Ion cyclotron range of frequencies heating and high-energy particle production in the Large Helical Device

Abstract

Significant progress has been made with ion cyclotron range of frequencies (ICRF) heating in the Large Helical Device. This is mainly due to better confinement of the helically trapped particles and less accumulation of impurities in the region of the plasma core. During the past two years, ICRF heating power has been increased from 1.35 to 2.7 MW. Various wave-mode tests were carried out using minority-ion heating, second-harmonic heating, slow-wave heating and high-density fast-wave heating at the fundamental cyclotron frequency. This fundamental heating mode extended the plasma density range of effective ICRF heating to a value of 1×1020 m−3. This use of the heating mode was its first successful application in large fusion devices. Using the minority-ion mode gave the best performance, and the stored energy reached 240 kJ using ICRF alone. This was obtained for the inward-shifted magnetic axis configuration. The improvement associated with the axis-shift was common for both bulk plasma and highly accelerated particles. For the minority-ion mode, high-energy ions up to 500 keV were observed by concentrating the heating power near the plasma axis. The confinement properties of high-energy particles were studied for different magnetic axis configurations, using the power-modulation technique. It confirmed that with the inward-shifted configuration the confinement of high-energy particles was better than with the normal configuration. By increasing the distance of the plasma to the vessel wall to about 2 cm, the impurity influx was sufficiently reduced to allow sustainment of the plasma with ICRF heating alone for more than 2 min.

Stability and Confinement Studies of High-Performance NBI Plasmas in the Large Helical Device Toward a Steady-State Helical Fusion Reactor

“…The ICRF heating system facilitates long-pulse operatio, which is capable of a steady-state injection of 1 MW with four antennas, while the achieved power is 2.9 MW in short-pulse injection [16].…”

Characterization of Ion Cyclotron Wall Conditioning Using Material Probes in LHD

“…Among the advantages of ICWC are to be operated under a strong magnetic field for fully torus area which was different from local area cleanings such as a flush lump and a laser ablation. The Large Helical Device (LHD) [6,7] is provided with superconducting coils for its confinement magnetic field and DC-glow discharge cleaning for its wall conditioning in most cases of operation [8]. As supplemental wall conditioning methods, boronization and titanium gettering are carried out for a few times during an experimental campaign.…”

“…Ion cyclotron range of frequency (ICRF) heating system has been developed for a high power and long pulse discharge plasma in LHD [6,7]. Using the same ICRF antenna sets, an initial operation of ICWC was done successfully in 2005 [9] and additional ICWC test experiments were operated in 2006 and 2007.…”

Thirty-Minute Plasma Sustainment by ICRF, EC and NBI Heating in the Large Helical Device

“…The average input power was 680 kW (ICRF [2][3][4] 520 kW, ECH 100 kW and NBI 60 kW) and the plasma duration was 31 min 45 sec. Before the 2003 campaign, the plasma sustainment time of the ICRF longpulse experiment was limited by local temperature rises of the divertor carbon plates near the ICRF antenna section, and gradual increase of out-gassing from the wall finally terminated the plasma operation [5]. After that experiment, a new mechanical structure and new carbon material sheets were used to suppress the temperature rise and the out-gassing rate.…”

“…They exhibited outof-phase behavior when the axis position was changed. For a constant axis position, some carbon divertor plates were locally heated extensively, and their temperatures increased more than linearly in time until they finally caused a sudden termination of the plasma [5].…”