Large helical device
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The Large Helical Device (abbreviated LHD , Japanese 大型 ヘ リ カ ル 装置 ōgata herikaru sōchi ) is an experiment on nuclear fusion that has been operated in Toki , Japan , since 1998 . As an experiment for basic research and technology development, LHD does not generate any energy. Alongside Wendelstein 7-X, LHD is currently one of the two largest fusion experiments based on the stellarator principle. Like Wendelstein 7-X, LHD is equipped with superconducting coils. This makes it possible in principle to generate field line cages that are stationary over time with high magnetic field strengths. In practice, the experiments are limited to about half an hour of operation. This period of time is sufficient to also clarify some technological questions for an energy-producing reactor such as ITER or DEMO .
background
The aim of fusion research is to generate energy from the fusion of light atomic nuclei, similar to what happens in the sun . In order for the fusion nuclear reaction to take place, two atomic nuclei must come extremely close; only then can the attractive nuclear force work. Since both nuclei are positively charged, they repel each other if they are further apart. The necessary approximation in a sufficient proportion of the random impacts can, however, be achieved if the kinetic energy of the nuclei and thus the temperature is high enough (around 1 million degrees). The matter then forms an ionized gas, a plasma .
The energy-providing fusion reaction that is possible at the relatively low temperatures is the so-called DT reaction. A heavy hydrogen nucleus - deuteron (D) - collides with a super-heavy hydrogen nucleus, a triton (T). The nuclei merge to form a helium nucleus ( alpha particle ) and a neutron is released. The development work in fusion technology today basically applies to this reaction. The main purpose of the experiments is to develop the plasma confinement with sufficient confinement time so that a net energy gain is possible.
Goals and questions
The aim of the LHD project is to clarify whether a fusion reactor based on the Heliotron principle can be realized. This gives rise to questions of a technological and physical nature:
- Technologically, it is about the construction and permanent operation of key components of a fusion power plant. LHD offers the possibility of realistically testing such components. Material issues and the ability to continuously operate high-performance components such as plasma heating can be specifically examined.
- Physically, it is about whether the insulation properties of a heliotron are sufficient for an energy-supplying reactor. It is equally important whether the LHD plasma is stable at the plasma pressures of such a reactor and how well the confinement of the fusion products works.
With this project goal, LHD fits into the worldwide studies on energy generation from fusion. In addition to questions relating to the special design of the heliotron, the technological and physical program provides results that can also be transferred to other construction principles.
technology
Like all systems for magnetic fusion research, LHD consists of a toroidal vacuum chamber in which a plasma is generated. Before this, the chamber is evacuated to around one ten billionth of normal pressure. This chamber has an outer diameter of 7.8 m. The vertical cross-sectional area has a diameter of 1.2 m. The plasma volume is therefore comparable to a medium-sized machine based on the tokamak principle, such as ASDEX Upgrade .
As a special feature of the LHD construction principle, an imaginary full rotation of the torus would rotate the elliptical vertical cross-section ten times - it forms a heliotron . This creates a helical (helical) magnetic field geometry. The magnetic field reaches field strengths of 3 T, which are generated by two helical coils that surround the vacuum vessel.
Magnetic field coils
The superconducting coils are operated at temperatures close to absolute zero. In total, LHD components with a mass of 820 t are cooled to 3.9–4.4 K. The cooling capacity of the helium condenser is about 5.7 kW at 4.4 K. The central coil system - the described helical, helical coil - consists of 450 turns. Overall, this results in a length of over 11 km superconductors. The coil current is around 11,000 A. LHD also has so-called poloidal field coils. Six of these ring-shaped coils, each with a diameter of 7-22 m, lie parallel to the ring-shaped axis of the torus. They serve to stabilize the plasma ring. With these coils and a controlled supply of current to the helical coil, the position of the plasma can be varied over a wide range.
heater
Since LHD is used for basic research, no operation with the fusion fuel tritium is planned. Since the plasma itself does not generate any energy, external heating must be used to maintain it. LHD has powerful microwave transmitters whose operating frequency is selected in such a way that the movement of ions or electrons in the magnetic field is stimulated: ion (ICRH) or electron cyclotron resonance heating (ECRH). In addition, LHD has fast neutral particle beams (NBI) that are injected into the plasma, ionize there and then release their directed high kinetic energy through collisions with the plasma particles.
