Multi-step rollouts are essential for model-based reinforcement learning (RL) and predictive control, yet learned dynamics models often become unstable when recursively applied, leading to divergence and unreliable policy updates. This paper proposes a model-agnostic hybrid dynamics framework that blends a provably contracting nominal model with a flexible excursion model through an uncertainty-guided switching law. The switching signal is derived from calibrated epistemic uncertainty and activates only when the system leaves the nominal region, ensuring that each model operates within its reliability regime. Under clearly stated smoothness and boundedness assumptions, we show that the resulting hybrid predictor yields globally bounded recursive multi-step rollouts: trajectories remain Lyapunov-stable in the nominal region and exhibit at most affine growth during excursions. To illustrate the theory in practice, we instantiate the hybrid dynamics framework within a model-based RL scheme that uses real one-step transitions for value learning and hybrid rollouts for policy improvement. Experiments on a nonlinear Duffing oscillator demonstrate stable long-horizon prediction and improved cost-effort trade-offs relative to a stabilizing baseline.

Superconducting radio frequency cavities with a high quality factor enable energy-efficient accelerator operation but are very sensitive to mechanical disturbances that detune their resonance. Accurate detuning estimation is therefore essential for efficient resonance control and stable beam conditions. This paper introduces Kalman-Inspired Neural Decomposition (KIND), a data-driven estimator that fuses a Dynamic Mode Decomposition model for stationary modal behavior with a Transformer-based predictor for transient dynamics. KIND further outputs learned uncertainty signals that indicate regime changes, enabling anomaly detection. Using operational cavity data, we compare KIND with a classical Kalman filtering baseline and discuss its potential as a foundation for future uncertainty-aware, forecast-based control.

The versatile 1.3 GHz superconducting radio-frequency (SRF) gun at HZB succesfully generated first photoemission beam from a high quantum efficiency (QE) multi-alkali photocathode. This demonstrates worldwide first beam operation of a SRF gun at high repetition rate and with a robust multi-alkali Na-based photoemissionn source. The setup of the test and all sub-systems is described. The latest results of SRF commissioning, cavity performance, photocathode QE measurements and beam parameter exploration campaigns is presented in the paper.

This work shows the results obtained with a novel modified active disturbance rejection control (MADRC) algorithm when controlling the detuning of a 9-cell TESLA superconducting radio frequency cavity. This type of control is essential in SRF cavities with high loaded quality factors since they are extremely sensitive to detuning due to their extremely narrow bandwidth. The modification of the active disturbance rejection control is based on loop shaping techniques and confers ease of design and implementation, as well as an improved behavior in presence of delay. To begin with, a theoretical design of the controller based on an experimental transfer function of the cavity is carried out and its performance is compared with that of a proportional integral control. In this preliminary test, the simulation results indicate a remarkable performance superiority of the MADRC. Later, the controller is tested on a TESLA cavity validating both the MADRC algorithm and its design methodology, and achieving a peak detuning reduction of 63%, as well as an rms reduction of 80% with respect to the open loop.

Two niobium elliptical 1.3 GHz superconducting radio frequency (SRF) electron photoinjector cavities were successfully recovered after mechanical inner surface damage. Both cavities had deep imprints in the critical high surface electric field area around the photoelectric cathode position. The lengthy repair procedure, which consists of surface inspection and defect characterization, mechanical polishing and light chemical etching is described in detail. In the process, a new high pressure rinsing (HPR) nozzle system optimized for the special photoinjector geometry was also developed. Subsequent cold RF tests demonstrate complete performance recovery. This is the first time that photoinjector cavities damaged in the high electric-field region could be recovered.

