7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Simulation study of particle transport by weakly coherent mode in the Alcator
C-Mod tokamak
C. Dong1, Y. Lang2, X. Q. Xu3, Z. B. Guo2, B. Li4, X. G. Wang5 and T. F. Tang6
1
Institute of Physics, Chinese Academy of Sciences2
School of Physics, Peking University3
Lawrence Livermore National Laboratory
4
School of Physics, Beihang University
5
Department of Physics, Harbin Institute of Technology
6
College of Physics and Optoelectronic Engineering, Shenzhen Universitye-mail (speaker): chaodong@iphy.ac.cnI-mode, featuring a temperature pedestal but without adensity pedestal, has been explored on many tokamaks.[1]It is typically accessed with the ion 𝐁 × 𝛁𝐵 drift awayfrom the active X-point. Contrary to its H-mode-likeenergy confinement, I-mode exhibits L-mode-like particle
confinement,[2] which avoids the impurity accumulation.
Due to the lack of density pedestal, the pressure gradientand bootstrap current in the I-mode pedestal are smaller
than those in the ELMy H-mode pedestal. This makes Imode naturally stable to the edge-localized modes(ELMs).[3] Combining both the advantages of L- and Hmodes, I-mode is a promising high-performance operation
regime for reactor-scale tokamak devices.
The transition from L-mode to I-mode is accompaniedby the changes in edge density, temperature, and magnetic
field fluctuations. The mid-frequency fluctuations arestrongly suppressed, which was observed experimentallyto correlate well with the reduction in the effective thermal
diffusivity in the pedestal region.[4] Meanwhile, a highfrequency weakly coherent mode (WCM) appears and itsamplitude was found to correlate positively to the particle
flux across the last closed flux surface (LCFS),[3]suggesting that the WCM is responsible for maintaining
high particle and impurity transport in the I-mode pedestal.
Up to now, there is still no consensus on the fundamental
mechanism behind the WCM and the comparison of its
produced transport in simulations with the experimental
results is not so satisfactory.
We conduct a simulation study of the physicalmechanisms behind the WCM and its produced particle
transport in the I-mode edge plasmas by using theBOUT++ code.[5] The WCM is identified in oursimulations by its poloidal and radial distributions as wellas its frequency and wavenumber spectra. Its produced
radial particle flux is calculated and compared with theexperimental value. The good agreement indicates that theWCM is an important particle transport channel in the Imode pedestal. It is found that the WCM can transportparticles across the strong outer shear layer of the 𝐸𝑟
well established in the formation of I-mode, based on
which a possible explanation is provided why I-mode does
not feature a density pedestal. The key point lies in thechange of the cross-phase between the electric potential
and density fluctuations induced by the 𝐄 × 𝐁 Doppler
shift. In the strong shear layer, although the electric
potential fluctuation is significantly suppressed, the crossphase is close to 𝜋/2, resulting in a strong drive of thedensity fluctuation and particle transport. To identify thephysical nature of the WCM, the linear dispersion relation
equation is derived in the slab geometry. We find a drift
Alfvén wave (DAW) instability driven by the electron
density and temperature gradients. The growth rate andfrequency of this DAW instability show similar
dependence on the parallel resistivity and the parallel
electron pressure gradient and thermal force terms in theparallel Ohm’s law as the simulated linear instability
under a typical toroidal mode number 𝑛 for the WCM.
Therefore, we argue that the DAW instability is possible
the main component of the WCM.
This work is supported by the National MCF Energy
R&D Program under Grant No. 2018YFE0311300.
References[1] A. E. Hubbard et al, Nucl. Fusion 56, 086003 (2016)[2] D. G. Whyte et al, Nucl. Fusion 50, 105005 (2010)[3] J. R. Walk et al, Phys. Plasmas 21, 056103 (2014)[4] A. E. Hubbard et al, Phys. Plasmas 18, 056115 (2011)[5] Y. Lang et al, Nucl. Fusion 62, 086018 (2022)