Speaker
Description
The H-1 flexible heliac is a medium-sized helical axis stellarator of major radius R=1m, and average minor radius <r> $ \simeq
0.15-0.2$ m. Precise control of the ratio $k_h$ of the helical winding current to the ring coil current provides access to a wide range of magnetic configurations. This provides rotational transform $\bar\iota$ in the range $0.9 < \bar\iota < 1.5$ for $B_0 < 1$ T, with both stellarator-like and tokamak-like shear, and magnetic well from $\simeq 5$% to $-2\%$ (i.e. hill).
The coil-in-tank construction technique allowed the TF, ring, helical and inner VF conductors be assembled on a 4m $\phi$ precision rotating table for accurate positioning using the measured magnetic axes of the electromagnets. A recent tomographic electron-beam magnetic field mapping exercise has shown that the location accuracy after installation and years of use is < 2mm. Under excitation there is a small distortion of the support structure, which is designed to resist overturning and other forces ~ 5 Tonne per TFC. Worst case displacements of ~2mm and rotational transform change of -0.5% are found, extrapolating to -2% at 1T.
Fluctuations ranging from drift waves to Alfvenic have been observed in the 1-100s of kHz range. Propagation, dispersion and fluctuation-induced transport data will be presented, including synchronously imaged 2D reconstructions of density fluctuations. Comparison of the profiles with 3D compressible MHD predictions (CAS3D) show best agreement with a beta induced Alfven eigenmode. Density fluctuation profiles are also observed by a 21 channel 2mm interferometer using imaging techniques to improve resolution. Recently a third current controller has enabled automated two-parameter configuration scans of rotational transform and magnetic well, revealing a general increase in fluctuations as the well depth is decreased.
Several unique optical diagnostics have been developed on H-1 and deployed on larger international devices. Coherence imaging is a very high resolution interferometric technique used initially on H-1 to determine ion temperature and flow by the Doppler effect. Further developments include isotope ratio imaging and polarisation effects. Imaging in synchronism with the applied RF heating has revealed images of RF propagation in H-1, suggesting an electrostatic mode clearly dependent on $\omega /\omega_{ci}$. Tests of a recently installed 'compact Alfven' antenna and adjustable side shields are underway. Results from recent investigations will be presented together with selected results from the 25 year history of H-1.
| Eligible for student paper award? | No |
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