Time evolutions of the scrape-off-layer particle widths and the divertor strike-line positions in low (L-) and high confinement (H-) mode plasmas with modulated lower hybrid current drive power on EAST

Abstract In the EAST tokamak, the divertor particle flux profiles in L- and H-mode plasmas with modulated lower hybrid current drive (LHCD) power in the upper single null configuration are analyzed using a theoretical model profile by a nonlinear least square fitting method. The fitting analysis successfully derives time evolutions of the strike-line positions as well as the effective widths of the scrape-off-layer (SOL) and private flux region for the particle flux, distinguishing respective effects of the LHCD power and edge localized modes (ELMs). For the studies, power deposition area of the LHCD power is estimated by using extreme ultraviolet radiation signals from the SOL. When an appreciable amount of the LHCD power is deposited in the SOL region, a significant expansion of the divertor particle flux profiles on the upper-outboard side by the LHCD power input is observed in an ELMy H-mode plasma. The upstream electron density profile measured is also considerably expanded by the LHCD power. The observed divertor particle flux profile is qualitatively explained by the additional expansion of the upstream density profile due to the LHCD effects. However, the particle flux profiles on the upper-inboard side do not show noticeable changes. The divertor strike-line positions on both sides and the loop voltage do not show any responses to the LHCD power. On the other hand, when the LHCD power is dominantly deposited in the core region of the L-mode plasma, the SOL particle widths are not simply increased. The SOL widths and the divertor strike-line positions on both sides are clearly modulated by the LHCD power modulation. In the H-mode plasma, low frequency density fluctuations less than ∼50 kHz in the SOL having low poloidal wavenumber less than <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>2</mml:mn> <mml:mstyle scriptlevel="0"/> <mml:mrow> <mml:mtext>c</mml:mtext> </mml:mrow> <mml:mrow> <mml:msup> <mml:mrow> <mml:mtext>m</mml:mtext> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:mrow> </mml:math> are noticeably suppressed in the SOL during the LHCD power-on phase. This fact suggests that the observed expansion of the divertor particle flux profiles is not caused by enhanced SOL turbulence. <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mi mathvariant="bold-italic">E</mml:mi> </mml:mrow> <mml:mo>×</mml:mo> <mml:mrow> <mml:mi mathvariant="bold-italic">B</mml:mi> </mml:mrow> </mml:mrow> </mml:math> drifts by electrical biasing of the SOL flux tube realized by appreciable LHCD power deposition in the SOL are proposed as a likely candidate mechanism for the interpretation of the experimental observations.