Quasi-optical Design Research Articles

Quasi-optics design of the dual-array ECE imaging system on the EAST Tokamak

A novel large aperture quasi-optical imaging system is designed for the new dual-array electron cyclotron emission (ECE) imaging (ECEI) instrument on the EAST tokamak. The zoom doublet scheme is used in the microwave imaging system on a super-conducting tokamak for the first time, and the focal plane can reach the high magnetic field side region even in the narrowest zoom configuration. The best spatial resolution in the vertical direction is 1.1 cm and the maximum vertical coverage can reach 80 cm. The field curvature is largely reduced in the narrow zoom configuration by the parabolic correction of a single lens surface. The imaging performance is fully characterized in the laboratory, and the characterized beam patterns show good agreements with the Gaussian beam specifications in the simulation results of the design.

Heterodyne detection at 300 GHz using neon indicator lamp glow discharge detector

A miniature neon indicator lamp, also known as a glow discharge detector (GDD), costing about 50 cents, was found to be an excellent room temperature terahertz radiation detector. Proof-of-concept 300 GHz heterodyne detection using GDD is demonstrated in this paper. Furthermore, a comparison to direct detection was carried out as well. Previous results with the GDD at 10 GHz showed 40 times better sensitivity using heterodyne detection compared to direct detection. Preliminary results at 300 GHz showed better sensitivity by a factor of 20 with only 56 μW local-oscillator power using heterodyne compared to direct detection. The higher the local-oscillator power (P(lo)), the better the sensitivity of the detector. Further improvement can be achieved by employing better quasi-optical design.

The Realization of Terahertz Image Reconstruction with High Resolution Based on the Amplitude of the Echoed Wave by using the Phase Retrieval Algorithm

An indirect imager working at terahertz band is presented and implemented, which is suitable for high-resolution planar object detection. The proposed imager employs a simple quasi-optics design to transmit and to receive terahertz waves, and adopts incoherent detection technology to extract the intensity of echoed signal, which results in a relatively low complexity and cost. Moreover, the Fienup Fourier phase-retrieval algorithm is successfully modified and is applied to retrieve the phase of the echoed signal and reconstruct the target image. Imaging experiments on typical planar objects are performed with the imager working at 0.2 THz, and the experimental results demonstrate the good performance of the proposed imager and validate the effectiveness of the reconstruction algorithm.

Study of image reconstruction for terahertz indirect holography with quasi-optics receiver

In this paper, an indirect holographic image reconstruction algorithm was studied for terahertz imaging with a quasi-optics receiver. Based on the combination of the reciprocity principle and modified quasi-optics theory, analytical expressions of the received spatial power distribution and its spectrum are obtained for the interference pattern of target wave and reference wave. These results clearly give the quantitative relationship between imaging quality and the parameters of a Gaussian beam, which provides a good criterion for terahertz quasi-optics transceivers design in terahertz off-axis holographic imagers. To validate the effectiveness of the proposed analysis method, some imaging results with a 0.3 THz prototype system are shown based on electromagnetic simulation.

A Method for Gyrotron Beam Coupling into a Corrugated Waveguide

In the radio-frequency (rf) power transmission system of an electron cyclotron heating and current drive (EC H&CD) system, the gyrotron power should couple with the fundamental mode of the corrugated waveguide (HE11 mode) because unwanted higher-order modes affect the beam radiation characteristics, which is a problem in the quasi-optical launcher design. To achieve high HE11 mode purity, a beam coupling method that measures the transmission mode in the waveguide was examined using a 170-GHz high-power gyrotron for the first time. In beam coupling, the offset and tilt angle of the input beam at the waveguide inlet were minimized by controlling the angles of the mirrors in the matching optical unit (MOU) to minimize unwanted LP11 modes in the waveguide. The rf field profile in free space after 1.3 m of the waveguide from the MOU was measured, and the transmission mode content was analyzed. According to the analyzed mode content, the HE11 mode content was optimized by remote adjustment of the mirror angles with a digital controller. The optimization procedure of beam coupling achieved 95% of HE11 mode purity at the entrance of transmission line, which is the first demonstration that meets the criteria of the ITER EC H&CD system.

