[en] National Spherical Torus Experiment (NSTX) has recently completed its first phase of operation. The primary objective for the baseline diagnostic1 set during this phase has been to aid in plasma control and machine operation, which successfully led to attainment of 1 MA of plasma current and some encouraging initial results with coaxial helicity injection (CHI). This first set of diagnostics relied on techniques previously established on tokamaks and related plasma devices. In the next phase of operation, strong auxiliary heating will become available in the form of rf heating through high harmonic fast waves (HHFWs) and neutral beam injection (NBI) for a combined total power of 11 MW. With intense auxiliary heating, accurate and reliable measurements of the plasma parameters for both machine operation and characterization of the plasma performance over a wide range of conditions will require an extended set of diagnostics. Profile diagnostics will be particularly important in this phase, and in some cases this capability can be achieved by upgrading existing diagnostics. However, many new diagnostic approaches must be implemented which take into account the constraints of a spherical torus device. An overview of the full complement of diagnostics planned for NSTX will be presented, and issues related to implementing each diagnostic will be discussed

[en] A frequency modulated Coherent Laser Radar ranging diagnostic is being used on the National Spherical Torus Experiment (NSTX) for precision metrology. The distance (range) between the 1.5 microm laser source and the target is measured by the shift in frequency of the linearly modulated beam reflected off the target. The range can be measured to a precision of < 100microm at distances of up to 22 meters. A description is given of the geometry and procedure for measuring NSTX interior and exterior surfaces during open vessel conditions, and the results of measurements are elaborated

[en] A frequency modulated Coherent Laser Radar ranging diagnostic is being used on the National Spherical Torus Experiment (NSTX) for precision metrology. The distance (range) between the 1.5 μm laser source and the target is measured by the shift in frequency of the linearly modulated beam reflected off the target. The range can be measured to a precision of <100 μm at distances of up to 22 m. A description is given of the geometry and procedure for measuring NSTX interior and exterior surfaces during open vessel conditions, and the results of measurements are elaborated

[en] A millimeter-wave reflectometry system for electron density profile and fluctuation measurements is being developed and installed on the National Spherical Torus Experiment. The initial frequency coverage will be in the bands 12--18, 20--32, and 33--50 GHz, provided by frequency-tunable solid-state sources. These frequencies correspond to O-mode cutoff densities ranging from 1.8x10¹² to 3.1x10¹³cm^-3, which will span both the plasma core (ρ=r/a<0.8) and edge (ρ>0.8) regions. Operated as a broadband swept-frequency (frequency-modulated continuous-wave) reflectometer, the diagnostic is expected to provide routine (shot-to-shot) time- (≤50 μs) and spatially resolved (∼1 cm) density profiles. The previous hardware can be easily reconfigured as a fixed-frequency reflectometer for density fluctuation measurements. The combination of measurements would be valuable for studying phenomena such as possible L- to H-mode transitions and edge-localized modes

[en] The novel hypervelocity dust injection diagnostic will facilitate our understanding of basic aspects of dust-plasma interaction and magnetic field topology in fusion plasma devices, by observing 'comet tails' associated with the injected micron-size dust particles. A single projection of the tail onto an image plane will not provide sufficient information; therefore, we plan to use two views, with intensified DiCam-Pro cameras on two NSTX ports. Each camera can furnish up to five overlaying sequential images with gate times greater than 3 ns and 1280x1024 pixel resolution. A coherent fiber bundle with 1500x1200 fibers will relay the image from an imaging lens installed directly on the port to the camera optics. The lens receives light from the outer portion of the NSTX cross section and focuses a 1 cm tail onto at least 60 fibers for adequate resolution. The estimated number of photons received by the camera indicates signal-to-noise ratios of 10²-10⁴, with the use of a 10 nm bandwidth filter. The imaging system with one camera was successfully tested on NSTX in 2005. Photographing lithium pellets yielded bright and distinctive pictures of the tails nearly aligned with B lines. We also observed that the bright 'filaments' - plasma cords with high density and temperature - are present in both top and bottom portions of the machine

[en] Measurement of the neutron energy spectrum above ∼16 MeV will yield information on the spatial and energy distributions of confined fast alphas in deuterium--tritium (DT) tokamaks (Fisher, Nucl. Fusion; Gorini Rev. Sci. Instrum.). The energetic neutrons result from fusion reactions involving the energetic ions created by alpha-fuel ion knock-on collisions. Standard two-gas bubble neutron detectors, designed to only detect neutrons with energies above a selectable threshold determined by the gas mixture, were used in preliminary attempts to measure the knock-on neutrons from DT plasmas in the Tokamak Fusion Test Reactor and Joint European Torus (JET). Subsequent measurements at accelerator neutron sources showed an unexpected below-threshold detector response that prevented observations of the alpha-induced neutron tails. Spontaneous bubble nucleation measurements show that the majority of this below-threshold response is due to slight variations in the gas mixture, and is not present in single-gas detectors. Single-gas detectors will be tested at the University of California Berkeley to determine the neutron energy threshold as a function of detector operating temperature and to confirm their suitability for alpha knock-on tail measurements. An array of single-gas detectors operating at different temperatures should allow measurements of the alpha knock-on neutron tail during planned DT experiments on JET

