Fast imaging of global eigenmodes in the H-1 heliac

1. Fast imaging of global eigenmodes in the H-1 heliac NandikaThapar, Shuiliang Ma, Jesse Read, Boyd Blackwelland John Howard Plasma Research Laboratory, Research School of Physical Sciences, Australian National University, IMAGING ATOMIC HELIUM LINE RATIOS To investigate the associated fluctuation-driven particle transport we propose to measure the relative phases and amplitudes of the fluctuating electron temperature and density perturbations using a tomographic helium spectral line ratio technique. We have developed a first-order collisional-radiative model for He I emission that indicates that atomic emission ratios are sensitive to local electron temperature and density fluctuations in the 0-30kHz range. As a first step, we require the reconstructed dc radial profiles of electron density and temperature. SYNCHRONOUS BROADBAND IMAGING Synchronous observations of the plasma fluctuations have been obtained at multiple fixed phases with respect to the magnetic fluctuation signal, in both radial and toroidal view directions. By assuming a harmonic structure in magnetic coordinates, tomography of the multiple fixed-phase radial projections can reveal the radial structure of the helical eigenmodes. ABSTRACT We report a study of coherent plasma instabilities in the H-1 plasma using a synchronous gated intensified camera. Due to its tight geometry, the inhomogeneities in the magnetic field give rise to mode coupling in H-1. The experiments have been carried out in radio frequency heated 0.5 T H/He discharges to study the coupling between low frequency acoustic and Alfven waves. We compare the results from a radially viewing tomographic imaging system and a toroidally viewing camera that images an injected neon gas jet injected in a toroidally-displaced poloidal cross-section. We report the variation of the eigenmode structure with magnetic configuration and present first results of tomography of the helium atomic emissions. DC profile reconstruction Inferred ne and Te CRM model RADIAL VIEW IMAGING SYSTEM The mode structure has been observed in broadband visible emission and also various helium atomic transitions for inferring the temperature and density fluctuationsin 0.5 T H/He low power discharges (60 kW). Left: Block diagram for performing synchronous imaging Right: Camera gates (red) synchronised with magnetic fluctuations (mirnov coil ) signal (black) Projection measurements for different configurations 5/4 branch kh=0.33 kh=0.44 Left: Raw image at 667nm and below: Abel inverted emission intensity profiles. Centre: Steady-state CRM calculation of intensity ratios for representative Te, ne Right: Inferred density and temperature profiles and comparison with interferometer. The edge electron temperature is higher than centre. Left: Radial camera viewing geometry and collection optics. Right A photo of the system together with calibration integrating sphere. Time dependent CRM and HeI synchronous imaging Registering the radial view camera with respect to the magnetic coordinate system To achieve accurate tomographic reconstructions of the phase-resolved brightness, it necessary to accurately locate the camera with respect to the H-1 magnetic coordinate system. We describe a calibration procedure based on imaging the fluorescence from argon atoms excited by electron beams injected into the confinement region and matching the observed electron beam tracks against a validated model of the electron beam trajectories Temperature dependence Density dependence 4/3 branch kh=0.63 kh=0.83 Step 1: Establish the electron-beam trajectory Tomographically reconstruct the e-beam Poincare image Fit the e-beam trajectory by using a validated model of the H-1 magnetic field Computed dependence of He I fractional emissivities (amplitude and phase) on oscillation frequency and for representative background densities and temperatures. Alfven mode frequency versus rotational transform (parameterized by coil current ratio Kh) in H-1 [1]. Above and below: Radial view images of average emission intensity and mode amplitude and phase projections for different configurations. The structure changes only when changing branches and the phase difference between magnetic and pressure fluctuations flips on either side of the resonance. HeI 668 nm HeI 706 nm (b) (a) Toroidal view: direct mode imaging using poloidal gas puff (c) HeI 728 nm 4/3 branch, kh=0.83 Fluctuation amplitudes and phases are markedly different. (a) Multi-wire rotating grid allows to reconstruct the e-beam image in the plane of the grid. (b) Sinogramof collected currents (c) Reconstruction of the Poincare e-beam image using simple back projection Left: Experimental setup: 1: camera, 2: internal mirror, 3: gas injector. Right: Gas Injection System Step 2: Fitting the model and observed images gives camera position and orientation Next step: reconstruct the mode structures in each colour to obtain fluctuating densities and temperatures. Use Doppler imaging to obtain displacement vector x and then compare with MHD expectations: Direct imaging performed by synchronously observing (in the toroidal direction) atomic emission from a neon gas supersonic jet injected across a poloidal cross-section. 2 1 Comparison with radial view tomographic inversion 3 Extraneous Reflections 4/3 branch (n=4,m=3) kh=0.83 References: Configurational Effects on Stability and Confinement in the H-1NF Heliac, B.D Blackwell et al. S. Ma, J Howard and N Thapar, “Relations between light emission and electron density and temperature fluctuations in a helium plasma”, Physics of Plasmas Vol 18, 083301 (2011) S. Ma, J Howard, B. Blackwell and N Thapar, “Measurements of electron density and temperature in the H-1 heliac plasma by helium line intensity ratio” Accepted RSI 2012 Left: Raw image of the electron-beam trajectories Right: Observed e-beam tracks (black) superimposed on the calculated trajectories (red) following optimization of the camera viewing parameters (position, view and field of view).