Quasi-optical System Research Articles

Design of an electron cyclotron emission diagnostics suite for COMPASS Upgrade tokamak.

COMPASS Upgrade is a medium size and high field tokamak that is capable of addressing key challenges for reactor grade tokamaks, including power exhaust and advanced confinement scenarios. Electron cyclotron emission will be available among the first diagnostics to provide measurements of high spatial and temporal resolution of electron temperature profiles and electron temperature fluctuation profiles through a radial view. A separate oblique view at 12° from normal will be utilized to study non-thermal electrons. Both the radial and oblique views are envisioned to be located in a wide-angle midplane port, which has dimensions that enable simultaneous hosting of the front-end of their quasi-optical (QO) designs. Each QO design will have an in situ hot calibration source in the front-end to provide standalone and calibrated Te (R,t) measurements. The conceptual design for each QO system, the Gaussian beam analysis, and the details of the diagnostic channels are presented.

Small-Signal Theory of the Gyro-BWO With the Zigzag Quasi-Optical System

We describe the small-signal theory of a backward-wave gyro-oscillator with a sectioned quasi-optical system with the zigzag path of the operating wave beam. This theory explains peculiarities (namely, a piecewise character) of changing the operating frequency and power of the output signal in the process of the broadband frequency tuning provided by changing the operating magnetic field.

Focused Connected Array Antenna as a Broadband Beam-Steering Feed for Quasi-Optical System

This communication proposes a broadband and efficient integrated focal plane array solution based on a near-field focused connected array of slots. The focused aperture provides: 1) broadband and highly efficient illumination of the quasi-optical system and 2) scanning capabilities within a focusing system. The connected array antenna in turn allows for a fully integrated solution that can synthesize a focused aperture while providing broadband impedance matching. Focused connected array antennas enable the coupling to a reflector system over bandwidths in excess of one octave and with aperture efficiencies in excess of 60%. To demonstrate the concept, we present two printed circuit board (PCB) prototypes operating in the band 3–6 GHz and yielding more than 60% reflector aperture efficiency under broadside illumination and allowing to scan one beamwidth at the lowest frequency with a frequency-averaged scan loss of 0.2 dB. The feasibility of scaling this concept to THz frequencies and with dynamic beam-steering capabilities is discussed in the context of a superconducting device.

A Design Method of Three-Dimensional Multireflector Quasi-Optical Systems Based on Gaussian Beam Mode Analysis

Two-dimensional (2-D) quasi-optical systems have been widely applied. However, the analysis and design of 3-D systems still need to be further developed. In this letter, an approach to rapidly analyze the performance of 3-D multireflector systems based on Gaussian beam mode analysis is proposed. The influence of reflector parameters, such as input distance, incidence angle, and so on, on the co- and cross polarization is discussed. Furthermore, a novel design method of high-performance 3-D multireflector systems is proposed. High computational efficiency is one of the advantages of this method due to the application of particle swarm optimization algorithm. 3-D systems with low cross polarization or other electromagnetic properties can be designed rapidly by this novel method. The proposed methods greatly improve the calculation efficiency, while providing calculation accuracy similar to that of physical optics (PO) in GRASP10.

Accurate S-Parameter Modeling and Material Characterization in Quasi-Optical Systems

In the article, we present an analytical model of a beam interaction with a sample in a quasi-optical system suitable for high accuracy S-parameter prediction and material extraction. Instead of assuming a plane wave illumination of the sample, our model works with the fundamental Gaussian-beam mode (TEM <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{00}$</tex-math></inline-formula> ) incident on a homogeneous parallel sided slab with unknown refractive index. Using the Gaussian-beam coupling theory the sample’s parameters can be determined from the level of decoupling (defocusing) in the optical system caused by the sample’s presence. The complex transmission and reflection coefficients can both be calculated using the model for normal as well as for oblique incidence. Moreover, the model also allows for the inclusion of imperfections in the quasi-optical system such as frequency-dependent properties of source and detector antennas and nominal beam misalignment. We successfully test the model by measuring a high quality reference sample (refractive index independently characterized with an open resonator technique) in the frequency band 110–170 GHz. A comparison in terms of S-parameters and extracted refractive indices between our Gaussian-beam coupling model and a standard plane wave model is also given showing the relevance of our work.

