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In this paper, an $\mathscr{H}_2$ norm-based model reduction method for linear quantum systems is presented, which can obtain a physically realizable model with a reduced order for closely approximating the original system. The model reduction problem is described as an optimization problem, whose objective is taken as an $\mathscr{H}_2$ norm of the difference between the transfer function of the original system and that of the reduced one. Different from classical model reduction problems, physical realizability conditions for guaranteeing that the reduced-order system is also a quantum system should be taken as nonlinear constraints in the optimization. To solve the optimization problem with such nonlinear constraints, we employ a matrix inequality approach to transform nonlinear inequality constraints into readily solvable linear matrix inequalities (LMIs) and nonlinear equality constraints, so that the optimization problem can be solved by a lifting variables approach. We emphasize that different from existing work, which only introduces a criterion to evaluate the performance after model reduction, we guide our method to obtain an optimal reduced model with respect to the $\mathscr{H}_2$ norm. In addition, the above approach for model reduction is extended to passive linear quantum systems. Finally, examples of active and passive linear quantum systems validate the efficacy of the proposed method., Comment: 13 pages,3 figures

Electropolishing process and cathodes have undergone modification and optimization for both low- and high-beta 650 MHz five-cell niobium cavities for PIP-II. Cavities treated with these modified electropolishing conditions exhibited smooth surfaces and good performance in baseline tests. Nonetheless, due to administrative constraints on project cavities, maximum gradient performance testing was not conducted. This paper presents a study conducted on a single-cell 650 MHz cavity utilizing the optimized electropolishing conditions, highlighting the maximum performance attained for this specific cavity. The cavity tested at 2 K in a vertical cryostat reached a superior accelerating field gradient of 53.3 MV/m at Q0 of 1.6x1010, which is the highest gradient attained for this type of large-sized cavities., Comment: 32nd Linear Accelerator Conference (LINAC 2024)

This review paper describes the energy-upgraded CEBAF accelerator. This superconducting linac has achieved 12 GeV beam energy by adding 11 new high-performance cryomodules containing eighty-eight superconducting cavities that have operated CW at an average accelerating gradient of 20 MV/m. After reviewing the attributes and performance of the previous 6 GeV CEBAF accelerator, we discuss the upgraded CEBAF accelerator system in detail with particular attention paid to the new beam acceleration systems. In addition to doubling the acceleration in each linac, the upgrade included improving the beam recirculation magnets, adding more helium cooling capacity to allow the newly installed modules to run cold, adding a new experimental hall, and improving numerous other accelerator components. We review several of the techniques deployed to operate and analyze the accelerator performance, and document system operating experience and performance. In the final portion of the document, we present much of the current planning regarding projects to improve accelerator performance and enhance operating margins, and our plans for ensuring CEBAF operates reliably into the future. For the benefit of potential users of CEBAF, the performance and quality measures for beam delivered to each of the experimental halls is summarized in the appendix., Comment: 66 pages, 73 figures, 21 tables

Electropolishing (EP) of 1.3 GHz niobium superconducting RF cavities is conducted to achieve a desired smooth and contaminant-free surface that yields good RF performance. Achieving a smooth surface of a large-sized elliptical cavity with the standard EP conditions was found to be challenging. This work aimed to conduct a systematic parametric EP study to understand the effects of various EP parameters on the surface of 650 MHz niobium cavities used in the Proton Improvement Plan-II (PIP-II) linear accelerator. Parameters optimized in this study provided a smooth surface of the cavities. The electropolished cavity showed significantly a higher accelerating gradient meeting baseline requirement and qualified for further surface treatment to improve the cavity quality factor., Comment: SRF2023

A prototype cryomodule containing eight Single Spoke Resonators type-1 (SSR1) operating at 325 MHz and four superconducting focusing lenses was successfully assembled, cold tested, and accelerated beam in the framework of the PIP-II project at Fermilab. The impact of induced residual magnetic fields from the solenoids on performance of cavities is presented in this contribution. In addition, design optimizations for the production cryomodules as a result of this impact are highlighted., Comment: SRF2023 Conference

Surface preparation of niobium superconducting RF cavities is a critical step for achieving good RF performance under the superconducting state. Surface defect, roughness, and contamination affect the accelerating gradient and quality factor of the cavities. We report surface inspection methods used to control the surface processing of 650 MHz cavities designated for the pre-production and prototype cryomodules for PIP-II linac. The cavity surface was routinely inspected visually, with an optical camera, and by microscopic scanning of surface replicas. This article covers details on the surface inspection methods and surface polishing process used to repair the surface.

