This paper discusses different methods to improve the radiation efficiency of plasma antennas in the Reconfigurable Central Plasma Antenna Array (RCPAA). The simulation is carried out in CST Microwave Studio, which shows that optimising the orientation and size of the gap introduced between the ground plane and surface wave coupler (SWC), representing a novel application of the defected ground plane (DGP) technique to plasma antennas, improves the gain and radiation efficiency. This paper proposes a modified equivalent circuit model incorporating additional lumped elements ( and ) to analyse the coupling effect on the plasma antenna. The RCPAA achieves an of -32.72 dB at 850 MHz with a improved gain of 3.3 dB and impedance bandwidth of 500 MHz. The gap also contributes to more than a 10% increase in the radiation efficiency of plasma antennas at resonant frequency, with a maximum efficiency of 25%.

The study presents the design and development of a plasma-based antenna array system for the UHF band. The system features an omnidirectional plasma antenna as its active radiator, a novel approach that has not been previously explored. Surrounding plasma reflectors around the plasma antenna form a reconfigurable array. The plasma antenna is the only radiating element in the array, whereas the surrounding plasma reflectors enhance the gain and directivity of the array, facilitating beam shaping and steering. This new design creates a fully plasma based system enabling dual reconfigurability in frequency and radiation pattern. Electronic beam scanning is possible in the azimuthal plane from 0° to 360° in 18° steps. The resonance frequency of the array is optimised to 820 MHz, resulting in a significant increase in plasma antenna gain from −5 to 0 dB within the operating frequency range. Activating the plasma reflectors reduces cross polarisation by 20 dB. The proposed array achieves a 25 dB front-to-back ratio and a bandwidth of 600 MHz. The array's RCS varies from 6 dBm 2 ${\text{dBm}}^{2}$ to − 30 ${-}30$ dBm 2 ${\text{dBm}}^{2}$ over the 1–12 GHz range, rendering it invisible from 8 to 12 GHz. This plasma-based array is suitable for radar, defence and navigation systems.

Microwave reflectometry is a non-destructive and non-invasive active diagnostic technique which involves a sweep of the probing microwave frequencies to determine the density of the plasma signal. In Aditya-Upgrade, the reflectometry plasma signal is acquired using an instrument called Automated Reflectometry Diagnostics Instrument (ARDI). The signal is acquired as Inphase (I) and quadrature (Q) phase components, contaminated by the noisy environment of the tokamak. This paper uses a combination of Empirical mode decomposition (EMD) and Maximal overlap discrete Wavelet Packet transform (MODWPT) to denoise the acquired signal. The significant Intrinsic Mode Functions (IMFs) are selected using energy-based thresholding and decomposed using MODWPT. The beat frequency (f(b)) is extracted using the spectrogram for all the sweeps in the shot. From the extracted f(b), the density profile is calculated using the Bottolier-Curtet algorithm. The density profile helps to understand the stability of the plasma, which helps develop technology for better machines for controlled thermonuclear fusion.

Disruption in a tokamak nuclear reactor refers to the rapid extinction of the plasma confinement. This is often an uncontrolled event that involves the loss of plasma stability and can potentially cause damage to the reactor itself. To ensure the safety of fusion reactors, precise disruption prediction for early identification is crucial. While numerous data-driven time-series models have been developed and are continuously evolving to enhance disruption prediction in tokamaks, these models however often rely on fixed time windows for predictions. Because of the dynamic nature of plasma discharge, traditional models like LSTM, Bi-LSTM, and Stacked LSTM often produce premature alarms that make forecasts too early to determine if a signal reliably indicates a disruption. In this study, we propose a novel dynamic time window aggregation mechanism integrated with a sequential Bi-LSTM model (Bi-LSTM-DTWA), for predicting disruptions. By dynamically adapting to each signal time, this approach enhances prediction performance and effectively addresses the issue of premature alarms. The implemented model is trained using data from the medium-sized Aditya tokamak. Experimental validation on the Aditya dataset, comprising 153 disruptive shots and 67 normally terminated shots with nine diagnostic signals each, shows that the predictive model efficiently forecasts disruptions within 10 to 23 ms in advance without premature alarms, making it suitable for real-time deployment with minimal computational overhead.

