Experimental Studies of Confinement Improvement, Disruption Mitigations and Runaway Electrons Mitigations in ADITYA and ADHITYA- U Tokamak

Abstract

Despite conquering numerous challenges for demonstrating controlled thermonuclear fusion in tokamaks, several key obstacles remain to be overcome for realizing a commercially viable fusion reactor. Current tokamaks have achieved the necessary temperatures for fusion, but improving energy confinement time (𝜏𝐸 ) is crucial for achieving the required Lawson criteria. Furthermore, preventing and mitigating plasma disruption and runaway electrons (REs) in tokamaks is essential for steady-state and safe tokamak operations. The thesis details the methodologies required for successful plasma operation in ADITYA U, an upgraded version of ADITYA tokamak, and experimental studies on ADITYA/ADITYA U, focusing on the understanding of 𝜏𝐸 enhancement and techniques for disruption and RE mitigation. The experimental demonstrations of techniques and approaches presented in this thesis are important and can be utilized for the safe operation of ITER and future fusion devices. The ADITYA (major radius (𝑅0 ) = 0.75 𝑚, minor radius (𝑎) = 0.25 𝑚, and a peak toroidal field (𝐵𝑇 ) of ~ 1.5 𝑇) had been operated in only circular limiter plasmas with 𝐻2 fuel. The ADITYA-U (𝑅0 = 0.75 𝑚, 𝑎 = 0.25 𝑚, 𝐵𝑇 of ~ 1.5 𝑇), with the addition of 3 new sets of divertor coils, a new graphite toroidal limiter, and a new vacuum vessel (VV), equipped with a real-time position control system, can produce shaped plasmas in 𝐻2 and 𝐷2 fuel in an open divertor configuration. To obtain designed plasma parameters in ADITYA-U, various operational parameters including loop voltage, toroidal magnetic field, pre-fill pressure, external gas puffing, wall conditioning, impurity recycling, radial error field compensation, and real-time horizontal plasma position regulation have been optimized. Using different scenario developments, such as operating the vertical field coils in parallel mode to raise the plasma current relatively faster. The achieved plasma parameters in ADITYA-U include peak plasma current of ~ 210 kA, pulse length of ~ 400 ms, with a flattop duration exceeding 200 ms, chord averaged density (𝑛̅𝑒0 ) ~ 1 − 6 × 1019𝑚−3 , and electron temperature (𝑇̅ 𝑒0 ) ~ 250 − 500 𝑒𝑉 have been obtained with 𝐻2 plasmas in limiter configuration. Similarly, the machine parameters are optimized to obtain the circular plasmas in 𝐷2 fuel, which have ~ 1.5 times higher 𝜏𝐸 as compared to 𝐻2 equivalents. Furthermore, the machine parameters are tuned to conduct preliminary plasma shaping experiments by energizing the upper and lower divertor coils using H2 and D2 fuels. Synchronized charging of both divertor coils provided better control over the plasma shape and extended the plasma pulse length with improved discharge characteristics. It is well known that the RE can damage the VV and the in-vessel components and pose a significant risk to the safety of a tokamak reactor. Methods to avoid and mitigate REs are actively researched worldwide and testing various techniques for RE mitigation on tokamaks ith different sizes and plasma parameters is highly desirable to experimentally validate their effectiveness. A novel method has been developed for RE mitigation and tested successfully in ADITYA/ADITYA-U tokamaks. In this method, a local vertical magnetic field (LVF) perturbation has been applied to mitigate REs during the start-up, ramp-up, and disruption phases. The LVF perturbations are produced using a set of magnetic field coils, arranged in a Helmholtz-like configuration. With the application of this perturbation field beyond a threshold value of ~ 1 – 3% of the toroidal magnetic field for a minimum duration of ~ 5 ms, the REs are successfully mitigated. This experiment is repeated in ADITYA-U and REs are successfully mitigated, which indicates that this technique is not specific to a particular tokamak device and can be considered as a tool for RE mitigation in large-scale tokamaks, including ITER. Sudden plasma disruptions remain another major cause of concern for the safe operation of a steady-state tokamak reactor. Hence, disruption avoidance and mitigation technologies along with the physics understanding of the disruptions are required to be developed. Extensive analysis of discharges from the ADITYA has revealed that a predominant cause of disruptions is the emergence of the m/n = 2/1 MHD mode. Building on this insight, various strategies have been implemented in ADITYA to prevent disruptions, such as utilizing a biased electrode and applying Ion Cyclotron Resonance (ICR) waves to counteract deliberate disruptions induced by 𝐻2 gas puffing. Injecting gas in relatively high quantities leads to the generation of resistive tearing modes causing plasma disruptions. However, applying a positive bias voltage to an electrode placed inside LCFS can inhibit the formation of magnetic islands, thereby averting plasma current quenching. The biased electrode creates a radial electric field that induces poloidal rotation, suppressing MHD activity and preventing disruptions. Since a biased electrode cannot be installed in a tokamak reactor, disruptions are also prevented by injecting an Ion Cyclotron wave pulse with a frequency of 24.8 MHz and a power of 50 to 70 kW through a fast wave poloidal type antenna located outside LCFS. For mitigating the disruption generated by heat load through radiations, a new method has been demonstrated in ADITYA-U by injecting micro-particles using an electromagnetic drive. The thesis investigates different scenarios to attain designed plasma parameters in ADITYA U by optimizing several machine parameters. A novel technique has been developed to mitigate REs by applying an LVF perturbation pulse. The successful mitigation of REs is achieved in all phases of plasma current. The results are described by developing a simple model based on increasing the radial diffusion of REs by the applied field perturbation. To understand the physical mechanisms behind plasma disruption, thousands of discharges have been analyzed. Based on this analysis, techniques are developed to prevent disruption. Moreover, experiments are conducted to demonstrate the injection of micro-particles through electromagnetic means in a tokamak, aiming to mitigate the heat loads generated by disruption through radiation.