Thirty-minute plasma sustainment by real-time magnetic-axis swing for effective divertor-load-dispersion in the Large Helical Device

Abstract

Achieving steady-state plasma operation at high plasma temperatures is one of the important goals of worldwide magnetic fusion research. A high temperature of approximately 2 keV, and steady-state plasma-sustainment operation of the Large Helical Device ͑LHD͒ ͓O. Motojima, K. Akaishi, H. Chikaraishi et al., Nucl. Fusion 40, 599 ͑2000͔͒ is reported. High-temperature plasmas were created and maintained for more than 30 min with a world record injected heating power of 1.3 GJ. The three-dimensional heat-deposition profile of the LHD helical divertor was modified and during long-pulse discharges it effectively dispersed the heat load using a magnetic-axis swing technique developed at the LHD. A sweep of only 3 cm of the major radius of the magnetic axis position ͑less than 1% of the major radius of the LHD͒ was enough to disperse the divertor heat load. The modification of the heat-load profile was explained well by field-line tracing. The steady-state plasma was heated and sustained mainly by hydrogen minority ion heating using ion cyclotron range of frequencies. The operation lasted until a sudden increase of radiation loss occurred, presumably because of wall metal flakes dropping into the plasma. The sustained line-averaged electron density was approximately 0.7-0.8ϫ 10 19 m −3 . The average input power was 680 kW, and the plasma duration was 31 min 45 s. This successful long operation shows that the heliotron configuration has a high potential as a steady-state fusion reactor.

Steady-state operation and high energy particle production of MeV energy in the Large Helical Device

“…Figure 5 shows the effect of this axis swing technique [24]. The axis radius was changed and was reciprocally swung between 3.65 and 3.70 m. The input ICRF power and plasma density are 800 kW and 0.8 × 10 19 m −3 .…”

“…Using the R ax swing technique, the particle deposition profile on one divertor tile was also effectively dispersed. The Langmuir-probe arrays were embedded in several tiles, and the ion saturation current profile was measured [24]. The result shows that the axis swing also dispersed the particle flux on each divertor tile in a direction perpendicular to the leg traces.…”

Modification of the helical divertor footprint induced by the net toroidal plasma current in the LHD heliotron

“…The L c profile on the divertor, and thus the particle and heat fluxes, vary with the operational magnetic configuration, such as the magnetic axis position [10]. This characteristic has been applied to reduce the heat load by using the magnetic axis scanning in the longpulse discharge experiment in the LHD [11,12].…”