[en] The goal of this paper is to characterize the effects of small non-axisymmetric divertor plate electrodes on the local scrape-off layer plasma. Four small rectangular electrodes were installed into the outer divertor plates of NSTX. When the electrodes were located near the outer divertor strike point and biased positively, there was an increase in the nearby probe currents and probe potentials and an increase in the Li I light emission at the large major radius end of these electrodes. When an electrode located farther outward from the outer divertor strike point was biased positively, there was sometimes a significant decrease in the Li I light emission at the small major radius end of this electrode, but there were no clear effects on the nearby probes. No non-local effects were observed with the biasing of these electrodes. (paper)

[en] Spatially resolved measurements of deuterium Balmer and Paschen line emission have been performed in the divertor region of the National Spherical Torus Experiment using a commercial 0.5 m Czerny-Turner spectrometer. While the Balmer emission lines, as well as the Balmer and Paschen continua in the ultraviolet and visible regions have been extensively used for tokamak divertor plasma temperature and density measurements, the diagnostic potential of infrared Paschen lines has been largely overlooked. We analyze Stark broadening of the lines corresponding to 2-n and 3-m transitions with principal quantum numbers n=7-12 and m=10-12 using recent model microfield method calculations [C. Stehle and R. Hutcheon, Astron. Astrophys., Suppl. Ser. 140, 93 (1999)]. Densities in the range (5-50)x10¹⁹ m^-3 are obtained in the recombining inner divertor plasma in 2-6 MW neutral beam heated H-mode discharges. The measured Paschen line profiles show good sensitivity to Stark effects and low sensitivity to instrumental and Doppler broadenings. The lines are situated in the near-infrared wavelength domain, where optical signal extraction schemes for harsh nuclear environments are practically realizable and where a recombining divertor plasma is optically thin. These properties make them an attractive recombining divertor density diagnostic for a burning plasma experiment

[en] A new high speed infrared camera has been successfully implemented and produced first set of heat flux measurements on the lower divertor tiles in the NSTX tokamak. High spatial and temporal resolutions, 6.4 mm and 1.6-6.3 kHz, respectively, enable us to investigate detailed structure of heat flux deposition pattern caused by transient events such as edge localized modes. A comparison of the data with a slow infrared camera viewing the same region of interest shows good agreement between the two independent measurements. Data analysis for various plasma conditions is in progress.

[en] Full text: We report on ELM triggering and pacing experiments in NSTX-U, including comparisons to pellet ablation models. Multiple sizes and types of solid impurity granules are injected into the low field side of the plasma to determine their ELM triggering and pacing capability. Examining the penetration depths, mass deposition locations, and ELM triggering efficiencies of sub-millimetre lithium, boron carbide (B4C) and carbon granules, we assess the optimal size and composition for minimally perturbative high frequency ELM pacing. By utilizing a neutral gas shielding model, benchmarked with lithium granule ablation experiments performed on DIII-D, the pedestal atomic deposition characteristics for the three different species of granule have been modelled for NSTX H-mode discharges with low natural ELM frequencies. Variations in the depositional barycentre can range from 5 cm for lithium to 17 cm for the same size and velocity carbon granule. We estimate that these penetration depths will be reduced by a factor proportional to q_s∼n_eT^3/2_e as the full NSTX-U capabilities are realized. In addition, by reducing the rotation speed of the impeller, the mass deposition location is translated closer to the top of the pedestal allowing further tuning of the pressure perturbation. At this location the pressure profile generated by the granule can be added to the preexisting pedestal pressure gradient, leading to a set of characteristics advantageous for ELM triggering while affecting a minimal perturbation to the core plasma. Using multiple high-speed cameras to precisely track the granule injections and monitor the ablation duration and penetration depths in NSTX-U, a fractional mass deposition location can be extrapolated. Fast infrared camera measurements are used to characterize the variations between triggered ELMs and the inter-ELM period. In addition, comparisons are also made between stimulated and spontaneously occurring ELMs. These measurements provide a comparison of the ELM peak heat flux mitigation factor, as well as variations in the ELM footprint due to the triggering mechanism. The results of ELM pacing and comparisons with the constructed ablation model in NSTX-U will be reported. (author)