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The High Duty Cycle (HDC) upgrade is a proposed improvement to the existing European X-ray Free Electron Laser (EuXFEL) to extend the pulsed RF duty factor from the actual value of around 1% to more than 5% up to Continuous Wave (CW). To implement this upgrade, the loaded quality factor (QL) of the superconducting cavities will increase by more than one order of magnitude. This will result in shrinking the cavity bandwidth to values as low as a few Hertz. Since the Lorentz Force Detuning (LFD) experienced during the accelerating field buildup is of hundreds of Hertz, the Low-Level RF (LLRF) system has to accurately track and control the cavity resonance frequency to obtain the desired accelerating gradient. Moreover, ponderomotive instabilities have to be suppressed to achieve stability during beam acceleration. Since LFD is a repetitive disturbance in cavity frequency, the correction to its effects can be implemented as a feedforward compensation on the piezoelectric tuners of the cavity. Initial results on the simulation of feedforward resonance control in the HDC regime are discussed in this proceeding.

A dependable and consistent electron source is a crucial requirement for the achievement of high-power free electron lasers (FELs). Over the past two decades, it has been demonstrated that photoinjectors based on SRF technology (SRF guns) are suitable for continuous wave (CW) beam generation. SRF guns possess both the high accelerating field gradients of normal conducting RF photoinjectors and the low power dissipation thanks to mature SRF cavity technology, and therefore have the potential to provide the high-brightness, high-current beams required for CW-XFELs. After the demonstration of the first SRF gun in Dresden-Rossendorf, several SRF gun programs based on different approaches have achieved promising progress and even succeeded in routine operation. SRF guns are expected to play an important role in XFEL facilities in the near future. In this paper, we give an overview of design concepts, important parameters and development status of the worldwide SRF gun projects.

Modern superconducting radio-frequency linear accelerators require the cavity bandwidth and detuning to be within a specified range to maximize the efficiency of the machine. To correctly estimate these states during operation, the measured RF signals should be calibrated. Due to the finite isolation of the waveguide directional couplers, cross-coupling effects in the forward and reflected channels complicate the calibration of the RF signals in Low-Level RF control systems. Past work proposed a compensation method employing least-squares optimization. This method requires the directivity of the directional couplers to be much higher than one. Additionally, the algorithm requires a tuning parameter to optimize the calculated calibration. However, for some accelerating systems, finding an acceptable value for such a parameter is challenging and time-consuming. Other methods strongly depend on the forward-reflected ratio during the field decay phase. This, in some circumstances, may decrease the accuracy due to noise or systematic errors. In this paper, we present a way to overcome these limitations by performing a global nonlinear least square optimization constrained by energy conservation laws. The method is tested with L-band superconducting resonators at loaded quality factors of 4. 6 ⋅ 1 0 6 and 2. 8 ⋅ 1 0 7 . [ABSTRACT FROM AUTHOR]

Nitrogen-doping and furnace-baking are advanced high-Q 0 recipes developed for 1.3 GHz TESLA-type cavities. These treatments will significantly benefit the high-Q 0 linear accelerator community if they can be successfully adapted to different cavity styles and frequencies. The strong frequency- and geometry-dependence of these recipes makes transferring the technology amongst different cavity styles and frequencies far from straightforward, requiring rigorous study. Upcoming high-Q 0 continuous-wave linear accelerator projects, such as the proposed Michigan State University Facility for Rare Isotope Beam Energy Upgrade, and the underway Fermilab Proton Improvement Plan-II, could benefit enormously from successful adaptation of these techniques to their β opt = 0.6∼650 MHz 5-cell elliptical superconducting rf cavities, operating at an accelerating gradient of around ∼17 MV/m. This is the first investigation of the adaptation of nitrogen doping and medium temperature furnace baking to prototype 644 MHz β opt = 0.65 cavities, with the aim of demonstrating the high-Q 0 potential of these recipes in these novel cavities for future optimization as part of the FRIB400 project R&D. We find that nitrogen-doping delivers superior Q 0 , despite the sub-GHz operating frequency of these cavities, but is sensitive to the post-doping electropolishing removal step and experiences elevated residual resistance. Medium-temperature furnace baking delivers reasonable performance with decreased residual resistance compared to the nitrogen doped cavity, but requires further recipe refinement. The gradient requirement for the FRIB400 upgrade project is comfortably achieved by both recipes. [ABSTRACT FROM AUTHOR]