A low particulate vacuum and purging system was developed to support PIP-II cryomodule string assembly. The overpressure can be controlled at a precision of 1 mbar above the atmospheric pressure regardless of the cavity or string assembly air volume. The system minimized the risk of uncontrolled nitrogen flow during the string assembly. Design features are presented., Comment: 21st International Conference on Radio-Frequency Superconductivity (SRF 2023)

The PIP-II project's LB650 cavities could potentially be vulnerable to mechanical deformation because of the geometric shape of the cavity due to reduced beta. The mechanical property of the niobium half-cell was measured following various heat treatments. The 5-cell cavities were tested in a controlled drop test fashion and the real-world road test. The result showed that the 900 $\deg$C heat treatment was compatible with cavity handling and transportation during production. The test provides the bases of the transportation specification and shipping container design guidelines., Comment: 21st International Conference on Radio-Frequency Superconductivity (SRF 2023)

Six 650 MHz high-power couplers will be integrated into the prototype High Beta 650 MHz (HB650) cryomodule for the PIP-II project at Fermilab. The design of the coupler is described, including design optimizations from the previous generation. This paper then describes the coupler life-cycle, including manufacturing, assembly, testing, conditioning, and the lessons learned at each stage.

The PIP II linear accelerator includes different types of niobium SRF cavities including 650 MHz elliptical low (0.61) and high (0.92) beta cavities. The elliptical cavity surface is processed with the electropolishing method. The elliptical cavities especially the low-$\beta$ 650 MHz cavities showed a rough equator surface after the EP was per-formed with the standard EP conditions. This work was focused to study the effect of different EP parameters, including cathode surface area, temperature and voltage, and optimize them to improve the cavity surface.

Cold electropolishing (EP) of a nitrogen-doped (N-doped) niobium (Nb) superconducting RF (SRF) cavity was found to improve its quality factor. In order to understand the effect of EP temperature on N-doped and undoped surfaces, a systematic EP study was conducted with 2/0 N-doped and heat-treated Nb samples in a beaker. The Nb samples were electropolished at different surface temperatures ranging from 0 to 42 C. The results showed that the doped surface was susceptible to the sample temperature during EP. EP resulted in the surface pitting on the doped samples where the number density of pits increased at a higher temperature. The surface results were compared with the surface of cutouts from a 9-cell cavity which was 2/0 N-doped and electropolished. This paper shows de-tailed surface features of the N-doped and undoped Nb surfaces electropolished at different temperatures.

Radio frequency superconductivity is a cornerstone technology for many future HEP particle accelerators and experiments from colliders to proton drivers for neutrino facilities to searches for dark matter. While the performance of superconducting RF (SRF) cavities has improved significantly over the last decades, and the SRF technology has enabled new applications, the proposed HEP facilities and experiments pose new challenges. To address these challenges, the field continues to generate new ideas and there seems to be a vast room for improvements. In this paper we discuss the key research directions that are aligned with and address the future HEP needs., Comment: contribution to Snowmass 2021

Field emission (FE) is one of the main limiting factors of superconducting radio-frequency (SRF) cavities operating in accelerators and it occurs whenever contaminants, like dust, metal flakes or even absorbates, are present on the surface of the cavity high electric field region. Field emission reduces the maximum achievable accelerating field and generates free electrons that may interact with the beam, damage or activate the beamline. One practical method that can be used to mitigate this problem is in-situ plasma cleaning, or plasma processing. The development of a processing that can be applied in-situ is extremely advantageous, since it enables the recovery of the cryomodule performance without the need of disassembling the whole cryomodule, which is an extremely expensive and time-consuming process. On the other hand, plasma processing only requires the cryomodule warm-up to room-temperature and the subsequent processing of the contaminated cavities. The entire process is reasonably quick and involves a limited number of personnel. For these reasons we would like to advocate for continuing to invest in the R\&D of plasma processing to optimize its applicability in cryomodules and for extending the technique to other frequency ranges and cavities geometries., Comment: contribution to Snowmass 2021

