A specific heat treatment at 300 C, named medium-temperature baking (Mid-T baking) is applied to superconducting radio-frequency (SRF) accelerating cavities to improve the quality factor (Qo) at medium accelerating fields (10 to 20 MV/m). This treatment is very successful when done properly as it can reduce by almost a factor of two the power dissipations in this field range. However, surface contamination can lead to the degradation of Qo instead. Plasma-based surface treatment provides an effective approach to eliminate contaminants from the Niobium surface. In this study an insitu plasma cleaning process with argon containing 10 % O2 was performed to remove hydrocarbons from Niobium surface. The treatment was applied before and after a heat treatment at 500 C under ultra-high vacuum conditions (Mid-T baking). Changes in chemical speciation and oxide layer alteration induced by plasma processing were analyzed using insitu X-ray photoelectron spectroscopy (XPS) and exsitu scanning electron microscopy (SEM). The results show that plasma treatment modifies the composition of Niobium oxides, converting a Nb2O5 layer into NbO2. Furthermore, a plasma treatment before Mid-T baking helps reduce unstable oxides such as NbxO and significantly increases the proportion of metallic Niobium at the surface. The Niobium sample treated by plasma prior to Mid-T baking showed a 53 % reduction in carbide formation. Moreover, the C1s component attributed to NbC bonds shifts toward lower binding energy, indicating the formation of a more metallic NbC phase. Whereas without plasma treatment, the higher binding energy component observed after Mid T baking is consistent with Nb2C.

Sub-femtosecond electron beams are powerful probes of ultrafast electronic, atomic, and nuclear dynamics, and promising drivers for ultrashort radiation generation from the extreme-ultraviolet to gamma-ray regimes. However, producing such beams at hundred-MeV energies with pC-level charge remains challenging. Here we propose a two-dimensional beam-compression scheme based on transverse--longitudinal coupling, in which dispersive beam optics convert the small transverse emittance of modern electron beams into an ultrashort longitudinal duration. Linear analysis and particle tracking show that, after the dominant longitudinal and energy-spread contributions are cancelled, the compressed bunch length is governed primarily by transverse beam quality and collective-effect growth. We further derive and verify a scaling law showing that, in the relevant parameter range, collective-effect-induced bunch-length degradation increases approximately linearly with bunch charge and decreases with beam energy. Start-to-end simulations of a realistic injector-to-compressor beamline produce a 200 MeV, pC-level bunch with an rms duration of 0.45 fs and a peak current of about 3.5 kA. Jitter studies indicate that sub-femtosecond performance is maintained for most error seeds. These results suggest a feasible route toward compact, high-energy attosecond electron beam sources and may provide a basis for future sub-femtosecond radiation sources based on undulator emission or inverse Compton scattering.

In this paper, we study Mahalanobis-guided latent out-of-distribution (OOD) detection for test-time RL controller switching in nonlinear time-varying systems. RL controllers can quickly control high-dimensional systems within the training distribution, but their performance can degrade when time-varying dynamics produce unseen observations. We consider a combined ES--DRL controller, where RL provides fast in-distribution actions and bounded extremum seeking (ES) provides robust model-independent control under OOD operation. The key challenge is deciding when to switch. We train a variational autoencoder (VAE) on in-distribution beam-profile observations and use Mahalanobis distance in the VAE latent space to detect OOD beam profiles at test time. This OOD decision sets a binary switch that selects either the RL controller or the ES controller. We evaluate the approach in safety-critical particle accelerator control. In this setting, spatial magnet motion creates OOD beam profiles that were not seen during RL training. Visualization of the VAE latent space shows that the proposed method identifies this OOD scenario and provides an interpretable signal for switching between RL and ES in the combined controller.

Energy-transfer efficiency is an important quantity in plasma-wakefield acceleration, especially for applications that demand high average power. Conventionally, the efficiency is measured using an electron spectrometer; an invasive method that provides an energy-transfer efficiency averaged over the full length of the plasma accelerator. Here, we experimentally demonstrate a novel diagnostic utilizing the excess light emitted by the plasma after a beam-plasma interaction, which yields noninvasive, longitudinally resolved measurements of the local energy-transfer efficiency from the wake to the accelerated bunch; here, as high as (58 $\pm$ 3)%. This method is suitable for online optimization of individual stages in a future multistage plasma accelerator, and enables experimental studies of the relation between efficiency and transverse instability in the acceleration process.