Low power tests on the new front steering EC launcher for FTU

Abstract A new electron cyclotron (EC) antenna for real time control experiments on FTU tokamak has been tested at IFP-CNR laboratory. mm-Wave characterization was performed at the operating frequency of 140 GHz with low power measurements, primarily to test the internal optics of the launcher in terms of focusing properties and beam directivity. The antenna is devoted to the injection of high power EC waves in different working configurations using a front steering setup. The beam direction is controlled with a fast mirror (steerable both toroidally and poloidally) and the available launching angles have been determined to fix the steering mirror working space. Beam dimension can also be controlled, using a sliding structure in the plug-in system, equipped with a focusing mirror that provides zooming capabilities. This paper contains results of low power tests performed on the antenna in order to characterize the launcher in its quasi-optical design. Beams are found to be slightly astigmatic (maximum ratio w y / w x = 1.1 ) and with an expected beam radius in the plasma ranging from 18.6 to 26.0 mm due to cited zooming effects. Additional beam patterns and linear scans of the launched mm-wave power for different injection angles have been measured, in some cases in realistic conditions (using a mock-up of the FTU port provided with metallic side-walls) to evaluate diffraction and beam truncation effects.

The ITER EC H&CD Upper Launcher: Transient mechanical analysis

Four Electron Cyclotron Heating and Current Drive (EC H&CD) Upper Launchers (UL) are foreseen for plasma stabilization purposes in ITER. The mission of the UL is to mitigate neoclassical tearing modes (NTM) and the sawtooth instability by localized heating and current drive with high precision and accuracy. Special attention is to be aimed to the structural design of the UL, because of highly restrictive space requirements. The ECH upper launcher has a length close to 6 m and the nominal gap to the neighbouring components is 20 mm. Electromagnetic perturbations could induce forces on the conductive materials and result in an excessive displacement of the portions that are located far from the fixation. During the last years, the design of the UL has been optimized by means of static simulations of the structure. The peak loads obtained by the electromagnetic simulation of the eddy currents induced during an upward Vertical Displacement Event (VDE) (considered as the worst case disruption scenario for the UL) were used. Static simulations are a fast tool for comparing different structure configurations and optimization of the UL design, but they do not provide a complete picture of the real situation, since they do not take into account for dynamic effects that could lead to larger displacements. In this paper, modal and transient analyses of the structure of the quasi-optical design (Preliminary Design Review status) of the upper launcher are presented. The results of transient and static analysis have been compared together and the Dynamic Amplification Factor (DAF) with respect to the transient loads have been assessed. The transient study and the modal analysis have shown that the EC upper launcher has a stiff structure with relatively high natural oscillation frequencies (compared to the duration of the VDE) and that the dynamic effects do not constitute a critical issue.

Quasioptical design of integrated Doppler backscattering and correlation electron cyclotron emission systems on the DIII-D tokamak

The quasioptical design of a new integrated Doppler backscattering (DBS) and correlation electron cyclotron emission (CECE) system is presented. The design provides for simultaneous measurements of intermediate wavenumber density and long wavelength electron temperature turbulence behavior. The Doppler backscattering technique is sensitive to plasma turbulence flow and has been utilized to determine radial electric field, geodesic acoustic modes, zonal flows, and intermediate scale (k∼1-6 cm(-1)) density turbulence. The correlation ECE system measures a second turbulent field, electron temperature fluctuations, and is sensitive to long poloidal wavelength (k≤1.8 cm(-1)). The integrated system utilizes a newly installed in-vessel focusing mirror that produces a beam waist diameter of 3.5-5 cm in the plasma depending on the frequency. A single antenna (i.e., monostatic operation) is used for both launch and receive. The DBS wavenumber is selected via an adjustable launch angle and variable probing frequency. Due to the unique system design both positive and negative wavenumbers can be obtained, with a range of low to intermediate wavenumbers possible (approximately -3 to 10 cm(-1)). A unique feature of the design is the ability to place the DBS and CECE measurements at the same radial and poloidal locations allowing for cross correlation studies (e.g., measurement of nT cross-phase).

Nuclear-Safety-Related and Shielding Analyses of the ITER Quasi-Optical ECH Launcher

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Nuclear-safety-related and shielding analyses were performed for the quasi-optical design of the electron-cyclotron-heating launcher installed in the ITER upper port. The critical nuclear responses, affecting the nuclear safety and technical performance of the launcher and nearby components, were calculated using the MCNP5 3-D radiation transport code. The computer-aided design (CAD) interface program McCad was applied for the conversion of the CAD model into MCNP geometry description. The radiation effects of the launcher's halved internal shield were estimated on the chemical-vapor-deposit diamond windows, vacuum vessel, and toroidal field coils. It was revealed that the nuclear-safety requirements were satisfied. </para>

Two-dimensional phase unwrapping to help characterize an electromagnetic beam for quasi-optical mode converter design

An improved two-dimensional phase unwrapping procedure is discussed that uses a weighted least-squares algorithm, a congruence operation, and a filter to unwrap the phase distribution of an electromagnetic beam. These improvements make possible several advances for mirror designs used in gyrotron quasi-optical mode converters. The improved phase unwrapping procedure is demonstrated by applying it to a measured beam and a simulated beam that are used to design mirrors. The unwrapping procedure produces a smooth unwrapped phase that does not change the characteristics of the beam. The smooth unwrapped phase distribution is also used to find an estimate for the wavenumber vector distribution that is needed to design the mirrors.

