TJ-II:Neutral Beam Injection

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TJ-II disposes of two neutral beam injectors (NBI), each of which can produce ≤ 300 ms pulses of neutral hydrogen accelerated to 40 keV, to provide up to 1.2 MW of absorbed additional heating for central electron densities up to 1.6×1020 m-3. [1] [2] [3] [4] [5]

NBI1 is located in sector D8. NBI2 is located in sector C1.

TJ-II vacuum vessel and the two NBI injectors. Left: NBI1; right: NBI2
Diagram of an NBI injector, coupled to TJ-II

Power supply

The two high voltage power supplies feeding the acceleration grids of the injectors are of the transformer–rectifier type, taking their primary energy from a pulsed flywheel generator, and are coupled to the acceleration grids through a switching device. [6]

Control system

The NBI system has its own control system, based on VMEbus, with controller boards running the OS9 real-time operating system. [7] [8]

Vacuum system

In order to keep reionization losses at an acceptable level, the maximum allowed pressure during the beam pulse is 10−4 mbar, which requires an installed pumping speed of 350,000 l/s. Titanium getter pumps have been chosen as the primary vacuum system for these injectors. Base pressures in the range of 5 × 10−9 mbar are attainable with titanium pump operation. [9]


The target calorimeter (red) in its operating position. Only part of the vacuum vessel is shown. The neutral beam is injected from the right. The infrared (IR) camera (below) views the calorimeter via a mirror

A carbon fiber composite (CFC) target calorimeter has been installed to study the power density distribution of the neutral beams in situ. The thermographic print of the beam can be recorded using an infrared camera, permitting a reliable analysis due to the highly anisotropic thermal conductivity of the target material. Direct conversion from temperature to power density is possible due to independent thermocouple measurements on two adiabatically mounted copper buttons. With the combined thermographic and calorimetric measurements it has been possible to determine the power density distribution of the beams. [10] [11] [12]

Power and particle balance

In the analysis of the confinement and transport properties of NBI heated plasmas, a proper evaluation of the power and particle balance is necessary. In order to achieve that, a number of experimental studies have been carried out to determine the beam power transmission and beam contribution to plasma fuelling. [13] [14]


  1. J. Guasp, M. Liniers, Theoretical evaluations of Neutral beam Injection efficiency for the TJ-II helical-axis stellarator, Fusion technology 24, 3, 251 (1993)
  2. J. Guasp, M. Liniers, Loss cone structure for ions in the TJ-II helical axis stellarator. Part I: properties without a radial electric field, Nucl. Fusion 40 (2000) 397-409
  3. J. Guasp, M. Liniers, Loss cone structure for ions in the TJ-II helical axis stellarator. Part II: radial electric field effects, Nucl. Fusion 40 (2000) 411-427
  4. J. Guasp, M. Liniers, C. Fuentes, and G. Barrera, Thermal load calculations at TJ-II vacuum vessel under NBI, Fusion Tech., 35, pp. 32-41 (1999)
  5. C. Fuentes et al, Neutral beam injection optimization at TJ-II, Fusion Engineering and Design 74, Issues 1-4 (2005) 249-253
  6. J. Alonso et al, High voltage power supplies for the neutral beam injectors of the stellarator TJ-II, Fusion Engineering and Design 56-57 (2001) 693-697
  7. L. Martínez-Laso et al, TJ-II neutral beam injectors control and data acquisition system, Fusion Engineering and Design 56-57 (2001) 477-480
  8. R. Carrasco et al, The NBI control system for the TJ-II, Fusion Engineering and Design 81, Issues 15-17 (2006) 1813-1816
  9. M. Liniers et al, Vacuum system of the neutral beam injectors at the stellarator TJ-II, Vacuum 67, Issues 3-4 (2002) 379-384
  10. C. Fuentes et al, Thermographic calorimetry of the neutral beam injectors heating beams at TJ-II, Rev. Sci. Instrum. 77 (2006) 10E519
  11. C. Fuentes et al, Power transmission of the neutral beam heating beams at TJ-II, Fusion Engineering and Design 82, Issues 5-14 (2007) 926-932
  12. J. Damba, Beam Transmission Monitoring of the TJ-II Neutral Beam Injectors, European Master of Science in Nuclear Fusion Physics and Engineering (Erasmus Mundus Programme, 2014)
  13. M. Liniers, G. Wolfers, et al., Beamline duct monitoring of the TJ-II Neutral Beam Injectors, Fusion Engineering and Design 88, Issues 6-8 (2013) 960-963
  14. M. Liniers, J. Damba, et al., Beam transmission dependence on beam parameters for TJ-II Neutral Beam Injectors , Fusion Engineering and Design (2017)