During the past decade, considerable experimental and theoretical progress has been made in the development of the Spherical Torus (ST) configuration. A second generation of 'proof of principal' devices, including GLOBUS-M, MAST and NSTX, are now operating with plasma currents in the 1 -2 MA range. The ST has already been the basis of a reactor study, and the concept has also been considered in the context of a volume neutron source. As the aspect ratio of a conventional circular tokamak is reduced towards one, the cross-section naturally assumes a shape with high elongation and triangularity, with only modest requirements for shaping fields. This configuration can sustain high plasma current with high values of edge q, meaning that the toroidal field can be much lower than that in a conventional tokamak. Compared to a standard tokamak, the magnetic field pitch can be very large (pitch angles > 50° at outer midplane). High values of toroidal beta can reached, and the combination of high β, high κ, and high q(a) leads to the potential for a large bootstrap current sustainment. The National Spherical Torus Experiment (NSTX) is a low-aspect-ratio torus (R/a≤1.3) designed to produce and study high β_T (25-40%) plasmas that are non-inductively sustained with high bootstrap fraction (≤70%). Heating and current drive is done with a 3-source neutral beam (5 MW) and a 12 strap, high-harmonic-fast-wave (HHFW) antenna (6 MW). In addition, coaxial-helicity-injection (CHI) is used for non-inductive plasma current initiation. Basic device capabilities are R=0.85 m, a=0.68 m, I_P≤1.4 MA, B_T≤0.45 T with elongation ≤ 2.0 and triangularity ≤ 0.5. Central plasma parameters in the range of n_e=4-5x10~(19) m~(-3) and T_e,T_i=3-4 keV are obtained for high power heating experiments, which began in late 2000. This paper will highlight some of the challenges and opportunities present in the diagnosis of ST plasmas on NSTX, and discuss the corresponding diagnostic development that is currently underway. After a brief description of diagnostic systems currently installed, examples of ST-specific diagnostic challenges will be highlighted, as will another case, where the ST configuration offers opportunities for new measurements.
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