RF Measurements Research Articles

Damping trapped modes in an in-vacuum undulator at a synchrotron radiation light source

In-vacuum undulators have been widely operated in many synchrotron radiation facilities across the world. They usually are required to be operated at a smaller magnet gap than those of other undulators. Thus, operating challenges including impedance effects on the stored electron beam are introduced by these devices. In this paper, we report the efforts in solving the problem of coupled-bunch instabilities caused by an in-vacuum undulator in the SPEAR3 storage ring. Using beam based measurements, cold rf measurements, and numerical simulations, the source of the beam instabilities is characterized as trapped modes in the vacuum chamber. Using numerical models, we explored several approaches to reduce the strength of the trapped modes and found that ferrite dampers were the most effective and simplest way for mode damping in our SPEAR3 in-vacuum undulator. The results of the first rf cold measurement on an in-vacuum undulator equipped with these ferrite dampers agree well with numerical simulations.

Design, RF measurement, tuning, and high-power test of an X-band deflector for Soft X-ray Free Electron Lasers (SXFEL) at SINAP

Abstract An X-band transverse-deflecting structure is a crucial RF unit for bunch-length measurement conducted at the Shanghai Soft-X-ray Free Electron Laser Facility (SXFEL) at the Shanghai Institute of Applied Physics (SINAP), Chinese Academy of Sciences. The target of this prototype deflector is to operate at 30 MW to have a deflection of 10 MV/structure to realize 20-fs resolution in a full-size structure (0.6 meters long, used on user facility of SXFEL), which satisfies the requirement of the linac beam size measurement at 840 MeV. The program was started in 2010 focusing on X-band deflectors, including design, fabrication, and RF measurement. The electrical tuning of the deflector was successfully done using a unique “cage”-type bead. Recently, a high-power test was conducted at KEK on the first prototype deflector. The high-gradient features at the nominal gradient was proven, but the further increase of power and pulse width was limited at 45 MW with a 173-ns pulse width due to approaching the feasible limit of the modified pointing vector value 5 MW/mm2. In this final stage, frequent breakdowns near the badly brazed point occurred and further conditioning had to be suspended owing to limited available time.

OpenEM – Electromagnetic field mapping robot for microwave and RF measurements

Abstract A low-cost, fully automated, electromagnetic field probing robot is described in this paper. The platform, called OpenEM Field Robot, is capable of mapping the spatial electromagnetic fields in free space. The system supports multiple antenna configurations and is scalable to large measurement domains. 3D printed antenna holders are also incorporated in order to allow for the near field measurements with minimal scattering and interference with the measurement. The goal of the system is to design a fully integrated platform for measuring electromagnetic fields that can be used with various other RF measurement devices and is configurable and easily modified for various applications in scientific research and education.

Development and beam commissioning of a continuous-wave window-type radio-frequency quadrupole

The radio-frequency quadrupole (RFQ) group at Peking University has built a window-type RFQ, operating at 162.5 MHz in continuous-wave (cw) mode. It is designed to accelerate a 50 mA deuteron beam from 50 keV to 1 MeV with a vane length of 1.809 m. The cavity was fabricated in two segments using 100% oxygen-free electronic (OFE) copper. Using an iterative assembly and measurement procedure for the precise alignment of the two segments, we reduced the assembly errors to within $\ifmmode\pm\else\textpm\fi{}0.05\text{ }\text{ }\mathrm{mm}$. The radio frequency (rf) measurements of the whole cavity show excellent rf properties, with the measured intrinsic Q-value of 8962, which equates to 96% of the simulated value for OFE copper. We also investigated field fluctuations caused by misalignment between the two segments, and studied their impact on the beam transmission using beam dynamics simulations. During field tuning, we compiled a set of unique tuning rules for the window-type RFQ. After tuning, the maximal field unflatness of the single quadrant is within $\ifmmode\pm\else\textpm\fi{}2%$, and the asymmetry of the four quadrants is within $\ifmmode\pm\else\textpm\fi{}1%$. During rf conditioning, the cw power of the cavity reached 55 kW within 32 hours, and we have recorded nearly seven hours of stable running at a cw power of 50 kW. The measured bremsstrahlung spectrum shows that the accelerator needs 49.9 kW to generate the intervane voltage of 60 kV, with a specific shunt impedance of $130.5\text{ }\text{ }\mathrm{k}\mathrm{\ensuremath{\Omega}}\text{ }\mathrm{m}$. An ${\mathrm{H}}_{2}^{+}$ ion beam extracted from an electron cyclotron resonance ion source was used for the beam commissioning, because deuteron beam acceleration will bring a serious radiation field. We achieved stable and robust acceleration of about 1.5 mA cw ${\mathrm{H}}_{2}^{+}$ beam for one hour.

