Imaging Diagnostics at the H-1 National Plasma Fusion Research Facility

1. Imaging Diagnostics at the H-1 National Plasma Fusion Research Facility • Because of its relatively unhindered viewing access, the H-1 heliac is well suited for the development of plasma imaging systems. • To obtain spatial profiles, the complex plasma geometry requires multiple viewing angles and the application of inverse methods (tomography). • Practical considerations require the use of various temporal, spatial and frequency domain multiplexing methods. • This poster describes imaging systems for key plasma parameters • Scanning interferometry for electron density • Coherence imaging for spectroscopy (Doppler, ratios, polarization etc) • Supersonic He injector and atomic emission line ratios for electron temperature Coherence imaging systems (Ti, vf, B, …) Helium line ratios for Te The ratio of the brightness of transitions from triplet and singlet atomic states of helium is sensitive to plasma electron temperature. A supersonic helium jet has been developed by USyd for probing Te profiles in the H-1 heliac. Static (spatial multiplex) coherence imaging • Coherence imaging systems are high though-put temporal and/or spatial multiplex polarization interferometers for high resolution, high-speed spectroscopic imaging. • Applications include • Doppler spectroscopy (CXRS, divertor) • Emission line ratios (Isotopes, He line ratios) • Polarization spectroscopy (MSE, Zeeman) • Thomson scattering • Systems are installed at the Australian National University, University of Sydney, Consorzio RFX, IPP Greifswald, NFRC Korea, JT-60 JAEA Quad-coherence imaging systems produce quadrature 2-d interference fringe images at the 4-corners of a CCD array. This allows a snapshot of the complex optical coherence. Left: Layout of the helium supersonic gas injection system and viewing geometry. Below: integrated 16-channel PMT detector/amplifier package developed at ANU for imaging spectroscopy Left: Viewing geometry for quadrant coherence imager. Toroidal field coils and helical field coil internal to vacuum tank are visible. The H-1 multi-view density interferometer Above: Layout for quadrant coherence imaging system The original multi-view scanning far-infrared interferometer utilized a rotating scanning grating to generate four independent views (sets of chords in a given viewing direction) of the plasma.  The multiple views allow phase shift data to be tomographically unfolded to obtain the plasma density contours Modulated (temporal multiplex) coherence imaging 16 channel time-resolved image of H-1 ECH plasma pulse in helium emission (471nm). The spatial intensity profile for the gas pulses becomes more edge localised as the temperature increases. Clutter in the filter passband precludes the use of the intrinsic He atomic emission for temperature estimation. Above: Typical 4-quadrant coherence image (leftmost) and extracted 2-d snapshots of the spectral line brightness, temperature and flow speed. Note hollow ion temperature profile and rigid rotation flow profile for low field argon discharge. (Compare with images to left) Above: Photograph of the first electro-optically modulated Doppler coherence-imaging spectrometer installed on the H-1 heliac and drawing showing the plasma cross-section and imaging arrangement. The 55 channel tomographic coherence imaging system Above: Photo of grating-based scanning interferometer and Gaussian beam model. Left: Tomographic reconstruction of saturated global instability in low-field (0.1T) argon discharge Left to right: Doppler images obtained using the 16-channel coherence imaging camera, showing the time-evolution of the plasma brightness profile, the plasma flow and ion temperature. Low field argon discharge, 488nm Left: The coherence tomography system Above: Plasma emission reconstructions compared with calculated magnetic isosurfaces Measuring isotope ratios Measurement of H-D isotope intensity ratios in Hb light gives information about particle fuelling in H-1. As the intensity ratio changes, the interferogram phase shifts and fringe contrast changes (see figures left). The technique is useful for 2-d imaging and when Doppler broadening blurs the lineshape. Above left: Measured pulse brightness versus plasma major radius for 471nm and 504nm transitions. Solid lines are expected brightnesses based on CRM model and measured Te and ne profiles. Right: The electron temperature profile inferred from the relative line intensities. Left: An electronic wideband sweep and fixed grating now replaces the grating-scanned laser beam. This is a turn-key solution that can provide a more rapid plasma sweep. Above: Temporally-multiplexed plasma sweeps of ECH plasma Left: The wheel located in the H-1 tank Above: sequence or reconstructions of large scale plasma instability