Page 4 - Hybrid MHD Model for a Driven, Ion-Current FRC

9th fce harmonic layer for 18 GHz, and with a maximum 60% efficiency at the 13th fce harmonic layer for 28 GHz. Comparing with the 8 GHz case shown in Fig. 4, the mode conversion process for 28 GHz (Fig. 5) shows different features: the EBW does not propagate, and its power is deposited at the 13th fce harmonic layer right after the X-B mode conversion. Due to the facts that the mode conversion layers are very close to each other, and the highly compacted high harmonics (> 10 fce) are close to the conversion layers and also across the separatrix, it is really difficult to verify the actual process of mode conversion near the O-mode cut-off and the UHR layer. When rays propagate along the distance between the O-mode cut-off layer and the UHR layer, what really happens in that region might be more complicated than our original assumptions. The power absorption through the 13th fce harmonic resonance is expected to reduce significantly while the Landau damping should be taken into account.
FIGURE 6. (a) Mode conversion and power absorption along the distance of ray propagation; (b) the tracing of ray propagation in X-Z plane for 50 GHz.
For 50 GHz, the O-mode cut-off density (~3.0 1019 m-3) is barely below the given maximum density (~3.2 1019 m-3). In this case, simulation results indicate that there is no mode conversion or power absorption, as shown in Fig. 6 (a). This is consistent with Fig. 6 (b) that part of rays are reflected at the O-mode cut-off layer and propagate outwards, and part of rays just shine through the plasma without any absorption. This scenario should be avoided because either way could cause damage to the components of in-vessel instruments or windows, unless the plasma density is increased significantly above the O-mode cut-off density.
CONCLUSIONS
For a given antenna position and the fixed profiles of plasma density and equilibrium field, different microwave heating scenarios with the focus on EBW excitation have been studied extensively with Genray ray tracing code. Results indicate that among the six selected frequencies, the 8 GHz case is the most promising scenario, which has higher mode conversion and power absorption efficiencies, as well as the relatively deeper power deposition. The 28 GHz case also shows very interesting results from a physics point of view, since the subsequent power absorption (above 50%) at the 13th fce harmonic resonance has not been reported in any fusion relevant plasmas.
ACKNOWLEDGMENTS
We would like to acknowledge the support and contributions from the rest of the TAE team and collaborators. A debt of gratitude is also due to our shareholders for their continuous support.
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