Measurement of beam steering and RF defocusing effect for a quarter-wave resonator

Quarter-wave resonators (QWRs) are being widely used in linear accelerators (linac) for acceleration of ions with low-[Formula: see text] velocity. Two effects of this kind of cavities are beam steering effect and RF defocusing effect caused by geometric asymmetry. Measurement for the two effects has been conducted by using beam position monitors (BPMs) and wires for a QWR type buncher whose frequency is 162.5[Formula: see text]MHz at Institute of Modern Physics (IMP), Chinese Academy of Sciences (CAS). Since experiment and simulation matches well, beam central position and beam envelope could be predicted in simulation code, which is beneficial for high-intensity proton accelerator design and commissioning.

Field tuning and rf measurements of the four-vane radio frequency quadrupole with ramped inter-vane voltage

This paper describes the physical design, rf measurement, and tuning result of a 325 MHz four-vane radio frequency quadrupole (RFQ) with a ramped inter-vane voltage. The 3-meter-long RFQ will accelerate the negative hydrogen ion beam from 50 keV to 3 MeV with a peak current of 5 mA, a pulse width of $10--40\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$, and a maximum repetition rate of 0.5 Hz. During tuning we reveal that the coupling loop of the coaxial coupler adopted for this RFQ enhances the interference between the dipole field distribution and the coupling coefficient. Therefore, an iteration method is adopted to simultaneously achieve the desired field distribution and coupling coefficient. After tuning, the relative error between the measured field distribution and the designed curve for the operating quadrupole mode is within $\ifmmode\pm\else\textpm\fi{}2.7%$, and the dipole components are about $\ifmmode\pm\else\textpm\fi{}1.9%$ of the quadrupole mode. The measured coupling coefficient of the rf power coupler equals 1.03, with the desired value of 1.04.

Compressive Sensing Based Radio Tomographic Imaging with Spatial Diversity.

Radio tomographic imaging (RTI) has emerged as a promising device-free localization technology for locating the targets with no devices attached. RTI deduces the location information from the reconstructed attenuation image characterizing target-induced spatial loss of radio frequency measurements in the sensing area. In cluttered indoor environments, RF measurements of wireless links are corrupted by multipath effects and thus less robust to achieve a high localization accuracy for RTI. This paper proposes to improve the quality of measurements by using spatial diversity. The key insight is that, with multiple antennae equipped, due to small-scale multipath fading, RF measurement variation of each antenna pair behaves differently. Therefore, spatial diversity can provide more reliable and strong measurements in terms of link quality. Moreover, to estimate the location from the image more precisely and make the image more identifiable, we propose using a new reconstruction regularization linearly combining the sparsity and correlation inherent in the image. The proposed reconstruction method can remarkably reduce the image noise and enhance the imaging accuracy especially in the case of a few available measurements. Indoor experimental results demonstrate that compared to existing RTI improvement methods, our RTI solution can reduce the root-mean-square localization error at least 47% while also improving the imaging performance.

Determination of the Bardeen–Cooper–Schrieffer material parameters of the HIE-ISOLDE superconducting resonator

