Operations of the Helium Subcooling System for the LHD Helical Coils during Ten Plasma Experimental Campaigns

Abstract

The Large Helical Device has a helium subcooling system with two cold compressors for helical coils to enhance the magnetic fields and improve the cryogenic stability of the coils by lowering the coil temperature. The system was installed in 2006 and then it has stably supplied 3.2 K subcooled helium at the nominal mass flow rate of 50 g/s to the coils during ten plasma experimental campaigns. The running time of the cold compressors exceeds 30,000 hours and the total time of subcooling operations exceeds 20,000 hours. In the system, the supplied helium is subcooled in a heat exchanger of a saturated helium bath. The bath pressure and temperature are reduced by a series of two centrifugal cold compressors with gas foil bearing. In the steady state subcooling operation, the bath temperature is stabilized within range of 0.02 K with automatic flow control of helium gas through the cold compressors by a heater in the bath. The control method is also useful to protect the system by mitigating large disturbance of the pressure and the mass flow rate. In the present study, the thermal hydraulic behavior of the system and the operational performance during ten plasma experimental campaigns are reported.

Improved performance of electron cyclotron resonance heating by perpendicular injection in the Large Helical Device

“…Perpendicular injection will be also applicable to FFHR-c1, a candidate for helical DEMO reactors [23], along with the concept of a remote steering antenna [24], suitable for the blankets. When the 77 GHz EC wave is injected from the outside of the horizontally elongated cross-section, on-axis ECRH with perpendicular propagation is available only in the magnetic field increased with sub-cooled helical coils such as the magnetic field strength of B t = 2.85 T at the magnetic axis of R ax = 3.6 m [25]. In contrast to the fundamental O-mode heating with 77 GHz, 154 GHz EC waves for the second-harmonic X-mode heating were obliquely injected as usual from the O port for the experiments introduced in the following sections, because , where e z , e tor , and e inj denote the unit vector in the vertical direction, that in the toroidal direction at the 2-O port center, and that in the direction from the launching antenna mirror center to the target point.…”