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A novel four-port circulator is presented which uses an inline ferromagnetic element to allow for transmission or reflection depending on the sense of the incident circularly polarized wave. This configuration has been shown to reduce the dependency on ferrite anisotropy and support higher power, low rf loss, operation. An analytic analysis of this device is presented alongside corroborating cold test data of the first prototype. High-power operation was performed at 2.856 GHz, with input power levels up to 8 MW for $3.5~\mu \text{s}$ in a pressurized nitrogen environment. The results from this research not only demonstrate ability to eliminate the use of greenhouse insulators, such as SF6, but also provide conceptual groundwork for a new class of ultrahigh power (50 MW+) nonreciprocal networks including circulators, isolators, phase shifters, and rf switches.

Finite element representations of Maxwell's equations pose unusual challenges inherent to the variational representation of the `curl-curl' equation for the fields. We present a variational formulation based on classical field theory. Borrowing from QED, we modify the Lagrangian by adding an implicit gauge-fixing term. Our formulation, in the language of differential geometry, shows that conventional edge elements should be replaced by the simpler nodal elements for time-harmonic problems. We demonstrate how this formulation, adhering to the deeper underlying symmetries of the four-dimensional covariant field description, provides a highly general, robust numerical framework., Comment: 13 pages, 16 figures

A new variational theory is presented for beam loading in microwave cavities. The beam--field interaction is formulated as a dynamical interaction whose stationarity according to Hamilton's principle will naturally lead to steady-state solutions that indicate how a cavity's resonant frequency, $Q$ and optimal coupling coefficient will detune as a result of the beam loading. A driven cavity Lagrangian is derived from first principles, including the effects of cavity wall losses, input power and beam interaction. The general formulation is applied to a typical klystron input cavity to predict the appropriate detuning parameters required to maximize the gain (or modulation depth) in the average Lorentz factor boost, $\langle \Delta\gamma \rangle$. Numerical examples are presented, showing agreement with the general detuning trends previously observed in the literature. The developed formulation carries several advantages for beam-loaded cavity structures. It provides a self-consistent model for the dynamical (nonlinear) beam--field interaction, a procedure for maximizing gain under beam-loading conditions, and a useful set of parameters to guide cavity-shape optimization during the design of beam-loaded systems. Enhanced clarity of the physical picture underlying the problem seems to be gained using this approach, allowing straightforward inclusion or exclusion of different field configurations in the calculation and expressing the final results in terms of measurable quantities. Two field configurations are discussed for the klystron input cavity, using finite magnetic confinement or no confinement at all. Formulating the problem in a language that is directly accessible to the powerful techniques found in Hamiltonian dynamics and canonical transformations may potentially carry an additional advantage in terms of analytical computational gains, under suitable conditions., Comment: 18 pages, 6 figures (2nd version: corrected typos and notation; added one more example in Section V). To appear in Physical Review Applied

We are developing a high-gradient, THz-driven electron gun. THz-frequency accelerator structures can enable the development of compact, bright, and efficient particle sources. Our two cell standing-wave gun is powered by a 110 GHz gyrotron and will produce 350 keV electrons with 500 k $W$ of input power. The field emission cathode is a copper tip located in the first cell. High power measurements will include diagnostics to measure the energy spread, charge, and size of the resulting bunch.

We present a topology for linear accelerators (linacs) in which the power is distributed to the cavities through a waveguide with periodic apertures that guarantees the correct phases and amplitudes along the structure length. Unlike conventional traveling and standing-wave linacs, the presented topology allows the cavity shapes to be designed without the constraints applied to the coupling between cells to transfer power from one cell to the next. Therefore, the topology permits more degrees of freedom for the optimization of individual cavity shapes in comparison with conventional linacs. The cavity shapes can be optimized for power consumption and efficiency, and/or the manipulation of the surface fields for high gradient operation. This topology also provides a possibility for low-temperature manufacturing techniques that prevent the annealing of the material during typical brazing processes; hence, the material could retain its original properties such as hardness. We present a design and an experimental demonstration of this linac.

In the field of beam physics, two frontier topics have taken center stage due to their potential to enable new approaches to discovery in a wide swath of science. These areas are: advanced, high gradient acceleration techniques, and x-ray free electron lasers (XFELs). Further, there is intense interest in the marriage of these two fields, with the goal of producing a very compact XFEL. In this context, recent advances in high gradient radio-frequency cryogenic copper structure research have opened the door to the use of surface electric fields between 250 and 500 MV/m. Such an approach is foreseen to enable a new generation of photoinjectors with six-dimensional beam brightness beyond the current state-of-the-art by well over an order of magnitude. This advance is an essential ingredient enabling an ultra-compact XFEL (UC-XFEL). In addition, one may accelerate these bright beams to GeV scale in less than 10 meters. Such an injector, when combined with inverse free electron laser-based bunching techniques can produce multi-kA beams with unprecedented beam quality, quantified by ~50 nm-rad normalized emittances. These beams, when injected into innovative, short-period (1-10 mm) undulators uniquely enable UC-XFELs having footprints consistent with university-scale laboratories. We describe the architecture and predicted performance of this novel light source, which promises photon production per pulse of a few percent of existing XFEL sources. We review implementation issues including collective beam effects, compact x-ray optics systems, and other relevant technical challenges. To illustrate the potential of such a light source to fundamentally change the current paradigm of XFELs with their limited access, we examine possible applications in biology, chemistry, materials, atomic physics, industry, and medicine which may profit from this new model of performing XFEL science., 80 pages, 24 figures

