State-of-the-art additive manufacturing technologies are not only finding ever-wider applications in everyday life, but also assuming an increasingly important role in scientific research. This kind of advanced manufacturing method eliminates many of the constraints of conventional processes in fabricating components with complex external shapes or intricate internal structures, thereby providing enhanced flexibility for the design and realization of a new generation of more efficient particle accelerators and storage rings. The RACERS team initiated by the Stochastic Cooling Group at GSI, Germany, is worldwide one of the first teams working on this topic. Based on the metal 3D-printing technology, two novel accelerating structures and one efficient cooling plate for a future stochastic cooling system are under development at GSI and for the FAIR project, respectively. Some successful experience as well as learnt lessons will be presented.

The dynamic process of a laser or particle beam propagating from vacuum into underdense plasma has been investigated theoretically. Our theoretical model combines a Lagrangian fluid model with the classic quasistatic wakefield theory. It is found that background electrons can be injected into wakefields because sharp vacuum-plasma transitions can reduce the injection threshold. The injection condition, injection threshold as well as the injection length can be given theoretically by our model and are compared with results from computer simulations. Moreover, electron beams of high qualities can be produced near the injection thresholds and the proposed scheme is promising in reducing the injection threshold and improving the beam qualities of plasma based accelerators.

In this article we discuss a peculiar regime of Compton Scattering that assures the maximum transfer of energy and momentum from free electrons propagating in vacuum to the scattered photons. We name this regime Full Inverse Compton Scattering (FICS) because it is characterized by the maximum and full energy loss of the electrons in collision with photons: up to 100 % of the electron kinetic energy is indeed transferred to the photon. In the case of relativistic electrons, characterized by a large Lorentz factor (gamma >> 1), FICS regime corresponds to an incident photon energy equal to mec^2/2 . We interpret such an astonishing result as FICS being the time reversal of direct Compton Scattering of very energetic photons (of energy much greater than mec2) onto atomic electrons. Although the cross section of Compton scattering is decreasing with the energy of the incident photon, making the process less probable with respect to other reactions (pair production, nuclear reactions, etc) when high energetic photons are bombarding a target, the kinematics straightforwardly implies that the back-scattered photons would have an energy reaching asymptotically me^2c^2 . FICS is instead the unique suitable working point in Compton scattering for achieving the total transfer of (kinetic) energy exactly from the electron to the photon. Experiencing transitions from the initial momentum to zero in the laboratory system, in FICS the electron is also subject to very large negative acceleration; this fact can lead to possible experiments of sensing the Unruh temperature and related photon bath. On the other side of the energy dynamic range, low relativistic electrons can be completely stopped by moderate energy photons (tens of keV), leading to full exchange of temperature between electron clouds and photon baths.

A predominantly electric E\&m storage ring, with weak superimposed magnetic bending, is shown to be capable of storing two different particle type bunches, such as helion (h) and deuteron (d), or h and electron ($e^-$), co-traveling with different velocities on the same central orbit. Rear-end collisions occurring periodically in a full acceptance particle detector/polarimeter, allow the (previously inaccessible) direct measurement of the spin dependence of nuclear transmutation for center of mass (CM) kinetic energies (KE) ranging from hundreds of keV up toward pion production thresholds. With the nuclear process occurring in a semi-relativistic moving frame, all initial and final state particles have convenient laboratory frame KEs in the tens to hundreds of MeV. The rear-end collisions occur as faster stored bunches pass through slower bunches. An inexpensive facility capable of meeting these requirements is described, with several nuclear channels as examples. Especially noteworthy are the $e^{+/-}$-induced weak interaction triton (t) $\beta$-decay processes, t + $e^+ \rightarrow$ h + $\nu$ and h + $e^- \rightarrow$ t + $\nu$. Experimental capability of measurement of the spin dependence of the induced triton case is emphasized. For cosmological nuclear physics, the experimental improvement will be produced by the storage ring's capability to investigate the spin dependence of nuclear transmutation processes at reduced kinetic energies compared to what can be obtained with fixed target geometry.