The Quasi-Optical Design of the QUaD Telescope

The Quasi-Optical Design of the QUaD Telescope

The quasi-optical design of the QUaD telescope

We describe the optical design and performance of ’QUEST and DASI’ or ’QUaD’, a ground-based high-resolution experiment designed to measure the polarisation properties of the cosmic microwave background radiation. QUaD uses bolometric detectors at 100 and 150GHz on a 2.6m Cassegrain telescope. The QUaD optics are designed to minimise systematic effects as well as to maximise sensitivity, and we report here on the comprehensive quasi-optical analysis used to achieve this design. We also present initial optical performance measurements achieved in operation, and discuss changes made to the optics to overcome some errors in the mechanical construction of the primary mirror. The QUaD experiment is now fully operational and taking world-leading data at the South Pole.

Low Noise 1 THz–1.4 THz Mixers Using Nb/Al-AlN/NbTiN SIS Junctions

We present the development of a low noise 1.2 THz and 1.4 THz SIS mixers for heterodyne spectrometry on the Stratospheric Observatory For Infrared Astronomy (SOFIA) and Herschel Space Observatory. This frequency range is above the limit for the commonly used Nb quasi particle SIS junctions, and a special type of hybrid Nb/AlN/NbTiN junctions has been developed for this project. We are using a quasi-optical mixer design with two Nb/AlN/NbTiN junctions with an area of 0.25. The SIS junction tuning circuit is made of Nb and gold wire layers. At 1.13 THz the minimum SIS receiver uncorrected noise temperature is 450 K. The SIS receiver noise corrected for the loss in the LO coupler and in the cryostat optics is 350-450 K across 1.1-1.25 THz band. The receiver has a uniform sensitivity in a full 4-8 GHz IF band. The 1.4 THz SIS receiver test at 1.33-1.35 THz gives promising results, although limited by the level of available LO power. Extrapolation of the data obtained with low LO power level shows a possibility to reach 500 K DSB receiver noise using already existing SIS mixer.

The quasi-optical design of telescopes to image the early universe

The cosmic microwave background (CMB), relic radiation from the Big Bang, had very faint temperature and polarization fluctuations imprinted on it as a structure formed in the early universe. By mapping the temperature fluctuations (∆T/T ≈ 10−5), cosmologists have learned a great deal. But in order to determine cosmological parameters uniquely and to rigorously test theories of the very early universe, the even fainter polarization fluctuations must be mapped. This demands a significant increase in sensitivity over earlier experiments and an in-depth understanding and control of telescope optics, in particular their effect on polarized signals. In order to maximize the CMB signal against foregrounds such as the atmosphere and galactic emission, most observations are made at frequencies between 0.02 and 0.9THz, which present their own set of unique challenges. Optical design in the terahertz waveband can be challenging and difficult to do with any confidence using techniques developed for visible wavelengths, especially for high-precision applications such as CMB astronomy. The Space Optics Design and Analysis group at the National University of Ireland, Maynooth (NUI Maynooth), specializes in the field of quasi-optical design for astronomical instrumentation.1 Optical design is concerned with the problem of calculating an electromagnetic field over a surface in an optical system when the field, or currents, over some other surface is known. The full solution to Maxwell’s equations is usually extremely difficult to find, and in practice approximations have to be made. Although physical optics (PO) can be used to characterize electromagnetic systems to high accuracy, it is computationally intensive at terahertz frequencies and often Figure 1. A quasi-optical telescope (the QUaD telescope) set up in MODAL. The model includes 31 corrugated conical horns at the focal plane (modeled using electromagnetic mode matching), two lenses, and a Cassegrain telescope. The fundamental Gaussian mode is used for beam visualization.

