Summary of Experimental Results in LHD

Since the successful production of the first plasma in 1998, outstanding plasma parameters have been achieved. The electron temperature of 10 keV, ion temperature of 13.5 keV, plasma stored energy of 1.44 MJ and beta value of 5% have been recorded independently during 9 years of operation in LHD. These results show that LHD has plasma confinement properties comparable to those of the world's largest fusion machines. The specific feature of steady state operation of LHD also has been demonstrated by a discharge of over 1 hour or by sustaining of a long pulse discharge with 1.6 GJ of input energy. During the fiscal year of 2006 the ion temperature of a hydrogen plasma was heated up to 5.2keV with the ion density of 1.2x1019m-3, where the maximum confined election density reached 1x1021m-3.


Attained Plasma Parameters

Beta value is the figure of merit which indicates the efficiency of the plasma confinement.It is defined as the ratio of the plasma pressure to the magnetic pressure.Economical magnetic fusion confinement devices are required to achieve a beta values of more than 5%.The figure on the right shows the recorded beta values in worldwide steady plasma confinement devices.The beta value in LHD extends year by year and is breaking new ground, and achieved 5% during the experimental campaign in FY 2006.

  Electron
Temperature
Ion
Temperature
Confinement
Time
Absorbed
Heating
Power
Electron
Density
Maximum
Electron
Temperature
10 keV 2 keV 0.06 s 1.2 MW 5.0×1018m-3
Maximum
Ion
Temperature
4.5 keV 13.6 keV 0.06 s 3.1 MW 3.5×1018m-3
Maximum
Confinement
Time
1.3 keV   0.36 s 1.5 MW 4.8×1018m-3
Fusion triple
products
nτTi=4.4×1019m-3keV
Record Maximum
Stored Energy
1.44 MJ

Maximum Beta
l
5.0%(at 0.425 T)
Maximum
electron
density
1.0×1021m-3
Maximum
heating input
energy
1.6 GJ(3268 s)
plasma
duration time
3900 s(by ECH)



Experimental Highlight

One of the main goal of the LHD project is to achieve and maintain the high beta value of more than 5%. Along with the upgrading of the heating systems, several optimizations effective for maintaining the high beta state, in particular the optimization of the magnetic axis position and plasma minor radius are executed. During the last experimental campaign, the instantaneous maximum beta value attained 5%. The figure on the right shows an example of the wave forms of the shot in which a high beta value of 4.8% is maintaind for about 50 times the energy confinement time. It is well known that the plasma excites instabilities to escape across the confining magnetic field, when the pressure exceeds a critical beta value. The most dangerous m=1/n=1 instability appears around the time when the beta value exceeds 3%,but the beta value continues to increase up to more than 4.5%. An m=2/n=3 instability which is supposed to be excited in the region beyond that of the n=1/m=1 mode remains but does not seem to affect the beta value. An m=1/n=2 instability which might be excited near the periphery of the plasma does not appear. Thus a high beta state is successfully maintained without any disruptive instabilities.

The figure on the left shows the electron temperature and density profile when the maximum beta value of 4.8% is attained during this shot. Electron temperature in the center reaches 4.5 keV and has triangular profile. Electron temperature profile is almost flat. There are no significant flattenings in the temperature profile which would occur in the presence of dangerous instabilities.


Recent Results of Theory and Computer Simulation Researches


Magnetohydrodynamics(MHD) simulation for LHD Plasma
Macroscopic behaviour of plasmas in LHD are studied by means of simulation of fully three-dimensional, compressible and nonlinear MHD equations. Formation of mushroom-like structures due to the instability is observed. The elongation of the stream line the toroidal direction shows excitation of strong toroidal flows.


Simulations of Alfvén eigenmodes in LHD plasmas
Resonant interactions between energetic particles and Alfvén eigen modes are studied with computer simulations. In the figure shown are the electric field of Alfven eigen modes and the outermost magnetic surface of an LHD plasma.


Simulation of anomalous transport caused by the
ion temperature gradient turbulence
Turbulence driven by inhomogeneity of a magnetically-confined toroidal plasma causes anomalous transport of particles and heat. Simulations performed on the Earth Simulator at JAMSTEC have revealed a generation process of zonal flows, and have confirmed the steady state of turbulent transport.


Molecular Dynamic Simulation of Collsions between Hydrogen and Graphite
Erosion mechanism of graphite by hydrogen injection is examined by classical molecular dynamics simulation using a modified Brenner REBO potential. From our simulation, it is found that the momentum transfer from injected hydrogen-atoms to a crystal graphene derives the erosion of the graphite and then hydrocarbon molecules are produced.
Result > Summary of Experimental Results in LHD
Result > Summary of Experimental Results in LHD