Development of impurity seeding and radiation enhancement in the helical divertor of LHD

Abstract

Impurity seeding to reduce the divertor heat load was conducted in the large helical device (LHD) using neon (Ne) and krypton (Kr) puffing. Radiation enhancement and reduction of the divertor heat load were observed. In the LHD, the ratio between the total radiated power and the heating power, f rad = P rad /P heating , is limited up to around 30% in hydrogen plasmas even for high density plasma just below the radiative collapse (n e, bar > 1 × 10 20 m −3 ), where n e, bar is the line averaged density. With Ne seeding, the ratio could be raised to 52% at n e, bar ~ 1.3 × 10 19 m −3 , albeit with a slight reduction in confinement. f rad ~ 30% could be sustained for 3.4 s using multi-pulse Ne seeding at n e, bar ~ 4 × 10 19 m −3 . The localized supplemental radiation was observed along the helical divertor X-points (HDXs) which is similar to the estimated structure by the EMC3-EIRENE code. Kr seeding was also conducted at n e, bar ~ 3.1 × 10 19 m −3 . f rad ~ 25% was obtained without a significant change in stored energy. The radiation enhancement had a slower time constant. The supplemental radiation area of the Kr seeded plasma moved from the HDXs to the core plasma. Highly charged states of Kr ions are considered to be the dominant radiators from the plasma core region.

Influence of impurity seeding on the plasma radiation in the EAST tokamak

“…For the future magnetic confinement fusion device ITER or DEMO, the exhaust of excessive heat power flux deposited on the plasma facing components (PFCs) is a crucial issue to avoid the PFCs damage. Power dissipation by seeding radiative cooling impurities has been investigated in the present tokamak devices for many years [1][2][3][4][5][6], which has been confirmed to be an effective technology. However, some impacts on the plasma energy confinement and the main plasma dilution caused by seeded impurities were also observed [5][6][7][8][9].…”

Radiative edge cooling experiments in Wendelstein 7-X start-up limiter campaign

“…Only in the Large Helical Device (LHD) impurity seeding experiments were carried out. Here, Ne, N2 and Kr were used as most effective species and enhancement of plasma radiation as well as reduction of the divertor heat load were observed without significant confinement degradation [14] [15]. Ne was observed to radiate considerably from the confined region, although in its periphery, in addition to the strong edge radiation.…”

Experimental Studies of and Theoretical Models for Detachment in Helical Fusion Devices

“…In the LHD, studies on the divertor detachment are performed by seeding impurities deliberately [17][18][19] and by applying RMP from special coils [20][21][22]. In this chapter, we focus on the detachment with the RMP application, which is a unique feature of the LHD.…”