The Rasnik 3-point alignment system, now widely applied in particle physics experiments and in the instrumentation of gravitational wave experiments, can be used as N-point alignment system by daisy chain N individual 3-point systems. The conceptual implementation of Rasnik chains in C3 is presented. The proper operation of a laser diode and a CMOS image sensor in liquid nitrogen has been verified. Next plans for testing a small but complete system, immersed in liquid nitrogen, are presented.

Developing HTS dipole inserts producing fields larger than 5 T within 15 T Nb3Sn outserts is necessary to generate 20 T or higher fields for future high energy colliders. Dipole inserts based on the cos-theta coil geometry with various stress management concepts and Bi2212 superconducting strand and cable are being developed at Fermilab both within and beyond the U.S.-MDP effort. The ultimate goal is to develop coil technology and an approach to manage azimuthal and radial strains of high temperature superconductor inserts when integrated within Nb3Sn outserts as a hybrid magnet system. This white paper reviews Bi2212 conductor properties and coil technologies, and proposes new ideas to face the challenges that Bi2212 still presents as an accelerator magnet conductor.

Bitter coil is an electromagnet used for the generation of exceptionally strong magnetic fields. The upper bound of magnet flux density is restricted by several factors. One principal restriction is the high stresses due to Lorentz forces in the coil. The Lorentz forces generate the distributed body force, which acts as the pressure of magnetic field. The common radial thickness profile of the Bitter coil is constant. In this paper the possibility of optimization by means of non-constant radial thickness profile of the Bitter coil is studied. The close form expression for optimal thickness profile is obtained. Both designs are compared and the considerable improvement of magnetic flux density is demonstrated. Moreover, the optimal design improves the shape of cooling channels. Namely, the highest cross-section of cooling channel is at the most thermally loaded inner surface of the coil.

The FNAL accelerator complex is poised to reach MW neutrino beams on target for the exploration of the dark sector physics and rare physics program spaces. Future operations of the complex will include CW linac operations at beam intensities that have not been seen before \cite{PIP2,RCS_LOI}. The ambitious beam program relies on multi-turn H$^{-}$ injection into the FNAL Booster and then extracted into delivery rings or the Booster Neutrino Beam (BNB) 8 GeV HEP program. A new rapid-cycling synchrotron (RCS) will be required to reach the LBNF goal of 2.4 MW because of intense space-charge limitations. There are many accelerator engineering challenges that are already known and many that will be discovered. This proposal calls for an intermediate step that will both facilitate the operation of Booster in the PIP-II era and gain operational experience associated with high power injection rings. This step includes the design, construction and installation of a 0.8 GeV accumulator ring (upgradeable to 1+ GeV) to be located in the PIP-II Booster Transfer Line (BTL). The PIP-II accumulator ring (PAR) may be primarily designed around permanent magnets or use standard iron core magnet technology with an aperture selected to accommodate the desired high intensity protons at 0.8 GeV.

We introduce a novel Proximal Policy Optimization (PPO) algorithm aimed at addressing the challenge of maintaining a uniform proton beam intensity delivery in the Muon to Electron Conversion Experiment (Mu2e) at Fermi National Accelerator Laboratory (Fermilab). Our primary objective is to regulate the spill process to ensure a consistent intensity profile, with the ultimate goal of creating an automated controller capable of providing real-time feedback and calibration of the Spill Regulation System (SRS) parameters on a millisecond timescale. We treat the Mu2e accelerator system as a Markov Decision Process suitable for Reinforcement Learning (RL), utilizing PPO to reduce bias and enhance training stability. A key innovation in our approach is the integration of a neuralized Proportional-Integral-Derivative (PID) controller into the policy function, resulting in a significant improvement in the Spill Duty Factor (SDF) by 13.6%, surpassing the performance of the current PID controller baseline by an additional 1.6%. This paper presents the preliminary offline results based on a differentiable simulator of the Mu2e accelerator. It paves the groundwork for real-time implementations and applications, representing a crucial step towards automated proton beam intensity control for the Mu2e experiment.

We derive a closed-form expression of the magnetic field of a finite-size current sheet and use it to calculate the field of permanent magnets, which are modeled through their surface current densities. We illustrate the method by determining the multipoles and the effective length due to fringe fields of a finite-length dipole constructed of magnetic cubes.