At a distance of 2.4kpc, W33 is an outstanding massive and luminous 10pc sized star forming complex containing quiescent infrared dark clouds as well as highly active infrared bright cloud cores heated by young massive stars. We report measurements of ammonia (NH$_3$) inversion lines in the frequency range 18--26GHz, obtained with the 40" resolution of the 100 m Effelsberg telescope. We have detected the ($J$, $K$)=(1,1), (2,2), (3,3), (4,4), (5,5), (6,6), (2,1) and (3,2) transitions. There is a maser line in the (3,3) transition towards W33 Main. Brightness temperature and line shape indicate no significant variation during the last $\sim$36yr. We have determined kinetic temperatures, column densities and other physical properties of NH$_3$ and the molecular clouds in W33. For the total-NH$_3$ column density, we find for 40"(0.5pc) sized regions 6.0($\pm$2.1)$\times$10$^{14}$, 3.5($\pm$0.1)$\times$10$^{15}$, 3.4($\pm$0.2)$\times$10$^{15}$, 3.1($\pm$0.2)$\times$10$^{15}$, 2.8($\pm$0.2)$\times$10$^{15}$ and 2.0($\pm$0.2)$\times$10$^{15}$cm$^{-2}$ at the peak positions of W33 Main, W33 A, W33 B, W33 Main1, W33 A1 and W33 B1, respectively. W33 Main has a total-NH$_3$ fractional abundance of 1.3($\pm$0.1)$\times$10$^{-9}$ at the peak position. High values of 1.4($\pm$0.3)$\times$10$^{-8}$, 1.6($\pm$0.3)$\times$10$^{-8}$, 3.4($\pm$0.5)$\times$10$^{-8}$, 1.6($\pm$0.5)$\times$10$^{-8}$ and 4.0($\pm$1.2)$\times$10$^{-8}$ are obtained at the central positions of W33 A, W33 B, W33 Main1, W33 A1, and W33 B1. From this, we confirm the already previously proposed different evolutionary stages of the six W33 clumps and find that there is no hot core in the region approaching the extreme conditions encountered in W51-IRS2 or Sgr B2. The ortho-to-para-NH$_3$ abundance ratios suggest that ammonia should have been formed in the gas phase or on dust grain mantles at kinetic temperatures of $\gtrsim$20K., Comment: 30 pages, 21 figures, accepted by A&A

High Q-factors are of utmost importance to minimize losses of superconducting radio-frequency cavities deployed in continuous wave particle accelerators. This study elucidates the surface treatment that can maximize the Q-factors in high-beta 650 MHz elliptical niobium cavities. State-of-the-art surface treatments are applied in many single-cell cavities, and surface resistance studies are performed to understand the microwave dissipation at this unexplored frequency. The nitrogen doping treatment is confirmed to be necessary to maximize the Q-factors at medium RF fields. We applied this treatment in five-cell high-beta 650 MHz cavities and demonstrated that extremely high Q-factors were obtained at medium RF fields with this treatment. We also demonstrated that adding a cold electropolishing step after N-doping is crucial to push the quench field of multicell cavities to higher gradients.

The non-collinear antiferromagnetic (AFM) structure makes Mn3Sn exhibit exotic properties. At present, it has been found that both the hydrostatic pressure and the strain introduced by interstitial N atoms have a great influence on this magnetic structure. Here, the effect of the residual strain (RS) on it is investigated. AC and DC magnetic measurement results suggest that Mn3Sn without RS has the non-collinear AFM structure only in the temperature range of 285 K to 400 K; while Mn3Sn with RS has a non-coplanar AFM structure in the entire temperature range from 5 K to 400 K. Both anomalous Hall effect and topological Hall effect appears in Mn3Sn with RS, supporting the anticipated non-coplanar AFM structure. Our findings point out a method to realize the chiral non-coplanar AFM structure through the engineering, thereby providing a path for the construction of topological antiferromagnets., Comment: 18 pages, 5 figures

We have measured trigonometric parallaxes for four water masers associated with distant massive young stars in the inner regions of the Galaxy using the VLBA as part of the BeSSeL Survey. G026.50$+$0.28. is located at the near end of the Galactic bar, perhaps at the origin of the Norma spiral arm. G020.77$-$0.05 is in the Galactic Center region and is likely associated with a far-side extension of the Scutum arm. G019.60$-$0.23 and G020.08$-$0.13 are likely associated and lie well past the Galactic Center. These sources appear to be in the Sagittarius spiral arm, but an association with the Perseus arm cannot be ruled out., Comment: 17 pages, 3 figures, 5 tables

Manipulation of the intersublattice interaction strengh (JRE-TM) in rare earth (RE)-transition metal (TM) alloys is a key issue to understand how efficiently the laser-induced angular momentum transfers from 3d to 4f spins and to have a better control of the ultrafast spin dynamics. In this work, the relationships between laser-induced demagnetization process and the intersublattice 3d-4f interaction for the GdCoFe alloys were systematically studied. The ultrafast two-stage demagnetization process could change into a one-stage mode as the angular momentum transferring channel between 3d and 4f spins is switched off, which could be modulated by JRE-TM. Furthermore, both the effective g-factor and damping constant deduced by the subsequently laser-induced magnetization precession process diverge at the angular momentum compensation point based on the ferromagnetic resonance method with the LLG equations. The results provide an alternative way to efficiently manipulate the ultrafast demagnetization time for practical applications.