This paper reviews the current state-of-the-art in understanding the phenomena of ultra-high vacuum radio-frequency (rf) breakdown in accelerating structures and the efforts to improve stable operation of the structures at accelerating gradients above 100 MV/m. Numerous studies have been conducted recently with the goal of understanding the dependence of the achievable accelerating gradients and breakdown rates on the frequency of operations, the geometry of the structure, material and method of fabrication, and operational temperature. Tests have been conducted with single standing wave accelerator cells as well as with the multi-cell traveling wave structures. Notable theoretical effort was directed at understanding the physical mechanisms of the rf breakdown and its statistical behavior. The achievements presented in this paper are the result of the large continuous self-sustaining collaboration of multiple research institutions in the United States and worldwide.

Large arrays of cryogenic sensors for various imaging applications ranging across x-ray, gamma-ray, Cosmic Microwave Background (CMB), mm/sub-mm, as well as particle detection increasingly rely on superconducting microresonators for high multiplexing factors. These microresonators take the form of microwave SQUIDs that couple to Transition-Edge Sensors (TES) or Microwave Kinetic Inductance Detectors (MKIDs). In principle, such arrays can be read out with vastly scalable software-defined radio using suitable FPGAs, ADCs and DACs. In this work, we share plans and show initial results for SLAC Microresonator Radio Frequency (SMuRF) electronics, a next-generation control and readout system for superconducting microresonators. SMuRF electronics are unique in their implementation of specialized algorithms for closed-loop tone tracking, which consists of fast feedback and feedforward to each resonator's excitation parameters based on transmission measurements. Closed-loop tone tracking enables improved system linearity, a significant increase in sensor count per readout line, and the possibility of overcoupled resonator designs for enhanced dynamic range. Low-bandwidth prototype electronics were used to demonstrate closed-loop tone tracking on twelve 300-kHz-wide microwave SQUID resonators, spaced at $\sim$6 MHz with center frequencies $\sim$5-6 GHz. We achieve multi-kHz tracking bandwidth and demonstrate that the noise floor of the electronics is subdominant to the noise intrinsic in the multiplexer., 7 pages, 5 figures, Submitted to the Journal of Low Temperature Physics (Proceedings of the 17th International Workshop on Low Temperature Detectors)

Recent studies of rf breakdown physics in cryogenic copper X-band accelerating structures have shown a dramatic increase in the operating gradient while maintaining low breakdown rates. The TOPGUN project, a collaboration between UCLA, SLAC, and INFN, will use this improvement in gradient to create an ultra-high brightness cryogenic normal conducting photoinjector [16] . The brightness is expected to be higher by a factor of 25 relative to the LCLS photogun [9] . This improvement in the brightness will lead to increased performance of X-Ray free electron lasers (FELs) and ultrafast electron diffraction devices [16] . We present the rf design for this S-band photogun, which will be a drop-in replacement for the current LCLS photogun.

This paper explores the physics of vacuum rf breakdowns in high gradient mm-wave accelerating structures. We performed a series of experiments with 100 GHz and 200 GHz metallic accelerating structures, at the Facility for Advanced Accelerator Experimental Tests (FACET) at the SLAC National Accelerator Laboratory. This paper presents the experimental results of rf tests of 100 GHz travelling-wave accelerating structures, made of hard copper-silver alloy. The results are compared with pure hard copper structures. The rf fields were excited by the FACET ultra-relativistic electron beam. The accelerating structures have open geometries, 10 cm long, composed of two halves separated by a variable gap. The rf frequency of the fundamental accelerating mode depends on the gap size and can be changed from 90 GHz to 140 GHz. The measured frequency and pulse length are consistent with our simulations. When the beam travels off-axis, a deflecting field is induced in addition to the decelerating longitudinal field. We measured the deflecting forces by observing the displacement of the electron bunch and used this measurement to verify the expected accelerating gradient. We present the first quantitative measurement of rf breakdown rates in 100 GHz copper-silver accelerating structure, which was 10 −3 per pulse, with peak electric field of 0.42 GV/m, an accelerating gradient of 127 MV/m, at a pulse length of 2.3 ns. The goal of our studies is to understand the physics of gradient limitations in order to increase the energy reach of future accelerators.

