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Research Needs for Magnetic Fusion Energy Sciences - US Burning ...

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tHEME 5: oPtiMiziNG tHE MaGNEtic coNFiGuRatioN<br />

introduction<br />

ScopE aNd FocuS<br />

a key strategy <strong>for</strong> the Us and world fusion program has been the investigation and development of<br />

a variety of magnetic configurations that are able to confine a hot, stable plasma. of these, the tokamak<br />

configuration has achieved parameters that are nearest those required in a fusion reactor.<br />

The tokamak exhibits good qualities <strong>for</strong> plasma confinement, and this has spurred its intense development,<br />

ultimately leading to iteR. nevertheless, the optimum magnetic configuration <strong>for</strong> solving<br />

the simultaneous challenges of fusion plasma physics and engineering is not yet known. each<br />

of the magnetic configurations being studied brings opportunities to optimize the magnetic fusion<br />

system in unique ways. each also has challenges that reflect tradeoffs in physics and engineering.<br />

as part of its strategic planning process, the department of energy asked the <strong>Fusion</strong> energy sciences<br />

advisory committee (Fesac) to critically evaluate the status of, and scientific opportunities<br />

<strong>for</strong>, major alternate magnetic confinement configurations. This led to the Report of the Fesac<br />

toroidal alternates Panel (taP), which <strong>for</strong>ms the starting point <strong>for</strong> the optimizing the magnetic<br />

configuration Theme covered in this chapter. (The taP report is available at http://fusion.<br />

gat.com/tap/). in this context, “alternate” means toroidal magnetic configurations other than<br />

the conventional aspect ratio tokamak. The scope of the evaluation was limited to the toroidal<br />

magnetic configurations that are most developed: the stellarator, the spherical torus, the reversed<br />

field pinch, and compact tori. compact tori are two distinct configurations, the spheromak<br />

and the field-reversed configuration, that share the common feature of having a simply connected<br />

wall geometry with no physical structure linking the plasma torus.<br />

The toroidal alternates offer particular scientific and technological benefits that could substantially<br />

improve the vision of a fusion reactor, if their physics challenges can be met. indeed, this<br />

is a primary motivation <strong>for</strong> research on alternate configurations. While the nomenclature used<br />

to identify specific toroidal configurations is important <strong>for</strong> historical and programmatic reasons,<br />

the tokamak and toroidal alternates are in reality closely related. together they span an essentially<br />

continuous spectrum of magnetic configurations, varying in major variables such as the aspect<br />

ratio of the torus, degree of internal versus external magnetization, and degree of 3-d shaping of<br />

the magnetic field. Figure 1 illustrates this configuration space. The axes are the major variables<br />

noted above, and “spots” are located at the combination of variables that represent the typical or<br />

historical view of the named configurations. This is not intended to be an exact representation of<br />

the configurations, rather a visual to help expose the opportunity and need to understand and<br />

predict toroidal confinement over a wide range of parameters. continuing exploration of this configuration<br />

space is a central idea of ReneW <strong>Research</strong> Thrusts 16-18.<br />

Figure 1 also illustrates that multi-configuration fusion research advances the understanding of<br />

fusion plasma physics much more completely than can be achieved by any one configuration on its<br />

own, by broadening the scientific approach to grow and validate fusion science over a wide range<br />

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