Millimeter-wave Interferometer Research Articles

Characterizing Lossy Dielectric Materials in Shock Physics by Millimeter-Wave Interferometry Using One-Dimensional Convolutional Neural Networks and Nonlinear Optimization

When a dielectric material undergoes mechanical impact, it generates a shock wave, causing changes in its refractive index. Recent demonstrations have proven that the modified refractive index can be determined remotely using a millimeter-wave interferometer. However, these demonstrations are based on the resolution of an inverse electromagnetic problem, which assumes that the losses in the material are negligible. This restrictive assumption is overcome in this article, in which a new approach is proposed to solve the inverse electromagnetic problem in lossy and shocked dielectric materials. Our methodology combines a one-dimensional convolutional neural network architecture, namely Inverse problem of Lossless or Lossy Shocked Wavefront Network (ILSW-Net), with a nonlinear optimization technique based on the Nelder–Mead algorithm to estimate losses within dielectric materials under a mechanical impact. Experimental results for both simulated and real signals show that our method can successfully predict the velocities and the refractive index while accurately estimating the shock wavefront.

Shock Properties Characterization of Dielectric Materials Using Millimeter-Wave Interferometry and Convolutional Neural Networks.

In this paper, a neural network approach is applied for solving an electromagnetic inverse problem involving solid dielectric materials subjected to shock impacts and interrogated by a millimeter-wave interferometer. Under mechanical impact, a shock wave is generated in the material and modifies the refractive index. It was recently demonstrated that the shock wavefront velocity and the particle velocity as well as the modified index in a shocked material can be remotely derived from measuring two characteristic Doppler frequencies in the waveform delivered by a millimeter-wave interferometer. We show here that a more accurate estimation of the shock wavefront and particle velocities can be obtained from training an appropriate convolutional neural network, especially in the important case of short-duration waveforms of few microseconds.

Imaging the Inner Astronomical Unit of the Herbig Be Star HD 190073

The inner regions of protoplanetary disks host many complex physical processes such as star–disk interactions, magnetic fields, planet formation, and the migration of new planets. To study directly this region requires milliarcsecond angular resolution, beyond the diffraction limit of the world's largest optical telescopes and even too small for the millimeter-wave interferometer Atacama Large Millimeter/submillimeter Array (ALMA). However, we can use infrared interferometers to image the inner astronomical unit. Here, we present new results from the CHARA and VLTI arrays for the young and luminous Herbig Be star HD 190073. We detect a sub-astronomical unit (sub-AU) cavity surrounded by a ring-like structure that we interpret as the dust destruction front. We model the shape with six radial profiles, three symmetric and three asymmetric, and present a model-free image reconstruction. All the models are consistent with a near face-on disk with an inclination ≲20°, and we measure an average ring radius of 1.4 ± 0.2 mas (1.14 au). Around 48% of the total flux comes from the disk with 15% of that emission appearing to emerge from inside the inner rim. The cause of emission is still unclear, perhaps due to different dust grain compositions or gas emission. The skewed models and the imaging point to an off-center star, possibly due to binarity. Our image shows sub-AU structure, which seems to move between the two epochs inconsistently with Keplerian motion and we discuss possible explanations for this apparent change.

Richard Hills

Richard Hills (1945–2022) Simon Mitton remembers a world-class pioneer and innovator of submillimetre astronomy. Emeritus Professor Richard Edwin Hills FRS, a world leading innovator of millimetre and submillimetre radio astronomy died on 5 June 2022. Richard was born on 30 September 1945 and attended Bedford School. He was admitted to Queens' College, Cambridge in 1964 to study for the Natural Sciences Tripos and he received his BA in Physics in 1967. Then he headed west, to the University of California, Berkeley (UCB) as a doctoral student in William John ‘Jack’ Welch's radio astronomy group, which had begun in the late 1950s when UCB established the Hat Creek Radio Observatory (HCRO). Within a couple of years, Hill had co-authored papers on time-of-arrival measurements of NP 0352, the pulsar in the Crab Nebula. Meanwhile, his backstory concerned the development of a novel radio interferometer at millimetre wavelengths. Richard's five years in California shaped his career. His doctoral thesis on interferometric observations of galactic water vapour was made with a first-generation millimetre wave interferometer that he and other students had constructed. At the 50th anniversary celebrations of HCRO Richard regaled attendees with a recollection of what it was like to work with Jack Welch: “Naturally Jack figured out how the interferometer should work, but we had a great time making some of the bits and pieces. I particularly recall making the delay lines out of a series of coaxial cables, which were all tied up with rope inside a box. I didn't realize it at the time, but it turned out that what Jack taught me about telescopes and interferometers, plus how to set about actually making a technical project happen, was essentially everything I needed to know for all the subsequent things I have worked on.”