The extrapolation strategy raised by Nesterov, which can accelerate the convergence rate of gradient descent methods by orders of magnitude when dealing with smooth convex objective, has led to tremendous success in training machine learning tasks. In this article, the convergence of individual iterates of projected subgradient (PSG) methods for nonsmooth convex optimization problems is theoretically studied based on Nesterov's extrapolation, which we name individual convergence. We prove that Nesterov's extrapolation has the strength to make the individual convergence of PSG optimal for nonsmooth problems. In light of this consideration, a direct modification of the subgradient evaluation suffices to achieve optimal individual convergence for strongly convex problems, which can be regarded as making an interesting step toward the open question about stochastic gradient descent (SGD) posed by Shamir. Furthermore, we give an extension of the derived algorithms to solve regularized learning tasks with nonsmooth losses in stochastic settings. Compared with other state-of-the-art nonsmooth methods, the derived algorithms can serve as an alternative to the basic SGD especially in coping with machine learning problems, where an individual output is needed to guarantee the regularization structure while keeping an optimal rate of convergence. Typically, our method is applicable as an efficient tool for solving large-scale $l$1-regularized hinge-loss learning problems. Several comparison experiments demonstrate that our individual output not only achieves an optimal convergence rate but also guarantees better sparsity than the averaged solution., Comment: 12 pages, 12 figures

The mechanism of spontaneous exchange bias (SEB) and the dominant factor of its blocking temperature are still unclear in Heusler alloys. Here, the related investigations are performed in Mn2Ni1.5Al0.5 Heusler alloys with SEB. The results of both magnetic measurements and first-principles calculations confirmed that spin frustrated and unfrustrated antiferromagnetic (AFM) states coexist there and they have different magnetic anisotropies, which are essential for SEB. Based on a series of measurement strategies, we demonstrate that the frustrated AFM state undergoes a first-order magnetic transition to the superferromagnet (SFM) state with the help of an external magnetic field, and SFM is retained due to the first-order property of the magnetic transition. SEB originates from the interface coupling of multiple sublattices between the unfrustrated AFM state and SFM state. By analyzing the Arrott plot using the Landau model, we found that the internal field of the system dominates the blocking temperature of SEB, which paves the way for improving the blocking temperature.

A systematic investigation about the structure and magnetism of Fe75-xCr25Gax (11

Quality factor is a primary cost driver for high energy, continuous wave (CW) SRF linacs like the LCLS-II X-ray free electron laser currently under construction. Taking this into account, several innovations were introduced in the LCLS-II cryomodule design to push substantially beyond the previous state-of-the-art quality factor achieved in operation. This includes the first ever implementation of the nitrogen doping cavity treatment, the capability to provide high mass flow cooldown to improve expulsion of magnetic flux based on recent R&D, high performance magnetic shielding, and other critical subcomponents. To evaluate the implementation of these new cryomodule features, two prototype cryomodules were produced. In this paper, we present results from the prototype cryomodule assembled at Fermilab, which achieved unprecedented cavity quality factors of 3.0e10 at a nominal cryomodule voltage. We overview cavity performance, procedures to achieve ambient magnetic field < 5 mG at the cavity wall, and the successful demonstration of high mass flow cooldown in a cryomodule. The cavity performance under various cool down conditions are presented as well to show the impact of flux expulsion on Q0.

When a superconducting radiofrequency cavity is cooled through its critical temperature, ambient magnetic flux can become "frozen in" to the superconductor, resulting in degradation of the quality factor. This is especially problematic in applications where quality factor is a cost driver, such as in the CW linac for LCLS-II. Previously, it had been unknown how to prevent flux from being trapped during cooldown in bulk niobium cavities, but recent R&D studies showed near-full flux expulsion can be achieved through high temperature heat treatment and cooling cavities through the superconducting transition with a spatial thermal gradient over the surface. In this paper, we describe the first accelerator implementation of these procedures, in cryomodules that are currently being produced for LCLS-II. We compare the performance of cavities under different conditions of heat treatment and thermal gradient during cooldown, showing a substantial improvement in performance when both are applied, enabling cryomodules to reach and, in many cases, exceed a Q0 of ~3x10^10., Comment: 11 pages