| heater | power | |
|---|---|---|
| Tangential NBI | 3 × 5 = 15 MW | at 180 keV |
| Radial NBI | 1 × 6 = 6 MW | at 40 keV |
| ICRH | 6 × 0.5 = 3 MW | (cw) at 38.47 MHz |
| 6 x 1.2 = 7.2 MW | (5 s) at 38.47 MHz | |
| ECRH | 2 × 0.5 = 1 MW | (2 s) at 82.7 GHz |
| 2 × 1 = 2 MW | (5 s) at 77 GHz | |
| 1 × 0.8 = 0.8 MW | (3 s) at 84 GHz | |
| 2 × 0.5 = 1 MW | (0.5 s) at 168 GHz | |
Fuel supply and removal
In addition to the supply and removal of energy, the controlled supply and removal of fuel must also be guaranteed for a fusion plasma. With LHD, gas can be "blown" into the plasma through high pressure valves. In addition, LHD has an injection of pellets , small, frozen spheres of the working gas, which are pneumatically shot into the plasma. This allows you to penetrate deeper into the plasma than gas that is supplied through a high pressure valve. Pellet injection from LHD can inject beads 3 mm in diameter into the torus at speeds of 200-600 m / s 11 times per second.
The removal of particles and energy from the plasma is of central importance for a fusion reactor. For this purpose, LHD is equipped with baffle plate systems to which the particles are directed and carried away by the magnetic field lines of other coils. This divertor is also used for fusion machines based on the tokamak principle. High-performance pumps are installed behind the baffle plates, which suck off the incoming particles - corresponding to the “fusion ash” in the reactor.
Physical properties of LHD plasmas
One approach to fusion research is to infer reactor-sized fusion machines from smaller experiments. As with wind tunnel experiments, a dimensional analysis can be used to infer the behavior of objects in their original size. This procedure saves experimental effort and also allows various experiments to be assessed with regard to their relevance to the reactor.
The physical quantities that allow such an analysis are dimensionless parameters - for fusion plasmas the most important ones are the plasma beta , the collisionality and the normalized gyroradius .
With regard to the normalized gyroradius, LHD is limited in that it is about ten times too large for reactor operation. This size depends on the size of the machines and the achievable magnetic field strength, so it cannot be improved in LHD operation.
In addition, LHD achieved collisionalities and plasma betas in experiments, each of which individually reached the necessary reactor conditions. Together, reactor-relevant values are not achieved. One quantity that includes all three dimensionless parameters is the magnetic Reynolds number . In the case of LHD, this is about a factor of 200 away from reactor conditions (status: end of 2009).
The plasma beta values achieved are record values for fusion machines with magnetic confinement. Here LHD was able to achieve averaged values of 5%. However, with these values there is also a substantial reduction in the plasma volume, since a shift in the plasma occurs due to the high plasma beta ( Shafranov shift ).
Due to the size of LHD, the achieved energy inclusion times are the highest ever achieved in a stellarator experiment. If the size of the plasmas is taken into account, the best energy inclusion of LHD almost reaches that of Wendelstein 7-AS .
The high plasma densities that LHD was able to achieve through the targeted use of fuel pellets of up to 10 21 m −3 are also noteworthy for fusion machines with magnetic confinement . This is significantly more than is possible in fusion experiments based on the tokamak principle.
However, essential questions about stability and fuel evacuation remain the subject of research. Nevertheless, on the basis of the experimental results, it was proposed to operate a fusion reactor according to the stellarator principle at very high densities. This is attractive because the usable fusion power increases with the square of the plasma density and lower operating temperatures would be required.
Another important result of the LHD experiments was to show that certain instabilities of magnetohydrodynamics in stellarator plasmas are significantly milder than previously assumed on the basis of theoretical calculations. This results in greater flexibility for the design of the magnetic field for the stellarator principle.
Web links
- LHD website (Japanese, English)
- Website Kakuyū Gōkaku Kenkyūjo (Japanese, English: National Institute for Fusion Science )
Individual evidence
- ↑ "Wendelstein 7-X" generates the first plasma. Retrieved December 10, 2015 .
- ↑ Japanese LHD fusion device went into operation. Retrieved September 12, 2010 .
- ↑ Nuclear fusion research. JSPS Bonn, accessed on September 12, 2010 .
- ↑ JPFRS , In: FUJIWARA Masami, MOTOJIMA osamu, HAMADA yasuji, WATARI Tetsuo et al .: Overview of LHD (Large Helical Device) Project . ( PDF 373 kB )
Coordinates: 35 ° 19 ′ 34 ″ N , 137 ° 10 ′ 7 ″ E