replaced by an SRF injector, is planned for 2027. In both stages, the facility will include an injector and an HZDR/RI cryomodule housing two TESLA-type SRF cavities. In the second stage, it will operate in cw mode with an SRF linac, enabling repetition rates of up to 200 kHz for very short bunches of 1–100 fC.