High-Resolution 400 GHz Submillimeter-Wave Quasi-Optical Radar Imaging System

A submillimeter-wave quasi-optical high-resolution imaging system, based on a frequency-modulated continuous wave radar sensor operating between 343GHz and 423GHz utilizing a bandwidth of 80GHz, is presented. The system setup in-tandem with the essential building blocks is described and the generation of high-resolution power- and elevation-profile images of various samples under test is presented. Experimental results show a lateral resolution limit in the order of wavelength.

Design of the cross-polarization scattering diagnostic on the HL-2A tokamak

A new cross-polarization scattering (CPS) diagnostic has been developed on HL-2A, which aims to measure the local magnetic fluctuation inside the plasma. It is based on the scattering of an incident microwave beam into the perpendicular polarization by magnetic fluctuations. The CPS diagnostic has been designed in the Q-band (33–50 GHz), which consists of the electronic system, quasi-optical, and polarization rejector. The ray-tracing code is used to simulate the propagation of the probe and scattered rays. To test the performance of the quasi-optical system, a 3D test platform is built and detailed test results are shown. Two methods are developed for polarization rejector on HL-2A: wire grid polarizer and dual-polarized horn antenna (DPHA). The laboratory test result shows that the polarization rejection of both methods is better than 30 dB, which meets the needs for magnetic fluctuation detection. In the future, the CPS diagnosis will be used to study the electromagnetic turbulence behavior in the high-performance plasma of the HL-2A tokamak.

Compact, tunable polarization transforming reflector for quasi-optical devices used in terahertz science

We describe the design, fabrication, and characterization of a compact polarization transforming reflector (PTR). The device can be easily tuned over a broad frequency range, has very little insertion losses, and can easily be integrated into quasi-optical systems that are based on a half-cube design. By varying the distance between the wire grid and a flat mirror, the polarization state of an arbitrary polarized Gaussian incident beam can be set to an output Gaussian beam in either linear or circular polarization. In addition, by varying the orientation of the wire grid, the PTR can be used as a universal polarizer, a property that has not been discussed or demonstrated in the literature. The ability to control the electric field polarization at Terahertz (THz) frequencies is essential for many applications, such as THz spectroscopy and high-field electron paramagnetic resonance spectroscopy.

Frequency and mode measurement techniques for megawatt-class gyrotrons

Abstract State-of-the-art vacuum electron tubes such as gyrotrons, deliver RF output powers up to more than 2 MW at frequencies up to 170 GHz. In terms of the very high power levels, a proper verification of the gyrotron components itself and measurements during gyrotron operation are vital to prevent possible fatal errors. Several basic RF measurement setups, which are used at IHM/KIT, are discussed. Currently, their upper frequency limit is 175 GHz. In terms of future gyrotron operation above 200 GHz, upgrades of the measurement setups for operation up to 260–330 GHz are prepared. The experimental devices discussed herein are a quasi-optical mode generator for the verification of the quasi-optical gyrotron output system, the window measurement test stand to verify the ceramic gyrotron output window and the frequency diagnostic system to measure the operating frequency and thereby the excited mode.

The Results of the Experimental Study of the Method of Directional Radiation Pattern Transformation of a Conical Horn Radiator Operating in Millimetric (Sub-THz) Band

The article describes the experimental study of one of techniques of implementation of a machine-vision illuminant operating within the millimetre (sub-THz) band and forming a directional radiation pattern (DRP) with parameters compliant with the specifics of functioning of radio vision systems. In view of this, experimental research of the method of formation of a DRP configuration of a millimetre (sub-THz) machine vision system illuminant based on a conical horn radiator adapted to receiver field of view was studied experimentally using a quasi-optic system. The results of the study of the DRP configuration of such illuminant designed using a quasi-optic system installed in the aperture of a conical horn radiator are presented.

Laser requirements for the designof fast laser-driven semiconductor switches for THz and mm-waves.

A reduced parameter model of fast laser-driven semiconductor switches of THz and mm-waves has been developed. The model predicts peak reflectivity and minimum transmissivity of switches, showing good agreement with experimental data, while requiring fewer inputs than published models. This simplification facilitated a systematic survey of laser parameters required for efficient switching. Laser energy density requirements are presented as a function of laser wavelength, laser pulse width, switched frequency, reflection angle, and semiconductor material (silicon or gallium arsenide). Analytical expressions have been derived to explain the dependence of laser requirements on switch parameters and to derive practical minima of required laser energy density. Diffusion is shown to quickly negate the shallow absorption advantage of laser wavelengths shorter than about 500nm in silicon or 800nm in gallium arsenide. Decreasing laser pulse width, to a derived limit, and switching S-polarized THz or mm-wave signals are shown to be means of lowering required laser energy. This is an especially useful result for devices operating at high power levels or THz frequencies, where extended switches are used in quasioptical systems.

Quasi-optical design of a millimeter wave imaging system

We report a complete design and simulation of a quasi-optical millimeter wave imaging system using ZEMAX and FEKO software, respectively. A Fresnel lens and a horn antenna are combined in this design. Compared to spherical and aspherical lenses, a Fresnel lens can be fabricated much easier at millimeter wavelengths. For focusing millimeter wave radiation, a Fresnel lens can be used to reduce the thickness of the focusing element and to lighten its weight from 25 to 4.5 kg. A horn antenna with a Gaussian profile and corrugated walls is designed for feeding this system at a central frequency of 94 GHz. The symmetrical radiation pattern of the designed corrugated Gaussian horn in E- and H-orthogonal planes, its wide bandwidth, and low side lobe levels make it a good candidate for feeding a W-band millimeter wave imaging system. The designed quasi-optical imaging system is light-weighted, has high resolution, and can be used in detecting hidden objects within a distance of 5 m with a 30-mm resolution in W-band at a central frequency of 94 GHz.

Development of fast-ion collective Thomson scattering diagnostics for the GDT experiment

In this paper, we describe hardware developed for the collective Thomson scattering (CTS) diagnostic for the large open magnetic trap GDT (Budker Institute, Russia) used in fusion researches. The diagnostic utilizes 400 kW/54.5 GHz gyrotron as a source of probe radiation and is aimed at the reconstruction of fast ion distributions over transverse and longitudinal velocities driven by high-power neutral beam injection. Formation and manipulation of the probe beam and collection of the scattered microwave radiation are done with large-aperture quasi-optical mirror systems. The receiving complex consists of two identical channels for simultaneous detection of scattering in two complement geometries. Each channel includes a highly sensitive radiometer/spectrum analyzer with the band of 54.470 ± 0.550 GHz, the noise temperature of 0.35 eV, the sensitivity less than 0.025 eV, the frequency/time resolution of 5–50 MHz/10 μs, and narrow-band notch filter providing at least -40 dB rejection in the band of 25 MHz with a central frequency tunable in 54.470 ± 0.050 GHz for protection of the input RF circuit from a backlight of powerful gyrotron radiation.

Design validation of in-vessel mirrors and beam dump for first plasma operations in ITER

First plasma operation in ITER will start after completing the assembly of the tokamak vessel and the installation of the main sub-systems, but prior to the installation of the blanket modules and the divertor cassettes. Utilization of temporary limiters and divertor replacement structures will provide a poloidal and toroidal reference side position to the plasma edge to protect the vacuum vessel and other components already installed during operations. An additional set of mirrors is required to reflect the power injected from the upper launcher towards the plasma resonance for EC-assisted breakdown of the plasma up to a beam dump needed to trap and absorb the power of the beams after crossing the plasma in order to reduce the stray radiation escaping back into the vacuum chamber down to less than 10% of the total power. The quasi-optical system has been designed, with shape and size of the mirrors compliant with the requirements provided by ITER for their installation, realization and plasma performances, resulting in two standard focussing mirrors and one grating mirror. The beam dump consists of a box with five metal plates, the first providing a spreading of the high incident power and the others coated with absorbing material with thickness distribution studied to gradually reduce the power during the multiple reflections inside the box, avoiding damages to the coating itself. This work focuses on the validation of the quasi-optical design of the mirrors and the assessment of the dump performances, based on a multi-bounces model developed ad-hoc for this purpose. The study includes a tolerance analysis for the beam dump to include the effect of uncertainties in the thickness of the absorbing coating and misalignments of the mirrors, to verify the performances of the dump also when operating in different conditions with respect to the nominal ones.

A remote gain controlled and polarization angle tunable Doppler backward scattering reflectometer.