Ultrafast electron diffraction techniques that employ relativistic electrons as a probe have been in the spotlight as a key technology for visualizing structural dynamics which take place on a time scale of a few femtoseconds to hundreds femtoseconds. These applications highly demand not only extreme beam quality in 6-D phase space such as a few nanometer transverse emittances and femtosecond duration but also equivalent beam stability. Although these utmost requirements have been demonstrated by a compact setup with a high-gradient electron gun with state-of-the-art laser technologies, this approach is fundamentally restricted by its nature for compressing the electrons in a short distance by a ballistic bunching method. Here, we propose a new methodology that pushes the limit of timing jitter beyond the state-of-the-art by utilizing consecutive RF cavities. This layout already exists in reality for energy recovery linear accelerator demonstrators. Furthermore, the demonstrators are able to provide MHz repetition rates, which are out of reach for most conventional high-gradient electron guns.

Energy-recovery linacs (ERLs) have been emphasised by the recent (2020) update of the European Strategy for Particle Physics as one of the most promising technologies for the accelerator base of future high-energy physics. The current paper has been written as a base document to support and specify details of the recently published European roadmap for the development of energy-recovery linacs. The paper summarises the previous achievements on ERLs and the status of the field and its basic technology items. The main possible future contributions and applications of ERLs to particle and nuclear physics as well as industrial developments are presented. The paper includes a vision for the further future, beyond 2030, as well as a comparative data base for the main existing and forthcoming ERL facilities. A series of continuous innovations, such as on intense electron sources or high-quality superconducting cavity technology, will massively contribute to the development of accelerator physics at large. Industrial applications are potentially revolutionary and may carry the development of ERLs much further, establishing another shining example of the impact of particle physics on society and its technical foundation with a special view on sustaining nature.

Helmholtz Zentrum Berlin is currently finalizing the construction of the demonstrator Energy Recovery Linac (ERL) bERLinPro [1]. The first part, which will be commissioned, will be the injector consisting of a superconducting RF (SRF) photo-injector (Gun) and a Booster module made up of three two cell SRF cavities. For the latter the 2.3 MeV beam from the gun needs to be accelerated to 6.5 MeV, whereas one Booster cavity will be operated in zero-crossing mode for bunch-shortening. Thus, for the final stage with a 100 mA beam, the twin power couplers of the Booster cavity need to deliver up to 120 kW in travelling continous wave (CW) mode at 1.3 GHz each. To achieve that, a dedicated coupler conditioning setup was installed and commissioned. Here, we will present the first conditioning results with the bERLinPro Booster fundamental power couplers in pulsed and CW regime

The Helmholtz Zentrum Berlin is carrying out two accelerator projects which make use of high gradient Continuous Wave (CW) Superconducting Radiofrequency (SRF) cavities: bERLinPro and BESSY-VSR. In both projects, a prompt detection of a quench is crucial to avoid damages in the cryomodules and cavities themselves. In this paper, the response of real time estimation of the cavity parameters using the transmitted and forward RF signals is simulated, in order to perform the quench detection. The time response of the estimated half bandwidth is compared with the dissipated energy in the cavity walls for the different type of SRF cavities used in both projects, i.e., bERLinPro’s photoinjector, booster and linac, and BESSY-VSR 1.5 GHz and 1.75 GHz cavities. As an intermediate step prior to the implementation in an mTCA.4 system together with the LLRF control and test with a real cavity, the algorithm has been implemented using a National Instruments FPGA board to check its proper behavior.

The Energy Recovery Linac (ERL) paradigm offers the promise to generate intense electron beams of superior quality with extremely small six-dimensional phase space for many applications in the physical sciences, materials science, chemistry, health, information technology and security. Helmholtz-Zentrum Berlin started in 2010 an intensive R&D programme to address the challenges related to the ERL as driver for future light sources by setting up the bERLinPro (Berlin ERL Project) ERL with 50 MeV beam energy and high average current. The project is close to reach its major milestone in 2020, acceleration and recovery of a high brightness electron beam. The goal of bERLinProCamp 2019 was to discuss scientific opportunities for bERLinPro 2020+. bERLinProCamp 2019 was held on Tue, 17.09.2019 at Helmholtz-Zentrum Berlin, Berlin, Germany. This paper summarizes the main themes and output of the workshop.