Phase detection system based on digital signal processing in millimeter wave interferometer for fusion plasma diagnostics

Abstract The design and development of a processing unit for the microwave interferometer is proposed here. The Microwave interferometer is well known for its use in several applications, and one of them is to measure the electron density of fusion plasma. An indigenous effort has been made to build a phase detection system that can be used to calculate fusion plasma electron density for millimeter-wave interferometer. It consists of oven-controlled crystal oscillator (OCXO), frequency synthesizer, IQ mixer, analog to digital converter (ADC), field-programmable gate array (FPGA) for processing the task and other proximity electronics. The phase-detection algorithm is derived using MATLAB. Testing and hardware realization is achieved by implementing it on an FPGA-based system. All the relevant system development requirements are attained with more than 95 percent accuracy. The device will be used in ADITYA tokamak for calculating electron plasma density.

A millimeter-wave Bell Test using a ferrite parametric amplifier and a homodyne interferometer

A combined ferrite parametric amplifier and millimeter-wave homodyne interferometer are proposed as an ambient temperature Bell Test. It is shown that the non-linear magnetic susceptibility of the yttrium iron garnet (YIG) ferrite, on account of its narrow line-width Larmor precessional resonance, make it an ideal material for the creation of entangled photons. These can be measured using a homodyne interferometer, as the much larger number of thermally generated photons associated with ambient temperature emission can be screened out. The proposed architecture may enable YIG quantum technology-based sensors to be developed, mimicking in the millimeter-wave band the large number of quantum optical experiments in the near-infrared and visible regions which had been made possible by use of the nonlinear beta barium borate ferroelectric, an analogue of YIG. It is illustrated here how the YIG parametric amplifier can reproduce quantum optical Type I and Type II wave interactions, which can be used to create entangled photons in the millimeter-wave band. It is estimated that when half a cubic centimeter of YIG crystal is placed in a magnetic field of a few Tesla and pumped with 5 Watts of millimeter-wave radiation, approximately 0.5 × 1012 entangled millimeter-wave photon pairs per second are generated by the spin-wave interaction. This means an integration time of only a few tens of seconds is needed for a successful Bell Test. A successful demonstration of this will lead to novel architectures of entanglement-based quantum technology room temperature sensors, re-envisioning YIG as a modern quantum material.

ЭКСПЕРИМЕНТАЛЬНОЕ ИССЛЕДОВАНИЕ ПРОНИКАНИЯ ЦИЛИНДРИЧЕСКИХ УДАРНИКОВ В СУХОЙ ПЕСОК

For the most complete study of the laws of shock interaction of solids with soil barriers, further development of experimental techniques for recording the parameters of the interaction process in direct and inverse experiments is necessary. In this work, to determine the parameters of the movement of the projectile (displacement and speed) in a direct experiment, a millimeter-wave radio interferometer is used. This method allows continuous recording of the movement of the rear end of the striker with high accuracy over a wide range of movements. Using the proposed technique, experiments were carried out to record the motion parameters of cylindrical impactors made of steel and aluminum alloy when interacting with an obstacle made of dry sand. At the same time, the movement of the rear end of the striker was also controlled using high-speed filming until the full immersion of the striker. The experiments showed that the measurement results obtained using two methods coincide within the measurement error. Based on the experiments, it can be concluded that the methodology for determining the displacement and velocity of a projectile in a ballistic experiment using a millimeter-wave radio interferometer allows continuous measurement of large displacements (100 mm or more), including when completely immersed in a target with sufficient practical goals accuracy. Based on the results of the experiments, the dependences of the movement of the projectile and its speed on time are constructed. A change in the penetration law was found with a decrease in the penetration velocity to values less than 100 m/s.