We present the design of a rf undulator at 91.392 GHz that has a period of 1.75 mm, while the minimum beam aperture seen by the beam is 2.375 mm. To confine the fields inside the undulator a corrugated waveguide is connected through a matching section to a linear taper and a mirror. After the mirror, a Bragg reflector and a matching section are used to reflect back all the fields leaking out of the mirror opening. This undulator requires approximately 1.4 MW for submicrosecond pulses to generate an equivalent K value of 0.1. Transferring such amounts of power at mm-wave frequencies requires overmoded corrugated waveguides, and coupling through irises creates excessive pulsed heating. We have designed a coupling scheme that allows coupling power from a highly overmoded corrugated waveguide to the undulator cavity through the beam pipe without disturbing the undulator fields. This system of the rf undulator and its coupling scheme allows for fast dynamic control of the polarization of the emitted light.

We report initial high-gradient testing results of a 110 GHz single-cell standing wave accelerating cavity powered by a 1 MW gyrotron. The cavity is fed with 10 ns, 100s of kilowatt pulses, and achieved a field gradient up to 225 MV/m. We also report the observation of rf breakdowns and cavity processing for $\lt /p\gt \gt 10 ^{5}$ pulses.

We propose an approach to search for axion dark matter with a specially designed superconducting radio frequency cavity, targeting axions with masses $m_a \lesssim 10^{-6} \text{ eV}$. Our approach exploits axion-induced transitions between nearly degenerate resonant modes of frequency $\sim$ GHz. A scan over axion mass is achieved by varying the frequency splitting between the two modes. Compared to traditional approaches, this allows for parametrically enhanced signal power for axions lighter than a GHz. The projected sensitivity covers unexplored parameter space for QCD axion dark matter for $10^{-8} \text{ eV} \lesssim m_a \lesssim10^{-6} \text{ eV}$ and axion-like particle dark matter as light as $m_a \sim 10^{-14} \text{ eV}$., Comment: 30 pages, 7 figures

The next generation of accelerators, from the compact to the large infrastructure dedicated to high energy physics, is highly demanding in terms of accelerating gradients. To upgrade performances of X band linacs at 11.424 GHz many resources are devoted to achieve high accelerating gradients and at the same time to obtain a high reliability. In the framework of a three-year funded project by the Vth Committee of the INFN to the Laboratori Nazionali di Frascati (LNF) and to the Laboratori Nazionali di Legnaro (LNL). Within a broad international collaboration the LNF has been involved in the design, manufacture and test of compact high power standing wave (SW) sections operating at high frequency while LNL is actively involved in the development of new materials and multilayers using PVD (Physical Vapor Deposition) methods. We will report about the status of the accelerating device and of the different ongoing R&D activities and characterization procedures such as tests of different materials and metallic coatings.

Novel types of dual circular polarizer are developed to convert TE10 mode into two different polarized TE11 modes in a circular waveguide. These designs have MHz bandwidth and high power transmission capability.They can be used for broadcasting and receiving circular polarized signals.

We report the experimental demonstration of a mm-wave electron accelerating structure powered by a high-power rf source. We demonstrate reliable coupling of an unprecedented rf power—up to 575 kW into the mm-wave accelerator structure using a quasi-optical setup. This standing wave accelerating structure consists of a single-cell copper cavity and a Gaussian to TM01 mode converter. The accelerator structure is powered by 110 GHz, 10-ns long rf pulses. These pulses are chopped from 3 ms pulses from a gyrotron oscillator using a laser-driven silicon switch. We show an unprecedented high gradient up to 230 MV/m that corresponds to a peak surface electric field of more than 520 MV/m. We have achieved these results after conditioning the cavity with more than 105 pulses. We also report preliminary measurements of rf breakdown rates, which are important for understanding rf breakdown physics in the millimeter-wave regime. These results open up many frontiers for applications not only limited to the next generation particle accelerators but also x-ray generation, probing material dynamics, and nonlinear light-matter interactions at mm-wave frequency.

Many numerical solvers model the general nonlinear problem of particle motion in self-generated fields through the computation of particle trajectories and fields in a Lagrangian and Eulerian framework, respectively. The particle-weighting step mapping the source terms between these frameworks is a primary source of noise and computation time. Herein we eliminate this costly step by treating the trajectories as bounds on the particle flow, applying charge conservation to interpolate the source distribution. The result is an order of magnitude improvement in accuracy and an 80 to 120 fold speed-up.

