Large Helical Device Project

The Large Helical Device (LHD) project involves construction of the world's largest superconducting helical device, which employs a heliotron magnetic field originally developed in Japan. The objectives are to conduct fusion-plasma confinement research in a steady-state machine and to elucidate important research issues in physics and engineering for helical plasma reactors.

Overview of the Large Helical Device (LHD)

The LHD comprisesa Plasma confinement device that employs superconducting coils, plasma heating systems and devices to measure and record plasma propeties and phenomena.

  1. LHD
  2. Neutral beam injection heating device
  3. Ion cyclotron range-of-frequency heating device
    (coaxial conduit for power transmission and stub tuner)
  4. Electron cyclotron heating device
    (waveguide for power transmission)
  5. Local Island Divertor (LID)
  6. Vacuum pumps
  7. Diagnostic ports
  8. Superconducting helical coils
  9. Superconducting poloidal coils
  10. Plasma
Specifications of the LHD
Outer diameter of the machine 13.5m
Troidal plasma diameter Approx. 8m
Poloidal plasma diameter 1.0 to 1.2m
Magnetic field Bo/Bmax 3/6.6T (4/9.2T)
Helical pitch number l/m 2/10
Net weight of the machine 1500t
Sectional view of LHD

Goals of the LHD Plasma Experiments

Fusion research．which is an integrated-system project, has advanced step by step with larger devices that were designed and connstructed based on the previous achievements, improved plasma performance and advances in theory and engineering technique. One objective of the LHD project is to identify physics and engineering issues in helical type fusion reactors. Research on Steady state operation, Which is required for a commercial fusion reactor is another imporant challenge for the LHD device.

In the next step, it is planned to increase the strength of confinement magnetic field, to boost heating power and to extend operation time. These efforts will lead to elucidation of the physics of steady-state currentless plasma.

Plasma control

Achieving high-Performance fusion plasmas requires research in such areas as control of confinement field configuration, optimization of fueling, control of impurities in peripheral plasmas, sustainment of steady-state plasmas and plasma-wall interaction. A wide range of research and development in plasma control is conducted in parallel with experimental research at the Large Helical Device.

To increase plasma denslty, flozen hydrogen pellets are injected into plasma at high speed, creating a high-density confined plasma.

Increase in plasma density and stored energy at the time of a five-shot series or pellet injections
CAD illustration of a pellet injector installed at a port on the LHD vessel
Hα light emission from ablating pellet in plasma

Upgrade R&D of Superconducting

The superconducting helical coils of LHD are presently cooled with lquid helium at 4.4K (-268.8°C). Sub-cooled liquid helium of 3K and/or pressurized superfluid hellium of 1.8K will be used in the future to increase the achievable magnetic field so that the better plasma performance will be obtained. For this purpose, research is being conduded to examine the cryostability of supercondutors cooled by sub-cooled liquid hellium and to investigate the heat transport characteristics of superfluid helium. An advanced current lead systen using high temperature super conductors is also being developed for the future higher current applications.

Pressurized superfluid helium experimental device

Plasma Diagnostics

Many types of diagnostic devices are used to measure high temperature plasmas in the LHD. These are used to investigate a wide variety of plasma characteristics. Measurement by multiple devices improves reliability of information about the space and time variation of plasma parameters.

Plasma Diagnostics Equipment
Large focusing mirror for Thomsom scattering Temporal evolution of electron temperature distribution

Plasma Heating

Three heating methods are being applied to attain higher temperature plasmas required for nuclear fusion reactions（100 million degrees, 1x1020/m3）. These methods are neutral beam injection (NBI), Which injects electrically neutral high-energy particles; ion cyclotron range-of-frequency (ICRF）heating and electron cyclotron resonance heating (ECH). which resonantly heat ions and electrons in a plasma by radio frequency waves or millimeter waves. Research for higher power injection technique and optimization of absorption in plasmas is under way.

Liquid stub tuner
Tuning device for adjusting coupling of plasma with radiation fron an ICRF antenna.
ICRF antenna installed in the vacuum Vessel
ICRF antenna has a complex three-dimensional configuration matched to the shape of twisted plasma.
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