ИЗМЕРЕНИЕ ВЕЛИЧИНЫ ДИНАМИЧЕСКОГО КОЭФФИЦИЕНТА ПУАССОНА ДРЕВЕСИНЫ С ПОМОЩЬЮ РАДИОИНТЕРФЕРОМЕТРА

A new method is proposed for determining the dynamic Poisson's ratio of both isotropic and anisotropic materials. The sample was loaded with a pulsed compressive load on a setup that implements the Kolsky technique using a split Hopkinson bar. The development in time of the longitudinal compressive deformation of the sample was determined by the signals recorded with the help of low-base strain gauges glued to the measuring bars. A millimeter-wave interferometer was used to measure the development of the radial components of the sample deformation in time. To assess the possible asymmetry of the radial expansion of the sample, measurements were carried out using two independent channels irradiating diametrically opposite zones of the lateral surface of the sample. The tests were carried out on a sample of pine with air humidity in the form of a cylinder 54 mm in diameter and 30 mm in height. A pulsed compressive load was loaded along the fibers. Using two channels of the radio interferometer, the separate displacement of the lateral surfaces of the sample was recorded both along and across the annual layers. It was determined that the displacements of the regions of the lateral surface of the sample during expansion along the annual layers are quite close, while during expansion across the annual layers they are very different. The relative transverse deformation of the sample in both cases was determined as the sum of lateral displacements divided by the sample diameter. As a result, two components of the dynamic Poisson's ratio were obtained, which amounted to ~0.2 (in the direction along the annual layers) and ~0.24 (in the direction across the annual layers).

Building blocks of dust: A coordinated laboratory and astronomical study of the archtype AGB carbon star IRC+10216

This article provides an overview of recent astronomical studies and a closely coordinated laboratory program devoted to the study of the physics and chemistry of carbon-rich Asymptotic Giant Branch stars, as illustrated by the archtype star IRC+10216. The increased sensitivity and angular resolution of high altitude, ground-based millimeter-wave interferometers in the past few years has enabled molecular astronomers to determine the excitation and spatial distribution of molecules within a few stellar radii of the central star where the molecular seeds of dust are formed, and thus to critically assess the physicochemical mechanisms of dust formation and growth. However the astronomical studies are crucially dependent on precise laboratory measurements of the rotational spectra — both in the ground and vibrationally excited states of the normal and rare isotopic species as well — of the principal molecules in the inner region which appear to contain only two or three heavy atoms.Much remains to be done by laboratory spectroscopists as evidenced by the large number of unassigned millimeter-wave rotational lines that are observed in the inner envelope of IRC+10216. To assess whether vibrationally excited SiC2 and HCN are the carriers of some of the unassigned features observed in this source, millimeter-wave laboratory spectra are compared with Cycle 0 observations from the Atacama Large Millimeter-wave Array (ALMA) under experimental conditions that produce both species. Also highlighted are ongoing laboratory studies of the silicon carbides SiC2 and SiCSi in their ground and excited vibrational states, and c-SiC3 in its ground vibrational state. Following the initial detection of c-SiC3 and SiCSi in the outer molecular envelope of IRC+10216, the laboratory spectroscopy was extended to higher frequency in support of the recent interferometric measurements. Thirty-two new millimeter-wave rotational transitions of SiCSi with J⩽48,Ka⩽3 and upper level energies Eu⩽484 K in the range from 178 to 391 GHz, and 35 new transitions of c-SiC3 with J⩽38,Ka⩽20 and Eu⩽875 K between 315 and 440 GHz were measured in the laboratory. In addition, five or six rotational transitions in one quanta of each of the three fundamental vibrational modes of SiCSi and the two lowest rotational transitions in the previously unexplored C–C stretching mode (ν1) of SiC2 were measured in the normal and doubly substituted 13C isotopic species. Areas of continued emphasis and the importance of complementary experimental methods and production techniques to detect and assign vibrational transitions are also highlighted.

Fringe jump compensation techniques for the time-averaging zero-crossing phase measurement in the KSTAR millimeter-wave interferometer.

A fringe jump compensation algorithm has been developed for a phase measurement that measures the phase within a single fringe. The algorithm is extremely useful in the case of the time-averaging zero-crossing phase detector on noisy environments. When the noise level on the measurements is not sufficiently suppressed, the signals near the fringe jump show a negative slope instead of a sharp drop. The slope brings an ambiguity over the compensation process. An algorithm with an additional channel that measures the phase of a half fringe shift has been applied in the millimeter-wave interferometer on the Korea Superconducting Tokamak Advanced Research device. These techniques removed the ambiguity in most cases. The algorithm can provide a most simple, robust, and cost-effective solution for the phase measurement system in various fields.

