MLM-based monochromator for molybdenum impurity monitoring on the Alcator C-MOD tokamak in the 30–130 Å range

Abstract

Molybdenum, from the vessel wall armor tiles and divertor plates, is expected to be the dominant high-Z impurity in the Alcator C-MOD tokamak plasma. To monitor the molybdenum emission in the extreme ultraviolet (XUV), a monochromator utilizing multilayer mirrors (MLMs) as dispersive elements will be installed on the C-MOD tokamak with a 10° above the mid-plane view across the plasma’s center. Much of the strong Mo emission under the C-MOD experimental conditions will be emitted between 30–50 Å and 65–90 Å by M-shell ions (n=3–3 and n=3–4 transitions), at 116 Å from MoXXXI and at 127 Å from MoXXXII. The monochromator will have three MLMs and either three-channel electron multipliers or three XUV silicon photodiodes mounted on a single θ-2θ goniometer. Each MLM and detector unit will simultaneously monitor a separate molybdenum emission range with a 1–10-ms temporal resolution. The resolution of the MLMs range from 1 to 5 Å depending on the wavelength, and each MLM will be optimized for one of the above spectral regions. This low-resolution monitoring of many charge states will enable radiative power loss estimates and can be used in impurity transport analysis. The design of the monochromator and the expected Mo emission will be presented and discussed.