Sherwood 2019

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Author: Patrick H Diamond
Requested Type: Poster
Submitted: 2019-02-22 20:22:01

Co-authors: Ting Long,Rui Ke,M. Malkov

Contact Info:
UC San Diego
9500 Gilman Drive
La Jolla, Ca   92093
USA

Abstract Text:
Several recent experiments have linked the phenomenology of the L-mode density limit (𝑛→𝑛𝑔) to the collapse of the edge shear layer and an associated increase in the particle transport. These results support the idea that the density limit is due primarily to an increase in particle transport, and that MARFE’s, disruptions, etc. are secondary consequences of edge cooling. Theoretical work (1) has identified the degradation of zonal flow shear with the transition from adiabatic to hydrodynamic electron response (i.e. 𝑘∥2𝑉𝑡ℎ2𝜔𝜈>1 to <1), which leads to a drop in shear flow production. The key physics is a decoupling of the Reynolds stress and the drift wave energy density flux in the hydrodynamic regime. The latter is observed in experiments
A major question remains — namely of how to connect the shear layer collapse to the Greenwald scaling 𝑛𝑔~𝐼𝑝. An answer lies in the zonal flow polarization length (screening length) which is effectively set by the poloidal gyro-radius. Thus, larger 𝐼𝑝⟶ larger 𝐵𝜃 ⟶ larger zonal potential, for fixed drift wave turbulence drive. As a consequence, increased current tends to mitigate the reduced production which occurs at higher density. These conclusions are supported by a study of zonal flows, turbulence and the associated feedback loops which incorporates the realistic neoclassical polarization. The interesting theoretical question here is the interplay of the inverse transfer process with the proper neoclassical screening. In a related vein, we note that in RFPs — which exhibit 𝑛~𝐼𝑝 scaling — neoclassical effects are negligible, but the ‘classical’ screening length is set by 𝜌𝑖 calculated with 𝐵𝜃. This also leads to favorable zonal flow scaling with currentWe also report on experimental studies of a test of the shear layer collapse scenario by using external bias to support the edge shear layer against collapse at high density.

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