First indigenously built tokamak ADITYA, operated over 2 decades with circular poloidal limiter has been upgraded to a tokamak named ADITYA Upgrade for the purpose having shape plasma operation with open divertor geometry. Experiment research in ADITYA-U has made significant progress, since last FEC 2016. After installation of PFC and standard tokamak diagnostics, the Phase-I plasma operations were conducted from December 2016 with graphite toroidal belt limiter. Purely Ohmic discharges in circular plasmas supported by Filament pre-ionization was obtained. The plasma parameters, Ip ~ 80-95 kA, duration ~ 80-180 ms with toroidal field (max.) ~ 1T and chord-averaged electron density ~ 2.5 x 10^19 m^-3 has been achieved. Being a medium sized tokamak, runaway electron (RE) generation, transport and mitigation experiments have always been one of the prime focus of ADITYA-U. MHD activities and density enhancement with H2 gas puffing studied. The Phase-I operation was completed in March 2017. The Phase-II operation preparation in ADITYA-U includes calibration of magnetic diagnostics followed by commissioning of major diagnostics and installation of baking system. After repeated cycles of baking the vacuum vessel up to ~ 130°C, the Phase-II operations resumed from February 2018 and are continuing to achieve plasma parameters close to the design parameters of circular limiter plasmas using real time plasma position control. Hydrogen gas breakdown was observed in more than ~2000 discharge including Phase-I and Phase-II operation without a single failure. Several experiments, including the primary RE control with lower E/P operation and secondary RE control with fuelling of Supersonic Molecular Beam Injection as well as sonic H2 gas puffing during current flat-top and Neon gas puffing for better plasma confinement are undergoing. The dismantling of ADITYA and reassembling of ADITYA-U along with experimental results of Phase-I and Phase-II operations from ADITYA-U will be discussed.
The Ohmically heated circular limiter tokamak ADITYA (R 0 =75 cm, a=25 cm) has been upgraded to a tokamak named the ADITYA Upgrade (ADITYA-U) with an open divertor configuration with divertor plates. The main goal of ADITYA-U is to carry out dedicated experiments relevant for bigger fusion machines including ITER, such as the generation and control of runaway electrons, disruption prediction, and mitigation studies, along with an improvement in confinement with shaped plasma. The ADITYA tokamak was dismantled and the assembly of ADITYA-U was completed in March 2016. Integration of subsystems like data acquisition and remote operation along with plasma production and preliminary plasma characterization of ADITYA-U plasmas are presented in this paper.
Since the 2018 IAEA-FEC conference, in addition to expanding the parameter horizons of the ADITYA-U machine, emphasis has been given to dedicated experiments on inductively driven particle injection (IPI) for disruption studies, runaway electron (RE) dynamics and mitigation, plasma rotation reversal, radiative-improved modes using Ne and Ar injection, modulation of magneto–hydrodynamic modes, edge turbulence using periodic gas puffs and electrode biasing (E-B). Plasma parameters close to the design parameters of circular plasmas with H2 and D2 as fuel have been realized, and the shaped plasma operation has also been initiated. Consistent plasma discharges having I P ∼ 100–210 kA, t ∼ 300–400 ms, n e ∼ 3–6 × 1019 m−3, core T e ∼ 300–500 eV were achieved with a maximum B T of ∼1.5 T. The enhanced plasma parameters are the outcome of repeated cycles of baking (135 °C), followed by extensive wall conditioning, which includes pulsed glow discharge cleaning in H, He and Ar–H mixture, and lithiumization. A higher confinement time has been observed in D2 compared to H2 plasmas. Furthermore, shaped plasmas are attempted for the first time in ADITYA-U. A first of its kind inductively driven particle injection for disruption mitigation studies has been developed and operated. The injection of solid particles into the plasma core leads to a fast current quench. Two pulses of electron cyclotron resonance wave at 42 GHz are launched in a single discharge: one pulse is used for pre-ionization and the second for heating. In a novel approach, a positively biased electrode is used to confine REs after discharge termination. E-B is also used for controlling the rotation of drift-tearing modes by changing the plasma rotation. Cold pulse propagation and signatures of detachment are observed during the injection of short gas puffs. A correlation between the plasma toroidal rotation and the total radiated power has been observed with neon gas injection-induced improved confinement modes.
