Fast imaging of global eigenmodes in the H-1 heliac

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.

Nandika Thapar, Shuiliang Ma, Jesse Read, Boyd Blackwell and John Howard Plasma Research Laboratory, Research School of Physical Sciences, Australian National University,

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

(a) Multi-wire rotating grid allows to reconstruct the e-beam image in the plane of the grid. (b) Sinogram of collected currents (c) Reconstruction of the Poincare e-beam image using simple back projection

Step 1: Establish the electron-beam trajectoryi) Tomographically reconstruct the e-beam Poincare imageii) Fit the e-beam trajectory by using a validated model of the H-1

magnetic field

(b)

(c)

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.

Step 2: Fitting the model and observed images gives camera position and orientation

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.

Extraneous Reflections

Comparison with radial view tomographic inversion

Toroidal view: direct mode imaging using poloidal gas puff

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.

(a)

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 fluctuations in 0.5 T H/He low power discharges (60 kW).

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).

DC profile reconstruction

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

Left: Block diagram for performing synchronous imaging Right: Camera gates (red) synchronised with magnetic fluctuations (mirnov coil ) signal (black)

Projection measurements for different configurations

kh=0.33 kh=0.44

kh=0.63 kh=0.834/3 branch

5/4 branch

1 2

3

4/3

4/3

5/4 5/4

6/5

7/6

6/5

5/4

Configuration parameter kh

4/3 branch

(n=4,m=3)

kh=0.83

Left: Radial camera viewing geometry and collection optics. Right A photo of the system together with calibration integrating sphere.

Left: Experimental setup: 1: camera, 2: internal mirror, 3: gas injector. Right: Gas Injection System

Direct imaging performed by synchronously observing (in the toroidal direction) atomic emission from a neon gas supersonic jet injected across a poloidal cross-section.

CRM model Inferred ne and Te

Time dependent CRM and HeI synchronous imaging

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.

Computed dependence of He I fractional emissivities (amplitude and phase) on oscillation frequency and for representative background densities and temperatures.

Temperature dependence Density dependence

HeI 668 nm HeI 706 nm

4/3 branch, kh=0.83

Fluctuation amplitudes and phases are markedly different.

HeI 728 nm

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: