Figure 1 shows a top-down view of NSTX-U with FIReTIP channels overlaid. Channel
1 (G to B, solid line) will supply the core density measurement for density
feedback control and Thomson scattering calibration. Channels 2 (L to I,
dashed line) and 3 (B to L, dotted line) will observe plasma fluctuations.
The semi-transparent lines represent the NBI beam paths. Reproduced with
permission from Menard et al., Nucl. Fusion 52, 083015 (2012). Copyright
2012 International Atomic Energy Agency.

Figure 2 shows the schematic of the visible light heterodyne interferometer with the
zeroth-order beam (solid) and the first-order diffracted beam (dotted) from
the acousto-optic modulator (AOM). Beam splitters are shown here as in a
fully optical system for clarity; fiber optics are used in the physical set-up.