HFSS Code Research Articles

Design and optimization of THz coupling in zirconia MAS rotors for dynamic nuclear polarization NMR

We present 3D electromagnetic simulations of the coupling of a 250 GHz beam to the sample in a 380 MHz DNP NMR spectrometer. To obtain accurate results for magic angle spinning (MAS) geometries, we first measured the complex dielectric constants of zirconia, sapphire, and the sample matrix material (DNP juice) from room temperature down to cryogenic temperatures and from 220 to 325 GHz with a VNA and up to 1 THz with a THz TDS system. Simulations of the coupling to the sample were carried out with the ANSYS HFSS code as a function of the rotor wall material (zirconia or sapphire), the rotor wall thickness, and the THz beam focusing (lens or no lens). For a zirconia rotor, the B1 field in the sample was found to be strongly dependent on the rotor wall thickness, which is attributed to the high refractive index of zirconia. The optimum thickness of the wall is likely due to a transmission maximum but is offset from the thickness predicted by a simple calculation for a flat slab of the wall material. The B1 value was found to be larger for a sapphire rotor than for a zirconia rotor for all cases studied. The results found in this work provide new insights into the coupling of THz radiation to the sample and should lead to improved designs of future DNP NMR instrumentation.

Operational requirements of the ion cyclotron wall conditioning in DTT

The Divertor Tokamak Test facility (DTT), a new superconducting tokamak (B0=6T, Ip=5.5MA, R0=2.19 m, a = 0.7 m, pulse length=100 s) (Ambrosino et al., 2021) under realization at the ENEA Frascati Research centre, needs an efficient first wall conditioning technique suitable for the use in the presence of a permanent magnetic field. The Ion Cyclotron Wall Conditioning (ICWC), based on Radio-Frequency (RF) discharges (Lyssoivan et al., 2012), with waves in the ion cyclotron range of frequencies, is one of the most efficient technique available (between discharges) for routine inter–pulse and overnight conditioning. ICWC will be performed using the first Ion Cyclotron Resonance Heating (ICRH) RF module foreseen during the first phase of DTT and constituted by two Antennas, four or two External Matching Units (EMUs) and four RF solid-state transmitters, operating in the frequency range (60–90) MHz. In the paper by means of the HFSS and CST codes, an eigenvalue analysis of the electromagnetic waves (E/H modes) excited inside the vacuum DTT volume has been carried out. Then full-wave simulations have been run for the entire DTT torus injecting a RF signal with a single and a pair of three–strap antennas, currently in the design phase for DTT, evaluating the S-parameters and the parallel electric field (E//) aligned to the toroidal magnetic field, responsible of the initial gas breakdown. On this basis the most appropriate choice for the RF feeding, in monopole phasing, as well as for the frequency of ICWC discharges in order to maximize E// has been identified and presented in the paper, demonstrating for the first time the usefulness of using an H mode at the frequency of 62.5 MHz instead of the lower frequency of 60 MHz minimizing the RF power required to activate the ionization process. Finally operational parameters for DTT, extrapolated from the analysis of ICWC experiments documented in literature, will be presented.

Preliminary analysis of the ICRF launcher for DTT

The paper reports the preliminary analysis of different typologies of ICRH launchers for chosing the most efficient solution for the ICRH system of the Divertor Tokamak Test facility (DTT), designed by the Italian DTT Limited Liability Consortium (S.C. a r.l.). In its final configuration this system will couple to the DTT plasma a nominal power of 6 MW in the 60 – 90 MHz frequency range by means of four launchers. This very preliminary analysis has been done with the ANSYS HFSS code.

RF network analysis of the WEST TWA mock-up in TITAN

WEST represents the ideal device to test a reactor relevant travelling wave array (TWA) due to its long pulse capability and full metal wall configuration. Moreover, the already installed ICRF high power launchers will allow a direct comparison between a classical in-port antenna and the TWA. The design and exploitation of a high power (2 MW) TWA mock-up to be tested in TITAN represent the first step towards its integration in WEST. This paper presents the progress in the RF characterization of this high power mock-up. An integrated modelling scheme based on the full-wave code HFSS and on Python packages for RF circuit simulations is developed and used to parametrically analyse the response of the mock-up inside TITAN. The sensitivity of the antenna to geometrical deformations, e.g. thermal dilatation or manufacturing tolerances, is analysed. The use of trimmers to compensate those deformations is discussed and results from a manufactured trimmer test-bed are presented. A diagnostic system is proposed to measure electric field and current on the straps. Aspects like sensitivity and calibration are briefly discussed. In conclusion, the next steps of the project are outlined.