Superconducting material parameters of the Nb film coatings on the quarter-wave resonator for the HIE-ISOLDE project were studied by fitting experimental results with the Bardeen–Cooper–Schrieffer Mattis–Bardeen (BCS-MB) theory. We pointed out a strong correlation among fitted estimators of material parameters in the BCS-MB theory, and used a procedure to reduce the uncertainty by merging two χ2 distributions of the surface resistance and effective penetration depth. In this procedure, unlike previous studies, BCS coherence length (ξ0) and London penetration depth (λL) were not fixed at their literature values in the calculation because in our film, whose thickness is from 2 to 13 μm, they may take altered values as a consequence of microstructual defects. Since surface resistance and penetration depth have similar dependencies on coherence length and mean free path, the effects of the correlation between the estimators of these two parameters could not be mitigated by just combining surface resistance and penetration depth data. We used upper critical field measurement by SQUID magnetometry to provide a complementary constraint to these RF measurements, and this allowed all the material parameters to be obtained by fitting the experimental data. In the best performing cavity, the determined parameters are ξ0 = 29 nm, λL = 26 nm, mean free path l = 99 nm, and coupling strength Δ0/kBTc = 1.7. The coherence length is slightly shorter than for clean bulk Nb (39 nm) in the literature although the Nb film is thick and rather bulk-like. A poorly performing cavity showed weaker coupling constant Δ0/kBTc = 1.5 which may indicate film contamination during the coating process.

Best Practices for Using Electrostatic Discharge Protection Techniques for Single-Event Transient Mitigation

We provide “best practices” for single-event transient (SET) mitigation using electrostatic discharge (ESD) protection techniques. We investigate the correlation between SET suppression and RF performance when ESD protection circuits for SET mitigation are combined with various RF switches. The three different single-pole single-throw (SPST) switch configurations (conventional design, floating body, and floating body/n-well) were implemented to provide insight on how to best apply dc ESD protection techniques for also achieving robust SET mitigation. Based on two-photon absorption laser experiments and RF measurements, a conventional SPST switch configuration shows relatively poor RF performance but is effective for SET mitigation due to the SET discharge current path provided by ESD protection circuit. On the other hand, the combination of the SPST switch and the ESD circuit, which exhibits the best RF performance among the three configurations, is relatively vulnerable to SETs. Based on these results, strategies and methodologies for ESD protection techniques to effectively mitigate SET are proposed.

Studies for high energy air shower identification using RF measurements with the ASTRONEU array

The Hellenic Open University (HOU) Cosmic Ray Telescope (ASTRONEU) consists of 9 large scintillator detectors and 3 RF antennas arranged in three autonomous stations operating at the Hellenic Open University campus in the city of Patras. High energy showers that are detected simultaneously by two distant stations and in coincidence with the RF antennas are used to study the RF signature of cosmic events. In previous studies we have shown that the timing of the RF signals as well as the measured electric field at the antennas position are in very good agreement with the simulation predictions. In this work we concentrate on the transfer functions of the antennas which are strongly frequency and angular dependent. We show that the RF spectra (at frequencies 30-80 MHz) of the detected showers are exhibiting features of the antenna response as predicted by detailed Monte Carlo simulation suggesting that a single antenna RF spectrum gives access to the cosmic ray arrival direction.

Methods of design a multiport measurements

The main implementation options for multiport RF and microwave measurements using n-port vector network analyzers, as well as switching measuring expansion units, are considered. A switching measuring unit and a port expander unit manufactured are presented by RPC Micran.

Procollagen III amino terminal propeptide (PIIINP): A marker of MTX induced liver fibrosis in rheumatoid arthritis patients?

Background: to evaluate the reliability of serum Procollagen III aminoterminal peptide (PIIINP) in the screening for hepatic fibrosis induced by long term Methotrexate Therapy (MTX) in Rheumatoid Arthritis (RA) patients. Methods and Findings: The study included 90 RA patients divided into 60 on MTX therapy (group I) and 30 on other medications (group II). All subjected to history taking, rheumatological examination and lab investigations including CBC, ESR, CRP, AST, ALT, RF and PIINP levels measurement, DAS score and Fibrosis-4 (FIB-4) score were calculated. Results showed significant lower disease duration, HCT, platelet count and higher MCV, AST, ALT, FIB-4 score, PIIINP level and more patients’ complaint of right hypochondrial pain in MTX intake group (p 170 can detect MTX induced hepatic fibrosis with (51.52%) sensitivity and (92.59%) specificity, 89.5% PPV and 61% NPV. Correlation between PIIINP and all studied variables showed a significant positive correlation between PIIINP level and disease duration, MTX intake duration, MTX current and cumulative doses, MCV, AST, ALT, AST/ALT, FIB-4 score and significant negative correlation with folic acid intake, platelet count (P<0.05). A positive correlation found between cumulative MTX dose and FIB-4 score (P<0.001). Conclusion: PIIINP can be used as screening noninvasive marker for MTX induced hepatic fibrosis in RA patients; it is positively correlated with AST/ALT ratio, FIB-4 score and cumulative MTX dose. Folic acid is protective against liver fibrosis in RA patients on MTX therapy.

