Large Helical Device

The Large Helical Device ( LHD abbreviated, Japanese大型 ヘリカル 装置, Ogata herikaru Sochi ) is an experiment to nuclear fusion, which is operated since 1998 in Toki in Japan. As an experiment for basic research and technology development LHD wins no energy. LHD is currently (as of February 2010), the largest fusion experiment operated by the stellarator principle. A similar system is in 2014 taken with the Wendelstein 7 -X in operation in Germany.

A special feature is LHD - such as Wendelstein 7 -X - equipped with superconducting coils. Thus, it is possible in principle to produce field line cages indefinitely at high magnetic field strengths. In practice, the experiments are limited to about half-hour operation. However, this period is perfectly sufficient to also clarify technological issues for an energy producing reactor such as ITER or DEMO.

Background

The objective of fusion research is to gain from the fusion of atomic nuclei energy, like it happens in the sun. Thus, the fusion nuclear reaction can take place, two atomic nuclei to have come extremely close; only the attractive nuclear force can act. Since both nuclei are positively charged, they come at a greater distance from each other. The required approximation to a sufficient proportion of random collisions can also be achieved when the kinetic energy of the cores and thus the temperature is high enough (of the order 1 million degrees). The material then form an ionized gas plasma.

Energizing the fusion reaction, which is possible at the relatively low temperatures, is the so- DT response. A heavy hydrogen nucleus - deuteron (D) - pushes this along with a super- heavy hydrogen nucleus, a Triton (T). The nuclei fuse to form a helium nucleus ( alpha particle ) and a neutron is free. The development work in fusion technology is now basically this reaction. The experiments are designed primarily to develop the plasma confinement with sufficient inclusion of time so that a net energy gain is possible.

Objectives and questions

The goal of the LHD project is to clarify whether a fusion reactor may be realized by the Heliotron principle. This results in issues of technological and physical nature arise:

• The technological issues include construction and permanent operating facilities of key components of a fusion power plant. LHD provides the opportunity to test such components realistic. Specifically, there may be material issues and the capability of continuous operation of high performance components such as the plasma heating was investigated.

• The physical problems include whether the insulating properties of a Heliotrons are sufficient for power generation. Equally important is whether the LHD plasma is stable at the plasma Press a fusion power plant and how well the inclusion of the fusion products work.

This project aim to LHD fit into the global studies on energy from fusion. In addition to questions that relate to the special design of the Heliotrons, provides the technological and physical program results that are transferable to other design principles.

Technology

Like all systems for magnetic LHD fusion research consists of a donut-shaped vacuum chamber in which a plasma is generated. Before the chamber is evacuated to about 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 comparable to a medium-sized machine after the tokamak principle, as about ASDEX Upgrade.

A special feature of the design principle of LHD, the elliptical vertical cross-section rotates ten times during a full rotation of the torus - it forms a Heliotron. This creates a helical ( helical ) magnetic field geometry. The magnetic field strength reaches 3 T, which are generated by two helical coils, which include the vacuum vessel.

Magnetic field coils

The superconducting coils to be operated at temperatures near absolute zero. Overall, LHD components with a mass of 820 t at 3.9 to 4.4 K cooled. The cooling power is about 5.7 kW. The central coil system - the described helical, helical coil - consists of 450 turns. Overall, this results in a length of about 11 km superconductor. The coil current is about 11,000 A. In addition, LHD has called poloidal field coils. Six of these ring-shaped coils with a diameter of 7-22 m, respectively are parallel to the annular axis of the torus. They serve to stabilize the plasma ring. These coils and a controlled current loading of the helical coil can be the location of the plasma vary within wide ranges.

Heating

Since LHD basic research is, no operation is provided with the fusion fuel tritium. Since the plasma thus itself does not generate energy, an external heating must be used for its maintenance. LHD has powerful microwave transmitter, whose frequency of operation is chosen such that in each case the movement of the ions or electrons in the magnetic field is fanned: ion ( ICRH ) or Elektronzyklotronresonanzheizung ( ECRH ). In addition, LHD has fast neutral beams (NBI ), which are injected into the plasma, where they ionize and then cast their directional high kinetic energy through collisions to the plasma.

Fuel supply and discharge

In addition to the supply and discharge of energy, even the controlled supply and discharge of fuel must be guaranteed for a fusion plasma. In LHD gas can be " blown " by high-pressure valves for plasma. LHD in addition has an injection of pellets, small frozen beads of the working gas, which are pneumatically fired in the plasma. They can thus penetrate more deeply into the plasma as the gas which is supplied through a high pressure valve. The pellet injection can LHD / s per second inject beads 3 mm in diameter at speeds of 200-600 m eleven times in the torus.

The particle and energy dissipation in the plasma is of central importance for a fusion reactor. LHD is equipped with a baffle plate to systems in which the particles are selectively fed and discharged through the magnetic field lines of further coils. This divertor is also used for fusion machine according to the Tokamak principle. Behind the baffle plates high capacity pumps are attached, which the incoming particles - according to the " fusion - ash " in the reactor - a vacuum cleaner.

Physical Properties of LHD plasmas

One approach in fusion research is of smaller experiments on fusion machines to close in reactor size. As with wind tunnel experiments, one can conclude with a dimensional analysis on the behavior of objects in the original size. This procedure saves the experimental effort and also makes it possible to evaluate various experiments in regard to their relevance reactor.

The physical quantities which allow such an analysis, are dimensionless parameters - for fusion plasmas are the major plasma beta which Kollisionalität and normalized gyroradius.

In terms of the normalized gyroradius LHD is limited in so far as this is about 10 times too big for a reactor operation. This size depends on the size of the machines and the achievable magnetic field strength, so therefore can not be improved in the LHD operation.

Besides reached in LHD experiments Kollisionalitäten and plasma betas, which reached the necessary reactor conditions individually. Together reactor - relevant values ​​are not reached. One size that includes all three dimensionless parameters, the magnetic Reynolds number. This is about a factor of 200 from reactor conditions in LHD (as of end of 2009) removed.

The achieved plasma beta values ​​are record values ​​for fusion machines with magnetic confinement. Here LHD could reach averaged values ​​of 5 %. However, at these values ​​also lead to a substantial reduction in plasma volume as a result of the high plasma beta a shift of the plasma occurs ( Shafranov shift).

The achieved energy confinement times are the highest ever achieved in a stellarator due to the size of LHD. Taking into account the size of the plasmas, so achieved the best energy confinement of LHD almost the Wendelstein 7 -AS.

Remarkable for fusion machines with magnetic confinement, even the high plasma densities that could reach LHD, by judicious use of fuel pellets to 1021 m 3rd This is significantly more than for the tokamak principle is possible in fusion experiments.

However, significant questions as to the stability and the fuel removal remain the subject of research. However, it has been proposed on the basis of experimental results, to operate a fusion reactor according to the Stellarator principle at very high densities. This is attractive because the useful fusion power increases with the square of the plasma density and lower operating temperatures would be required.

An important result of LHD experiments it was further demonstrate that certain instabilities of magnetohydrodynamics in Stellaratorplasmen are significantly milder than was previously suspected on the basis of theoretical calculations. This results in a greater flexibility for the design of the magnetic field for the stellarator principle.