Remote control of the diagnostic systems is the basic requirement for the high performance plasma operation in a fusion device. This work presents the development of the remote control system for the multichannel Doppler backward scattering (DBS) reflectometers. It includes a remote controlled quasi-optical system and a remote intermediate frequency (IF) amplifier gain control system. The quasi-optical system contains a rotational polarizer, its polarization angle is tunable through a remote controlled motor, and it could combine the microwave beams with a wide frequency range into one focused beam. The remote IF gain control system utilizes the digital microcontroller (MCU) technique to regulate the signal amplitude for each signal channel. The gain parameters of amplifiers are adjustable, and the feedback of working status in the IF system will be sent to MCU in real time for safe operation. The gain parameters could be controlled either by the Ethernet remote way or directly through the local control interface on the system. Preliminary experimental results show the effectiveness of the remote controlled multichannel DBS system.

350-GHz Bandpass Filters Using Superconducting Coplanar Waveguide

On-chip filters play an invaluable role in most detectors and instruments, blocking unwanted, harmonic, or mirrored signals. In this article, two coplanar waveguide bandpass filters working at 350 GHz are designed based on quarter-wavelength stepped impedance resonators, which can be easily integrated in on-chip systems. A second-order filter with a classical Chebyshev response and a fourth-order filter with quasi-elliptical characteristic is developed together to define the roll-off performance. Both filters are patterned based on superconducting niobium film by the optical lithography and lift-off process, which could reduce the significant metallic loss in high terahertz band. All the chips integrated with transitions are evaluated through a quasi-optical system, which is built based on the sensitive superconductor-insulator-superconductor tunnel junction detector at cryogenic temperature. The calibrated transmission responses, which are matched with simulations, have indicated that such superconducting coplanar waveguide filters can be enabled in future terahertz astronomical spectrometer.

Continuous terahertz wave imaging of microelectronics objects

The results of using a terahertz imaging system for microelectronic flaw detection are presented. The distribution of the transmitted radiation through the plastic leaded chip carrier microchip at frequencies of 206 and 806 GHz is obtained. The possibility of using quasi-optical systems with a continuous terahertz radiation source for microelectronic flaw detection has been evaluated.

A Gaussian Beam Mode Analysis Method for 3-D Multi-Reflector Quasi-Optical Systems

3-D quasi-optical systems have a more comprehensive range of application scenarios, and their analysis and design are more complicated than those of 2-D systems. In this work, we improve Gaussian beam mode analysis (GBMA) to analyze 3-D multi-reflector systems. The expressions of co- and cross-polarization and their derivations are given and discussed in detail. Furthermore, several 3-D dual-reflector systems with different rotation angles are chosen as simulation examples to assess the validity and precision of 3-D GBMA compared with physical optics (PO) in the commercial software GRASP10. Furthermore, a 3-D double ellipsoidal reflector system with a π/2 rotation angle operating at 183 GHz is designed, manufactured, and tested. Measured results of the system demonstrate that it is in good agreement with the simulated results of 3-D GBMA and PO for both the co- and cross-polarization. By comparing the computing time performance of 3-D GBMA and PO in GRASP10, the high efficiency of 3-D GBMA is clarified. With 3-D GBMA, the field in 3-D quasi-optical systems can be calculated preciously and rapidly.

Quasi-optical design of EC H&CD launcher for CFETR

The China Fusion Engineering Test Reactor (CFETR) is the next-generation device for China magnetic controlled fusion program. A high power (36 MW) and continuous millimeter-wave injected by two Upper Launchers (ULs) are planned for Electron Cyclotron Heating and Current Drive (EC H&CD) in CFETR. A 3D visual quasi-optical design tool for the UL has been developed based on MATLAB that is capable of accurate visualization of beam propagations and the evaluations of quasi-optical system layout in the UL. One UL has two identical sets of quasi-optical modules, each one is mainly composed of nine fixed focusing mirrors and one flat steering mirror. By using the design tool to optimize the quasi-optical system for the UL, the minimum concentration radius (∼ 110 mm) of the nine mm-wave beams is obtained at the resonance layer. The evaluation results of the peak heat load on the FMs and SM obtained were 0.71 MW/m2 and 1.46 MW/m2, respectively.

A Fourier Optics Tool to Derive the Plane Wave Spectrum of Quasi-Optical Systems [EM Programmer's Notebook

Quasi-optical (QO) antennas employ feeds in combination with lenses or reflectors. Design optimization of such antennas can be expensive due to the high computational cost associated with the analysis of electrically large and multiscale structures. This issue's “EM Programmer's Notebook” column describes a freely available MATLAB-based tool for the fast analysis of QO antennas. It facilitates the design of QO antenna feeds through the introduction of a field-matching methodology. It could also be useful in the codesign of feeds and QO components. The tool provides valuable complimentary capabilities to commercial packages.