India's newly commissioned compact spherical tokamak at IPR has reached first plasma — a major commissioning milestone — but its early tests have not yet shown a sustained plasma current, the electrical current carried by the plasma. First plasma was achieved on 11 December 2025, and the conclusion reports 114 initial plasma discharges. In a detailed discharge, the current signal lasted 1 ms and peaked near 99 ms.
The report describes a compact spherical tokamak intended to study low-aspect-ratio physics and technologies, with first-plasma experiments as its initial operational focus. Vacuum, gas-discharge cleaning, power supplies, electronics, diagnostics and electron-cyclotron heating were validated locally before integration under the centralized control system, and commissioning data were archived in ST-DAS.
Building a working machine
The design lists a major radius of 0.28 metres, a minor radius of 0.16 metres and an aspect ratio of 1.75. Its toroidal-field system uses six TF coils with three turns. Phase 1 lists approximate targets of 30 kA for plasma current, 5 × 10^18 m^-3 for density, 50 eV for electron temperature and 10 ms for Ohmic duration.
The vacuum work produced a series of reported milestones. After baking at 150°C for 48 hours, the base pressure was 1 × 10^-6 mbar, with a leak rate below 2 × 10^-9 mbar l/s. After 10 days of continuous pumping, the base pressure was 1 × 10^-7 mbar; after wall conditioning, the reported ultimate base pressure reached 2.1 × 10^-8 mbar. The baking system was not installed for current operation.
The TF assembly contained 40 interfaces. After refined joint-tightening, measured total joint resistance was 2.42 mΩ, while terminal resistance, including 1.2 mΩ of busbar resistance, was 8.04 mΩ. The reported terminal resistance was within the allowable operating range of the power supply.
Magnetic checks also tracked the expected field behavior. A toroidal-field pulse lasting about 0.4 seconds reached 8.3 kA and produced a measured field of 876 G. A radial profile taken at 0.5 kA showed a reasonably good inverse-radius dependence. The same measurements indirectly estimated about 15 kA of toroidal vessel current on the outboard side during current ramp-up.
The ST-DAS was configured for 69 signals, with listed sampling rates from 10 kHz or lower through 1 MHz.
First plasma, then a harder test
The first-plasma sequence reached TF flattop in approximately 50 ms and applied 600 W of ECR power. Plasma was produced on the first attempt, with field-aligned filaments and a ring-shaped toroidal plasma column, alongside the expected diagnostic signals.
That early result did not settle plasma-current formation. In one early discharge, no plasma current formed and the configuration remained open-field-line. The report says electron-cyclotron absorption was expected to be multipass and sensitive to relatively poor wall conditions; it does not quantify the absorption fraction or the evolution of density and electron temperature.
In shot no. 30, a plasma-current signal appeared for 1 ms, rose during the ascent of loop voltage and peaked near 99 ms. The vacuum comparison remained near the noise floor, while its 2σ envelope stayed close to zero.
To examine what was happening around the vessel, the analysis used a filament-based model and preliminary fiber-optic measurements. The model estimated 100 kA in the high-field-side inner wall, 31 kA in the top and bottom plates and 13 kA in the outer wall during peak loop-voltage formation. The measurements agreed with the model in timing but differed in magnitude by 20%, and calibration was still under way.
Field analysis points to a second issue in the same brief interval. Between 98.5 and 99.5 ms, the upward vertical-field component was larger than the applied negative component; the negative component decreased until 98.86 ms, after which the modeled net vertical field became positive.
A focused commissioning agenda
The report leaves the next phase focused on reducing the outboard vessel current and compensating induced currents through coil-configuration optimization or additional sources. It also leaves open the behavior of electron-cyclotron absorption, density and electron-temperature evolution under improved wall conditions; those quantities were not quantified in the early discharge.
The paper's conclusion describes the ST as operational after commissioning, with all subsystems integrated and plasma experiments under way. That status belongs to an initial commissioning phase: the reported plasma-current result is transient, and the induced-vessel-current estimates remain under calibration.
Paper data and sources
Original title: First Plasma Commissioning and Operational Highlights from India's First Spherical Tokamak at IPR
Authors: Kishore Mishra, Aditya Verma, N. Mansoori et al.
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-26
DOI: Not available
Original paper · Full text