A high-energy electron-positron collider has been widely recognized by the particle physics community to be the next crucial step for detailed studies of the Higgs boson and other fundamental particles and processes. Several proposals for such colliders, either linear or circular, are currently under evaluation. Any such collider will be required to reach high lumimosities, in order to collect enough data at a reasonable time scale, while at the same time coping with high rates of background particles produced from beam-beam interactions during the collisions. In this paper, we analyze the luminosity and beam-beam interaction characteristics of the Cool Copper Collider (C$^3$) and perform a comparison with other linear collider proposals. We conclude that C$^3$ can reach the same or higher collision rates as the other proposals, without having to cope with higher beam-induced background fluxes. Thus, C$^3$ emerges as an attractive option for a future electron-positron collider, benefiting from the collective advancements in beam delivery and final focus system technologies developed by other linear collider initiatives.

New forms of electron beams have been intensively investigated recently, including vortex beams carrying orbital angular momentum, as well as Airy beams propagating along a parabolic trajectory. Their traits may be harnessed for applications in materials science, electron microscopy and interferometry, and so it is important to measure their properties with ease. Here we show how one may immediately quantify these beams' parameters without need for additional fabrication or non-standard microscopic tools. Our experimental results are backed by numerical simulations and analytic derivation.

Computer modeling is essential to research on Advanced Accelerator Concepts (AAC), as well as to their design and operation. This paper summarizes the current status and future needs of AAC systems and reports on several key aspects of (i) high-performance computing (including performance, portability, scalability, advanced algorithms, scalable I/Os and In-Situ analysis), (ii) the benefits of ecosystems with integrated workflows based on standardized input and output and with integrated frameworks developed as a community, and (iii) sustainability and reliability (including code robustness and usability).

Plasma wakefields offer high acceleration gradients, orders of magnitude larger than conventional RF accelerators. However, the achievable luminosity remains relatively low, typically limited by repetition rate and the charge accelerated per shot. In this work, we show that a train of drive bunches can be harnessed to accelerate multiple witness bunches in a single shot. We demonstrate that periodically loading the wakefields removes the limit on the energy transfer from the drive beam to the plasma, which allows the luminosity to be increased. Proof-of-concept simulations for the AWAKE scheme are carried out to demonstrate the technique, achieving a doubling of the accelerated charge while exploiting only a fraction of the drive train.

Recently, a high-energy superconducting $e^+e^-$ linear collider with energy recovery (ERLC) has been proposed, using twin RF structures to avoid parasitic collisions in linacs. Such a collider could operate in DC (duty cycle) or CW (continuous) mode, if sufficient power is available, with a luminosity of ${\cal O}(10^{36})$ cm$^{-2}$s$^{-1}$ at $2E_0=$250-500 GeV. In this paper, we find that the luminosity of the ERLC operating in DC mode does not depend on the accelerating gradient (for the same total power), allowing the ERLC to operate at the maximum available accelerating gradients. We also consider for the first time an $e^-e^-$ twin collider with energy recovery and estimate its achievable luminosity. Such an $e^-e^-$ collider is simpler than an $e^+e^-$ one. It does not require beam recycling, since electrons can be produced anew each time. Travelling-wave RF structures allow higher gradients and reduced thermal loading. It is shown that an ERLC with an acceleration gradient of $G=40$ MeV/m can be operated in CW mode with $L_{e^+e^-} =$ (1-2.5)$10^{36}$ and $L_{e^-e^-} =$ (3-7)$10^{36}$ cm$^{-2}$s$^{-1}$ for reasonable powers of 150 and 300 MW at $2E_0=250$ and 500 GeV, respectively. Such a machine is a promising candidate for a Higgs factory.

