Ion Heat Pulse Propagation Experiments Using High Speed Charge Exchange Spectroscopy in W7-X

Clamping of ion temperature (T_i) profiles in electron-cyclotron resonant heated (ECRH) plasmas in the W7-X stellarator limits plasma performance and is found across magnetic configurations [1]. Power balance studies in ECRH step down in combination with local GENE runs show ion channel turbulent transport is likely dominated by ion temperature gradient (ITG) turbulence. To further characterize the turbulence transport properties, we performed ion heat pulse propagation studies using neutral beam injection (NBI) modulation experiments in W7-X to quantify T_i profile stiffness. To measure T_i at timescales relevant to the ion heat pulse, a charge exchange (CX) diagnostic has been employed, allowing measurements with time resolutions below 1 ms [2]. Electron temperature (T_e) measurements were made with the electron cyclotron emission (ECE) diagnostic in combination with ECRH modulation to study the electron heat transport as in previous electron heat pulse propagation experiments [3]. The CX and ECE diagnostics enabled successful experiments during the OP 2.1 campaign using both NBI and ECRH modulation, with clear modulation observed in T_e and T_i. Initial estimates of diffusive and convective transport coefficients using simplified physics models are presented, in addition to physics model development to include the effects of NBI modulation of lower order fluid moments on transport.

[1] M.N.A. Beurskens et al 2021 Nucl. Fusion

[2] C. Swee et al 2022 Rev. Sci. Instrum.

[3] G.M. Weir et al 2021 Nucl. Fusion