New Analytical derivation of Group Velocity in TW accelerating structures

Ultra high-gradient accelerating structures are needed for the next generation of compact light sources. In the framework of the Compact Light XLS project, we are studying a high harmonic traveling-wave accelerating structure operating at a frequency of 35.982 GHz, in order to linearize the longitudinal space phase. In this paper, we propose a new analytical approach for the estimation of the group velocity in the structure and we compare it with numerical electromagnetic simulations that are carried out by using the code HFSS in the frequency domain.

Quasi-3-D Modeling of Higher Order Mode Instability in a Two-Gap Input Cavity of Broadband MBK

We present the results of our comprehensive study of the stability of a two-gap input cavity used in an experimental broadband multiple-beam klystron (MBK) with 18 beams and 7 cavities, which was designed for projected >500 kW maximum output power and ~13% bandwidth (so-called NRL MBK-3 design). The device was experimentally observed to be prone to self-excitation at all beam voltages below its designed operational value of ~42 kV. Postexperiment examination of the two-gap input cavity revealed traces of breakdown on its face. Detailed simulations performed using a 3-D computational electromagnetic code HFSS have shown presence of a few high-Q higher order modes (HOMs), which could explain self-excitation of the two-gap input cavity in a wide range of beam voltages. To explore HOM instability of the two-gap input cavity of the MBK in greater details, we applied the code TESLA-MB, which represents a parallel extension of the 2-D large-signal klystron code TESLA and which is suitable for an accurate modeling of multiple-beam devices with unique distributions of R/Qs over gaps in all its beam-tunnels.

Design studies on an s-band hybrid accelerating structure

In an electron linac, the standard bunching system is generally composed of a standing-wave (SW) pre-buncher (PB), a travelling-wave (TW) buncher and a standard TW accelerating structure. To keep the beam performance similar to the standard bunching system but reduce the cost as much as possible, the hybrid accelerating structure (HAS) was proposed by integrating the PB, the B and the accelerating structure together. The RF design of the HAS was conducted by 2D code SUPERFISH, which is used to determine the dimensions of the SW and TW cells, and 3D code HFSS, which is used to optimize the input and output RF coupler cells. The on-axis electric field distribution simulated by HFSS can fully meet the dynamic requirement. The phase jump in SW section is 180 $$^\circ $$ while the phase advance in TW section is 120 $$^\circ $$ . The RF design of the HAS is completed, and the engineering design of the prototype is being conducted.

Design of mid-plane passive active multijunction antenna for 5-GHz KSTAR LHCD system

The major upgrade of KSTAR LHCD system to 4MW is planned in 2021. 4-MW RF power will be provided by 8×0.5MW klystrons. Highly oversized circular waveguide transmitting TE01° mode will be adopted for the low loss transmission line longer than 50m. The design of the 4-MW mid-plane Passive Active Multijunction launcher has been performed. Considering parallel refractive index N|| for the efficient current drive and the maximum power density at the launcher mouth, the launcher is composed of 256 active waveguides whose dimension is 58mm x 7mm and 18mm of spatial period for N||0=2.5. Multijunction with Emax <3.5kV/cm and high power waveguide components have been designed using HFSS code. The antenna properties were evaluated using ALOHA. Prototype PAM launcher with 32 active waveguides is under development for 0.5-MW pulsed operation.

Modeling Vacuum Electronic Devices Using Generalized Impedance Matrices

We have developed a general approach to modeling a large class of vacuum electronic devices (VEDs) by using impedance matrices to characterize the circuit structure. Our approach can treat VEDs that have an arbitrary number of interaction gaps, severs, and input and/or output ports that may incorporate arbitrarily complex matching and/or tuning elements and windows. To find the impedance matrix for a given structure, we use the computational electromagnetics 3-D finite-element code HFSS to compute the response of the entire structure to an excitation of each individual port and gap. We define voltages and currents as certain integrals over the electric and magnetic fields, respectively, the ratios of which are elements of the generalized impedance matrix. This matrix is then imported into a beam-wave interaction code, which is used to compute VED performance (gain, output power, bandwidth, and so on). We have implemented this capability in a new 2-D code TESLA-Z, which has been verified by comparison with the large-signal code TESLA-FW and then validated by comparison with measured data from a Ka-band folded-waveguide power-booster TWT. Similar capability was also implemented in the 1-D interaction code CHRISTINE-CC.

