Session E07: Advanced SRF, THz and Laser Acceleration

Analysis of Low Purity Niobium SRF Cavities

Superconducting radio-frequency (SRF) cavities are resonant structures that generate the accelerating electric field inside many particle accelerators. Introducing certain impurities into high-purity niobium can improve quality factors and accelerating gradients, allowing for lower cryogenic costs which may enable the next generation of accelerators. Success has been found in nitrogen-doping, infusion of the native oxide into the niobium surface, and thin films upon a niobium bulk. We question why some impurities improve RF performance while others hinder it. The purpose of this study is to characterize the impurity profile of low purity niobium, and correlate these intrinsic impurities with the performance of cavities with low residual resistance ratio (RRR) so that the mechanism of these impurity-based improvements can be better understood and improved upon. We have found behavioral similarities between low RRR cavities and to high RRR cavities that are weakly nitrogen-doped. Additionally, we performed surface treatments such as low temperature baking and nitrogen-doping on low RRR cavities to evaluate how the intentional addition of more impurities to the RF layer affects performance. Our results have the potential to unlock a new understanding on SRF materials.   

Investigating the Role of Oxygen Diffusion in SRF Cavity Performance

Superconducting radiofrequency (SRF) cavities are resonators with extremely low resistivity that have applications in particle accelerators and superconducting qubits. One performance metric of SRF cavities is its quality factor (Q₀). Q₀ is inversely proportional to resistance, and higher Q₀ corresponds to less power loss, thus lowering capital required to keep the accelerators cold. The performance of SRF cavities is largely governed by the surface composition of the first 100 nm of the cavity surface. Nitrogen doping and oxygen doping are two empirically derived surface treatments that yield high Q₀ by altering the surface composition. We compare the performances of nitrogen doped and oxygen doped cavities in terms of Q₀, heating profiles, accelerating gradients, and surface composition. A simulation of the diffusion of oxygen into the bulk of the cavity was built using COMSOL Multiphysics software. Simulated results are compared to the actual surface composition of the cavities as determined from secondary ion mass spectrometry analysis. Understanding how oxygen diffusion affects the performance of these cavities allows us to have further insight into the underlying mechanisms that enable these surface treatments to yield high Q₀ performance.   

Temperature mapping analysis of Nitrogen-doped Niobium radio-frequency cavity

Nitrogen-doping of niobium superconducting radio-frequency (SRF) cavity has been shown to significantly improve the quality factors as compared to the standard treated niobium cavities. However, the nitrogen-doped cavities generally quench at lower accelerating gradients. To understand the possible sources of rf losses in nitrogen-doped cavities that could  reduce the quench field, we used 576 thermometer sensors to map the temperature distribution of the outer cavity surface while increasing the accelerating field. Our measurements have revealed a strong effect of the cavity cooldown rate on the intensity of hot spots and their spatial distribution, indicating a significant contribution from trapped vortices on the RF losses.   

Magnetic field mapping of superconducting radio frequency cavities

Superconducting radio frequency (SRF) cavities are the fundamental building blocks of modern particle accelerators. Niobium is the material of choice to build such cavities.  These cavities require a cryogenic cool-down to ~2 – 4 K for optimum performance minimizing RF losses on the inner cavity surface. However, temperature-independent residual losses in SRF cavities cannot be prevented entirely. One of the major sources of residual losses is trapped magnetic flux. The flux trapping mechanism depends on different factors, such as surface preparations and cool-down conditions. We have developed a diagnostic magnetic field scanning system (MFSS) using Hall probes and anisotropic magneto-resistance sensors to study the spatial distribution of trapped flux in 1.3 GHz single-cell cavities. The first result from this newly commissioned system revealed that the trapped flux on the cavity surface might redistribute with increasing RF power. The MFSS was also able to capture significant magnetic field enhancement at specific cavity locations after a quench.   

Nb3Sn superconducting radiofrequency cavities: development and applications

Nb3Sn superconducting radiofrequency (SRF) cavities are a promising new technology for particle accelerators. Nb3Sn's critical temperature ~18 K is approximately twice as high as the standard SRF material niobium ~9 K, allowing Nb3Sn cavities to have high Q ~10^10 even at 4 K for frequencies ~1 GHz. This could allow for significantly improved efficiency for large accelerators and could enable new applications for small industrial accelerators using small cryocoolers and conduction cooling instead of large liquid helium refrigerators. Fermilab's Nb3Sn SRF program involves material development including studies of growth, post-treatment, and microstructure, as well as prototyping for applications. Recently, Fermilab's Nb3Sn cavities have started development also towards quantum sensing for dark matter searches. In this contribution, we overview the program and recent results.   