Magnetic fields are a big issue for SRF cavities, especially in areas with strong electromagnets or ferromagnetic materials. Magnetic shieldings consisting of metal alloys with high magnetic permeability are often used to reroute the external magnetic flux from the cavity region. Those Mu metal shields are typically designed for weak magnetic fields like Earth’s magnetic field. Next to strong magnetic field sources like superconducting (SC) solenoids, those shields can be easily saturated, resulting in a degradation of the shielding efficiency and a permanent magnetization. For the photoinjector of bERLinPro a new SC solenoid will be installed inside the cryomodule next to the SRF gun cavity. Calculations show that the fringe fields of the solenoid during operation can saturate the cavity Mu-metal shields. Therefore we designed an SC magnetic shield placed between the solenoid and the cavity shield to protect the latter during magnet operation. In this paper we will present the design and first testings of this SC magnetic shield.

bERLinPro is an Energy Recovery Linac (ERL) project, currently being set up at the Helmholtz-Zentrum Berlin fur Materialien und Energie, Berlin, Germany. bERLinPro is designed as - and for - experiments in accelerator physics and as a test bed for novel ERL components. MESA is an ERL project under construction at the Johannes Gutenberg- Universitt, Mainz, Germany. MESA is designed as a user facility to perform experiments in dark matter physics and precision measurements of natural constants. Despite the diverse goals, the main linac, providing the larger part of the particles energy, is fairly compatible. It is planned to test and run the MESA linac module in bERLinPro, prior to its usage in MESA. The goals and benefits of this unique cooperation for both projects are outlined in this paper. The necessary adaptions in bERLinPro, including hardware aspects, the new optics, and the scope of performance are described.

X-ray tomography has been established as a nondestructive three-dimensional analysis tool, commercially offered by industrial vendors. Typical applications cover shape control and failure detection (voids, cracks) deep inside of complicated bulk pieces like engine blocks, bearings, turbine blades etc.. We evaluated the applicability of the process for superconducting radio frequency cavities, in particular a generic 1.3 GHz single test cell cavity, the 1.4-cell 1.3 GHz bERLinPro electron gun cavity and the 1.5 GHz VSR-1-cell-prototype cavity. The gun cavity experienced severe shape modifications during its tuning process and features a complicated internal stiffening construction. Thus it is a challenge to measure its actual internal cavity surface shape after the complete preparation process with a resolution, sufficiently high (better than 0.2 mm) to serve as input for meaningful comparative field simulations. First tests with a vendor’s on-site X-ray source, operating at X-ray energies up to 590 keV revealed an insufficient resolution of the inner surface, attributed to the unfavorable X-ray damping characteristics of niobium. This was overcome with the aid of an accelerator-based source (X-ray spectrum up to 7.5 MeV), operated by Fraunhofer IISEZRT, Fürth, Germany. Results show significant, while understood, shape changes and indicate partial inner surface modifications of the gun cavity. Further, the data evaluation process, which was needed to provide input for field simulations, raised issues because of data set size and complexity and illustrated further some typical artefacts, which are discussed in the paper.