EPICS data acquisition system for FPGA-based plasma electron density measurement on KSTAR

The Korea Superconducting Tokamak Advanced Research (KSTAR) has several diagnostics devices, including a millimeter-wave interferometer (MMWI), far-infrared interferometer (FIR), two-color interferometer, etc., for measuring plasma electron densities. The MMWI diagnostic device measures the electron density of the plasma from the phase difference generated by transmitting a 1 mm (280 GHz) electromagnetic wave into it. The FIR device is similar to the MMWI, but uses a far-infrared (119 um) laser as its electromagnetic wave source. The MMWI is the primary plasma density measurement system in KSTAR. However, its signal contains significant noise and hundreds of fringe-jump events from the phase comparator circuit. The original analogue circuit of the phase comparator is limited in noise handling since it needs to average the raw IF signal to be treated in analogue circuit with limited bandwidth. To handle noise with more flexibility and to reduce occurrence of fringe-jump errors, we developed a new data acquisition (DAQ) system using a high-speed FPGA-based digitizer. The MMWI signal resembles a 10 MHz sine wave. We acquired data at 100 MSPS using a new digitizer. The phase difference per sample was 36°, which lead to inaccurate results due to noise. To increase the accuracy, the new DAQ system has interpolation logic within the FPGA that increases the phase resolution. In addition, the phase comparative value is transmitted to two channels by inverting the signal. The signal that does not travel through the plasma has a phase difference of 180°. The FPGA-based system was constructed using the same algorithm as existing phase comparator circuit [2]. The detected phase was averaged to obtain a phase value, and the result was transmitted as 1280 phase values per msec. In the EPICS Input/Output Controller (IOC), the density is calculated using data received from the direct memory access (DMA) via peripheral component interconnect (PCI) Express. The above FPGA logic was successfully used for real-time feedback control in the ITER CODAC environment. This paper introduces a new, detailed MMWI DAQ system design concept using FPGA, and its operational results.

Protostellar Outflows

Outflows from accreting, rotating, and magnetized systems are ubiquitous. Protostellar outflows can be observed from radio to X-ray wavelengths in the continuum and a multitude of spectral lines that probe a wide range of physical conditions, chemical phases, radial velocities, and proper motions. Wide-field visual and near-IR data, mid-IR observations from space, and aperture synthesis with centimeter- and millimeterwave interferometers are revolutionizing outflow studies. Many outflows originate in multiple systems and clusters. Although most flows are bipolar and some contain highly collimated jets, others are wide-angle winds, and a few are nearly isotropic and exhibit explosive behavior. Morphologies and velocity fields indicate variations in ejection velocity, mass-loss rate, and in some cases, flow orientation and degree of collimation. These trends indicate that stellar accretion is episodic and often occurs in a complex dynamical environment. Outflow power increases with source luminosity but decreases with evolutionary stage. The youngest outflows are small and best traced by molecules such as CO, SiO, H2O, and H2. Older outflows can grow to parsec scales and are best traced by shock-excited atoms and ions such as hydrogen-recombination lines, [Sii], and [Oii]. Outflows inject momentum and energy into their surroundings and provide an important mechanism in the self-regulation of star formation. However, momentum injection rates remain uncertain with estimates providing lower bounds.

Operation results of the KSTAR far infrared interferometer.

The 2015 KSTAR experimental campaign was the first year of routine measurement with a far infrared interferometer (FIRI) utilizing 118.87 μm CH3OH lasers at maximum 200 mW CW beam power. By using rtEFIT reconstruction, the path lengths of interferometers can be calculated and so the line-averaged electron densities n¯e from the FIRI and a millimeter-wave interferometer were in excellent agreement. In this way, the number of successfully diagnosed discharges is counted: 1003 shots or 83.7% of sustained discharges, defined as shots of plasma current IP ≥ 0.3 MA with pulse lengths tf ≥ 2.0 s, have good-quality FIRI data within a few fringe jump errors. In addition, real-time H-mode density feedback control based on the FIRI was also successfully achieved with supersonic molecular beam injection as an actuator. Both constant density and controlled linear increment with a ramp-up rate of 1.0 × 1019 m-3 s-1 were achieved.

Upgraded millimeter-wave interferometer for measuring the electron density during the beam extraction in the negative ion source.

The upgraded millimeter-wave interferometer with the frequency of 70 GHz is installed on a large-scaled negative ion source. Measurable line-averaged electron density is from 2 × 1015 to 3 × 1018 m-3 in front of the plasma grid. Several improvements such as the change to shorter wavelength probing with low noise, the installation of special ordered horn antenna, the signal modulation for a high accuracy digital phase detection, the insertion of insulator, and so on, are carried out for the measurement during the beam extraction by applying high voltage. The line-averaged electron density is successfully measured and it is found that it increases linearly with the arc power and drops suddenly at the beam extraction.

Validation experiment of a numerically processed millimeter-wave interferometer in a laboratory.

We propose a new interferometer system for density profile measurements. This system produces multiple measurement chords by a leaky-wave antenna driven by multiple frequency inputs. The proposed system was validated in laboratory evaluation experiments. We confirmed that the interferometer generates a clear image of a Teflon plate as well as the phase shift corresponding to the plate thickness. In another experiment, we confirmed that quasi-optical mirrors can produce multiple measurement chords; however, the finite spot size of the probe beam degrades the sharpness of the resulting image.

