TJ-II is a flexible heliac installed at the National Fusion Laboratory in Spain. Its first plasma run was in 1997, and as of 2022 is still operational. It is a four period low magnetic shear stellarator with major radius R = 1.5 m, average minor radius a < 0.22 m, and magnetic field on axis up to 1.2 T. It is ‘flexible’ because varying the currents in the central circular and helical coils changes the plasma shape and sizes (plasma volume ≈ 0.6 – 1.1 m3). It has 32 toroidal coils (which create the toroidal field), and 4 poloidal coils, and 2 helical coils around the ‘central conductor’. The circular and the helical coils generate a three-dimensional twist of the central axis of the configuration. The central conductor is inside the toroidal coils and the plasma and vacuum vessel form a helix around it. The horizontal position of the plasma is controlled by the vertical field coils. The combined action of these magnetic fields generate bean-shaped magnetic surfaces that guide the particles of the plasma so that they do not collide with the vacuum vessel wall. TJ-II can produce a 0.25-seconds pulse every 7 minutes.
The objective of the experimental program of TJ-II is to investigate the physics of plasmas in a device with a helical magnetic axis having a great flexibility in its magnetic configuration, and to contribute to the international effort regarding the study of magnetic confinement devices for fusion.
In a series of papers, we have performed a comprehensive study of the neoclassical transport of TJ-II:
- J L Velasco, J A Alonso, I Calvo and J Arévalo. Vanishing neoclassical viscosity and physics of the shear layer in stellarators. Physical Review Letters,109:135003, 2012. arXiv / PDF
- J L Velasco and F Castejón. Study of the neoclassical radial electric ﬁeld of the TJ-I I ﬂexible heliac. Plasma Physics and Controlled Fusion, 54(1):015005, 2011. arXiv / PDF
- J L Velasco, K Allmaier, A López Fraguas, C D Beidler, H Maaßberg, W Kernbichler, F Castejón and J A Jiménez. Calculation of the bootstrap current proﬁle for the TJ-II stellarator. Plasma Physics and Controlled Fusion, 53(11):115014, 2011. arXiv / PDF
These theoretical predictions have been later thoroughly compared to the experiment for TJ-II discharges, e.g.:
- J L Velasco, J A Alonso, I Calvo, J Arévalo, E Sánchez, L. Eliseev, S Perﬁlov, T Estrada, A López Fraguas, C Hidalgo and the TJ-I I Team. Damping of radial electric field fluctuations in the TJ-II stellarator. Plasma Physics and Controlled Fusion, 55(12):124044, 2013. arXiv / PDF
- Tallents, D López-Bruna, J L Velasco, M A Ochando, B Ph Van Milligen, V I Vargas, J J Martinell, D Tafalla, J M Fontdecaba, J Herranz, E Blanco, F Tabarés, T Estrada, I Pastor and the TJ-II Team. Transport analysis in an ECH power scan of TJ-II plasmas. Plasma Physics and Controlled Fusion, 56(7):075024, 2014. PDF
The output of TJ-II neoclassical simulations is also the input for experimental or theoretical studies, e.g.:
- Á. Cappa, J. Varela, D. López Bruna, E. Ascasíbar, M. Liniers, L.G. Eliseev, J.M. Fontdecaba, J.M. García-Regaña, A. González-Jerez, N.K. Kharchev, F. Medina, A.V. Melnikov, S. Mulas, M. Ochando, D. Spong, J L Velasco, and TJ-II Team. Stability analysis of TJ-II stellarator NBI driven alfvén eigenmodes in ECRH and ECCD experiments. Nuclear Fusion, 61(6):066019, 2021. PDF
- S. Yamamoto, K. Nagasaki, K. Nagaoka, J. Varela, Á. Cappa, E. Ascasíbar, F. Castejón, J.M. Fontdecaba, J.M. García-Regaña, Á. González-Jerez, K. Ida, A. Ishizawa, M. Isobe, S. Kado, S. Kobayashi, M. Liniers, D. López-Bruna, N. Marushchenko, F. Medina, A. Melnikov, T. Minami, T. Mizuuchi, Y. Nakamura, M. Ochando, K. Ogawa, S. Ohshima, H. Okada, M. Osakabe, M. Sanders, J L Velasco, G. M. Weir, and M. Yoshinuma. Effect of ECH/ECCD on energetic-particle-driven MHD modes in helical plasmas. Nuclear Fusion, 60(6):066018, 2020. PDF
- E Sánchez, T Estrada, J L Velasco, I Calvo, A Cappa, J A Alonso, José Manuel García-Regaña, R Kleiber, and J Riemann. Validation of global gyrokinetic simulations in stellarator configurations. Nuclear Fusion, (59):076029, 2019. PDF
- E Sánchez, R Kleiber, R Hatzky, M Borchardt, P Monreal, F Castejón, A López-Fraguas, X Sáez, J L Velasco, I Calvo, A Alonso, and D López-Bruna. Collisionless damping of flows in the TJ-II stellarator. Plasma Physics and Controlled Fusion, 55(1):014015, 2013. PDF
An overview of the most recent results of TJ-II can be found in:
- C Hidalgo and the TJ-II team. Overview of the TJ-II stellarator research programme towards model validation in fusion plasmas. Nuclear Fusion, 62: 04202559, 2021. PDF