Engineering design of the steady state neutral beam injector for SST-1

Characteristics of the positive ion source at reduced gas feed

The neutral beam injector of steady state superconducting tokamak (SST1-NBI) at IPR is designed for injecting upto 1.7 MW of neutral beam (Hº, 30-55 keV) power to the tokamak plasma for heating and current drive. Operations of the positive ion source (PINI or Plug-In-Neutral-Injector) of SST1-NBI were carried out on the NBI test stand. The PINI was operated at reduced gas feed rate of 2-3 Torr l/s, without using the high speed cryo pumps. Experiments were conducted to achieve a stable beam extraction by optimizing operational parameters namely, the arc current (120-300 A), acceleration voltage (16-40 kV), and a suitable control sequence. The beam divergence, power density profiles, and species fractions (H(+):H2(+):H3(+)) were measured by using the diagnostics such as thermal calorimetry, infrared thermography, and Doppler shift spectroscopy. The maximum extracted beam current was about 18 A. A further increase of beam current was found to be limited by the amount of gas feed rate to the ion source.

Steady state performance test analysis of actively cooled extractor grids for SST-1 neutral beam injector

Neutral beam injection (NBI) system is a workhorse to heat magnetically confined tokamak fusion plasma. The heart of any NBI system is an ion extractor system. Steady State Superconducting Tokamak-1 (SST-1) needs 0.5 MW of hydrogen beam power at 30 kV to raise the plasma ion temperature to ~1 keV and 1.7 MW of hydrogen beam power at 55 kV for future upgradation. To meet this requirement, an ion extractor system consisting of three actively cooled grids has been designed, fabricated, and its performance test has been done at MARION test stand, IPP, Julich, Germany. During long pulse (14 s) operation, hydrogen ion beam of energy 31 MJ has been extracted at 41 kV. In this paper, we have presented detailed analysis of calorimetric data of actively cooled extractor grids and showed that by monitoring outlet water temperature, grid material temperature can be monitored for safe steady state operation of a NBI system. Steady state operation of NBI is the present day interest of fusion research. In the present experimental case, performance test analysis indicates that the actively cooled grids attain steady state heat removal condition and the grid material temperature rise is ~18°C and saturates after 10 s of beam pulse.