Abstract SRF cavity quality factors can be accurately measured using RF-power based techniques only when the cavity is very close to critically coupled. This limitation is from systematic errors driven by non-ideal RF components. When the cavity is not close to critically coupled, these systematic effects limit the accuracy of the measurements. The combination of the complex base-band envelopes of the cavity RF signals in combination with a trombone in the circuit allow the relative calibration of the RF signals to be extracted from the data and systematic effects to be characterized and suppressed. The improved calibration allows accurate measurements to be made over a much wider range of couplings. Demonstration of these techniques during testing of a single-spoke resonator with a coupling factor of near 7 will be presented, along with recommendations for application of these techniques. [ABSTRACT FROM AUTHOR]

Minimization of Radio-Frequency (RF) dissipation on superconducting surfaces is of interest for many applications. A prominent one is the use of Superconducting RF (SRF) cavities for charged particle acceleration. In addition to our previous investigation which characterized the occurrence of high-field non-linear losses by microscopic surface topography, the topic of increased linear losses as a function of surface topographic character merits consideration. Surfaces with isotropic homogeneous surface topography may be well characterized by power spectral density (PSD) derived from systematic height measurements. PSD characterizations of representative niobium cavity surface treatments have been developed: Electro-Polishing (EP) , Nano-Mechanical Polishing (NMP) and Centrifugal Barrel Polishing (CBP) A perturbation model based on PSD statistical analysis is used to calculate additional RF loss on these surfaces when superconducting. The model assesses the penetration depth effects for a superconductor. We thus estimate the RF power dissipation ratio between these rough surfaces and an ideal smooth surface. [ABSTRACT FROM AUTHOR]

A superconducting radio frequency (SRF) cavity provides superior stability to power high intensity light sources and can suppress coupled-bunch instabilities due to its smaller impedance for higher order modes. Because of these features, SRF cavities are commonly used for modern light sources, such as the TLS, CLS, DLS, SSRF, PLS-II, TPS, and NSLS-II, with an aggressive approach to operate the light sources at high beam currents. However, operating a SRF cavity at high beam currents may result with unacceptable stability problems of the low level RF (LLRF) system, due to drifts of the cavity resonant frequency caused by unexpected perturbations from the environment. As the feedback loop gets out of control, the cavity voltage may start to oscillate with a current-dependent characteristic frequency. Such situations can cause beam abort due to the activation of the interlock protection system, i.e. false alarm of quench detection. This malfunction of the light source reduces the reliability of SRF operation. Understanding this unstable mechanism to prevent its appearance becomes a primary task in the pursuit of highly reliable SRF operation. In this paper, a Pedersen model, including the response of the LLRF system, was used to simulate the beam-cavity interaction of a SRF cavity under heavy beam loading. Causes for the onset of instability at high beam current will be discussed as well as remedies to assure the design of a stable LLRF system. [ABSTRACT FROM AUTHOR]

The multi-physics design of a five cell, β G = 0 . 61 , 644 MHz superconducting elliptical cavity being developed for an energy upgrade in the Facility for Rare Isotope Beams (FRIB) is presented. The FRIB energy upgrade from 200 MeV/u to 400 MeV/u for heaviest uranium ions will increase the intensities of rare isotope beams by nearly an order of magnitude. After studying three different frequencies, 1288 MHz, 805 MHz, and 644 MHz, the 644 MHz cavity was shown to provide the highest energy gain per cavity for both uranium and protons. The FRIB upgrade will include 11 cryomodules containing 5 cavities each and installed in 80-meter available space in the tunnel. The cavity development included extensive multi-physics optimization, mechanical and engineering analysis. The development of a niobium cavity is complete and two cavities are being fabricated in industry. The detailed design of the cavity sub-systems such as fundamental power coupler and dynamic tuner are currently being pursued. In the overall design of the cavity and its sub-systems we extensively applied experience gained during the development of 650 MHz low-beta cavities at Fermi National Accelerator Laboratory (FNAL) for the Proton Improvement Plan (PIP) II. [ABSTRACT FROM AUTHOR]

