Initial measurements of turbulence with a PCI diagnostic on Wendelstein 7-X*

A phase contrast imaging (PCI) diagnostic has been implemented on the W7-X stellarator at IPP Greifswald and operated during the 2017 OP1.2a campaign [1]. The diagnostic has been upgraded recently as described in a companion presentation [2], and its operation will be continued in the 2018 OP1.2b campaign. New results from OP1.2b will be presented, expanding on previous observations of broadband turbulence as well as Alfvén wave phenomena in ECR heated plasmas collected during OP1.2a. The low frequency broad-band turbulence is believed to be evidence of  nonlinearly saturated spectrum of ion scale turbulence driven by temperature gradients, while the band of nearly coherent modes observed in the Alfvén frequency range under different ECRH conditions may be manifestation of the presence of energetic electrons. Possible driving mechanisms include gradients of the thermal electron pressure or more likely, trapped electrons [3].

[1] E. Edlund, M. Porkolab, O. Grulke, et al, Proc. HTPD2018 Conference, San Diego, 2018.

[2] Z. Huang, et al, presented at this conference.

[3] M. Valovic, et al, Nuclear Fusion 40, 1569 (2000)