Setting up and debugging SRF support systems, such as LLRF control, quench detection, microphonics and Lorentz-force detuning control, etc., often requires extensive time spent operating the cavities. This results in time consuming and costly operation. Early into the development stages the actual cavity system may not even be available. It is therefore highly desirable to pre-evaluate these systems under realistic conditions prior to final testing with the SRF cavities. We devised an FPGA-based "virtual cavity" that takes a regular low-level RF input and generates the signals for RF-power reflection, transmission and detuning that mimic the response of a real cavity system. As far as the user is concerned, the response is the same as for a real cavity. This "black-box" model includes mechanical modes, Lorentz force detuning, a field depended quality factor, quenches and variable input coupling and is currently being expanded. We present the model and show some applications for operating the quench detection, LLRF and microphonics control for 1.3 GHz bERLinPro cavities. The same system can be used for other cavity types, including normal conducting cavities. INTRODUCTION The unavailability of SRF cavities and all the associated ancillary systems for testing them is a big issue when designing and debugging LLRF control algorithms and related techniques like quench detection, detuning compensation, etc. In [1] an FPGA-based virtual cavity was presented which could be used to perform hardware-in-theloop (HIL) simulations of the mentioned systems. This system, based on off-the-shelf National Instruments hardware, takes the forward RF signal coming from the LLRF system and models the electrical behaviour of an SRF cavity generating the RF transmitted and reflected signals. In addition to this basic electrical behaviour, some more advanced features were introduced to the model such as a quenching, field dependant Q and mechanical response fed by Lorentz force detuning and microphonics, making the system more realistic and close the real world. In this paper all these components of the virtual cavity are summarized and a more modern and more compact hardware is presented, which allowed the introduction of one more feature: the simulation of the piezo tuner with its associated mechanical transfer function. The resulting system offers the operator the flexibility to choose between different quality factors, couplings, quench thresholds, mechanical responses while the FPGA calculates in real time the transmitted and reflected RF voltages for a given RF input. Figure 1 depicts the overview of the working principle of this virtual cavity, Figure 1: Overview of the virtual cavity operation scheme. NEW HARDWARE The hardware used in [1] was the main limitation to add more features to the virtual cavity as almost all the FPGA resources were used. Therefore the whole design was migrated to a newer and more compact device from National Instruments [2]. This device is composed by a real time controller in charge of the communications with the user via Ethernet and a Kintex-7 FPGA where the cavity model is implemented. The analog module was also changed, [3], in order to have more number of inputs and outputs, 16-bit ADCs and a bigger dynamic range. Figure 2 shows both devices. Figure 2: National Instruments NI-7935R FlexRIO controller, [2] with the NI-5783 Analog Adapter Module, [3]. ___________________________________________ * Work supported by German Bundesministerium für Bildung und Forschung, Land Berlin, grants of Helmholtz Association and partially supported by Basque Country PPG17/25 project. † pablo.echevarria@helmholtz-berlin.de 19th Int. Conf. on RF Superconductivity SRF2019, Dresden, Germany JACoW Publishing ISBN: 978-3-95450-211-0 doi:10.18429/JACoW-SRF2019-WETEB4 WETEB4 772 Co nt en tf ro m th is w or k m ay be us ed un de rt he te rm so ft he CC BY 3. 0 lic en ce (© 20 19 ). A ny di str ib ut io n of th is w or k m us tm ai nt ai n at tri bu tio n to th e au th or (s ), tit le of th e w or k, pu bl ish er ,a nd D O I. SRF Technology Ancillaries LLRF Figure 3: Graphical user interface where the parameter input, analog output selection and data visualization are depicted. ELECTRICAL MODEL The basic cavity electrical behaviour can be modelled using an equivalent RLC-circuit, [4], which leads to the following equation: cav = (− / −∆ ∆ − / )cav + +( / / ) amp (1) where / is the half bandwidth, ∆ the cavity detuning, L is the load resistance, m is the transformer ratio (both dependent on r/Q, Q0 and the coupling factor) and cav = (cav , cav), amp = (amp , amp) are the real and imaginary parts of cavity voltage and the driven current respectively. The reflected voltage is calculated with the following equation: ref = cav − 0amp (2) where Z0 is the impedance of the RF system (normally 50 ohm). Equations (1) and (2), after a proper discretization, have been implemented in the FPGA, where the in-phase and in quadrature components of the input are obtained through an IQ-sampling block. The user interface allows the introduction of the following parameters: r/Q, Q0, Qext, frequency, and detuning and the program calculates all the derived variables as QL, coupling factor, half-bandwidth, rise time, etc. as it can be seen in Fig. 3. The range of these parameters are limited by the fixed point representation inside of the cavity and depend on each other. For instance at 1.3 GHz the minimum QL is around 1.75×105 and the maximum around 5×1011. MORE REALISTIC FEATURES The utilization of an FPGA allows that the parameters in equations (1) and (2) are not only given by the user but also changed dynamically and in real time. For instance the amplitude of the transmitted voltage can be calculated and used to change Q0 to the minimum possible value when the field is above a threshold given by the user, simulating this way a quench of the SRF cavity. The amplitude can be also used to address a look-up table where different Q0 values are stored and fed back to the cavity model, so the system presents a field dependent Q0. Finally, the square of the amplitude value can be added to a signal coming from the host simulating microphonics and fed to a block that implements the mechanical transfer function of the cavity. Up to five mechanical eigenmodes can be set by the user defined by the natural frequency, , the quality factor, , and the coupling factor, . Each of these mechanical modes are defined by: (∆ ∆̇ ) = (− ) (∆ ∆̇ ) + +(− ) (cav + ) (3) 19th Int. Conf. on RF Superconductivity SRF2019, Dresden, Germany JACoW Publishing ISBN: 978-3-95450-211-0 doi:10.18429/JACoW-SRF2019-WETEB4 SRF Technology Ancillaries LLRF WETEB4 773 Co nt en tf ro m th is w or k m ay be us ed un de rt he te rm so ft he CC BY 3. 0 lic en ce (© 20 19 ). A ny di str ib ut io n of th is w or k m us tm ai nt ai n at tri bu tio n to th e au th or (s ), tit le of th e w or k, pu bl ish er ,a nd D O I.