The acceleration of high-current beam in Superconducting Radio Frequency (SRF) cavities is a challenging but essential for a variety of advanced accelerators. SRF cavities should be carefully designed to minimize the High Order Modes (HOM) power generated in the cavities by the beam current. The reduction of HOM power we demonstrate in a particular case can be quite large. This paper presents a method to systematically control the HOM resonance frequencies in the initial design phase to minimize the HOM power generation. This method is expected to be beneficial for the design of high SRF cavities addressing a variety of Energy Recovery Linac (ERL) applications. [ABSTRACT FROM AUTHOR]

We present a study of an innovative scheme to generate high repetition rate (MHz-class) GeV electron beams by adopting a two-pass two-way acceleration in a super-conducting Linac operated in Continuous Wave (CW) mode. The beam is accelerated twice in the Linac by being re-injected, after the first pass, in opposite direction of propagation. The task of recirculating the electron beam is performed by an arc compressor composed by 14 Double Bend Achromat (DBA). In this paper, we study the main issues of the two-fold acceleration scheme, the electron beam quality parameters preservation (emittance, energy spread), together with the bunch compression performance of the arc compressor, aiming to operate an X-ray Free Electron Laser. The requested power to supply the cryogenic plant and the RF sources is also significantly reduced w.r.t a conventional one-pass SC Linac for the same final energy.

In this work we investigate the effect of the bunch pattern in a linac on the Higher Order Mode (HOM) power generation. The future ERL-based electron–ion collider eRHIC at BNL is used as an illustrative example. This ERL has multiple high current Superconducting Radiofrequency (SRF) 5-cell cavities. The HOM power generated when a single bunch traverses the cavity is estimated by the corresponding loss factor. Multiple re-circulations through the Energy Recovery Linac (ERL) create a specific bunch pattern. In this case the loss factor can be different than the single bunch loss factor. HOM power can vary dramatically when the ERL bunch pattern changes. The HOM power generation can be surveyed in the time and frequency domains. We estimate the average HOM power in a 5-cell cavity with different ERL bunch patterns. [ABSTRACT FROM AUTHOR]

Many modern and future particle accelerators employ high gradient superconducting RF (SRF) to generate beams of high energy, high intensity and high brightness for research in high energy and nuclear physics, basic energy sciences, etc. In this paper we report the record performance large-scale SRF system with average beam accelerating gradient matching the International Linear Collider (ILC) specification of 31.5 MV m ^−1 . Design of the eight cavity 1.3 GHz SRF cryomodule, its performance without the beam and results of the system commissioning with high intensity electron beam at Fermilab Accelerator Science and Technology (FAST) facility are presented. We also briefly discuss opportunities for further beam studies and tests at FAST including those on even higher gradient and more efficient SRF acceleration, as well as exploration of the system performance with full ILC-type beam specifications.

Q 0 determinations based on RF power measurements are subject to at least three potentially large systematic effects that have not been previously appreciated. Instrumental factors that can systematically bias RF based measurements of Q 0 are quantified and steps that can be taken to improve the determination of Q 0 are discussed. [ABSTRACT FROM AUTHOR]

Recent efforts towards production of high-quality magnesium diboride (MgB 2 ) coatings have raised the possibility of producing usable accelerating cavities. Work continues to reliably produce films of sufficient quality over the large, complex surface area of an accelerating cavity, but this technology would open many interesting technical opportunities. One of these is to replace the traditionally required liquid helium cryogenic systems with a dry system based on cryocoolers. This is made possible by the much higher T c of MgB 2 , allowing operation closer to 30 K where cryocooler efficiency becomes competitive with alternative systems. This removes the need for pressure vessels in the cryomodule as well as internal distribution systems, greatly simplifying cryomodule design and fabrication. The lack of uniform cooling over the cavity surface, however, complicates behavior by coupling RF losses, heat leak, and cooling design in a way not seen in traditional SRF cavities. In this paper, these complexities are explored, including realistic cryocooler performance, temperature dependant RF losses, and standard thermal management challenges. [ABSTRACT FROM AUTHOR]

Abstract: Frequency tuning systems for accelerating cavities are required to compensate static and dynamic frequency perturbations during beam operation. In the case of superconducting cavities, these are commonly tuned by deformation of the cavity wall in specific places of the geometry. Nevertheless, considering the mechanical properties and the frequency versus displacement sensitivity of some accelerating structures, tuning by deformation does not allow meeting the requirements. Inspired from the “room temperature technology”, an alternative tuning technique by insertion of a helium-cooled superconducting plunger has been studied and validated for the superconducting Spiral2 accelerator. [Copyright &y& Elsevier]

