RF Structures Research Articles

Design and Test of C-band Linac Prototypes for Electron FLASH Radiotherapy

Flash Therapy is a revolution in cancer cure since it spares healthy tissue from the damage of ionization radiations without decreasing its effectiveness in tumor control. To allow the implementation of the FLASH therapy concept into actual clinical use and treat deep tumors, Very High Electron Energy (VHEE) should be achieved in a range of 50-150 MeV. In the framework of the VHEE project carried out at Sapienza University, in collaboration with INFN, we investigate the main issues in designing a compact C band (5.712 GHz) electron linacs for FLASH Radiotherapy. In this paper, we describe the design strategy, the electromagnetic properties, and the first prototypes of the RF structures to be tested at Sapienza University.

Self-Excitation Thresholds in RF Structures

A theory and general method for calculating self-excitation thresholds for a large class of standing and traveling-wave structures used in amplifiers, klystrons, traveling-wave tubes, and other vacuum electronic devices (VED) is presented. One determines the circuit parameters of RF structures that are changed due to the presence of electron beam and analyze the stability of the obtained matrices. New determinant equations defined by such stability matrices are derived in a general form applicable to any RF structure and as an important special case for RF structures with electron beams. The theory and computational methods described here are essential for stability analysis of RF structures and VED.

Uncertainty Quantification of Printed Microwave Interconnects by Use of the Sparse Polynomial Chaos Expansion Method

The performance of printed RF interconnect structures for microwave applications is highly dependent on their geometric characteristics as well as manufactured pattern fabrication errors/variations, that can only be insufficiently modeled by current simulation techniques. Here, for the first time, the response of such systems is accurately investigated when random fabrication errors are introduced in the geometric-design variables. To analyze these structures in a computationally efficient way, sparse-basis-polynomial-chaos-expansions (SB-PCE) are used to approximate the quantities of interest using a limited number (sparse) of basis functions. The size of the PCE series is reduced by retaining the most important terms. These are computed using the orthogonal-matching-pursuit algorithms from compressed-sensing (CS). In CS, a relatively small number of deterministic model evaluations is needed and a sparse but reliable PCE is found. The SB-PCE method is proven accurate and much more efficient (roughly two orders of magnitude shorter execution time) compared to alternative methods such as the Monte-Carlo (MC) and least-squares-PCE (LS-PCE) enabling for the first time the accurate modeling of realistic RF structures for specific manufacturing process variations. Numerical results for a monolithic-microwave-integrated circuit (MMIC) verify the effectiveness of the method and describes the structure’s performance under uncertainty.

Characterization of Additively Produced RF-Structures on Low-Cost PCB Materials Based on Inkjet Technology for Industrial Applications up to 80 GHz

Usually, the printed circuit board industry has to use special subtractive manufacturing methods, if small line and space dimensions down to 50 μm are desired. This drastically raises production effort and costs. In this paper a method is shown, how the inkjet technology can be used to produce passive RF structures with minimum line and space dimensions of 25 μm. For the first time, this is possible over complete panels of low-cost PCB substrates, which have a relatively high surface roughness compared to ceramic or glass carriers. To ensure process reliability, printed lines must also be plated with an additional electroless copper layer to overcome the surface roughness with conductive layers and to lower insertion loss in the two-digit GHz range. Additionally, a surface impedance model is developed, which allows broadband first time right simulations. Finally, additively produced, passive RF filters are compared to their subtractive counterparts to show their benefits regarding production costs and electrical performance up to 80 GHz.

Design and Analysis of Width-Modulated Two-Section Sine Waveguide for G-Band Traveling-Wave Tube

A novel width-modulated sine waveguide traveling-wave tube (SWGTWT) is proposed for G-band operation. The broadband single-section and two-section SWGTWT are analyzed, which has the potential to address various technological challenges in the THz regime, such as fabrication difficulty, reflection, and attenuation of RF power. The two-section design has separate RF structures for input and output sections of the TWT, which helps to eliminate undesired reflection during high-power operation. The width modulation is employed to enhance the interaction impedance and also to realize the flat dispersion characteristic of the slow wave structure (SWS). The fabrication feasibility of the structure is demonstrated by fabricating the scaled structure at the Ku-band. A 20.4-kV, 50-mA cylindrical beam is employed in the particle-in-cell (PIC) simulation. The single-section structure offers a maximum power of 53 W with a 5.19% efficiency at 210 GHz. The peak power and a 3-dB bandwidth for the two-section structure are estimated to be 38.9 W and 32.5 GHz, respectively. The structure produces greater than 10 W of output power over a 46-GHz frequency band covering the range of 200–246 GHz.