Design of a Broadband E-Plane Power Combiner Based on Quarter-Arc Bent Rectangular Waveguides for Sub-THz and THz Wave**Supported by the National Natural Science Foundation of China under Grant No 11075032, and the Fundamental Research Funds for the Central Universities under Grant No ZYGX2014J033.

A method of designing an E-plane power combiner composed of two quarter-arc bent rectangular waveguides is proposed for sub-THz and THz waves. The quarter-arc bent-waveguide power combiner has a simple geometry which is easy to design and fabricate. By HFSS codes, the physical mechanism and performance of the power combiner are analyzed, and the relationship between the output characteristics and the structure/operating parameters is given. Simulation results show that our power combiner is suitable for the combining of two equal-power and reversed-phase signals, the bandwidth of the combiner is wide and can be adjusted by the radius of the quarter-arc, and the isolation performance of the combiner can be improved by adding thin film resistive septa at the junction of two quarter-arc bent waveguides. Meanwhile, an approximate method based on the analytic geometrical analysis is given to design this power combiner for different frequency bands.

Input Coupling for Photonic Bandgap Fiber Accelerators

Dielectric hollow-core optical fibers have been proposed as a means of confining a traveling wave mode in order to produce a micron-scale laser-driven particle accelerator. A calculation of the power coupling to the confined accelerator mode for this type of structure is complicated by the presence of unconfined or leaky modes. We propose a computational method for resolving this issue, and evaluate the coupling efficiency of laser light into several proposed structure geometries using the frequency-domain code HFSS.

Design of a Traveling-Wave Slot Array Power Divider Using the Method of Moments and a Genetic Algorithm

A 1-to-25-way uniform amplitude and linear phase power divider, using slotted rectangular waveguides in the traveling-wave mode, is designed to cover the 71–77-GHz band. The design employs the scattering parameters of three-port couplers computed from the method of moments (MoM) solution to the integral equation of the slot aperture field. Subsequent genetic algorithm (GA) optimization is aimed at maximizing the return loss and combining efficiency and flatness of coupling amplitudes in the whole frequency band. GA utilizes the MoM solution to the coupled integral equations of slot apertures in the entire power divider structure. Simulations indicate the optimized power divider yielding better than 26-dB return loss and greater than 86% efficiency in the 71–77-GHz band for the power divider combiner connected back to back. Computed MoM results have been validated by the commercial code HFSS and also by experimentally measured results. The measured insertion loss of two back-to-back divider–combiners is about 2 dB.

Modeling of the interaction of a volumetric metallic metamaterial structure with a relativistic electron beam

We present the design of avolumetric metamaterial (MTM) structure and its interaction with a relativistic electron beam. This novel structure has promising applications in particle beam diagnostics, acceleration, and microwave generation. The volumetric MTM has a cubic unit cell allowing structures of arbitrary size to be configured as an array of identical cells. This structure allows the exploration of the properties of a metamaterial structure without having to consider substrates or other supporting elements. The dispersion characteristics of the unit cell are obtained using eigenmode simulations in the HFSS code and also using an effective medium theory with spatial dispersion. Good agreement is obtained between these two approaches. The lowest-order mode of the MTM structure is found to have a negative group velocity in all directions of propagation. The frequency spectrum of the radiation from a relativistic electron beam passing through the MTM structure is calculated analytically and also calculated with the CST code, with very good agreement. The radiation pattern from the relativistic electron beam is found to be backward Cherenkov radiation, which is a promising tool for particle diagnostics. Calculations are also presented for the application of a MTM-based wakefield accelerator as a possible all-metal replacement for the conventional dielectric wakefield structure. The proposed structure may also be useful for MTM-based vacuum electron devices for microwave generation and amplification.

Analysis and Suppression of High-Order Mode Oscillation in an S-Band Klystron

High-order mode oscillation occurred in an S-band experimental klystron with 300 MHz instantaneous bandwidth. The cause of the oscillation was analyzed and simulated. A method was proposed to suppress the oscillation through adjusting the structure and size of a resonant cavity in the tube, in which the high-order mode frequency can obviously be changed and the variation of the fundamental mode frequency is very small. Improved designs for the resonant cavity with Ansoft HFSS code and corresponding experiment have been performed, which shows that the simulation results are in good agreement with the experimental measurements. The testing result of an experimental klystron with this improved resonant cavity indicates that the high-order mode oscillation can effectively be suppressed.