In ADITYA Upgrade Tokamak, glow discharge wall conditioning (GDC) is performed regularly during the hightemperature plasma operation cycle using Hydrogen (H) and Helium (He) gases. H GDC is carried out after plasma operation for long durations (5 to 12 hours) on every plasma operation day in automatic mode to control Oxygen [O] and Carbon [C] containing impurities. This leads to high retention of H gas on Graphite limiter plates and Stainless Steel (SS) vessel wall. Subsequently, high H outgassing rate requires increased pumping time and high H recycling during plasma discharges affect the plasma performance in respect to H fueling control of the plasma. Intermittent He GDC for shorter duration can be used to decrease the H retention mainly in graphite wall by ion-induced desorption and sputtering phenomena. However, the high sputtering yield and deep penetration properties of He ions lead to high He retention in graphite and metallic walls. The removal of Helium from the limiter and wall is more difficult than H due to its properties of non-reactive, hard-to-trap, vacuum pumping limitation etc. To overcome above-mentioned issues with extended H GDC, a new approach involving Pulsed Glow Discharge Wall Conditioning (P-GDC) has been introduced in Aditya-U tokamak to reduce the residual Hydrogen and Helium concentration in SS vessel walls and graphite limiter plates. It has been observed earlier with continuous GDC that the impurity removal rate is usually high in initial few seconds of GDC operation. The initial high reaction rate is due to the reaction of working gas ions with loosely bound outer most monolayers species. The removal rate then decreases exponentially as hard bonded O and C containing impurities come out slowly. Moreover, the released impurity gases are re-implanted in the wall materials partially deeper in presence of continuous GDC that they have been in its absence. Thus overall impurity removal rate decreases exponentially with time in presence of continuous gas feeding of working gas at high pressure in typical continuous GDC. Initiation of the glow discharge needs filling of H gas at high pressure ~ 10-2 mbar and ~1 kV biasing voltage between the electrodes and vessel in Aditya Upgrade GDC. In case of pulsed GDC the gas needs to be injected in pulsed mode and the discharge needs to be initiated during every pulse of gas-feed. Therefore in P-GDC, to facilitate the fast initiation of discharge, a source of free of electrons has been introduced in the vessel. A fast feedback pulsed gas-fueling control system and electrons emission system has been developed to initiate glow discharge in each gas-feed pulse at various operating pressure 1 x 10-4 mbar and above in presence of an applied DC voltage. The different Pulsed GDC experiments have been carried out with H, He, Ar as working gases and the results are compared with traditional continuous GDC. The series of experiments have been performed to optimize beneficial wall conditioning for plasma operation. The design, development, operation and resul...
An experimental set-up to investigate the effect of a biased electrode introduced in the edge region on ADITYA tokamak discharges is presented. A specially designed double-bellow mechanical assembly is fabricated for controlling the electrode location as well as its exposed length inside the plasma. The cylindrical molybdenum electrode is powered by a capacitor-bank based pulsed power supply (PPS) using a semiconductor controlled rectifier (SCR) as a switch with forced commutation. A Langmuir probe array for radial profile measurements of plasma potential and density is fabricated and installed. Standard results of improvement of global confinement have been obtained using a biased electrode. In addition to that, in this paper we show for the first time that the same biasing system can be used to avoid disruptions through stabilisation of magnetohydrodynamic (MHD) modes. Real time disruption control experiments have also been carried out by triggering the bias-voltage on the electrode automatically when the Mirnov probe signal exceeds a preset threshold value using a uniquely designed electronic comparator circuit. Most of the results related to the improved confinement and disruption mitigation are obtained in case of the electrode tip being kept at ~3 cm inside the last closed flux surface (LCFS) with an exposed length of ~20 mm in typical discharges of ADITYA tokamak.
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