Development of electron cyclotron emission imaging system on the HL-2A tokamak

Frontier system-on-chip (SoC) technology for microwave diagnostics (invited)

The next generation of fusion reactors, exemplified by projects such as the Demonstration Power Plant following the International Thermonuclear Experimental Reactor, faces the monumental challenge of proving the viability of generating electricity through thermonuclear fusion. This pursuit introduces heightened complexities in diagnostic methodologies, particularly in microwave-based diagnostics. The increased neutron fluence necessitates significant reductions in vessel penetrations and the elimination of internal diagnostics, posing substantial challenges. SoC technology offers a promising solution by enabling the miniaturization, modularization, integration, and enhancing the reliability of microwave systems. After seven years of research, our team successfully pioneered the V- and W-band system-on-chip approach, leading to the development of active transmitters and passive receiver modules applied in practical settings, notably within the DIII-D tokamak project. Arrays of these modules have supported microwave imaging diagnostics. New physics measurement results from the Electron Cyclotron Emission Imaging system on DIII-D provide compelling evidence of improved diagnostics following the adoption of SoC technology. Furthermore, we achieved a breakthrough in developing an F-band SoC, advancing higher frequency capabilities for fusion devices. These achievements represent a significant leap forward in fusion diagnostic technology, marking substantial progress toward establishing reliable and efficient plasma diagnostics for future fusion reactors.

Development of a toroidally resolved broadband ECE imaging system for measurement of turbulent fluctuations on the KSTAR

The two electron cyclotron emission imaging (ECEI) systems installed at adjacent ports (G and H) on the KSTAR tokamak incorporate large-aperture mm-wave optics, broadband electronics, and high speed digitization (up to 1 MSa/s) for 2D and quasi-3D visualization of MHD-scale fluid dynamics. Recently, the ECEI systems have been proved to be capable of visualization of smaller scale fluctuations albeit with a limited spatiotemporal resolution and even capable of measurement of ion cyclotron harmonic waves by direct high-speed sampling of the ECE IF signals. A four-channel prototype subsystem with a higher sampling rate up to 16 GS/s has been integrated into the G-port ECEI system, enabling the measurement of plasma waves in the GHz range in the form of modulated ECE signals and characterization of high-frequency turbulence during the evolution of pedestal. To achieve higher toroidal resolution in the turbulence measurement, the H-port ECEI system is now being upgraded to have a toroidally dual detector array of 2(toroidal) × 12(vertical) × 8(radial) channel configuration and a high-speed subsystem of 2(toroidal) × 4 channel configuration. The new mm-wave optics has been designed via beam propagation simulation, and the measured performance of the fabricated lens indicates a toroidal resolution of 8-10 cm depending on the focus position and zoom factor, allowing for the measurement of parallel wavenumber up to k‖ ∼ 0.8 cm-1.

Design of a 140 GHz waveguide notch filter for millimeter-wave receiver module protection in fusion plasma diagnostics

A carefully designed waveguide-based millimeter-wave notch filter, operating at 140 GHz, safeguards plasma diagnostic instruments from gyrotron leakage. Utilizing cylindrical cavity resonators with aperture coupling, the filter efficiently resonates 140 GHz wave-power into the TE11p mode, optimizing various geometrical parameters for practical fabrication and high-yield production. Thorough thermal analysis ensures its ability to handle power. The filter achieves outstanding performance with over 90 dB rejection at 140 GHz while providing low insertion loss over the passband (110-138 GHz), which is ideally suited for system-on-chip approach F-band diagnostic system applications.

MHD instability dynamics and turbulence enhancement towards the plasma disruption at the HL-2A tokamak

The evolutions of MHD instability behaviors and enhancement of both electrostatic and electromagnetic turbulence towards the plasma disruption have been clearly observed in the HL-2A plasmas. Two types of plasma disruptive discharges have been investigated for similar equilibrium parameters: one with a distinct stage of a small central temperature collapse ([Formula: see text] 5-10%) around 1 millisecond before the thermal quench (TQ), while the other without. For both types, the TQ phase is preceded by a rotating 2/1 tearing mode, and it is the development of the cold bubble from the inner region of the 2/1 island O-point along with its inward convection that causes the massive energy loss. In addition, the micro-scale turbulence, including magnetic fluctuations and density fluctuations, increases before the small collapse, and more significantly towards the TQ. Also, temperature fluctuations measured by electron cyclotron emission imaging enhances dramatically at the reconnection site and expand into the island when approaching the small collapse and TQ, and the expansion is more significant close to the TQ. The observed turbulence enhancement near the X-point cannot be fully interpreted by the linear stability analysis by GENE. Evidences suggest that nonlinear effects, such as the reduction of local [Formula: see text] shear and turbulence spreading, may play an important role in governing turbulence enhancement and expansion. These results imply that the turbulence and its interaction with the island facilitate the stochasticity of the magnetic flux and formation of the cold bubble, and hence, the plasma disruption.