Abstract: At the radiation source ELBE a superconducting radio-frequency photoinjector (SRF gun) was developed and put into operation. Since 2010 the gun has delivered beam into the ELBE linac. A new driver laser with 13MHz pulse repetition rate allows now to operate the free-electron lasers (FELs) with the SRF gun. This paper reports on the first lasing experiment with the far-infrared FEL at ELBE, describes the hardware, the electron beam parameters and the measurement of the FEL infrared radiation output. [Copyright &y& Elsevier]

Particle accelerators are widely used in scientific research and industry. Recently, techniques utilized for accelerator are developed rapidly in China. We briefly review the recent progress in this field and primarily focus on superconducting RF, innovative structures for particle production and acceleration and beam diagnosis. Several advanced compact user facilities are also introduced. [ABSTRACT FROM AUTHOR]

Abstract: Under the DOE SciDAC project on Accelerator Science and Technology, a suite of electromagnetic codes has been under development at SLAC that are based on unstructured grids for higher accuracy, and use parallel processing to enable large-scale simulation. The new modeling capability is supported by SciDAC collaborations on meshing, solvers, refinement, optimization and visualization. These advances in computational science are described and the application of the parallel eigensolver Omega3P to the cavity design for the International Linear Collider is discussed. [Copyright &y& Elsevier]

Abstract: At INFN-Legnaro the heavy ion injector PIAVE, based on two superconducting RFQ and eight quarter wave resonators (QWR), is at an advanced stage of beam commissioning. The RFQs (SRFQ1 and SRFQ2), built in full Nb within a stiffening Ti jacket, are 0.8m in diameter and 1.4 and 0.8m long respectively, with a resonant frequency of 80MHz. The PIAVE beam is bunched by a normal conducting 3-harmonic buncher upstream the SRFQs. The SRFQs are specified to work at a peak surface field of 25.5MV/m, a value which was exceeded in the test phase and has been recently confirmed in on-line tests. Phase and amplitude locking, versus both microphonics and pressure variations of the liquid helium bath, is the main issue. Since November 2004, the two SRFQs have been used quite extensively, for beam acceleration tests in PIAVE, showing a high degree of reliability. A 16O3+ pilot beam, received from an ECR ion source located on a high voltage platform, was used in the tests. The typical ion beam current was a few hundreds nA, even though it could be raised up to a few μA without any inconveniences. Beam tests with 132Xe18+ were made too. The paper reports the more recent results of on-line SRFQ tests and beam operation. [Copyright &y& Elsevier]

Abstract: The shape of a TeSLA inner cell can be improved to lower the peak surface magnetic field at the expense of a higher peak surface electric field by making the cell reentrant. Such a single-cell cavity was designed and tested at Cornell, setting a world record accelerating gradient [V. Shemelin et al., An optimized shape cavity for TESLA: concept and fabrication, 11th Workshop on RF Superconductivity, Travemünde, Germany, September 8–12, 2003; R. Geng, H. Padamsee, Reentrant cavity and first test result, Pushing the Limits of RF Superconductivity Workshop, Argonne National Laboratory, September 22–24, 2004]. However, the disadvantage to a cavity is that liquids become trapped in the reentrant portion when it is vertically hung during high pressure rinsing. While this was overcome for Cornell’s single-cell cavity by flipping it several times between high pressure rinse cycles, this may not be feasible for a multi-cell cavity. One solution to this problem is to make the cavity reentrant on only one side, leaving the opposite wall angle at six degrees for fluid drainage. This idea was first presented in 2004 [T.L. Grimm et al., IEEE Transactions on Applied Superconductivity 15(6) (2005) 2393]. Preliminary designs of two new half-reentrant (HR) inner cells have since been completed, one at a high cell-to-cell coupling of 2.1% (high-k cc HR) and the other at 1.5% (low-k cc HR). The parameters of a HR cavity are comparable to a fully reentrant cavity, with the added benefit that a HR cavity can be easily cleaned with current technology. [Copyright &y& Elsevier]