DC MAGNETIC HALL PROBE TECHNIQUE FOR CHARACTERIZATION OF THIN FILM AND MULTILAYER SUPERCONDUCTORS FOR ACCELERATING CAVITIES

The best superconducting RF resonator Nb cavities used in particle accelerators can produce the accelerating gradients up to 45-50 MV/m. Further advances in the SRF accelerator technology demand even higher accelerating gradients and lower RF losses, which could be achieved using superconductors with critical fields higher than that of Nb, or multilayer (S-I-S) structures consisting of alternating thin layers of superconductor(S) and insulator(I) deposited on a bulk Nb. The measurements of the RF breakdown field of thin films and multilayers is an important problem requiring an experimental setup which can emulate the field configuration of SRF cavities on test flat samples. This work reports the development of a simple and efficient technique to measure the magnetic field of full flux penetration, B_p into bulk, thin film and multilayer superconductors.  The experimental setup contains a small superconducting solenoid which can produce a parallel surface magnetic field up to 500 mT and Hall probes to detect penetrated magnetic field across the superconducting sample. This system has been calibrated and used for the measurements on B_p on bulk Pb and Nb, Nb₃Sn thin film and Nb₃Sn/Al₂O₃ multilayer samples.   

Temporal and Spatial Characterization of Ultrafast Terahertz Near-Fields for Particle Acceleration

THz-frequency accelerating structures could provide the accelerating gradients needed for next generation particle accelerators with compact, GV/m-scale devices. One of the most promising THz generation techniques for accelerator applications is optical rectification in LiNbO₃ using the tilted pulse front method. However, accelerator applications are limited by significant losses during transport of THz radiation from the generating nonlinear crystal to the acceleration structure. In addition, the spectral properties of high-field THz sources make it difficult to couple THz radiation into accelerating structures. A better understanding of the THz near-field source properties and how they are affected by changes in the generation setup is necessary for the optimization of THz transport and coupling. We have developed a technique for detailed measurement of the THz near-fields that reveals the effects caused by varying the optical rectification setup. Using this technique we have reconstructed the full temporal 3D THz near-field close to the LN emission face. Our results will help to inform designs of novel structures for use in THz particle acceleration.   

The Laser-hybrid Accelerator for Radiobiological Applications (LhARA)

The ‘Laser-hybrid Accelerator for Radiobiological Applications’, LhARA, is conceived as a uniquely flexible facility dedicated to the study of a completely new regime of radiobiology. The ambition of the multidisciplinary collaboration is that the technologies demonstrated in LhARA will be transformative in the delivery of ion beam therapy (IBT).  

The laser-hybrid approach offers enormous potential by providing more flexible, compact, and cost-effective high-energy particle sources while evading the space-charge limitations of current sources. LhARA uses a high-power laser to generate an ultrashort burst of protons or light ions from a target.  These are captured using strong-focusing electron-plasma (Gabor) lenses at energies up to 15 MeV, enabling ultra-high instantaneous dose rates of up to 10⁹ Gy/s in pulses as short as 10-40 ns.  Further acceleration up to 127 MeV is facilitated by a fixed-field alternating-gradient (FFA) accelerator designed to accommodate the source flexibility.  Measuring the extremely high flux, low energy proton and ion beams at LhARA presents significant challenges. Novel techniques such as beam-gas curtain profile monitors and ion-acoustic dose-profile monitors are being developed for use in proof-of-principle systems.   

Electromagnetic cascade evolution in the collision of two-color ultra-intense laser pulses

We consider the collision of laser pulses with Petawatt intensities, and study the statistics of the electron-positron-photon jets which arise due to strong-field QED effects. The nonlinear dynamics of the seeded electrons in the field is analysed, and analytical estimates are derived for the angular spectra/widths of the emitted jets. It turns out that varying the amplitude and frequency ratio of the laser pulses can help control these jet properties. These results are verified with three-dimensional Monte-Carlo and two-dimensional particle-in-cell simulations. It is shown that they could potentially be helpful in distinguishing between products borne out of cascades, and fermions formed due to Schwinger decay of the vacuum.