This article examines the impact of various view dump geometries on their electromagnetic (EM) behavior within the microwave-millimeter range, particularly at frequencies between 60 and 180 GHz. Since the view dump is to be placed inside the tokamak machine, it is expected that the material that is used for its fabrication should have certain required properties, such as low reflection in the desired range of frequencies, sustained at high temperature and high vacuum conditions, low outgassing rate, and low thermal expansion rate. The MACOR material meets all these desired mechanical requirements. For EM absorption capabilities in the desired frequency range, the slab geometry made of MACOR material was first analyzed using CST Microwave Studio. Further, to enhance the EM absorption capabilities of the slab geometry, the slab geometry was modified into various other geometries such as triangular groove, pyramid, inverted pyramid, cone, and inverted cone. In addition, an extensive simulation study was also performed to identify the structural parameters associated with these different geometries, which have the utmost impact on the EM behavior. Experimental investigations for the simulated geometries were obtained by fabricating a prototype of MACOR. The experimental and simulated results for the slab, pyramid, and inverted cone geometries are in good agreement.

Tokamak plasma diagnostics based on microwave reflectometry are used for analysing the plasma characteristics. One such characteristic is the radial density profile, which can be measured using microwave reflectometry. The plasma signal is acquired using the Automated Reflectometry Diagnostics instrument (ARDI). The acquired signal is contaminated by plasma turbulence and other noise sources. Hence, the acquired signal should be denoised to estimate the beat frequency ($f_{b}$) from which the density profile can be measured. This paper uses a combination of Variational mode decomposition (VMD), and Maximal overlap discrete wavelet packet transform (MODWPT) to analyse the tokamak plasma signal. Energy-based thresholding is used to select meaningful Intrinsic Mode Functions (IMFs). MODWPT is used to identify the viewport, back wall and plasma frequencies.

In this paper, first time, we present the analytical, computational and experimental characteristics of a Leaky mode dielectric loaded helix antenna (LWDHA), exhibiting large scanning and high radiation efficiency. An analytical formulation is reported that utilizes electromagnetic potential theory coupled with polarization current in the dielectric medium. A unique way has been adopted for computing the amount of radiated electric field arising due to dielectric and helix separately. The analytical characteristics are validated by designing an antenna in Leaky mode configuration using CST Microwave Computational software and by characterizing it in the laboratory. The fabricated antenna has an axial length of 4.33 lambda 0, (lambda 0 is the wavelength of free space at the center frequency, i.e. 6.5 GHz), wide functional beam scanning range up to 110 degrees, omnidirectional radiation in the azimuthal plane and high radiation efficiency (98%). In comparison with other reported Leaky wave antennas, the proposed antenna shows advantageous characteristics of the simple structure of compact length and fulfills the growing demand for the miniaturization and integration of frequency-dependent devices.

This paper presents the design and performance of a novel square corner reflector antenna for the UHF range. The proposed plasma corner reflector antenna (PCRA) for beamforming and beam shaping is designed in CST. In this setup, two flat sheet plasma reflectors are placed behind an omnidirectional metal antenna intersecting at 90(0). Three layers of such V-shaped plasma reflector is placed for electrical beam switching. The gain of this configuration has been optimized to reach up to 12 dBi and 14 dBi for different beam patterns, demonstrating significant performance improvement.

