TJ-II stellarator

Overview

The TJ-II is a medium-sized stellarator of the flexible heliac type, operated by the Laboratorio Nacional de Fusión at the Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT) in Madrid, Spain. Commissioned in 1997, TJ-II is a key device within the European fusion research program under the EUROfusion consortium. Its primary mission is not to achieve high-performance fusion records, but to serve as a physics experiment platform for understanding and improving the stellarator concept.

TJ-II's defining feature is its high degree of magnetic configuration flexibility. This allows researchers to systematically vary key magnetic properties—such as rotational transform (iota), magnetic shear, and magnetic well depth—and study their direct impact on plasma confinement, magnetohydrodynamic (MHD) stability, and turbulence. This capability makes TJ-II an essential tool for validating theoretical models and codes used to design next-generation stellarators like Wendelstein 7-X. Its research contributes to the fundamental understanding of 3D plasma physics, which is critical for optimizing the stellarator line as a potential fusion power plant.

Physics / Mechanism

The TJ-II is a four-period helical-axis advanced stellarator, or "heliac." Unlike classical stellarators or tokamaks with a planar magnetic axis, the TJ-II's magnetic axis follows a helical path around a central conductor ring. This complex geometry is generated by a set of external coils without requiring a large net toroidal plasma current, thus making it inherently immune to current-driven disruptions.

The coil system consists of:

• 32 toroidal field (TF) coils: These provide the main toroidal magnetic field.
• A central conductor (CC): A circular coil interlinked with the TF coils, which induces the helical shape of the magnetic axis.
• Two vertical field (VF) coils: These control the radial position of the plasma column.
• A helical coil: A single conductor winding around the central conductor, which provides the primary rotational transform.

This combination of coils provides exceptional control over the magnetic topology. By adjusting the ratios of currents in the central and vertical field coils, operators can modify the rotational transform profile from low, negative shear to high, positive shear. The average plasma radius can be varied from 0.1 to 0.22 meters, and the plasma volume can be changed by a factor of two. This flexibility is crucial for exploring different plasma confinement regimes and stability boundaries within a single device.

Plasma heating is primarily achieved through Electron Cyclotron Resonance Heating (ECRH) and Neutral Beam Injection (NBI). Two gyrotrons deliver up to 600 kW of ECRH power at 53.2 GHz, used for plasma startup and electron heating. Two co- and counter-injecting NBI lines provide up to 3 MW of power, enabling access to higher plasma densities and ion temperatures.

Historical development

The TJ-II project was conceived in the late 1980s as a successor to the TJ-I torsatron at CIEMAT. The design was a collaboration between CIEMAT, Oak Ridge National Laboratory (ORNL) in the US, and the Max Planck Institute for Plasma Physics (IPP) in Germany. The "TJ" designation honors the torsatron-heliac lineage, with the "II" indicating the second major device in the CIEMAT program. The choice of a flexible heliac design was motivated by the need to experimentally investigate the influence of magnetic configuration on plasma behavior, a critical open question in stellarator physics at the time.

Construction began in the early 1990s, with significant components fabricated by Spanish industry. The device was assembled at CIEMAT, and first plasma was achieved in 1997. The initial operational phase focused on characterizing the magnetic configurations and developing basic plasma scenarios using ECRH.

The early 2000s saw the installation of the first NBI system, which significantly expanded the device's operational space to higher densities and beta values. A second NBI was added later, providing balanced injection and momentum control capabilities. Throughout its operational history, TJ-II has been continuously upgraded with advanced diagnostics, including Thomson scattering, reflectometry, charge exchange recombination spectroscopy, and various plasma edge probes. These upgrades have maintained TJ-II's status as a leading facility for detailed physics studies.

Current status

As of 2026, TJ-II remains fully operational and is a central component of the EUROfusion work package on stellarator optimization. Its research program is focused on several key areas aligned with the European fusion roadmap.

One major research line is the study of plasma transport and turbulence. Experiments on TJ-II have provided clear evidence of the link between magnetic configuration and turbulent transport, demonstrating that confinement can be improved by tuning the rotational transform profile to avoid low-order rational surfaces. These studies are vital for validating gyrokinetic codes in complex 3D geometries.

Another focus is on MHD stability, particularly the control of energetic particle-driven modes. With its powerful NBI systems, TJ-II can generate significant fast-ion populations, allowing for the study of Alfvén Eigenmodes and their impact on fast-ion confinement. This research is directly relevant to burning plasma scenarios in future reactors, where alpha particles will constitute a significant energetic particle population.

Plasma-wall interaction and impurity transport are also actively investigated. Recent upgrades include the installation of a liquid lithium limiter, designed to test novel plasma-facing components and their effect on plasma performance and recycling. These experiments explore solutions for handling the intense heat and particle fluxes expected in a reactor environment.