In the past several years, Fermi National Accelerator Laboratory (Fermilab) has suffered a number of troublesome setbacks. In some cases, failures to respond effectively have only made things worse, resulting in the current crisis atmosphere at the laboratory. This is reflected in part in the poorly rated performance appraisals (or PEMP) that the DOE provides to the lab every year. Three entities are currently responsible for supporting, managing, operating, and leading the lab: the Department of Energy (DOE), Fermi Research Alliance (FRA), and the Laboratory Director Office. Two recent events provide opportunities for interested parties to reverse the recent decline: the release in December 2023 of the P5 Report and the DOE decision to rebid the Management & Operating (M&O) contract. The P5 Report a decadal update of recommendations for US particle physics, includes an ambitious and positive roadmap for projects and activities at Fermilab. Carrying out the P5 plan successfully, however, will also require addressing many of the other problems currently besetting the lab. The selection of a new M&O contractor is under the responsibility of the Deputy Director for Operations of the DOE Office of Science Dr. Juston Fontaine. A new M&O Contractor would replace FRA and name a new Director effective January 1, 2025. A new management team, one hopes, would be motivated to solve problems and would enjoy a honeymoon period, enabling them to make positive changes more easily. Stimulated by these recent developments, the authors of this document, all present or past employees, convened confidentially in order to collect testimonials on a series of events symptomatic of the lab problems, to provide constructive criticism and offer possible solutions. This document summarizes this work.

We demonstrate a silicon-based electron accelerator that uses laser optical near fields to both accelerate and confine electrons over extended distances. Two dielectric laser accelerator (DLA) designs were tested, each consisting of two arrays of silicon pillars pumped symmetrically by pulse front tilted laser beams, designed for average acceleration gradients 35 and 50 MeV/m respectively. The DLAs are designed to act as alternating phase focusing (APF) lattices, where electrons, depending on the electron-laser interaction phase, will alternate between opposing longitudinal and transverse focusing and defocusing forces. By incorporating fractional period drift sections that alter the synchronous phase between $\pm 60^\circ$ off crest, electrons captured in the designed acceleration bucket experience half the peak gradient as average gradient while also experiencing strong confinement forces that enable long interaction lengths. We demonstrate APF accelerators with interaction lengths up to 708 ${\mu}$m and energy gains up to 23.7 $\pm$ 1.07 keV FWHM, a 25$\%$ increase from starting energy, demonstrating the ability to achieve substantial energy gains with subrelativistic DLA.

In coherent radiation of an ensemble of electrons, radiation field from electrons resonantly drives the other electrons inside to produce stimulated emission. The radiation reaction force on the electrons accounting for this stimulated radiation loss is classically described by the Lienard-Wiechert potential. Despite its being the foundation of beam physics for decades, we show that using the "acceleration field'' in Lienard-Wiechert potential to describe radiative interactions leads to divergences due to its implicit dependence on instantaneous interactions. Here, we propose an alternative theory for electromagnetic radiation by decomposing the interactions into instantaneous part and retarded part. It is shown that only the retarded part contributes to the irreversible radiation loss and the instantaneous part describes the space charge related effects. We further apply this theory to study the coherent synchrotron radiation wake, which hopefully will reshape our understanding of coherent radiation and collective interactions.

Intense highly charged ion beam production is essential for high-power heavy ion accelerators. A novel movable Vlasov launcher for superconducting high charge state Electron Cyclotron Resonance (ECR) ion source has been devised that can affect the microwave power effectiveness by a factor of about 4 in terms of highly charged ion beam production. This approach based on a dedicated microwave launching system instead of the traditional coupling scheme has led to new insight on microwave-plasma interaction. With this new understanding, the world record highly charged xenon ion beam currents have been enhanced by up to a factor of 2, which could directly and significantly enhance the performance of heavy ion accelerators and provide many new research opportunities in nuclear physics, atomic physics and other disciplines.

Fermilab has traditionally not been an EPICS house; as such expertise in EPICS is limited and scattered. PIP-II will be using EPICS for its control system. When in operation, it will need to interface with the existing, modernized (see ACORN) legacy control system. Treating EPICS controls at Fermilab as a green field, we have developed and deployed a software pipeline which addresses these needs and presents to developers a tested and robust software framework, including template IOCs from which new developers can quickly deploy new front ends, aka IOCs. In this presentation, motivation for this work, implementation of a continuous integration/continuous deployment pipeline, testing, template IOCs, and the deployment of user services/applications will be discussed. This new infrastructure of IOCs and services is being developed and used in the PIP-II cryomodule teststand; our experiences and lessons learned will be also be discussed.