Antenna synthesis is becoming increasingly challenging with tight requirements for C-SWAP (cost, size, weight and power) reduction while maintaining stringent electromagnetic performance specifications. While machine learning approaches are increasingly being explored for antenna synthesis, they are still not capable of handling large shape sets with diverse responses. We propose a branched deep convolutional neural network architecture that can serve as a drop-in replacement for a full-wave simulator (it can predict the full spectral response of reflection co-efficient, input impedance and radiation pattern). We show the utility of such models in surrogate-assisted evolutionary optimization for antenna synthesis with arbitrary specification of targeted response. Specifically, we consider the large shape set defined by the set of 16-vertexes polygonal patch antennas and consider antenna synthesis by specifying independent constraints on return loss, radiation pattern and gain. In contrast to online surrogates, our approach is an offline surrogate that is objective-agnostic; trained once, it can be used over multiple optimizations whereby the model training costs become amortized across multiple synthesis requests. Our approach outperforms evolutionary optimizations relying on full-wave solver-based fitness estimation. Specifically, we report the design, fabrication and experimental characterization of three polygon -shaped patch antennas, each fulfilling different objectives (narrow band, dual-band & wide-band). The reported methodology enables rapid synthesis (in seconds), produces verifiable sound designs and is promising for furthering data-driven design methodologies for electromagnetic wave device synthesis.

This paper presents the design, development and characterization of a leaky wave plasma antenna at 2.45 GHz which has potential application in military Wi-Fi, in areas where a continuous change in frequency is required. The design of the cylindrical plasma antenna is optimized by varying the plasma tube and ground cylinder dimensions to achieve better impedance matching (S11) of the antenna. Furthermore, the axial length of the plasma generated inside the tube is directly proportional to the input excitation power, which also determines the plasma resonant frequency, making it possible to fine-tune the resonant frequency. The novelty of our work lies in two key achievements that surpass previously reported results. First, our designed antenna achieves an enhanced directivity of 4.31 dBi, coupled with a broad bandwidth of 441 MHz at 2.45 GHz. This represents a substantial improvement over prior designs. Second, and most notably, the antenna attains a high radiation efficiency of 73.8%, a benchmark not reached in earlier studies. These advancements underscore significant contributions to the field of plasma antenna technology. The designed plasma antenna is fabricated and characterized experimentally to determine its resonant frequency and scattering parameters. A 10 KHz AC power supply is used for plasma generation inside the tube. The experimental results obtained are consistent with the simulation results.

Disruptions in tokamak nuclear reactors, where plasma confinement is suddenly lost, pose a serious threat to the reactor and its components. Classifying discharges as disruptive or non-disruptive is crucial for effective plasma operation and advanced prediction. Traditional disruption identification systems often struggle with noise, variability, and limited adaptability. To address these challenges, we propose an enhanced stacking generalization model called the "Double-Phase Stacking Technique" integrated with Pool-based Active Learning (DPST-PAL) for designing a robust classifier with minimal labor cost. This innovative approach improves classification accuracy and reliability using advanced data analysis techniques. We trained the DPST-PAL model on 162 diagnostic shots from the Aditya dataset, achieving a high accuracy of 98% and an F1-score of 0.99, surpassing conventional methods. Subsequently, the deep 1D convolutional predictor model is implemented and trained using the classified shots obtained from the DPST-PAL model to validate the reliability of the dataset, which is tested on 47 distinct shots. This model accurately predicts the disruptions 7-13 ms in advance with 93.6% accuracy and exhibited no premature alarms or misclassifications for our experimental shots.

Poloidal asymmetries in neutral and impurity ion temperatures and particle and impurity transports have been observed in many tokamaks. To investigate these asymmetries, space-resolved visible spectroscopic diagnostic of the ADTIYA-U tokamak has been upgraded to measure the spatial profile of Hα and impurities spectral line profile, ion temperature, and plasma rotation from both low and high field sides of the plasma, simultaneously with a better spatial resolution. This has been done by developing a linear array of 15 optical fibers as compared to the present linear array of nine fibers having a core diameter of 400 μm and coupling it to the entrance slit of a 1 m long high-resolution spectrometer of the existing diagnostic. A sCMOS detector with a larger height has also been employed to record 15 tracks as compared to the nine tracks when the spectrometer was coupled to a CCD detector having a size of 26.6 × 6.6 mm. The spatial profile of the Hα spectral line from the low and high field sides of the ADITYA-U plasma has been now recorded at every 20 ms to investigate neutral particle dynamics.

