Experimental characterization of the edge fluctuations during I-phase at ASDEX Upgrade

Abstract The intermediate phase (I-phase) is a plasma regime that exists at the transition from the low-confinement mode (L-mode) to the fully developed high-confinement mode (H-mode), or vice versa. This regime is characterized by improved particle and energy confinement compared to the L-mode, but not as pronounced as in the fully developed H-mode. Generally, type-I edge localized modes are absent in I-phase plasmas and instead smaller burst-like events occur. Using the thermal helium beam spectroscopy and ball-pen probe diagnostic at ASDEX Upgrade, we observe precursors to the burst, which are studied in detail in this work. We find that the I-phase precursor mode, a high-frequency edge fluctuation, is localized in the confined region and propagates in the electron diamagnetic direction in the plasma frame. It has a normalized poloidal wavenumber in the range <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>0.01</mml:mn> <mml:mo>&lt;</mml:mo> <mml:msub> <mml:mi>k</mml:mi> <mml:mrow> <mml:mi>θ</mml:mi> </mml:mrow> </mml:msub> <mml:msub> <mml:mi>ρ</mml:mi> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">s</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> <mml:mo>&lt;</mml:mo> <mml:mn>0.06</mml:mn> </mml:mrow> </mml:math> , indicating that it is in the mesoscale between classical magnetohydrodynamic and turbulent micro-instabilities. The precursor mode exists for several 100 µ s, with a rapid drop in frequency and change in cross-phase between potential and density fluctuations <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mo>|</mml:mo> <mml:msub> <mml:mi>α</mml:mi> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi mathvariant="normal">n</mml:mi> <mml:mi mathvariant="normal">e</mml:mi> </mml:msub> <mml:mo>,</mml:mo> <mml:mi>ϕ</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> <mml:mo>|</mml:mo> </mml:mrow> <mml:mtext>⩾</mml:mtext> <mml:mn>0</mml:mn> </mml:mrow> </mml:math> , before a burst sets in, causing a substantial particle ejection. Our results suggest that the precursor mode is an electromagnetic drift-wave (DW)-like mode, such as a micro-tearing or a kinetic peeling-ballooning mode. The microscopic analysis of single precursors reveals, that heat transport increases first, followed by particle transport. This is why we favor the precursor being a micro-tearing mode. In particular, our results lead to the hypothesis that the burst is the full development of the precursor. More specifically it undergoes a change in its underlying nature as indicated by the change in cross-phase, transitioning from a DW-like to a more interchange-like mode, which ultimately leads to the ejection of heat and particles characteristic of the burst.