The initial commissioning of the Superconducting RF (SRF) photoinjector is achieved with a Cu photocathode due to its robustness with respect to interactions with the SRF cavity of the injector. Here we present the preparation and characterization of a Cu photocathode plug and the diagnostics to insert the photocathode in the back wall of the SRF cavity. A polycrystalline bulk Cu plug was polished, particle free cleaned and characterized by x-ray photoelectron spectroscopy. During the transfer of the photocathode insert into the gun module the whole process was controlled by several diagnostic tools monitoring the insert position as well as RF, vacuum and cryogenic signals. We discuss the challenges of the photocathode transfer into an SRF cavity and how they can be tackled.

BESSY VSR is set out to provide a variable pulse pattern to the BESSY II users. This project is now fully funded and heading into its implementation phase. The pulse pattern, consisting of long and short pulses, require inserting cavities providing a 3 and a 3.5 harmonic of the fundamental harmonic of the ring. Therefore 1.5 and 1.75 GHz cavities are developed with appropriate higher order mode damping spectrum. Similarly the BESSY II ring and injector chain has to be upgraded to provide appropriate diagnostics and increase the injection eiciency. In this paper we give the current status of the project and give an overview of scientiĄc challenges currently being tackled.

Helmholtz-Zentrum Berlin (HZB) is currently constructing a high average current superconducting (SC) ERL as a prototype to demonstrate low normalized beam emittance of 1 mm·mrad at 100 mA and short pulses of about 2 ps. To attain the required beam properties, an SRF based photoinjector system was developed and during the past year underwent RF commissioning and was setup within a dedicated diagnostics beamline called Gunlab to analyze beam dynamics of both, a Copper cathode and a CsK2Sb cathode as well as their quantum efficiency at UV and green light respectively. The medium power prototype a first stage towards the final high power 100 mA design presented here features a 1.4×λ/2 cell SRF cavity with a normal-conducting, high quantum efficiency Cs2KSb cathode, implementing a modified HZDR-style cathode insert. This injector potentially allows for 6 mA beam current and up to 3.5 MeV kinetic energy, limited by the modified twin TTF-III fundamental power couplers. In this contribution, the first RF commissioning results of the photo-injector module will be presented including dark current analysis as well as measured beam properties with an initially installed Copper cathode.