A first-of-its-kind, inductively driven micro-particle (Pellet) accelerator and injector have been developed and operated successfully in ADITYA-U circular plasma operations, which may ably address the critical need for a suitable disruption control mechanism in ITER and future tokamak. The device combines the principles of electromagnetic induction, pulse power technology, impact, and fracture dynamics. It is designed to operate in a variety of environments, including atmospheric pressure and ultra-high vacuum. It can also accommodate a wide range of pellet quantities, sizes, and materials and can adjust the pellets' velocities over a coarse and fine range. The device has a modular design such that the maximum velocity can be increased by increasing the number of modules. A cluster of lithium titanate/carbonate (Li2TiO3/Li2CO3) impurity particles with variable particle sizes, weighing similar to 50-200 mg are injected with velocities of the order of similar to 200 m s-1 during the current plateau in ADITYA-U tokamak. This leads to a complete collapse of the plasma current within similar to 5-6 ms of triggering the injector. The current quench time is dependent on the amount of impurity injected as well as the compound, with Li2TiO3 injection causing a faster current quench than Li2CO3 injection, as more power is radiated in the case of Li2TiO3. The increase in radiation due to the macro-particle injection starts in the plasma core, while the soft x-ray emission indicates that the entire plasma core collapses at once.

Electron cyclotron resonance heating (ECRH) two-pulse experiments are carried out on the tokamaks SST-1 and Aditya-U using single 42-GHz gyrotron. Initially, the system was used to carry out either breakdown or heating. A new anode modulator power supply with fast rise time and fall time (1 ms) has been integrated with 42-GHz gyrotron system, which facilitate to switch the gyrotron for more than one pulse within plasma shot. The first pulse is used for the plasma breakdown at low-loop voltage and second pulse is used for plasma heating. The power in the first pulse is maintained low (less than 150 kW) for the breakdown at fundamental harmonic, while power in the second pulse is more than 200 kW for plasma heating. In both the tokamaks, SST-1 and Aditya-U, two pulse experiments have been carried out and heating effect is observed clearly in Aditya-U tokamak during second ECRH pulse. This article discusses about the two ECRH pulse experiments on both the tokamaks SST-1 and Aditya-U.

This paper describes the experimental analysis and preliminary investigation of the predictability of pitch angle scattering(PAS) events through the electron cyclotron emission(ECE)radiometer signals at the ADITYA-Upgrade(ADITYA-U) tokamak. For low-density discharges at ADITYA-U, a sudden abnormal rise is observed in the ECE signature while other plasma parameters are unchanged. Investigations are done to understand this abrupt rise that is expected to occur due to PAS. The rise time is as fast as 100 μs with a single step and/or multiple step rise in ECE radiometer measurements. This event is known to limit the on-axis energy of runaway electrons. Being a repetitive event, the conditions of its repetitive occurrence can be investigated, thereby exploring the possibility of it being triggered and surveyed as an alternate runaway electron mitigation plan. Functional parameterization of such events with other discharge parameters is obtained and the possibility to trigger these events is discussed.PREDICT code is used to investigate the possible interpretations for the PAS occurrence through modeling and supporting the ECE observations. The trigger values so obtained experimentally are set as input criteria for PAS occurrence. Preliminary modeling investigations provide reliable consistency with the findings.