Experiments with high gradient traveling-wave mm-wave metallic accelerating structures were performed to research the physics of vacuum rf breakdowns. The accelerating structures are open, composed of two identical halves separated by an adjustable gap. The electromagnetic fields were induced by an ultrarelativistic electron beam at the Facility for Advanced Accelerator Experimental Tests (FACET) at SLAC National Accelerator Laboratory. These accelerating structures have a fundamental beam-synchronous mode with a frequency that varies from 95 to 140 GHz depending on the gap width. When the gap is opened by more than half of a freespace wavelength, the corresponding synchronous mode remains trapped demonstrating behavior consistent with the so called ``surface wave.'' This regime is of potential interest for femtosecond electron beam diagnostics and beam manipulation techniques. The behavior of the surface wave was characterized and the measured synchronous frequency, pulse length and beam deflection voltage are found to be in good agreement with simulations. The wave magnitude was changed by positioning the beam at different distances from the surface of a structure-half and the corresponding rf breakdown rate was recorded. The breakdown rate was $4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ per pulse at a peak surface electric field of $0.3\text{ }\text{ }\mathrm{GV}/\mathrm{m}$ and a pulse length of 60 ps.

The development of high brightness electron sources can enable an increase in performance and reduction in size of extreme X-ray sources such as free electron lasers (FELs). A promising path to high brightness is through larger electric fields in radio-frequency (rf) photoinjectors. Recent experiments with 11.4 GHz copper accelerating cavities at cryogenic temperatures have demonstrated $500\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ surface electric fields with low rf breakdown rates. However, when the surface electric fields are larger than $300\text{ }\text{ }\mathrm{MV}/\mathrm{m}$, the measured cavity quality factor, ${Q}_{0}$, decreases during the input rf pulse by up to 30%, recovering before the next rf pulse. In this paper, we present an experimental study of the rf losses, manifested as degradation of ${Q}_{0}$, in a copper cavity operated at cryogenic temperatures and high gradients. The experimental conditions range from temperatures of 10--77 K and rf pulse lengths of 100--800 ns, using surface electric fields up to $400\text{ }\text{ }\mathrm{MV}/\mathrm{m}$. We developed a model for the change in ${Q}_{0}$ using measured field emission currents and rf signals. We find that the ${Q}_{0}$ degradation is consistent with the rf power being absorbed by strong field emission currents accelerated inside the cavity.

Recent investigations of RF copper structures operated at cryogenic temperatures performed by a SLAC-UCLA collaboration have shown a dramatic increase in the maximum surface electric field, to 500 MV/m. We examine use of these fields to enable very high field cryogenic photoinjectors that can attain over an order of magnitude increase in peak electron beam brightness. We present beam dynamics studies relevant to X-ray FEL injectors, using start-to-end simulations that show the high brightness and low emittance of this source enables operation of a compact FEL reaching a photon energy of 80 keV. The preservation of beam brightness in compression, exploiting micro-bunching techniques is discussed. While the gain in brightness at high field is due to increase of the emission current density, further increases in brightness due to lowering of the intrinsic cathode emittance in cryogenic operation are also enabled. While the original proposal for this type of cryogenic, ultra-high field photoinjector has emphasized S-band designs, there are numerous potential advantages that may be conferred by operation in C-band. We examine issues related to experimental implementation in C-band, and expected performance of this type of device in a future hard X-ray FEL such as MaRIE., 10 pages, 5 figures, to appear in Proceedings of 2018 European Advanced Accelerator Conference, Nuclear Instruments and Methods

A new compact low-loss fast phase-shift high-power microwave (HPM) phase shifter (PS) is proposed, designed, and cold and high-power tested. Firstly, based on solving the scattering matrix and eigenvectors, we design a novel HPM dual circular polarizer, and then a dumbbell-like metal plug driven by a high-speed servomotor is used to slide a short circuit along the dual circular polarized port to adjust the output RF phase, which varies 180 $^{\circ}$ by moving the plug with a quarter of the guided wavelength. The X-band PS has a total length of 9.5 cm and a power capacity achieved 300 MW at 30-ns HPM pulse. A fast phase shift of 310 $^{\circ}$ was achieved within 0.1 s in test, a high precision of phase shift 1 $^{\circ}$ can be realized, and the tested insertion loss was $ 0.15 dB.

Traditional linear interaction RF sources (klystrons and traveling wave tubes) provide limited amounts of power at the mm-wave and terahertz part of the spectrum because their critical dimensions have to be small compared to the wavelength they operate. Due to these small dimensions, the amount of current that can go through the beam apertures is limited. We designed, built and hot-tested a new gyrocon-like mm-wave source, with an output interaction circuit based on a spherical sector cavity. This spherical sector cavity provides a large beam aperture, allowing for a significant amount of current to go through. This source operates as a 5th harmonic frequency multiplier, with an output frequency of 57.12 GHz. The measured peak output power was 54.26 W at the 5th harmonic, while power at the other harmonics was at least 33.28 dB lower than the 5th harmonic.

We are exploring the physics of vacuum rf breakdowns in high gradient metallic accelerating structures at sub-THz frequencies. We present the experimental results of rf tests performed in beam-driven travelling-wave accelerating structures. We measured the breakdown rates, the frequency of the emitted radiation, and the gradients. As the next step in this study, we are developing accelerating structures powered by an rf source. We report on the status of this project.