Removing Random Phase Contributions of Sweeping Local Oscillator From Modulated RF Measurements

This paper proposes a measurement strategy to remove the random phase contributions of the sweeping local oscillator (LO) from modulated radio frequency phase measurements based on downconversion. To obtain a stable phase spectrum for each run of tone-by-tone sweeping measurement, the LO used for downconversion is coupled out to an extra path and modulated to be a wideband multisine signal. By monitoring the phase change of multisine signal at a specific and fixed frequency point, random LO phase offsets during the frequency sweep can be measured and compensated. In this paper, two experimental test benches with the same core measurement strategy but different architectures are established for method validation. According to the experimental results using test bench one, the phase stability is ±(0.2°–0.4°) at 1.8 GHz for fundamental mixing-based downconversion and ±(0.6°–1°) for harmonic mixing at 6 GHz. Based on test bench two, relative phase deviations of ±(0.8°–1.2°) at 22 GHz for fundamental mixing and ±(2°–3°) at 66 GHz for harmonic mixing are reachable.

Improved RF measurements of SRF cavity quality factors

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

RF Test Article Experiment on the Impact of Non-Hexavalent Chromium-Based Conversion Coatings on Electrical Assemblies

Abstract Conversion coatings are treatments applied to aluminum structures to inhibit corrosion while maintaining electrical conductivity. In aerospace applications, the most common type of conversion coatings (MIL-DTL-5541 Type I) contain hexavalent chromium compounds as the corrosion-inhibiting agent. These Type I conversion coatings have a long pedigree and are highly effective in preventing corrosion; however, the hexavalent chromium compounds in these coatings are carcinogenic and water-soluble. Therefore, the use of these compounds is highly regulated in order to protect both workers and the public, leading to high cost in both use and disposal. Regulations are also beginning to restrict outright use, where new designs for the DOD are prohibited from using Type I coatings by DFARS 48 CFR Parts 223 and 252 and all use has been prohibited by Europe by REACH regulations since September 2017. In response, more environmentally friendly non-hexavalent chromium-based processes, referred to as MIL-DTL-5541 Type II conversion coatings, have become available. However, the long term reliability and performance impacts resulting from the use of these coatings are not fully understood and there is an ongoing effort in the aerospace industry organized by NASA to fully define these impacts while hardware is still in the design stage. While significant work has been performed to define the corrosion performance of various type-II conversion coatings, there has been minimal work performed to quantify the impact a type-II conversion coating would have on RF electrical assemblies. The standard methodologies defined by MIL-DTL-81706B have significant limitations and cannot fully capture the impact at microwave frequencies. For this reason, an investigation is underway at Northrop Grumman to quantify the impact these coatings may have on the quality, reliability, and performance of our electronic systems. At IMAPS 2017, Northrop Grumman introduced a RF test article designed to assess the longitudinal impact a Type II conversion coating would have on RF electrical assemblies where plated printed wiring boards (PWBs) and aluminum structures come in intimate contact. The test article features a specialized suspended stripline/microstrip stepped impedance filter designed to de-tune in the presence of galvanic corrosion. This talk follows the work presented in IMAPS 2017 by discussing an experiment where three different populations of test articles, each coated with a different type of conversion coating, are subjected to environmental testing. This talk also discusses how an initial measurement of these test articles before environmental exposure showed little difference between these populations. Finally, this talk reviews the initial results of this experiment, discussing environmental testing and the RF measurements captured during these tests.

Detailed study of RF properties of cold models for CW window-type RFQ

A 50 mA CW deuteron RFQ is being built for a joint 973 project between Peking University and the Institute of Modern Physics. This RFQ adopts a high-frequency window-type structure. To study its RF properties and to validate the reliability of an electromagnetic simulation, two full-length aluminum models with tuners were built in succession. RF measurements were obtained from the test bench and compared to the simulations, including frequencies, quality factors, and electric fields of different modes and the field in aperture. Through field tuning, the maximal field unflatness for a single quadrant and the average asymmetry of four quadrants were reduced from 8.7% and $$\pm \,$$ 3.6% to 5.8% and $$\pm \,$$ 1.7%, respectively. Moreover, a tuning method of adjusting the gap distance between the endplates and the vanes was also studied in this paper.