Sub-wavelength waveguide loaded by a complementary electric metamaterial for vacuum electron devices

We report the electromagnetic properties of a waveguide loaded by complementary electric split ring resonators (CeSRRs) and the application of the waveguide in vacuum electronics. The S-parameters of the CeSRRs in free space are calculated using the HFSS code and are used to retrieve the effective permittivity and permeability in an effective medium theory. The dispersion relation of a waveguide loaded with the CeSRRs is calculated by two approaches: by direct calculation with HFSS and by calculation with the effective medium theory; the results are in good agreement. An improved agreement is obtained using a fitting procedure for the permittivity tensor in the effective medium theory. The gain of a backward wave mode of the CeSRR-loaded waveguide interacting with an electron beam is calculated by two methods: by using the HFSS model and traveling wave tube theory; and by using a dispersion relation derived in the effective medium model. Results of the two methods are in very good agreement. The proposed all-metal structure may be useful in miniaturized vacuum electron devices.

Theory and Simulation of Arbitrarily Shaped Groove Staggered Double Grating Array Waveguide

A theoretical model for the arbitrarily shaped groove staggered double grating array waveguide (SDGAW) slow-wave structure is proposed in this paper. The boundary of the groove was approximately replaced by a series of steps and the formulas of dispersion and interaction impedance were obtained utilizing the field matching method combined with the continuity of transverse admittance. Considering the traditional single mode approximation (SMA) model is not so accurate for describing the high frequency (HF) characteristics, a multiple modes approximation (MMA) model is established, which takes account for the higher order terms in the grooves, thus greatly improves the accuracy of the numerical calculations. As an example, the HF characteristics of a cosine groove SDGAW for a 0.22-THz traveling wave tube were calculated utilizing the SMA model, the MMA model, and HFSS code, the results were compared and discussed. Comparison of the HF characteristics among several kinds of groove shaped SDGAWs has been made by numerical calculation.

Studies on the coupling transformer to improve the performance of microwave ion source.

A 2.45 GHz microwave ion source has been developed and installed at the Variable Energy Cyclotron Centre to produce high intensity proton beam. It is operational and has already produced more than 12 mA of proton beam with just 350 W of microwave power. In order to optimize the coupling of microwave power to the plasma, a maximally flat matching transformer has been used. In this paper, we first describe an analytical method to design the matching transformer and then present the results of rigorous simulation performed using ANSYS HFSS code to understand the effect of different parameters on the transformed impedance and reflection and transmission coefficients. Based on the simulation results, we have chosen two different coupling transformers which are double ridged waveguides with ridge widths of 24 mm and 48 mm. We have fabricated these transformers and performed experiments to study the influence of these transformers on the coupling of microwave to plasma and extracted beam current from the ion source.

Development of C band energy doubler

A C band(5712 MHz) energy doubler (SLED) is successfully developed, which is the first one in China. This paper presents the design, simulation and optimization about the resonant cavity, coupling irises and 3 dB directional coupler using HFSS and CST code. The SLED is tested in low RF power, the measurement results are very well and all the technical parameters are reached or exceeded.

Field theory of a terahertz staggered double-grating arrays waveguide Cerenkov traveling wave amplifier

Based on a rectilinear sheet electron beam propagating through the tunnel of a staggered double-grating arrays waveguide (SDGAW) slow-wave structure (SWS), a three dimensional field theory for describing the modes and the beam-wave interaction is presented, in which the higher order terms inside the grooves are retained. The fields' distribution and the conductivity losses are also calculated utilizing the theoretical model. With the optimized parameters of the SWS and the electron beam, a 1 THz SDGAW Cerenkov traveling wave amplifier may obtain a moderate net gain (the peak gain is 12.7 dB/cm) and an ultra 3 dB wideband (0.19 THz) considering the serious Ohmic losses. The theoretical results have been compared with those calculated by 3D HFSS code and CST STUDIO particle-in-cell simulations.

Substrate Integrated Waveguide Power Divider, Circulator and Coupler in [10-15] GHZ Band

The Substrate Integrated Waveguide (SIW) technology is an attractive approach for the design of high performance microwave and millimeter wave components, as it combines the advantages of planar technology, such as low fabrication costs, with the low loss inherent to the waveguide solution. In this study, a substrate integrated waveguide power divider, circulator and coupler are conceived and optimized in [10-15] GHz band by Ansoft HFSS code. Thus, results of this modeling are presented, discussed and allow to integrate these devices in planar circuits.