Development of Antenna System and Simulation of Analysis Method for Digitally-Controlled Millimeter-Wave Interferometer

We proposed a flexible electron density measurement system based on the interferometer for confinement study. An INRD guide antenna for spatial scan capability of a probe beam has been analytically designed. As a result of fabricating and evaluating the antenna, it was confirmed that the antenna has a predicted performance. We also evaluated the analysis method of a sampled data with numerical simulation in order to calculate phase shift due to the plasma density. A 3000 order FIR filter could successfully separate a sample signal with various frequencies into a signal with a single frequency when S/N is not critical.

Design and implementation of a 150 GHz single-channel millimeter wave interferometer on Joint TEXT tokamak

A simple, single-channel millimeter-wave interferometer system has been designed, fabricated, and installed on the J-TEXT tokamak. For the plasma density anticipated on J-TEXT, a solid-state source operating at 150 GHz has been chosen to minimize errors due to both vibration along the beam path and refraction in the plasma. The new aspect of the interferometer design is to use a subharmonic mixer for detection with a frequency doubled 150 GHz source. It employs a single source which is bias-tuned and modulated with a sawtooth wave form up to 100 kHz in order to generate the intermediate frequency. The 12.5 GHz voltage-controlled oscillator is multiplied to 75 GHz before a final doubler raises it to 150 GHz. A portion of the 75 GHZ power is used for the local oscillator (LO) and is directly connected to the LO input of the subharmonic mixer. The phase is evaluated by a digital phase comparator using a software-based algorithm. Detection noise limits the minimum resolvable phase change with the interferometer to ±0.05 fringe, which corresponds to an averaged electron density change along the chord of ±1.1 × 10(17) m(-2). The maximum measurable electron density is expected to be ∼9 × 10(19) m(-3). A comparison of preliminary results from the millimeter wave interferometer with that from the far-infrared hydrogen cyanide laser (wavelength of 337 μm) interferometer shows good agreement during the pulse flat-top period. The millimeter wave interferometer system will be used as a part of the density feedback control system in the future.

Remote sensing of patterns of cardiac activity on an ambulatory subject using millimeter-wave interferometry and statistical methods

Using a 94-GHz millimeter-wave interferometer, we are able to calculate the relative displacement of an object. When aimed at the chest of a human subject, we measure the minute motions of the chest due to cardiac activity. After processing the data using a wavelet multiresolution decomposition, we are able to obtain a signal with peaks at heartbeat temporal locations. In order for these heartbeat temporal locations to be accurate, the reflected signal must not be very noisy. Since there is noise in all but the most ideal conditions, we created a statistical algorithm in order to compensate for unconfident temporal locations as computed by the wavelet transform. By analyzing the statistics of the peak locations, we fill in missing heartbeat temporal locations and eliminate superfluous ones. Along with this, we adapt the processing procedure to the current signal, as opposed to using the same method for all signals. With this method, we are able to find the heart rate of ambulatory subjects without any physical contact.

Development of leaky-wave antenna for digitally controlled millimeter-wave interferometer

We propose a new interferometer concept that can realize electron-density distribution measurement with high spatial and moderate temporal resolution. The image non-radiative dielectric guide antenna can probe a wide measurement area simultaneously. We fabricated the antenna with an electromagnetic simulator and confirmed that the simulated and measured radiation patterns are consistent with each other. In addition, we found that the antenna shows the required characteristics such as scanning characteristics, which depend on the input frequency.

Characterization of Ion Cyclotron Resonance Acceleration for Electric Propulsion with Interferometery

This paper describes the experimental characterization of single-pass ion cyclotron resonance heating as applied to acceleration of ions for electric propulsion. A millimeter-wave interferometer system has shown to be a clear and simple method of quantifying ion acceleration due to ion cyclotron resonance heating. The experimental work was done on the VX-10 experiment of the variable specific impulse magnetoplasma rocket concept. The perpendicular velocity of the ions generated by ion cyclotron resonance heating was converted into axial velocity by the decreasing gradientoftheaxialmagnetic fieldattheexhaustofthepropulsionsystemfromconservationofthemagnetmoment. Thisincreaseinaxialvelocityispredictedtocauseadecreaseindensityduetoconservationofcurrentintheplasma. InterferometerdensitymeasurementsweretakenatthreedifferentlocationsontheVX-10experimentupstreamand downstream of the ion acceleration zone. A clear measurement of a 25% density drop for helium and a 40% density drop for deuterium was measured downstream of the ion resonance zone characteristic of ion acceleration.