Analysis of synthetic electron cyclotron emission from the high field side of HL-2M tokamak plasmas

A synthetic electron cyclotron emission (ECE) diagnostic is used to interpret ECE signals from preset plasma equilibrium profiles, including magnetic field, electron density, and electron temperature. According to the simulation results, the electron temperature (T_e) profile covering the harmonic overlap region can be obtained by receiving ECE signals at the high field side (HFS) of the HL-2M plasma. The third harmonic ECE at the low field side (LFS) cannot pass through the second harmonic resonance layer at the HFS unless the optical thickness (τ) of the second harmonic becomes gray (τ ≤ 2). In addition, the impact of the relativistic frequency down-shift has been evaluated and corrected. The measurable range of the HFS ECE has been calculated by scanning different parameters (electron density, temperature, and magnetic field). Higher plasma parameters allow a wider radial range of electron temperature measurements. The minimum inner measurable position can reach R = 120 cm (r/a = -0.89) when the product of core temperature (T_e0) and density (n_e0) is greater than 35 × 10¹⁹ keV m^-3, which is extended by more than 30 cm inward compared with that of the LFS measurement. The HFS ECE will greatly improve the diagnostic ability of ECE systems on the HL-2M tokamak.

Development of ECE/ECEI diagnostics and MHD-related studies on HL-2A tokamak

A novel 60-channel electron cyclotron emission (ECE) radiometer has been designed and tested for the measurement of electron temperature profiles on the HL-2A tokamak. This system is based on the intermediate frequency division technique, and has the features of wide working frequency range (60−90 GHz) and high temporal-spatial resolution (3 µs, 1 cm), which covers almost the entire plasma region. Also, an electron cyclotron emission imaging (ECEI) system has been developed for studying two dimensional electron temperature fluctuations. It is comprised of several front-end quasi-optical lenses, a 24 channel heterodyne imaging array with a tunable RF frequency range spanning 60−135 GHz, and a set of back-end ECEI electronics that together generate two 24×8 array images of the 2nd harmonic X-mode electron cyclotron emission from the HL-2A plasma. The measurement region can be flexibly shifted due to two independent local oscillator sources, and the field of view can be adjusted easily by changing the position of the zoom lenses as well. The temporal resolution is about 2.5 µs and the achievable spatial resolution is 1 cm. The ECE/ECEI diagnostics have been demonstrated to be powerful tools to study MHD-related physics including the multi-scale interaction between macro-scale MHD and micro-scale turbulence on the HL-2A tokamak.

Hyperbolic lens design of local oscillator optics system for electron cyclotron emission imaging on J-TEXT

An electron cyclotron emission imaging diagnostic system that contains two 16-antenna arrays is being developed on J-TEXT tokamak. In this heterodyne system, the mixers in the front microwave antenna are used to down-convert the electron cyclotron emission to a 2-12 GHz radio frequency. All of the 24 antenna mixers in the individual enclosure box are driven by shining local oscillator (LO) power via launching optics. The previous approach for LO optics was designed with spherical and cylinder lenses, which has limitations such as the inhomogeneity of the energy deposition on different channels and the difficulty of optics alignment. A new generation of LO optics has been designed and applied on J-TEXT with a hyperbolic lens for uniform power deposition across the entire antenna array. The robustness of the optical alignment will be significantly increased with three hyperbolic lenses. Furthermore, the simulation results and robustness analysis of these LO optics are discussed in this paper.

Liquid crystal polymer receiver modules for electron cyclotron emission imaging on the DIII-D tokamak

A new generation of millimeter-wave heterodyne imaging receiver arrays has been developed and demonstrated on the DIII-D electron cyclotron emission imaging (ECEI) system. Improved circuit integration, improved noise performance, and enhanced shielding from out-of-band emission are made possible by using advanced liquid crystal polymer (LCP) substrates and monolithic microwave integrated circuit (MMIC) receiver chips. This array exhibits ∼15 dB additional gain and >30× reduction in noise temperature compared to previous generation ECEI arrays. Each LCP horn-waveguide module houses a 3 × 3 mm GaAs MMIC receiver chip, which consists of a low noise millimeter-wave preamplifier, balanced mixer, and IF amplifier together with a local oscillator multiplier chain driven at ∼12 GHz. A proof-of-principle partial LCP instrument with 5 poloidal channels was installed on DIII-D in 2017, with a full proof-of-principle system (20 poloidal × 8 radial channels) installed and commissioned in early 2018. The enhanced shielding of the LCP modules is seen to greatly reduce the sensitivity of ECEI signals to out-of-band microwave noise which has plagued previous ECEI studies on DIII-D. The LCP ECEI system is expected to be a valuable diagnostic tool for pedestal region measurements, focusing particularly on electron temperature evolution during edge localized mode bursting.

