On heat loading, novel divertors, and fusion reactors

Abstract

The limited thermal power handling capacity of the standard divertors (used in current as well as projected tokamaks) is likely to force extremely high (∼90%) radiation fractions frad in tokamak fusion reactors that have heating powers considerably larger than ITER [D. J. Campbell, Phys. Plasmas 8, 2041 (2001)]. Such enormous values of necessary frad could have serious and debilitating consequences on the core confinement, stability, and dependability for a fusion power reactor, especially in reactors with Internal Transport Barriers. A new class of divertors, called X-divertors (XD), which considerably enhance the divertor thermal capacity through a flaring of the field lines only near the divertor plates, may be necessary and sufficient to overcome these problems and lead to a dependable fusion power reactor with acceptable economics. X-divertors will lower the bar on the necessary confinement to bring it in the range of the present experimental results. Its ability to reduce the radiative burden imparts the X-divertor with a key advantage. Lower radiation demands allow sharply peaked density profiles that enhance the bootstrap fraction creating the possibility for a highly increased beta for the same beta normal discharges. The X-divertor emerges as a beta-enhancer capable of raising it by up to roughly a factor of 2.

Magnetic geometry and physics of advanced divertors: The X-divertor and the snowflake

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Recent progress on spherical torus research

“…252 The q div, peak is reduced from 8 MW/m 2 to 2 MW/m 2 by increasing f exp from 10 to 40. The ideas for divertor heat flux mitigation through expanding the divertor flux include the snow-flake divertor (SFD) 253 and the X-divertor, 254,255 which are based on additional field null points close to the usual single null configuration of a conventional divertor. The multiple field nulls make the field null size larger, and cause greater field null expansion in the null region.…”

Downstream Heat Flux Profile vs. Midplane T Profile in Tokamaks

“…An encouraging conclusion from this analysis is that if there is radial thermal diffusion along the field lines distant from the midplane, concepts such as the X-divertor 17 , Snowflake divertor 18 and particularly Super-X divertor 4 may offer an enhanced opportunity for thermal diffusion to contribute to heat flux dispersion, as a complement to the magnetic flux expansion central to these designs.…”