Abstract: Experimental results on the characterization of the linear and non-linear microwave properties of niobium film produced by UHV cathodic arc deposition are presented. Surface impedance Z s as a function of RF field and intermodulation distortion (IMD) measurement have been carried out by using a dielectrically loaded resonant cavity operating at 7GHz. The experimental data show that these samples have a lower level of intrinsic non-linearities at low temperature and low circulating power in comparison with Nb samples grown by sputtering. These results make UHV cathodic arc deposition a promising technique for the improvement of RF superconducting cavities for particle accelerators. [Copyright &y& Elsevier]

Abstract: Kneisel, Ciovati, Myneni and co-workers at TJNAF have recently fabricated two superconducting cavities from the center of a large grain Nb billet manufactured by CBMM. Both cavities had excellent properties with one attaining an accelerating gradient of 45MV/m (2K) after a 48h and 120°C bake [P. Bauer et al., An investigation of the properties of BCP niobium for superconducting RF cavities, in: K.-J. Kim, C., Eyberger (Eds.), Proceedings of the Pushing the Limits of RF Superconductivity workshop, Argonne National Laboratory Report ANL-05/10, March 2005, pp. 84–93]. An investigation is underway to use magneto-optical (MO) imaging to observe the flux penetration behavior of a sheet sliced from this billet. The large grain size (some larger than 50mm) allowed us to isolate multiple bi-crystals and tri-crystals. In the first stage of the present study we have taken the as-received sheet (RRR ∼280), which has been etched to reveal the grain structure. By magneto-optical examination we observed preferential flux penetration at some grain boundaries of a bi-crystal where the grain boundary was almost perpendicular to the sample surface and there was <1μm surface step across the boundary. At other grain boundaries, with large steps or where the grain boundaries were not normal to the surface, we observed no preferential flux penetration. Preliminary transport measurements on a bi-crystal showed greater normal state resistance and lower superconducting critical current at the grain boundary. [Copyright &y& Elsevier]

Abstract: The first niobium-coated copper cavities were produced at CERN in the early 1980s. The sputter coating technology was chosen as a basis for development, first in the pure diode configuration and subsequently in the magnetron configuration, which was then adopted for the successful series production of the LEP and LHC cavities. In parallel, an intensive R&D effort was undertaken at CERN and other Laboratories in order to understand the advantages and limitations of this technique. Some highlights of the present understanding will be given. Several new developments in the coating technique are being pursued around the world, which will be discussed together with their motivations. [Copyright &y& Elsevier]

Abstract: We summarize the proceedings of Working Group 1 of the 2005 Energy Recovery Linac (ERL) Workshop. The subject of this working group, the electron gun and injector design, is arguably the most critical part of the ERL as it determines the ultimate performance of this type of accelerators. Working Group 1 dealt with a variety of subjects: The technology of DC, normal-conducting RF and superconducting RF guns; beam dynamics in the gun and injector; the cathode and laser package; modeling and computational issues; magnetized beams and polarization. A short overview of these issues covered in the Working Group is presented in this paper. [Copyright &y& Elsevier]

Abstract: The design, fabrication and commissioning of a 703.75MHz SRF photoinjector with a retractable multi-alkali photocathode designed to deliver 0.5A average current at 100% duty factor is the present undertaking of the electron cooling group in the Collider Accelerator Division of Brookhaven National Labs. This photoinjector represents the state of the art in photoinjector technology, orders of magnitude beyond the presently available technology, and should be commissioned by 2007. The R&D effort presently underway, and the focus of this paper, will address the numerous technological challenges that must be met for this project to succeed. These include the novel physics design of the cavity, the challenges of inserting and operating a multi-alkali photocathode in the photoinjector at these high average currents, and the design and installation of a laser system capable of delivering the required 10s of watts of laser power needed to make this photoinjector operational. [Copyright &y& Elsevier]

Abstract: Energy recovery linacs (ERL) have significant potential uses in high energy physics and nuclear physics. We describe some of the potential applications which are under development by our laboratories in this area, and the technology issues that are associated with these applications. The applications that we discuss are electron cooling of high-energy hadron beams and electron–nucleon colliders. The common issues for some of these applications are high currents of polarized electrons, high-charge and high-current electron beams and the associated issues of high-order modes. The advantages of ERLs for these applications are numerous and will be outlined in the text. It is worth noting that some of these advantages are the high brightness of the ERL beams and their relative immunity to beam–beam disturbances. [Copyright &y& Elsevier]