Development of a real time magnetic island identification system for HL-2A tokamak

A novel real time magnetic island identification system for HL-2A is introduced. The identification method is based on the measurement of Mirnov probes and the equilibrium flux constructed by the equilibrium fit (EFIT) code. The system consists of an analog front board and a digital processing board connected by a shield cable. Four octal-channel analog-to-digital convertors are utilized for 100 KHz simultaneous sampling of all the probes, and the applications of PCI extensions for Instrumentation platform and reflective memory allow the system to receive EFIT results simultaneously. A high performance field programmable gate array (FPGA) is used to realize the real time identification algorithm. Based on the parallel and pipeline processing of the FPGA, the magnetic island structure can be identified with a cycle time of 3 ms during experiments.

Bayesian tomography and integrated data analysis in fusion diagnostics

In this article, a Bayesian tomography method using non-stationary Gaussian process for a prior has been introduced. The Bayesian formalism allows quantities which bear uncertainty to be expressed in the probabilistic form so that the uncertainty of a final solution can be fully resolved from the confidence interval of a posterior probability. Moreover, a consistency check of that solution can be performed by checking whether the misfits between predicted and measured data are reasonably within an assumed data error. In particular, the accuracy of reconstructions is significantly improved by using the non-stationary Gaussian process that can adapt to the varying smoothness of emission distribution. The implementation of this method to a soft X-ray diagnostics on HL-2A has been used to explore relevant physics in equilibrium and MHD instability modes. This project is carried out within a large size inference framework, aiming at an integrated analysis of heterogeneous diagnostics.

New compact and efficient local oscillator optic system for the KSTAR electron cyclotron emission imaging system

Electron cyclotron emission imaging (ECEI) diagnostic on Korean Superconducting Tokamak Advanced Research utilizes quasi-optical heterodyne-detection method to measure 2D (vertical and radial) T_e fluctuations from two toroidally separated poloidal cross section of the plasma. A cylindrical lens local oscillator (LO) optics with optical path length (OPL) 2-2.5 m has been used in the current ECEI system to couple the LO source to the 24 vertically aligned array of ECE detectors. For efficient and compact LO optics employing the Powell lens is proposed so that the OPL of the LO source is significantly reduced from ∼2.0 m to 0.4 m with new optics. The coupling efficiency of the LO source is expected to be improved especially at the edge channels. Results from the optical simulation together with the laboratory test of the prototype optics will be discussed in this paper.

Design of the 2D electron cyclotron emission imaging instrument for the J-TEXT tokamak

A new 2D Electron Cyclotron Emission Imaging (ECEI) diagnostic is being developed for the J-TEXT tokamak. It will provide the 2D electron temperature information with high spatial, temporal, and temperature resolution. The new ECEI instrument is being designed to support fundamental physics investigations on J-TEXT including MHD, disruption prediction, and energy transport. The diagnostic contains two dual dipole antenna arrays corresponding to F band (90-140 GHz) and W band (75-110 GHz), respectively, and comprises a total of 256 channels. The system can observe the same magnetic surface at both the high field side and low field side simultaneously. An advanced optical system has been designed which permits the two arrays to focus on a wide continuous region or two radially separate regions with high imaging spatial resolution. It also incorporates excellent field curvature correction with field curvature adjustment lenses. An overview of the diagnostic and the technical progress including the new remote control technique are presented.

Note: Upgrade of electron cyclotron emission imaging system and preliminary results on HL-2A tokamak

The electron cyclotron emission imaging system on the HL-2A tokamak has been upgraded to 24 (poloidally) × 16 (radially) channels based on the previous 24 × 8 array. The measurement region can be flexibly shifted due to the independence of the two local oscillator sources, and the field of view can be adjusted easily by changing the position of the zoom lenses. The temporal resolution is about 2.5 μs and the achievable spatial resolution is 1 cm. After laboratory calibration, it was installed on HL-2A tokamak in 2014, and the local 2D mode structures of MHD activities were obtained for the first time.

Quasi 3D ECE imaging system for study of MHD instabilities in KSTAR

A second electron cyclotron emission imaging (ECEI) system has been installed on the KSTAR tokamak, toroidally separated by 1/16th of the torus from the first ECEI system. For the first time, the dynamical evolutions of MHD instabilities from the plasma core to the edge have been visualized in quasi-3D for a wide range of the KSTAR operation (B0 = 1.7∼3.5 T). This flexible diagnostic capability has been realized by substantial improvements in large-aperture quasi-optical microwave components including the development of broad-band polarization rotators for imaging of the fundamental ordinary ECE as well as the usual 2nd harmonic extraordinary ECE.