The use of infrared lasers to power optical-scale lithographically fabricated particle accelerators is a developing area of research that has garnered increasing interest in recent years. The physics and technology of this approach is reviewed, which is referred to as dielectric laser acceleration (DLA). In the DLA scheme operating at typical laser pulse lengths of 0.1 to 1 ps, the laser damage fluences for robust dielectric materials correspond to peak surface electric fields in the $\mathrm{GV}/\mathrm{m}$ regime. The corresponding accelerating field enhancement represents a potential reduction in active length of the accelerator between 1 and 2 orders of magnitude. Power sources for DLA-based accelerators (lasers) are less costly than microwave sources (klystrons) for equivalent average power levels due to wider availability and private sector investment. Because of the high laser-to-particle coupling efficiency, required pulse energies are consistent with tabletop microJoule class lasers. Combined with the very high (MHz) repetition rates these lasers can provide, the DLA approach appears promising for a variety of applications, including future high-energy physics colliders, compact light sources, and portable medical scanners and radiative therapy machines.

A novel type of dual circular polarizer for simultaneously receiving and transmitting right-hand and left-hand circularly polarized waves is developed and tested. It consists of a H-plane T junction of rectangular waveguide, one circular waveguide as an E- plane arm located on top of the junction, and two metallic pins used for matching. The theoretical analysis and design of the three-physical- port and four-mode polarizer were researched by solving Scattering- Matrix of the network and using a full-wave electromagnetic simulation tool. The optimized polarizer has the advantages of a very compact size with a volume smaller than 0:6‚ 3 , low complexity and manufacturing cost. A couple of the polarizer has been manufactured and tested, and the experimental results are basically consistent with the theories.

Permanent magnet insertion devices (IDs), which are the main radiation generating devices in synchrotron light sources and free-electron lasers, use a time-invariant but space-periodic magnetic field to wiggle relativistic electrons for short-wavelength radiation generation. Recently, a high power microwave based undulator has also been successfully demonstrated at SLAC which promises the advantage of dynamic tunability of radiation spectrum and polarization. Such IDs employ transverse elecromagnetic fields which are periodic in both space and time to undulate the electrons. In this paper we develop a detailed theory of the principle of electromagnetic IDs from first principles for both linear and circular polarization modes. The electromagnetic equivalent definitions of undulator period (${\ensuremath{\lambda}}_{u}$) and undulator deflection parameter ($K$) are derived. In the inertial frame where the average momentum of the electron is zero, we obtain the figure-8-like trajectory for the linear polarization mode and the circular trajectory for the circular polarization mode. The corresponding radiation spectra and the intensity of harmonics is also calculated.

We present a novel method of reducing the footprint and increasing the efficiency of the modern multi-MW rf pulse compressor. This system utilizes a high power rf polarizer to couple two circular waveguide modes in quadrature to a single resonant cavity in order to replicate the response of a traditional two cavity configuration using a 4-port hybrid. The 11.424 GHz, high-Q, spherical cavity has a 5.875 cm radius and is fed by the circularly polarized signal to simultaneously excite the degenerate $T{E}_{114}$ modes. The overcoupled spherical cavity has a ${Q}_{0}$ of $9.4\ifmmode\times\else\texttimes\fi{}{10}^{4}$ and coupling factor ($\ensuremath{\beta}$) of 7.69 thus providing a loaded quality factor ${Q}_{L}$ of $1.06\ifmmode\times\else\texttimes\fi{}{10}^{4}$ with a fill time of 150 ns. Cold tests of the polarizer demonstrated good agreement with the numerical design, showing transmission of $\ensuremath{-}0.05\text{ }\text{ }\mathrm{dB}$ and reflection back to the input rectangular WR 90 waveguide less than $\ensuremath{-}40\text{ }\text{ }\mathrm{dB}$ over a 100 MHz bandwidth. This novel rf pulse compressor was tested at SLAC using XL-4 Klystron that provided rf power up to 32 MW and generated peak output power of 205 MW and an average of 135 MW over the discharged signal. A general network analysis of the polarizer is discussed as well as the design and high power test of the rf pulse compressor.

A new design of dual circular polarizer is presented in this paper. This innovative design converts radiofrequency (rf) energy from TE10 mode in a rectangular waveguide to two polarized TE11 modes in a circular waveguide. A reflection less than $\ensuremath{-}20\text{ }\text{ }\mathrm{db}$ is achieved and breakdown field is less than $42\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ at input of 1 MW. Meanwhile, this polarizer has a megahertz bandwidth, and the thermal stability is also discussed. This device can be used for broadcasting and receiving the circular polarized signals.