Low-Cost Microfabrication for MEMS Switches and Varactors

This paper presents a low-cost microfabrication technique for manufacturing radio frequency microelectromechanical system (RF MEMS) switches and varactors without intensive clean-room environments. The fabrication process entails only laser microstructuring technique, non-clean-room microlithography, and standard wet-bench and hot-film emboss of SU-8 and ADEX polymers. MEMS movable structures were fabricated out of 14- $\mu \text{m}$ -thick aluminum foils and suspended above coplanar-waveguide transmission lines, which were implemented on top of FR4 substrates via 5- $\mu \text{m}$ -thick SU-8 dielectric anchors. Both MEMS structures and an FR4 substrate were integrated using micropatterned polymers, developed by using dry-film ADEX and SU-8 polymers, for a composite assembly. An average fabrication yield of higher than 60% was achieved, calculated from 10 fabrication attempts. The RF measurement results show that the RF MEMS devices fabricated by using the novel microfabrication process have good figures of merit, at much lower overall fabrication costs, as compared to the devices fabricated by conventional clean-room process, enabling it as a very good microfabrication process for cost-effective rapid prototyping of MEMS.

Development of a 750-mm-long stripline kicker for HEPS

The superfast kickers are required for the HEPS storage ring on-axis injection system due to its very small dynamic aperture. A 750-mm-long stripline pair type of kicker prototype was researched and developed to demonstrate achieved performance of bandwidth, impedance, kicker strength, field uniformity and beam power loss. The cross sections of the kicker main body and the end of taper part are optimized for a good impedance matching and field uniformity. 3D simulation further optimizes the taper part to minimize the beam power loss and maintain a lower reflection. The high-voltage feedthrough is also designed and optimized by 3D CST. RF and high-voltage measurements are taken to verify the design of kicker assembly. The testing transmission odd-mode impedance is 50 ± 0.5 Ω, the even-mode impedance is 60 ± 0.5 Ω, and return loss is less than − 13 dB. The peak voltage and the rise time of pulse width inserting kicker assembly just decrease 3% of 20 kV and slow down 80 ps, respectively. RF testing results agree well with the simulation ones, which meet the design specification. The kicker assembly works well at ± 20 kV pulse.

GaN nanodiode arrays with improved design for zero-bias sub-THz detection

GaN based self-switching diodes (SSDs) have been fabricated for the first time on SiC substrate. They have been characterized as RF power detectors in a wide frequency range up to 220 GHz, showing a cutoff frequency of about 200 GHz. At low-frequency, RF measurements exhibit a square law detection with a responsivity that well agrees with the calculations performed by means of a quasi-static model based on the shape of the I–V curve. Exploiting such a model, a simple DC characterization allows defining design rules for optimizing the practical operation of the diode arrays as RF power detectors. As strategy to improve the performance of SSDs operating as zero-bias detectors at room temperature, in terms of responsivity and noise equivalent power, we suggest: (i) the reduction of the channel width and (ii) the increase of the number of diodes in parallel in order to reduce the total device impedance to a value that coincides with 3 times that of the transmission line (or antenna) to which they are connected.

A 300-mK Test Bed for Rapid Characterization of Microwave SQUID Multiplexing Circuits.

Microwave SQUID multiplexing is a promising technique for multiplexing large arrays of transition edge sensors. A major bottleneck in the development and distribution of microwave SQUID multiplexer chips occurs in the time-intensive design testing and quality assurance stages. To obtain useful RF measurements, these devices must be cooled to temperatures below 500 mK. The need for a more efficient system to screen microwave multiplexer chips has grown as the number of chips requested by collaborators per year reaches into the hundreds. We have therefore assembled a test bed for microwave SQUID circuits, which decreases screening time for four 32-channel chips from 24 h in an adiabatic demagnetization refrigerator to approximately 5 h in a helium dip probe containing a closed cycle 3He sorption refrigerator. We discuss defining characteristics of these microwave circuits and the challenges of establishing an efficient testing setup for them.