X -band linac design

In order to explore physics in the EUV to soft x-ray region, we have designed a machine which is capable of accelerating a $\ensuremath{\sim}250\text{ }\text{ }\mathrm{pC}$ electron bunch to an energy of $\ensuremath{\sim}1\text{ }\text{ }\mathrm{GeV}$. The front end of the CLARA (Compact Linear Accelerator for Research and Applications) system at Daresbury Labs will be used as an S-band injector of $\ensuremath{\sim}180\text{ }\text{ }\mathrm{MeV}/\mathrm{c}$, sub-ps FWHM, $\ensuremath{\sim}250\text{ }\text{ }\mathrm{pC}$ electron bunches into the XARA (X-Band Accelerator for Research and Application) system. A rf feasibility study has been carried out for a structure operating in the $2\ensuremath{\pi}/3$ mode at a frequency of 11.9942 GHz which is fed by a SLED klyston setup. The average cell of this structure has an iris radius of $⟨a⟩=3.2\text{ }\text{ }\mathrm{mm}$ and a shunt impedance of $⟨{R}_{s}⟩=106.55\text{ }\text{ }\mathrm{M}\mathrm{\ensuremath{\Omega}}/\mathrm{m}$. A high target gradient of $80\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ for a single-bunch operation of the linac is necessary due to spatial constraints at Daresbury Labs. We have also implemented Gaussian detuning of the linac in order to future-proof the project for potential multibunch operation of the machine. After combining the rf study with an analysis of the uncoupled long-range wakefield and the short-range transverse wakefields, the optimal structure parameters are outlined as a compromise between the shunt impedance, electrical breakdown rate and wakefields in the structure. As novel designs will be tested using this free-electron laser (FEL) an increased beam charge may be useful. Therefore a beam dynamics study via the particle tracking code elegant has been performed to assess how the beam quality evolves while traveling through the XARA rf structures for different bunch charges and beam offsets. These simulations reveal how the bunch is disturbed for varying bunch charges and offsets and give an initial indication of how sensitive the beam parameters (beam centroid position, emittance, RMS beam size, etc.) are to the wakefields generated in XARA. An analytical formulation of the beam motion as it travels through the XARA linac has been utilized to calculate the emittance growth. This allows for comparison between analytical and numerical simulation of the beam dynamics to give confidence in the results. The beam dynamics study shows that for a bunch charge of ${Q}_{b}=250\text{ }\text{ }\mathrm{pC}$ and a beam offset of ${C}_{x}={\ensuremath{\sigma}}_{z}/2=0.253\text{ }\text{ }\mathrm{mm}$, the normalized emittance growth at the end of the XARA linac is $\mathrm{\ensuremath{\Delta}}{\ensuremath{\epsilon}}_{Nx}/{\ensuremath{\epsilon}}_{Nx}\ensuremath{\sim}270%$. This may be mitigated by the design of a magnetic lattice which would include kickers to recenter the beam on the electrical axis of the structure, reducing the effect of the wakefields on the bunch. Currently it is probable that a maximum bunch charge of $\ensuremath{\sim}250\text{ }\text{ }\mathrm{pC}$ will be utilized for this machine; however a future design of a magnetic focusing lattice may allow higher charges to be viable and will reduce the emittance growth.

Atomistic Modeling of the Coupling Between Electric Fields and Bulk Plastic Deformation in Rf Structures

Atomistic Modeling of the Coupling Between Electric Fields and Bulk Plastic Deformation in Rf Structures

Multi-Objective Optimization of RF Structures

Multi-Objective Optimization of RF Structures

Versatile, high brightness, cryogenic photoinjector electron source

Since the introduction of the radio-frequency (rf) photoinjector electron source over thirty years ago, peak performance demands have dictated the use of high accelerating electric fields. With recent strong advances in obtainable field values, attendant increases in beam brightness are expected to be dramatic. In this article, we examine the implementation of very high gradient acceleration in a high frequency, cryogenic rf photoinjector. We discuss in detail the effects of introducing, through an optimized rf cavity shape, rich spatial harmonic content in the accelerating modes in this device. Higher spatial harmonics give useful, enhanced linear focusing effects, as well as potentially deleterious nonlinear transverse forces. They also serve to strongly increase the ratio of average accelerating field to peak surface field, thus aiding in managing power and dark current-related challenges. We investigate two scenarios which are aimed at unique exploitation of the capabilities of this source. First, we investigate the obtaining of extremely high six-dimensional brightness for advanced free-electron laser applications. We also examine the use of a magnetized photocathode in the device for producing unprecedented low asymmetric emittance, high-current electron beams that reach linear collider-compatible performance. As both of the scenarios demand an advanced, compact solenoid design, we describe a novel cryogenic solenoid system. With the high field rf and magnetostatic structures introduced, we analyze the collective beam dynamics in these systems through theory and multi-particle simulations, including a particular emphasis on granularity effects associated with microscopic Coulomb interactions.