Vacuum electronic devices, such as rf sources for accelerator applications, must provide high rf power with high efficiency. To achieve these requirements, multi-beam klystron and sheet-beam klystron devices have been developed. Multi-beam klystrons, at high frequency employ separate output cavities; hence they have the disadvantage that combining externally all the rf pulses is challenging. Sheet-beam klystrons have problems with instabilities and with space charge forces that makes the beam not naturally confined. We are proposing an alternative approach that reduces space charge problems, by adopting geometries in which the space charge forces are naturally balanced. In this paper we will present the design and challenges of a radial klystron, composed by concentric pancake resonant cavities. In this case, space charge forces are naturally balanced in the azimuthal direction.

This paper explores a hybrid mode rf structure that served as a superconducting radio-frequency coupler. This application achieves a reflection ${\mathrm{S}}_{(1,1)}$ varying from 0 to $\ensuremath{-}30\text{ }\text{ }\mathrm{db}$ and delivers cw power at 7 KW. The coupler has good thermal isolation between the 2 and 300 K sections due to vacuum separation. Only one single hybrid mode can propagate through each section, and no higher order mode is coupled. The analytical and numerical analysis for this coupler is given and the design is optimized. The coupling mechanism to the cavity is also discussed.

We present the applications of advanced RF acceleration to X-ray sources. We give an overview of high-gradient research activities and its applications to extremely high brightness electron guns and short period rf undulators. Article not available.

We propose to couple two coaxial waveguides by 2D periodic perforation in the common wall in order to ensure selective coupling between two waves propagating in these waveguides. In the experiment at a frequency of 10 GHz, the H 01 mode of the internal waveguide was converted to the rotating H 51 mode of the external waveguide with a power loss of several percent.

A new compact and wide-band waveguide dual circular polarizer at Ka-band is presented and tested in this paper. This compact structure is composed of a three-port polarizing diplexer and a circular polarizer realized by a simple pair of large grooves. The polarizing diplexer includes two rectangular waveguides with a perpendicular H-plane junction, one circular waveguide coupled in E- plane. A cylindrical step and two pins are used to match this structure. For a LHCP or RHCP wave in the circular port, only one speciflc rectangular port outputs power and the other one is isolated. The accurate analysis and design of the circular polarizer are conducted by using full-wave electromagnetic simulation tools. The optimized dual circular polarizer has the advantage of compact size with a volume smaller than 1.5‚ 3 , broad bandwidth, uncomplicated structure, and is especially suitable for use at high frequencies such as Ka-band and above. The prototype of the polarizer has been manufactured and test, the experimental results are basically consistent with the theories.

A fundamental element of the CLIC concept is two-beam acceleration, where RF power is extracted from a high current, low energy drive beam in order to accelerate the low current main beam to high energy. The CLIC Power Extraction and Transfer Structure (PETS) is a passive microwave device in which bunches of the drive beam interact with the constant impedance of the periodically loaded waveguide and excite preferentially the synchronous mode. The RF power produced is collected downstream of the structure by means of the RF power extractor; it is delivered to the main linac using the waveguide network connecting the PETS to the main CLIC accelerating structures. The PETS should produce 135 MW at 240 ns RF pulses at a very low breakdown rate: BDR < 10{sup -7}/pulse/m. Over 2010, a thorough high RF power testing program was conducted in order to investigate the ultimate performance and the limiting factors for the PETS operation. The testing program is described and the results are presented.

Fabrication technologies for X-band high gradient accelerating structures have been studied at KEK with SLAC, INFN and CERN. A scanning field emission microscope has been developed at KEK for the observation of the microscopic surface defects which may be related to the rf breakdown trigger. We present the progress on the experimental results of studying field emission characteristics by scanning an arbitrary area of 0.5 mm×0.5 mm on OFHC copper surface using a newly developed scanning field emission microscope.

An analytical method to estimate the scattering of RF power into multiple modes inside a nonuniform waveguide using perturbation techniques is presented. This method is quite general, much faster than numerical methods, and may be applied to a wide variety of nonuniform waveguides. We have used this method to design and analyze a three-section mode transformer that converts a TE01 circular waveguide mode to a TE20 rectangular waveguide mode. This mode converter is specially suitable for use in ultra-high-power applications. Due to the complexity of the problem, we have used the symbolic solver Mathematica to produce most of our analytical results. The results match within reasonable accuracy with High Frequency Structure Simulator field solver simulations. We also present the application of this mode converter to variety of ultra-high-power microwave components and experimentally show the performance of these devices.

