Large Helical Device (LHD)

The Large Helical Device (LHD) is one of the world’s largest helical-type fusion experiments, operated by the National Institute for Fusion Science (NIFS) in Toki, Gifu Prefecture, Japan. Since producing its first plasma in 1998, it has led research on confining plasma with twisted magnetic fields. On this page, we first build an intuitive picture of what LHD is, and then walk through its physics and engineering, all the way to its new role in the 2020s.

The Device in a Nutshell (High School)

To ignite fusion, you need to confine a plasma (a gas in which atomic nuclei and electrons have come apart) hotter than 100 million degrees, hotter than the core of the Sun, keeping it suspended in mid-air without touching the walls of the vessel. Because a plasma is a collection of electrically charged particles, it can be caught with the force of magnets (a magnetic field).

LHD uses a method called the heliotron, which builds a cage out of twisted magnetic fields. Around a doughnut-shaped (toroidal) vessel, two coils are wound in a twisting helix. Simply running a current through these coils naturally creates the twisted magnetic field needed to confine the plasma.

This is LHD’s most easily understood strength. The other major approach, the tokamak, drives a large current through the plasma itself to twist the magnetic field. The heliotron approach used by LHD, however, completes the magnetic field with the external coils alone, so there is no need to run a current through the plasma. Not relying on a plasma current brings a big advantage: an accident in which the current suddenly disappears and confinement collapses (a disruption) is inherently unlikely to occur, and operation is easier to sustain for long periods. This ability to keep running stably for a long time is called steady-state capability, and it is where LHD excels most.

The Device’s Physics and Engineering (Undergraduate to Graduate)

The Heliotron Configuration and How the Magnetic Field Is Made

LHD uses an L=2, M=10 heliotron configuration. L is the pole number, expressing how many times the helix twists while a single helical coil goes once around the cross section of the torus, and M is the number of helical pitches (field periods) as the coil goes once around the torus. With this configuration, two continuous helical coils go ten times around the torus, producing the rotational transform needed for plasma confinement with external coils alone.

The main parameters are a major radius of about 3.9 m (the magnetic axis position can be varied), an average minor radius of about 0.6 m, and a maximum magnetic field of 3 T. By using the poloidal coils to shift the magnetic axis position inward or outward, the device can switch between a configuration that prioritizes confinement performance and one that secures a large plasma volume.

A representative measure of confinement quality is the energy confinement time $\tau_E$. It is defined as the stored plasma energy $W$ divided by the heating power $P$,

$$\tau_E = \frac{W}{P}$$

The longer $\tau_E$ is, the easier it is to maintain a high temperature with the same heating. The confinement time of helical systems is organized through international empirical rules (scaling laws) based on device size, magnetic field, density, and heating power, and LHD has provided a large amount of the underlying data.

The Engineering of Superconducting Coils

The heart of LHD is its superconducting helical coils. The conductors use NbTi/Cu superconducting materials, cooled to liquid-helium temperature (about 4 K, close to absolute zero) so that their electrical resistance becomes zero. Because the resistance is zero, once a current is set flowing the coils do not generate heat and can maintain a strong magnetic field for a long time. This steady magnetic field provided by superconductivity is the engineering foundation of LHD’s talent for long-duration operation.

To heat the plasma, several methods are combined to inject over 20 MW of high power. Neutral Beam Injection (NBI), which fires fast neutral atoms into the plasma to heat it, is the main method. In addition, Electron Cyclotron Heating (ECH) heats electrons with microwaves, and Ion Cyclotron Range of Frequency (ICRF) heating directly warms ions with radio waves.

The Track Record of Steady-State Operation

One of LHD’s representative achievements is long-duration steady-state discharge. Taking advantage of the ability to maintain the magnetic field with superconducting coils rather than relying on a plasma current, it has sustained a plasma over timescales of tens of minutes and shown high figures for the total heating energy injected during that time. For fusion to work as a power source, it must burn stably for a long time rather than in a brief pulse. The steady-state character unique to helical systems presents one concrete answer to this challenge.

Achieving High Temperature and the Deuterium Experiments

In 2017, LHD moved into experiments using deuterium (an isotope of hydrogen) and achieved an ion temperature of about 120 million degrees (roughly 10 keV). Temperature is expressed in units of energy because, in plasma physics, $T$ is treated as $k_B T$ (where $k_B$ is the Boltzmann constant), and 1 keV corresponds to about 11.6 million degrees. This high-temperature achievement was an important result showing that the ultra-high-temperature plasma needed for fusion can be realized even in a heliotron configuration.