Frequency jump using 704.4 MHz radio-frequency quadrupole and cross-bar H -type drift tube linear accelerators

A large-scale proton or ${\mathrm{H}}^{\ensuremath{-}}$ accelerator often requires a jump in radio frequency, typically in the beam velocity range of $\ensuremath{\beta}=0.4--0.6$. The MYRRHA linac has two frequency jumps: one at $\ensuremath{\beta}\ensuremath{\cong}0.2$ and another one in the typical beam velocity range at $\ensuremath{\beta}=0.56$. Using the MYRRHA linac as an example, this study investigates two new solutions, whereby the frequency jump in the range of $\ensuremath{\beta}=0.4--0.6$ is eliminated. A single frequency jump at $\ensuremath{\beta}\ensuremath{\cong}0.2$ can shorten the whole linac considerably and reduce the construction and operation costs. The proposed frequency jump sections will use 704.4 MHz radio-frequency quadrupole and cross-bar $H$-type drift tube linear accelerators. To ensure a safe and reliable cw operation at such very high frequency for these kinds of accelerating structures, careful design studies with respect to beam dynamics and rf structures including water-cooling channels have been performed. The results demonstrated the feasibility of both solutions.

Evaluating Optically Transparent Conductors at RF and Microwave Frequencies: A New Figure of Merit

Optically transparent conductors (TCs) have been implemented and adopted into next-generation transmission lines and other RF structures from UHF to millimeter-wave (mmWave) applications. A current method of evaluating the performance of optically TCs uses a figure of merit (FOM) which is frequency agnostic. A new FOM for optically TCs at RF, microwave, and millimeter-wave frequencies is presented and provides distinction to the TC FOM proposed in 1976 where it adds a frequency dependence, by incorporating the skin effect requirement. A detailed analysis of the new FOM for both transparent conducting oxides and mesh conductors is presented and compared using TC literature data analyzed at 24 and 240 GHz where large FOM variation is shown. The manuscript shows the new RF FOM and classical FOM can vary by up to a factor of 10 when the skin depth of the conductor is not considered.

Surface roughness and finishing techniques in selective laser melted GRCop-84 copper for an additive manufactured lower hybrid current drive launcher

Recent advances in selective laser melting (SLM) 3D printing technology allows additive manufacture of lower hybrid current drive (LHCD) RF launchers from a new material, Glenn Research Copper 84 (GRCop-84), a Cr2Nb (8 at. % Cr, 4 at. % Nb) precipitation hardened alloy, in configurations unachievable with conventional machining. Rough surfaces in additive manufactured components are a limiting factor in RF structures. Surface roughness increases RF losses and impedes conditioning by trapping gas and contaminants that induce arcing by evolving from the surface at high power. SLM printed GRCop-84 is post-processed with mass finishing to reduce surface roughness. Wet blasting, vibratory finishing, and chemical etching remove adhered powder grains from SLM printed GRCop-84. Profilometry and scanning electron microscopy were used to quantify resulting surface quality. Chemical etching is examined to remove zinc contamination from wire electrical discharge machining slag prior to vacuum use. Etch rates in SLM printed GRCop-84 are anisotropic, and etch times should be limited to prevent pitting corresponding to the laser hatch pattern. Vibratory mass finishing and mechanical polishing produced surfaces suitable for low RF loss. Use of chemo-mechanical finishing is recommended for the interior of SLM printed structures.

High gradient ultra-high brightness C-band photoinjector optimization

Cathode, laser system, high RF peak field, and gun solenoid are fundamental aspects that have to be optimized to produce high brightness beam in photoinjectors. In order to produce ultra-high brightness beams of about one order of magnitude more than the present devices, new studies have been done in the last years. One promising approach is to increase the peak field on the cathode surface. In this paper we will present a study on a C-band gun using a cathode peak field of 240 MV/m. A first RF study to reach such very high field was done, by means of shortening the RF pulse duration. A first design of the RF structures and solenoid was carried out. If we will not reach exactly the predicted gradient during the RF tests, the impact on the beam dynamics have been simulated reducing the peak field on the cathode up to the value of 130 MV/m, maintaining the same geometry. The convenience of such gun also at a lower gradient, opened the way to possible applications on light production devices. For example, this C-band injector is one of the main options to guide the X-band linac in the CompactLight design study, where an X-ray FEL is foreseen.