Purpose: To measure radiation dose in a water-equivalent medium from very high-energy electron (VHEE) beams and make comparisons to Monte Carlo (MC) simulation results. Methods: Dose in a polystyrene phantom delivered by an experimental VHEE beam line was measured with Gafchromic films for three 50 MeV and two 70 MeV Gaussian beams of 4.0–6.9 mm FWHM and compared to corresponding MC-simulated dose distributions. MC dose in the polystyrene phantom was calculated with the EGSnrc/BEAMnrc and DOSXYZnrc codes based on the experimental setup. Additionally, the effect of 2% beam energy measurement uncertainty and possible non-zero beam angular spread on MC dose distributions was evaluated. Results: MC simulated percentage depth dose (PDD) curves agreed with measurements within 4% for all beam sizes at both 50 and 70 MeV VHEE beams. Central axis PDD at 8 cm depth ranged from 14% to 19% for the 5.4–6.9 mm 50 MeV beams and it ranged from 14% to 18% for the 4.0–4.5 mm 70 MeV beams. MC simulated relative beam profiles of regularly shaped Gaussian beams evaluated at depths of 0.64 to 7.46 cm agreed with measurements to within 5%. A 2% beam energy uncertainty and 0.286° beam angular spread corresponded to a maximum 3.0% and 3.8% difference in depth dose curves of the 50 and 70 MeV electron beams, respectively. Absolute dose differences between MC simulations and film measurements of regularly shaped Gaussian beams were between 10% and 42%. Conclusions: The authors demonstrate that relative dose distributions for VHEE beams of 50–70 MeV can be measured with Gafchromic films and modeled with Monte Carlo simulations to an accuracy of 5%. The reported absolute dose differences likely caused by imperfect beam steering and subsequent charge loss revealed the importance of accurate VHEE beam control and diagnostics.

The radial klystron is a multi-dimensional rf source where the beam is generated by a cylindrical gun and it expands along the radial dimension. The space charge repulsion forces are balanced in the azimuthal direction. We present the design of a low-voltage klystron, showing the single cavity stability and high efficiency.

Future Linear Colliders and Accelerators require rf systems and components that are capable of handling hundreds of megawatts power levels at x-band frequencies and higher. Standard rf components that have been in use for a long time such as waveguide bends, directional couplers and hybrids, can not be used because of peak field considerations. Indeed, one has to reinvent most of these components taking into account the constraints imposed by ultra-high-power operation. Here, we present a new design for circular waveguides bends propagating the low-loss TE01 mode. The bend has smooth walls and low field levels. We present a simple synthesis process for designing such a device. The general philosophy of this technique can be applied to other components as well. We describe the detailed design of the bend and compare our design with finite element simulations and experimental data. The bend has very low ohmic losses, and the TE01 mode is transmitted with virtually perfect mode purity.

Purpose: We have experimentally characterized and simulated the dosimetric properties and spatial fidelity of a novel X-ray array source and collimation system called SPHINX that has the potential to generate complex intensity modulated X-ray beams by varying the electron beam intensity only, and without any moving parts like in multi-leaf collimators. Methods: We investigated the spatial fidelity and the X-ray performances of a SPHINX prototype in tungsten, using a Cyber Knife and the experimental high-energy electron beam line at XTA at SLAC National Laboratory. Dose distributions were recorded with gafchromic films, placed at the distal end of SPHINX and at several depths in a solid water phantom. The geometry of SPHINX and of the experimental set-ups was also modeled in Monte Carlo (MC) simulations with the FLUKA code, used to reproduce the experimental results and, after validation, to predict and optimize the performance and design of the SPHINX. Results: The results indicate significant particle leakage through the channels during a single-channel irradiation for high incident energies, followed by a rapid decrease for energies of clinical interest. When the collimator channels are used as target, the photon production increases, however at expense of the beam size that is also enlarged. The illumination of all channels simultaneously shows a fairly even transmission of the beam. Conclusion: With the measurements we have verified the MC models and the uniformity of beam transmission through SPHINX, and we have evaluated the importance of particle leakage through adjacent channels. These results can be used to optimize SPHINX design through the validated MC simulations. Funding: Weston Havens Foundation, Office of the Dean of Medical School and Office of the Provost (Stanford University). Loo, Maxim, Borchard, Tantawi are co-founders of TibaRay Inc. Loo and Tantawi are TibaRay Inc. board members. Loo and Maxim received grants from Varian Medical Systems and RaySearch Laboratory.

In this paper, we discuss a new compact converter, which converts the TE/sub 01/ circular mode into the TE/sub 20/ rectangular mode. Compared to the Marie converter, the new device has a significantly shorter length and is, therefore, more suitable for use in evacuated waveguide systems. Both the design efforts and the experimental testing of the prototype are described. Based on computer simulations, this converter has a center frequency of 17.15 GHz, and the predicted power transfer between the desired modes is 99.9%. Results from the low-power testing of the prototype indicate that when the TE/sub 20/ output mode is split into two equal parts (each in the TE/sub 10/ rectangular mode), the power transferred to each arm of the split differs by less than 0.15 dB over a 200-MHz bandwidth. This is the first experimental study of a converter based on this design. A high-power version of this device is now being fabricated for use under high vacuum conditions.