Effective control of impurities and precise regulation of the fueling gas are supreme prerequisites for optimal operation in any fusion device. Conventional wall-conditioning methods fall short of achieving optimal wall conditioning. Conventional wall-conditioning methods, such as vessel baking and H2/(D2)-fueled glow discharge cleaning (GDC), are generally required to remove wall-absorbed impurities in bulk after vessel venting. The excess amount of hydrogen, injected during H2 GDC, can be reduced by helium (He)-fueled GDC. However, He removal from the vessel is more challenging due to its low molecular mass, very low condensation temperature, and inert characteristics. In ADITYA-U, optimal wall conditioning cannot be achieved using H2 followed by He-fueled GDC when applied for extended periods spanning hours or days. A GDC with a mixture of argon and hydrogen (Ar-H2) is introduced in the ADITYA-U tokamak to obtain better wall conditioning than H2 followed by He GDC. In Ar-H2 GDC, long-lived ArH+ ions are formed in sufficient numbers and accelerated toward the vessel wall with high momentum. This results in the breaking of high energy bonds of impurities with the wall/plasma facing components, which is not possible by H+, H2+, H3+ ions in H2 GDC due to their lower momentum. An optimal blend ratio of Ar to H2 is established at 15%-20% for the mixture. This composition ensures that the introduction of high-Z Ar does not adversely affect tokamak plasma operations. The C- and O-containing impurities are reduced beyond the limit of the prolonged operation of H2 GDC. Relative low pressures of dominant impurities such as CO, CH4, and H2O are obtained due to the Ar-H2 GDC compared to routinely operated H2 GDC. A comparison study of H2 GDC and the developed Ar-H2 GDC is performed in terms of wall conditioning and tokamak plasma operation. The encouraging results of the Ar-H2 GDC are obtained in both wall cleaning and tokamak operation scenarios in the midsize tokamak ADITYA-U. This development and application of Ar-H2 GDC are beneficial for large-sized fusion devices, leading to improved impurity reduction, reduced operational fuel consumption (H2/D2/He), and enhanced control over fuel recycling/extraction.

The design, development, and characterization of a Silicon Carbide (SiC) based hightemperature black body source at 600 ºC for Electron Cyclotron Emission (ECE) measurements has been done. The design has been optimized for higher emissivity performance in 65-140 GHz frequency range using CST Microwave Studio. The innovative design features a pyramid-based structure, incorporating a heater and emitter surface, integrated with an electrical control system. The improvement in emissivity with variation in pyramid slant angle was analysed. The design was refined to ensure surface temperature consistency within a range of ± 15 ºC and rapid heating, taking less than 60 minutes to reach 600 ºC from room temperature. The developed black body calibration source was thermally characterized using IR camera for different set of temperatures and mean temperature distribution was determined. The microwave characterization of the calibration source has been performed in 65–220 GHz frequency range using Vector Network Analyser (VNA) and reflectivity of more than 20 dB has been obtained. The results highlight the synergy between advanced design methodologies, and precise engineering, leading to the development of an efficient SiC-based black body source. This research work not only contributes significantly to the field of engineering but also paves the way for enhanced accuracy and reliability in ECE measurements.

A wideband reconfigurable plasma antenna excited by a surface wave coupler (SWC) operating at 13.56 MHz is presented in this study. The plasma antenna and SWC are designed and modeled in CST Studio. A detailed analysis of the SWC is carried out using CST Studio, and experimental measurements supplement the performance. This work explores the signal transmission capabilities of a plasma antenna excited at 13.56 MHz for the first time. A SWC is a two-port device that is modified for radiating electromagnetic waves through a plasma column produced at a frequency of 13.56 MHz with a coupling efficiency of 99%. The power port and signal port of the SWC are isolated for better coupling efficiency and to reduce interference. The return loss and isolation between the ports of the SWC at 13.56 MHz are −22 dB and −46.81 dB, respectively. The plasma antenna excited by a surface wave has a wider bandwidth than its metallic counterpart. It varies from 280 to 540 MHz for input plasma power ranging from 2 W to 10 W. In addition, a reconfigurability frequency span of 240 MHz for power ranging from 2 to 10 W is achieved experimentally. Moreover, we attained an enhanced peak gain of -5 dBi. The radiation efficiency of the plasma antenna is around 20%. The experimental results of the SWC and plasma antenna are in coherence with its simulations.