Optimal synthesis of quasi-optical launchers for high-power gyrotrons

Quasi-optical launcher designs for high-power gyrotrons use perturbations of the waveguide surface following the cavity to convert the high order cavity mode to a Gaussian-like beam, which is then radiated from a spiral cut of the waveguide. The design technique of these launchers is based on an analytic theory that specifies the mode amplitudes to obtain a Gaussian-like distribution of the field in the azimuthal and longitudinal directions. This approach can generate a beam with high Gaussian content and low diffraction losses. However, by adjusting the mode amplitudes to obtain a Gaussian-like distribution of the field in the radial (instead of the azimuthal) direction, designs with significantly less diffraction loss, reduced beam divergence angles, and higher Gaussian beam content can be achieved

Diffraction Synthesis and Experimental Verification of a Quasi-Optical Power Splitter at 150 GHz

A general and flexible synthesis method based on the physical optics approximation is proposed for computing smooth-surface reliefs of reflectors. As a design example, the method has been applied to a quasi-optical power splitter consisting of two reflectors in a dual-offset configuration that couples a launched beam from a pyramidal horn antenna into a 2/spl times/4 horn antenna array. The two reflectors were treated as diffractive phase elements so that the proposed synthesis method allows reflector designs for many applications. The quasi-optical design has been confirmed at 150 GHz utilizing a vector field measurement system. The measured field distribution in the receiving antenna array plane is compared with the simulated one and shows a very good agreement.

A Two-Dimensional Quasi-Optical Power Combining Oscillator Array With External Injection Locking

A quasi-optical power combiner for a 4/spl times/4 IMPATT oscillator array has been designed and experimentally investigated. The combiner consists of a bi-periodic dielectric phase grating which transforms the near field of a rectangular horn array into a pseudoplane wave. The horn array is excited by oscillators which operate uniformly in both amplitude and phase. A parabolic mirror with a superimposed surface relief couples the pseudoplane wave into a rectangular output horn antenna. In principle, the combiner has no restriction in inter-element spacing and is hence scalable up to submillimeter wavelengths without degradation of power combining efficiency. The quasi-optical design has been verified by scalar field measurements in several planes. The oscillator matrix is injection-locked by a master oscillator from the output port. A continuous wave output power of 1.3 W with an overall power combining efficiency of 70% has been measured at 65 GHz.

Semiconductor Detectors, Mixers, and Frequency Multipliers for the Terahertz Band

The state of the art in the development of semiconductor detectors, mixers, and frequency multipliers based on Schottky-barrier diodes (SBDs) and heterojunction structures for uncooled terahertz receivers is reviewed. The present status of this field features a transition from quasi-optical designs based on dot-matrix, whisker-contacted SBDs to the designs with hybrid-integrated and monolithic constructions on the planar SBD base, which are positioned in a waveguide mount. The high-level performance of these planar devices is achieved by partially or completely removing or changing semiconductor substrates and/or using membrane constructions incorporated in the waveguide.

Single-crystal EPR studies of transition-metal ions in inorganic crystals at very high frequency

Electron paramagnetic resonance (EPR) single-crystal rotation studies at very high frequency (249.9 GHz) of transition metal ions with electron spins greater than one-half are reported. At 249.9 GHz, the spectra are in the high-field limit despite large zero-field splittings. This leads to a considerable simplification of the spectra, and aids in their interpretation. Single-crystal 249.9 GHz EPR spectra of Ni2+ in Ni2CdCl6· 12H2O, Mn2+ (0.2%) in ZnV2O7, and Fe3+ (2%) in CaYA104 were recorded at 253 K in an external magnetic field of up to 9.2 T, along with those at X-band and Q-band frequencies at 295 K and lower temperatures. The goniometer used at 249.9 GHz for single-crystal rotation is based on a quasi-optical design and is an integral part of a special Fabry-Perot resonator. The values of the spin-Hamiltonian parameters were estimated from a simultaneous fitting of all of the observed line positions at several microwave frequencies recorded at various orientations of each crystal with respect to the external magnetic field with least-squares fitting in conjunction with matrix diagonalization. Estimates of zero-field splitting parameterD at room temperature are: for Ni2+, about −31 GHz (site I) and about −7 GHz (site II); for Mn2+, about 6 GHz; and for Fe3+, about 29 GHz.

Superconducting integrated receiver as 400-600 GHz tester for coolable devices

A laboratory-purpose submillimeter wavelength receiver is developed and tested. The device can in situ detect the spectrum of an rf source working below 100 K within the frequency range 400-600 GHz. A sample is placed close to the receiver sensor; both are in vacuum inside the probe stick and cooled by liquid helium in the standard transport dewar. The quasioptical sensor design is based on the superconducting integrated receiver chip; its noise temperature below 300 K was measured at /spl ap/500 GHz with a variable-temperature black body. The output level of the tester is suitable for the direct readout by a spectrum analyzer. Details of the design and main test data are reported.