Using an electron beam to study CNT's magnetic field

For the experimental mapping of magnetic flux surfaces in CNT an electron beam was emitted by a 6V light bulb filament biased between -60 and -140V. The position of the filament in the chamber was adjusted in the radial direction using an edge-welded bellows. Various methods to visualize the magnetic flux surfaces were attempted, including using an electron gun and a steel wire mesh coated with phosphor. The optimal method was determined to be with emission from a bare filament, visualized using rotating phosphor-coated aluminum rods that together spanned nearly the entire cross section of the torus. The light emitted as the electrons collided with the phosphor rods detailed the shape of the magnetic flux surface, and was captured using a long exposure photograph. The final image of the mapped magnetic flux surfaces for a specified current ratio in the coils was obtained by superimposing 10 individual flux surface images using a step width of 3cm of the filament.


Figure 1: long-exposure photograph of a filament-emitted electron beam impinging on the phosphor-coated rod. The rod, which was swept back and forth during the exposure, can be seen faintly on the lower right. The resulting image is known as a Poincaré cross-section and indicates where a magnetic flux surface intersects the sweeping plane of the rod.
Figure 2: long-exposure shot of a beam from the electron gun impinging on the phosphor-coated rod.
Figure 3: long-exposure photograph of an electron beam impinging on the phosphor-coated mesh. The position of the emitter was moved during the exposure, causing the strike points on the mesh to sweep from the inside of the confinement volume toward the edge.
Figure 4: Composite image of Poincaré sections corresponding to multiple probe locations, showing CNT's nested flux surfaces. Each section was isolated from the background through image subtraction and colored black for clarity.  Note the existence of magnetic islands near the edge. It is possible to predict what this image should look like with the help of a numerical field-line tracer. By comparing the discrepancies between the experimental results shown here and the numerical predictions, error fields (deviations from the predicted magnetic field) can be diagnosed.
Figure 5: Image of an electron beam emitted by a bare filament and traveling within the confining volume of CNT's magnetic field. The glow is emitted by neutral gas particles upon colliding with electrons in the beam. The neutral gas pressure and beam current must both be increased from the cases in the previous images in order for the glow to be visible to the naked eye.
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