We report the experimental demonstration of a 5th harmonic mm-wave frequency multiplying vacuum electronic device, which uses an over-moded spherical sector output cavity. In this device, a pencil electron beam is helically deflected in a transverse deflecting cavity before entering the output cavity. No magnetic field is required to focus or guide the beam. We built and tested a proof-of-principle device with an output frequency of 57.12 GHz. The measured peak power was 52.67 W at the 5th harmonic of the drive frequency. Power at the 4th, 6th, and 7th harmonics was 33.28 dB lower than that at the 5th harmonic.

We are developing a standing wave structure with an rf feed to each individual cell. This approach minimizes rf power flow and electromagnetic energy absorbed by an rf breakdown. The objective of this work is a robust high-gradient (above 100 MV/m) X-band accelerator structure.

A design for a compact x-ray light source (CXLS) with flux and brilliance orders of magnitude beyond existing laboratory scale sources is presented. The source is based on inverse Compton scattering of a high brightness electron bunch on a picosecond laser pulse. The accelerator is a novel high-efficiency standing-wave linac and rf photoinjector powered by a single ultrastable rf transmitter at X-band rf frequency. The high efficiency permits operation at repetition rates up to 1 kHz, which is further boosted to 100 kHz by operating with trains of 100 bunches of 100 pC charge, each separated by 5 ns. The entire accelerator is approximately 1 meter long and produces hard x rays tunable over a wide range of photon energies. The colliding laser is a Yb∶YAG solid-state amplifier producing 1030 nm, 100 mJ pulses at the same 1 kHz repetition rate as the accelerator. The laser pulse is frequency-doubled and stored for many passes in a ringdown cavity to match the linac pulse structure. At a photon energy of 12.4 keV, the predicted x-ray flux is 5×10[superscript 11] photons/second in a 5% bandwidth and the brilliance is 2 × 10[superscript 12] photons/(sec mm[superscript 2] mrad[superscript 2] 0.1%) in pulses with rms pulse length of 490 fs. The nominal electron beam parameters are 18 MeV kinetic energy, 10 microamp average current, 0.5 microsecond macropulse length, resulting in average electron beam power of 180 W. Optimization of the x-ray output is presented along with design of the accelerator, laser, and x-ray optic components that are specific to the particular characteristics of the Compton scattered x-ray pulses., United States. Defense Advanced Research Projects Agency (Grant N66001-11-1-4192), National Science Foundation (U.S.) (Grant DMR-1042342), United States. Dept. of Energy (Grant DE-FG02-10ER46745), United States. Dept. of Energy (Grant DE-FG02-08ER41532), Center for Free-Electron Laser Science

Silicon woodpile photonic crystals provide a base structure with which to build a three-dimensional dielectric waveguide system for high-gradient laser-driven acceleration. To realize an on-chip woodpile laser accelerator, a key component is the power coupler to deliver laser power to the fundamental accelerating mode. The woodpile waveguide is periodically loaded in the longitudinal direction; therefore simple cross-sectional mode profile matching is not sufficient to launch the accelerating mode appropriately and will result in significant scattering loss. Several traveling-wave coupler design schemes developed for multicell radio frequency cavity accelerators can be adapted to the woodpile accelerator coupler design. This paper presents design procedures and results using these methods. We present simulations indicating near 100% power transmission between the transverse electric mode of a silicon-guide side coupler and the transverse--magnetic-like accelerating mode of a woodpile waveguide. The coupler launches a full traveling-wave propagation of the accelerating mode, which maintains its propagation quality over long waveguide structures, and provides better tolerance on the structure fabrication uncertainty and material breakdown than standing-wave coupling.

Static magnetic undulators used by x-ray light sources are fundamentally too limited to achieve shorter undulator periods and dynamic control. To overcome these limitations, we report experimental demonstration of a novel short-period microwave undulator, essentially a Thomson scattering device, that has yielded tunable spontaneous emission and seeded coherent radiation. Its equivalent undulator period (λu) is 13.9 mm while it has achieved an equivalent magnetic field of 0.65 T. For future-generation light sources, this device promises a shorter undulator period, a large aperture, and fast dynamic control.

We describe a new concept for a microwave circuit functioning as a charged-particle accelerator at mm wavelengths, permitting an accelerating gradient higher than conventional passive circuits can withstand consistent with cyclic fatigue. The device provides acceleration for multiple bunches in parallel channels, and permits a short exposure time for the conducting surface of the accelerating cavities. Our analysis includes scalings based on a smooth transmission line model and a complementary treatment with a coupled-cavity simulation. We also provide an electromagnetic design for the accelerating structure, arriving at rough dimensions for a seven-cell accelerator matched to standard waveguide and suitable for bench tests at low power in air at 91.392 GHz. A critical element in the concept is a fast mm-wave switch suitable for operation at high power, and we present the considerations for implementation in an H-plane tee. We discuss the use of diamond as the photoconductor switch medium.