Overview of the 7th cycle plasma experiment

Achieving a high beta value of 4.1% at the Large Helical Device (LHD)
-- Overview of the 7th cycle plasma experiment --

National Institute for Fusion Science

Director-General Dr. Osamu MOTOJIMA

LHD beta value achieved to 4.1% (renewed the world record)

We raised the electric power for a neutral particle beam injector to heat plasmas up to maximum 13,000 kW. By adjusting its heating distribution, a high beta value of 4.1% was drawn forth.
As a value achieved by a helical confinement system, our LHD value (4.1%) replaced the past world record (3.4% achieved with a German device -- W7-AS -- owned by the Max-Planck-Institute of Plasma Physics).
A beta value represents a ratio between the pressure of plasmas and that of magnetic field used to confine the plasmas. We conduct various LHD researches to achieve as high a beta value as possible. In the future nuclear fusion reactors, a beta value of 5% or higher is expected for economic reasons. NIFS will attempt to bring out a much higher value by intensifying the electric power for heating and adjusting the magnetic field for confinement.
When we achieved the beta value for 4.1%, the plasma core temperature was around 5,300,000 degC, the density 28 trillions/cc, and the magnetic field strength was 0.45 tesla. This beta value was computed using the average pressures of plasmas and magnetic field within the area where the 99% of plasma energy is present. Also, the plasma energy was measured from the magnetic field. Please refer to the diagram one and the diagram two for the detail.

NIFS also renewed a world record on the amount of data to be collected in one plasma generation cycle.

In the NIFS LHD experiment on January 22, the data collected in one plasma generation cycle amounted to over 3.4 billion bytes. NIFS overrode a previous record of 1.6billion bytes collected by JET.

Lots of major plasma parameter records were renewed as well.

Besides the plasma pressure experiment, we also renewed world records on many of major plasma parameters that were used in LHD researches this year. Please refer to the diagram one for the detail.

Plasma ion temperature achieved to 113 million degC (electron temperature 52 million degC, density 3.5 trillion/cc) with the use of argon gas.
- In the future, we will attempt to achieve our original goal that is to simultaneously heat ions and electrons up to 100 million degC or higher with the use of hydrogen plasmas that have a density of 20trillions or more/cc.
Electric discharge duration achieved to 756 sec (heating power 72kW, temperature 2.8 million degC, density 240 billions/cc).
- As for a long-term discharge, we will attempt to maintain the electric discharge for about 1000sec with 1000kW heating power; that is, our challenge is to generate plasmas of total 1billion J while heating input energy is maintaining the discharge.
Stored energy accumulated to 1.3 million J and electrons density achieved to 220 trillions/cc.
- We will keep challenging to renew those records even more and also will try to resolve some physical challenges such as what determines the maximum value for density.

[Description] Large Helical Device (LHD)

With the LHD, we can build the magnetic field configuration that is necessary to confine plasmas with external coils only. Therefore, in comparison with ITER that depends on the current run through plasmas or Tokamak type devices such as JT60, the LHD has a superior controllability. The LHD is told more appropriate a device for the steady operation, the quality that is strongly expected for future electric power generation reactors. Also in terms of magnetic field structures, the LHD fundamentally differs from Tokamak type devices. Therefore, by conducting complementary researches with the latter type devices, it is expected to become a supporting device in proceeding the ITER project.

The LHD experiment in the fiscal year 2003 (Apr.2003 thr Mar.2004) was started from Sep.24, 2003 and completed on Jan.22, 2004. During the experiment, we discharged plasmas for 7510 times.

Especially, we succeeded in efficiently confining the high-pressure plasmas this year. In order to generate plasmas that are more suitable for nuclear fusion, we will continuously attempt to intensify the electric power for heating. Also, we would like to establish the plasma control method on the basis of those previous results.

Diagram Three shows a record of major LHD plasma parameter developments for the past 6 years since the start of the first experiment. It indicates that the LHD experiments have been carried out prosperously.

Diagram One: It indicates the progress of beta values produced with helical type experiment devices.

Diagram Two: High-temperature, high-density plasmas possess high-pressure. Due to this pressure, those plasmas are known to cause instability so that they could escape across the magnetic field.

*In the LHD, some people were apprehensive about this instability that once the beta value exceeds 3% or more, the instability may restrict any further elevation of the value.
A result from the current experiment indicates that the instability in question did not happen and that the LHD could produce a high beta value. This also means that the biggest issue regarding the achievement of a high beta value was possibly resolved.

Diagram Two: Cont.

Diagram Two shows a typical waveform of a plasma discharge. At around the beta value of 3% or over, slight fluctuation in a magnetic field, a mode called m=1/n=1, becomes visible on the plasma surface. This fluctuation is what was concerned the most. Even though the fluctuation appears sometimes intermittently after one second, they eventually become restricted, and the beta value will achieve to 4%. The m=2/n=3 mode is observed further out of plasmas than m=1/n=1 mode; yet, there are not much influence to the plasma confinement. Also, m=2/n=1 mode that was observed within a lower beta area (2.5% or lower), located nearby the plasma core, has been stabilized. This experiment result shows the possibility of a presence of those machineries to stop the instability and calls for the necessity to come up with a new theoretical speculation. Moreover, it may also result in prompting researchers to review the current LHD beta value that has been considered at its upper limit. This draws a bright curtain for future nuclear fusion reactors.
NIFS will attempt to construct a physical model with a high predictive accuracy.

Diagram Three: Development of temperature and discharge duration. This is a major record regarding the LHD plasma confinement performance. NIFS performed 7 experiments for the past 6 years. Only for the first year, we conducted 2 short-term experiments. Thence, we've had a 4 to 5-month-cycle experiment once a year.

	2002	2003
Beta value	3.2% magnetic field 0.5T temperature 4.4 million degC density 29 trillions/cc	4.1% magnetic field 0.45T temperature 5.3 million degC density 28 trillions/cc
Electron temperature	120 million degC Ion temperature 23 million degC density 5 trillions/cc	Same as Left
Ion temperature	80 million degC Electron temperature 46 million degC density 3 trillions/cc	13 million degC Electron temperature 52 million degC density 3.5 trillions/cc
Density	160 trillions/cc temperature 46 million degC	220 trillions/cc temperature 40 million degC
Accumulated energy	1.16 million J temperature 12 million degC density 96 trillions/cc	1.3 million J temperature 15 million degC density 103 trillions/cc
Discharge duration	150 sec temperature 26 million degC density 5 trillions/cc heating power 500kW	756 sec temperature 2.8 million degC density 240 billions/cc heating power 72kW

Table 1: Comparison of the records between the year 2002 and 2003

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