Design and Fabrication of 3D Printed Reconfigurable Metamaterial-inspired Structures

Abstract In this paper, 3D printing is used as an alternative manufacturing technique to fabricate metamaterial-inspired RF structures for liquid profiling. A dual split-ring resonator (SRR) based sensor tag is designed and integrated with a microfluidic channel for detecting different liquid samples. The sensor is 3D printed using a high-temperature resin and metallized using a custom developed metal patterning process. The sensor requires a very small volume of 8.6 μL of sample under test for detection. The resonance frequency of the SRR changes with change in sample loading and the shift is monitored for sample profiling. Different volatile organic compounds are introduced and the shift is monitored demonstrating the sensitivity of the proposed tag. The low-cost, real-time nature of the tag makes it an ideal choice for monitoring liquids along the supply chain.

Inkjet-/3D-/4D-Printed Wireless Ultrabroadband Modules for IoT, Smartag and Smart City Applications

This publication provides an overview of additive manufacturing techniques including Inkjet, 3D and 4D printing methods. The strengths, opportunities and advantages of this array of manufacturing techniques are evaluated at different scales. We discuss first the applicability of additive manufacturing techniques at the device scale including the development of origami inspired tunable RF structures as well as the development of skin-like conformal, flexible systems for wireless/IoT, Smartag and smart city applications. We then discuss application at the package scale with on package printed antennas and functional packaging applications. Following this, there is a discussion of additive manufacturing techniques in applications at the die scale such as 3D printed interconnects. The paper is concluded with an outlook on future advancements at the component scale with the potential for fully printed passive components.

Numerical investigations into a THz-driven dielectric accelerator with a Bragg reflector

Abstract Using THz pulses to generate an accelerating field in a dielectric loaded accelerator structure is a promising way to mitigate the disadvantages of RF and Laser accelerator structures. This paper proposes an energy efficient side-coupling dielectric single-grating structure accompanied with a multi-layer Bragg reflector that is designed for THz pulse operation. To show the comparative advantages of the new design, we also numerically investigate a single-side coupling THz-driven dual-grating structure which is composed of Silicon Dioxide (SiO2). Simulation results show that the reflector structure effectively manipulates the THz field compared to dual-grating structure, not only boosts the field strength in the accelerating channel, but also optimizes the field distribution and generates a periodic field reversal to achieve better phase synchronicity for relativistic particles, thereby increasing the accelerating gradient by more than 40%. It is also found that the energy-recycling capability of a Bragg reflector highly depends on the operating THz pulse duration and beam channel width. Choosing a longer pulse duration and setting a narrower beam channel width with an optimum pillar height can increase the single pulse efficiency by more than 70% compared to the bare dual-grating structure.

3-D-Printing-Based Selective-Ink-Deposition Technique Enabling Complex Antenna and RF Structures for 5G Applications up to 6 GHz

This paper introduces a novel additive-manufacturing technique to obtain high-resolution selective-ink-deposition on complex 3-D objects, packages, and modules for 5G applications. The technique consists of embossing the desired pattern directly on the 3-D printed dielectric surface and then applying ink with a suitable tool. This approach is tested in combination with stereolithography 3-D printing technology to obtain selectively metallized 3-D circuits. In particular, the “clear” resin from FormLab is utilized for the 3-D printed dielectric, while the metallization is performed with silver nanoparticle ink from Suntronic. As a preliminary study, test samples containing lines with different widths are manufactured, demonstrating a pitch down to $135~\mu \text {m}$ and satisfactory sheet resistance of $0.011~\Omega /\text {sq.}$ (the electromagnetic characterization of the dielectric resin is reported in the Appendix). Then, two broadband multiport RF structures are developed to show the versatility of the proposed technology. First, an ultrawideband 3-D crossover, operating in the range 100 MHz–5 GHz, is conceived to test the suitability of the proposed technology to perform selective metallization on curved semienclosed areas. Then, the technology is applied to a multiple-input–multiple-output (MIMO) antenna system, based on four proximity-fed annular slot antennas, arranged on the lateral sides of a cube and decoupled by introducing a cross-shaped structure in the interior of the cube. This circuit offers a broad range of metallization challenges, as it features embossed and engraved parts, high-resolution patterns (line widths down to 0.7 mm) and sharp edges. Each slot radiates unidirectionally with the same polarization and uses the cube and its internal cross-shaped structure as a resonant cavity. The antenna system is designed to operate in the band 3.4–3.8 GHz, which is one of the sub-6-GHz 5G bands in Europe, and it is thought for hotspot and access-point applications. The final antenna topology is composed of only two blocks, weighs 21.29 g, and occupies a volume of $44.4\times 45.8\times 45.8\,\,\text {mm}^{3}$ , featuring an envelope correlation coefficient (ECC) lower than 0.005 and a total active reflection coefficient (TARC) lower than −6 dB in all the bands of interests.

