Large Helical Device (LHD)
The Large Helical Device (LHD) is the world’s largest heliotron-type fusion research device, operated by the National Institute for Fusion Science (NIFS) in Toki, Gifu Prefecture, Japan. Since its first plasma in 1998, LHD has been at the forefront of stellarator/heliotron research.
What is a Heliotron?
Section titled “What is a Heliotron?”A heliotron is a type of stellarator, which confines plasma using external magnetic coils without requiring plasma current for confinement. This fundamental difference from tokamaks offers several advantages:
| Feature | Tokamak | Heliotron/Stellarator |
|---|---|---|
| Plasma current | Required for confinement | Not required |
| Steady-state operation | Challenging | Inherently steady-state |
| Disruption risk | Significant | Very low |
| Magnetic field configuration | Created by plasma + coils | Created by coils only |
| Coil complexity | Simpler | More complex |
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value |
|---|---|
| Device type | Heliotron (L=2, M=10) |
| Major radius | 3.9 m |
| Minor radius | 0.6 m (average) |
| Plasma volume | 30 m3 |
| Magnetic field | 3 T (maximum) |
| Coil type | Superconducting (NbTi, 4.4 K) |
| Total weight | 1,500 tonnes |
| First plasma | 1998 |
Magnetic Configuration
Section titled “Magnetic Configuration”LHD uses a unique helical winding configuration:
- 2 helical coils wound helically around the torus
- 3 pairs of poloidal coils for plasma position control
- L=2 (pole pairs), M=10 (field periods)
This configuration creates a twisted three-dimensional magnetic field structure that confines plasma without plasma current.
Role of NIFS
Section titled “Role of NIFS”The National Institute for Fusion Science (NIFS), established in 1989, operates LHD and conducts comprehensive fusion research:
- Plasma confinement physics
- Plasma heating and control
- Materials science for fusion
- Fusion reactor engineering
- Theory and simulation
NIFS is an inter-university research institute, collaborating with universities and research institutions worldwide.
Research Achievements
Section titled “Research Achievements”World Records and Milestones
Section titled “World Records and Milestones”LHD has achieved numerous significant results:
Ion Temperature
- Achieved ion temperature of 120 million degrees Celsius (2017)
- Demonstrated excellent confinement in heliotron configuration
Electron Density
- Achieved central electron density of 1.2 x 10^21 m^-3
- Record-high density operation
Steady-State Operation
- Demonstrated plasma maintenance for over 47 minutes (2017)
- Total heating energy of 3.36 GJ in a single discharge
High Beta
- Achieved volume-averaged beta of 5%
- Important for compact reactor designs
Scientific Discoveries
Section titled “Scientific Discoveries”Improved Confinement Modes
- Super Dense Core (SDC) plasma with extremely high density
- Internal Transport Barrier (ITB) formation
Impurity Control
- Local Island Divertor (LID) concept
- Helical divertor for heat exhaust
Plasma Turbulence
- Detailed studies of turbulent transport
- Comparison with tokamak turbulence
Deuterium Experiment Campaign
Section titled “Deuterium Experiment Campaign”Since 2017, LHD has conducted deuterium plasma experiments, marking a significant advancement:
Objectives
Section titled “Objectives”- Achieve higher ion temperatures with deuterium
- Study isotope effects on confinement
- Investigate plasma-wall interactions with deuterium
Safety Measures
Section titled “Safety Measures”- Tritium handling facilities for trace tritium production
- Environmental monitoring systems
- Radiation safety protocols
Results
Section titled “Results”- Ion temperature exceeding 120 million degrees achieved
- Important data for understanding isotope effects
- Validation of heliotron reactor concepts
Comparison with Other Stellarators
Section titled “Comparison with Other Stellarators”LHD represents one approach to stellarator optimization:
| Device | Location | Type | Major Radius | Magnetic Field |
|---|---|---|---|---|
| LHD | Japan | Heliotron | 3.9 m | 3 T |
| Wendelstein 7-X | Germany | Optimized Stellarator | 5.5 m | 3 T |
| TJ-II | Spain | Heliac | 1.5 m | 1 T |
Each device explores different aspects of stellarator physics:
- LHD: Continuous helical coils, high-density operation
- Wendelstein 7-X: Computationally optimized modular coils
- TJ-II: Flexible magnetic configuration
Future Directions
Section titled “Future Directions”LHD Research Priorities
Section titled “LHD Research Priorities”- Further exploration of high-performance plasma regimes
- Development of advanced plasma control techniques
- Contribution to DEMO design through stellarator-specific research
Heliotron Reactor Concept
Section titled “Heliotron Reactor Concept”Based on LHD research, NIFS is developing the Force-Free Helical Reactor (FFHR) concept:
- Steady-state operation (no disruptions)
- Reduced maintenance requirements
- Potential for compact reactor design
Significance
Section titled “Significance”LHD has demonstrated that stellarators/heliotrons are a viable path to fusion energy:
- Proven steady-state capability
- Achieved reactor-relevant plasma parameters
- Contributed fundamental understanding of 3D magnetic confinement
As tokamak and stellarator research advance together, LHD continues to provide essential data for the global fusion effort.