Notable implementations

TJ-II's primary implementation is its role as a flexible physics experiment within the global fusion program. Its contributions are best understood through its key experimental campaigns and findings:

• Configuration Scans: TJ-II has performed extensive scans of its magnetic configuration space, providing a unique database on the relationship between magnetic topology (iota, shear) and plasma confinement. These experiments have confirmed theoretical predictions that confinement is degraded when the rotational transform profile contains low-order rational surfaces and improved in regions with low rational density. This work directly informs the design principles for stellarators aiming for a low-[/glossary/neoclassical-transport] state.

• Internal Transport Barriers (ITBs): The device was one of the first stellarators to demonstrate the formation of electron ITBs—regions of sharply reduced transport in the plasma core. These barriers are often linked to the rational surfaces in the rotational transform profile, highlighting the importance of profile control for achieving advanced confinement regimes.

• Zonal Flow and Turbulence Studies: Using advanced diagnostics, TJ-II has made significant contributions to understanding the role of self-generated zonal flows in regulating turbulence. It has provided some of the clearest experimental evidence of these sheared flows in a stellarator, confirming their crucial role in the transition to improved confinement regimes.

• Liquid Metal PFC Research: The installation and operation of a liquid lithium limiter represents a significant implementation of advanced plasma-facing component technology. Experiments are ongoing to assess the benefits of lithium in terms of impurity control, hydrogen recycling, and overall plasma performance, providing valuable data for future devices like ITER.

Open challenges

Despite its successes, TJ-II faces challenges inherent to its design and scale. As a medium-sized, moderate-field device, it cannot access the high-temperature, low-collisionality regimes of a reactor. Therefore, extrapolating its findings on transport and confinement to reactor-scale devices requires careful validation with sophisticated simulation codes.

One of the primary scientific challenges is to fully unravel the complex interplay between neoclassical and turbulent transport in 3D geometries. While TJ-II has shown that configuration matters, a complete predictive model that captures the multi-channel physics across different collisionality regimes remains an active area of research. The device's flexibility is both a strength and a challenge, as the vast operational space is difficult to explore exhaustively.

From an engineering perspective, operating and maintaining the complex coil system and aging power supplies presents an ongoing effort. Furthermore, integrating new diagnostics and hardware, such as the liquid lithium limiter, into the compact and intricate vacuum vessel requires innovative engineering solutions. The limited pulse length (typically < 300 ms) also constrains the study of phenomena that evolve on longer timescales, a limitation common to many non-superconducting fusion experiments.

Outlook

The 5-15 year outlook for TJ-II is to continue its mission as a dedicated physics platform for stellarator optimization, working in close concert with larger devices and theoretical modeling efforts. Its role is expected to evolve towards addressing specific questions for the design of next-step stellarators, including a potential European demonstration power plant (DEMO).

Key research priorities in the coming years will likely include:

1. Advanced Control Schemes: Developing and testing real-time control of plasma profiles using its multiple heating and diagnostic systems to sustain high-performance scenarios.
2. Energetic Particle Physics: Further exploiting its NBI capabilities to study fast-ion transport and MHD stability in reactor-relevant scenarios, providing crucial data for validating models used for alpha particle physics in a reactor.
3. Plasma-Wall Interaction: Continuing to pioneer the use of liquid metal plasma-facing components, which could be a critical technology for solving the material challenges of a fusion power plant.
4. Code Validation: Serving as a primary validation platform for advanced stellarator simulation codes (e.g., gyrokinetic, extended MHD), which are essential for designing future devices with confidence.

TJ-II is not planned to be a candidate for achieving net energy gain, but its scientific output is a critical investment in the knowledge base required to make the stellarator a viable path to commercial fusion energy. It will remain a vital training ground for the next generation of plasma physicists and engineers in the European fusion program.

References

1. First plasmas in the TJ-II stellarator — Nuclear Fusion (1999)
2. Overview of TJ-II experiments — Nuclear Fusion (2003)
3. Confinement transitions in the TJ-II stellarator — Plasma Physics and Controlled Fusion (2004)
4. Understanding of the role of rational surfaces and magnetic shear on transport in the TJ-II stellarator — Nuclear Fusion (2011)
5. Energetic particle physics in the TJ-II stellarator — Nuclear Fusion (2015)
6. Recent results from the TJ-II stellarator — Nuclear Fusion (2017)
7. The TJ-II Team. Overview of the TJ-II stellarator results — Fusion Engineering and Design (2007)
8. First liquid lithium limiter experiments in TJ-II — Nuclear Fusion (2019)