Calculation and Application of Impedance Matrices for Vacuum Electronic Devices

We present a computationally efficient method for the calculation of impedance (Z-) matrices for a large class of standing- and traveling-wave structures used in klystrons, traveling-wave tubes, and other RF vacuum electronic devices. We apply joining formulas by which the Z-matrix of a structure can be constructed from the Z-matrices of its component parts without a full-scale finite-element (FE) electromagnetic (EM) simulation of the whole structure. For the inverse operation, we apply subtraction formulas that define the Z-matrix of a structure from which a selected section has been removed. New Z-matrix modification equations are derived which determine the Z-matrix of a modified structure, without recomputing the entire structure Z-matrix. The approach and examples are extended to multibeam RF structures. The obtained Z-matrices are suitable for the large-signal 1-D and 2-D beam-wave interaction codes CHRISTINE-CC and TESLA-Z. Application of the techniques described here greatly facilitates the accurate calculation of Z-matrices and optimization of large, complex circuits that are difficult to model whole in a single FE simulation.

Next generation high brightness electron beams from ultrahigh field cryogenic rf photocathode sources

Recent studies of the performance of radio-frequency (rf) copper cavities operated at cryogenic temperatures have shown a dramatic increase in the maximum achievable surface electric field. We propose to exploit this development to enable a new generation of photoinjectors operated at cryogenic temperatures that may attain, through enhancement of the launch field at the photocathode, a significant increase in five-dimensional electron beam brightness. We present detailed studies of the beam dynamics associated with such a system, by examining an S-band photoinjector operated at $250\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ peak electric field that reaches normalized emittances in the 40 nm-rad range at charges (100--200 pC) suitable for use in a hard x-ray free-electron laser (XFEL) scenario based on the LCLS. In this case, we show by start-to-end simulations that the properties of this source may give rise to high efficiency operation of an XFEL, and permit extension of the photon energy reach by an order of magnitude, to over 80 keV. The brightness needed for such XFELs is achieved through low source emittances in tandem with high current after compression. In the XFEL examples analyzed, the emittances during final compression are preserved using microbunching techniques. Extreme low emittance scenarios obtained at pC charge, appropriate for significantly extending temporal resolution limits of ultrafast electron diffraction and microscopy experiments, are also reviewed. While the increase in brightness in a cryogenic photoinjector is mainly due to the augmentation of the emission current density via field enhancement, further possible increases in performance arising from lowering the intrinsic cathode emittance in cryogenic operation are also analyzed. Issues in experimental implementation, including cavity optimization for lowering cryogenic thermal dissipation, external coupling, and cryocooler system, are discussed. We identify future directions in ultrahigh field cryogenic photoinjectors, including scaling to higher frequency, use of novel rf structures, and enabling of an extremely compact hard x-ray FEL.

Observation of multipath effects in the detection of RF-modulated x-rays

As part of the Air Force Research Laboratory’s ongoing research in particle accelerators and beams, we recently began investigating the interaction of RF-modulated x-ray beams with RF structures. These bremsstrahlung x-rays, produced using an S-band RF electron linac and tungsten foil, were able to drive RF signals at the linac frequency and its integer harmonics in an S-band waveguide. This was interpreted as a result of secondary electron production inside the waveguide by the modulated x-ray beam and implied that the primary electron beam RF modulation was preserved, to at least some degree, through both the processes of x-ray and secondary electron production. The strength of these signals induced on the waveguide varied as the distance of the waveguide from the tungsten foil changed, with the nature of these changes suggesting the existence of an interference process resulting from multipath propagation of the modulated x-rays. Here, we report on our initial attempt to verify this multipath propagation by altering the configuration of shielding and reflective masses in the environment of the waveguide. These measurements support the existence of a multipath propagation effect and further indicate that the RF modulation of the x-rays is preserved as they undergo scattering events inside the accelerator vault.