The deuterium experiments have another physical aim. They allow a systematic study, by comparison with hydrogen plasma, of the isotope effect, which is how the confinement performance changes when the mass of the particles making up the plasma (the isotope) changes. Understanding this effect is indispensable for predicting the performance of the deuterium-tritium mixed plasma that an actual fusion reactor will use.

Research Frontier (PhD)

A major role that LHD has played is experimentally unraveling the physics of 3D magnetic configurations. Whereas the magnetic field of a tokamak is approximately axisymmetric, the magnetic field of a helical system inherently has a three-dimensional structure. This difference connects directly to the problem of transport, which is how particles and heat escape out of the device. In particular, in low-collisionality regimes, a form of transport called neoclassical transport, which is sensitive to the three-dimensional structure of the magnetic field, becomes significant. LHD has made detailed measurements of this neoclassical transport, the spontaneous formation of the radial electric field, and anomalous transport driven by turbulence, building a foundation for testing the transport theory of stellarator/heliotron systems.

Keywords often encountered in the literature include neoclassical transport, radial electric field, bootstrap current, International Stellarator Scaling, and the super-dense core that handles high-density regimes. These have become a shared language for understanding confinement in helical-system plasmas.

The positioning of the research has also shifted as the 2020s began. Having gone through its deuterium experiments, LHD is shifting its emphasis from a stage of chasing record updates on a single device toward a role as an academic research platform that broadly shares the knowledge of 3D plasmas gained in helical systems. The physics of 3D configurations accumulated here is being applied to stellarator research around the world, including Germany’s Wendelstein 7-X, and to the design of future fusion reactors. Unsolved challenges remain as well, such as the conditions for stably maintaining high plasma pressure (high beta) in a helical system, and how to optimize the control of heat and particles between the core and the wall, both of which are still under active research.

Check Your Understanding

Q1. Compared with the tokamak approach, what is characteristic about the plasma current in the heliotron approach used by LHD, and why is that advantageous for steady-state operation?

A It creates the twist of the magnetic field by making the plasma current large, so the current can be sustained and it suits steady-state operation B Because it makes the confining magnetic field with external helical coils alone and needs no plasma current, disruptions are unlikely to occur and long-duration operation is easy C It stabilizes by periodically switching the plasma current on and off, so it specializes in pulsed operation D By running the plasma current through superconducting coils, there is no current decay and steady-state operation is possible

Q2. How is the energy confinement time defined, and what does a long value mean?

A The heating power divided by the plasma energy; the longer it is, the worse the heating efficiency B The stored plasma energy divided by the heating power; the longer it is, the easier it is to maintain a high temperature with the same heating C The plasma temperature divided by the density; the longer it is, the lower the density D The time from when heating begins until ignition; the longer it is, the easier it is to ignite

Q3. State the achievement LHD reached in its 2017 deuterium experiments and the physical effect that using deuterium allows one to study.

A It achieved 120 million degrees in electron temperature and can study the effect of the density limit B It achieved an ion temperature of about 120 million degrees (roughly 10 keV) and can study the isotope effect due to the difference in particle mass C It achieved an ion temperature of about 15 million degrees and can reproduce the pp chain inside the Sun D It achieved 10 MW of fusion output and can study the efficiency of tritium breeding

Q4. How does the three-dimensional structure of the magnetic field in a helical system relate to plasma transport? Give one related keyword and explain.

A The 3D structure does not affect transport, and confinement performance is determined solely by the plasma current B The 3D magnetic field structure affects the transport by which particles and heat escape out of the device, and in particular neoclassical transport becomes significant in low-collisionality regimes C The 3D structure makes the magnetic field perfectly axisymmetric, so transport becomes ideally zero D The 3D structure governs the cooling efficiency of the superconducting coils and is unrelated to transport

Related Topics

• Stellarator / Helical Approach: Learn the confinement principles of the stellarator/heliotron approach that LHD belongs to, from the ground up.
• Plasma Transport: A detailed explanation of transport phenomena, such as neoclassical and anomalous transport, that lie at the heart of LHD’s research themes.