J. Miyazawa

K. Ikeda

K. Nagaoka

K. Tsumori

M. Osakabe

O. Kaneko

S. Morita

T. Mutoh

Y. Oka

Y. Takeiri

National Institute for Fusion Science (NIFS-Repository)

§ 2. Achievement of a High-IonTemperatu re in Argon-SeededDischarges by High-Energy NBI HeatingTakeiri, Y., Morita, S., Miyazawa, J., Tsumori, K.,Ikeda, K., Oka, Y., Osakabe, M., Nagaoka, K.,Kaneko, 0., Mutoh, T.The Large Helical Device (LHD) is equipped with anegative-ion-based neutral beam injection (NBI) system,which consists of three tangential injectors. Since theinjection energy of hydrogen is as high as 150 - 180 keY,most of the ionized beam power goes to electrons, and,moreover, a large part of the injected beam passes throughthe plasma without ionization in low-density hydrogenplasmas. As a result, the achieved ion temperature has beenless than 2.5 keVin such a hydrogen discharge. High-Zdischarges are effective to increase the direct ion heatingpower, because the ionization rate of the injected neutralbeam is higher especially in low densities and the ionnumber density is reduced compared with the electronnumber density. We already confirmed the increase in theion temperature in high-Z plasmas with neon-seededdischarges in the 5th experimental campaign [1,2].The high-Z plasma is diluted with the wall-absorbedhydrogen during the discharge, and a degree of the dilutionis large in the low-density plasmas. To realize thelow-density and high-Z plasmas, a glow discharge cleaningwas intensively performed with neon gas for reduction of thewall-absorbed hydrogen. Successive three nights of Ne-glowdischarge cleaning led to a gradual reduction of the residualhydrogen.To realize the higher-Z discharges with lower-densities,argon gas was puffed to low-density plasmas. Figure 1shows the time evolution of the various plasma parametersin an Ar-puffed plasma. The central ion temperature is muchincreased after the Ar gas puff and reaches 7 keV with aninjection power of9.8 MW at an electron density ofO.3xl019m-3. The toroidal rotation is correlated with the increase inthe ion temperature, and the toroidal rotation velocityreaches 40 km/s, which is about 30 % of the Ar-thermalvelocity.Figure 2 shows the central ion temperature as a functionof the density-normalized NBI absorption power. Althoughthe horizontal axis in the figure is the NBI power divided bythe electron density, it roughly corresponds to the ionheating power divided by the ion density. In the Ne-puffedplasma, the ion temperature is achieved to about 5 keV at anelectron density of around lxlO l9 m-3, and is decreasedbelow this density, probably because the ion number is notso relatively reduced due to the background hydrogen ions.In the Ar-puffed plasma, on the other hand, high-Z6Toroidal RotationJ0.5 1 1.5 2 2.5 3TIme (sec)Fig. 1 Time evolution ofthe various plasma parametersin an Ar-puffed plasma. The central ion temperature ismeasured with the Doppler broadening ofAr XVII.discharges are kept in lower densities less than 1x I0 19 m-3due to its higher charge number. As a result, thedensity-normalized power is extended to higher values andthe ion temperature is proportionally raised to 7 keV.The above results suggest that a high ion temperatureshould be achieved as well in hydrogen discharges if thedirect ion heating power is increased to the same power asthat in the Ar-seeded discharges.[1] Y. Takeiri, et al., Phys. Plasmas 10, 1788 (2003).[2] S. Morita, et aI., 19th IAEA Fusion Energy Conference,Lyon, 2002, EX/P2-18.83.6m, 2.81, y=1.254I •Ar-puff I7 0 Ne-puff •.,6 • •••att •500"> •Q) 4 0C• 0 IJ· •i=3CD <DJ2 o •Fig. 2 Central ion temperature as a function ofthe density-normalized NBI absorption power.

Achievement of a High-Ion Temperatu re in Argon-Seeded Discharges by High-Energy NBI Heating

Achievement of a High-Ion Temperatu re in Argon-Seeded Discharges by High-Energy NBI Heating

Abstract

Similar works

Full text

Available Versions

National Institute for Fusion Science (NIFS-Repository)