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

tHEME 5: oPtiMiziNG tHE MaGNEtic coNFiGuRatioN
introduction
ScopE aNd FocuS
a key strategy for the Us and world fusion program has been the investigation and development of
a variety of magnetic configurations that are able to confine a hot, stable plasma. of these, the tokamak
configuration has achieved parameters that are nearest those required in a fusion reactor.
The tokamak exhibits good qualities for plasma confinement, and this has spurred its intense development,
ultimately leading to iteR. nevertheless, the optimum magnetic configuration for solving
the simultaneous challenges of fusion plasma physics and engineering is not yet known. each
of the magnetic configurations being studied brings opportunities to optimize the magnetic fusion
system in unique ways. each also has challenges that reflect tradeoffs in physics and engineering.
as part of its strategic planning process, the department of energy asked the Fusion energy sciences
advisory committee (Fesac) to critically evaluate the status of, and scientific opportunities
for, major alternate magnetic confinement configurations. This led to the Report of the Fesac
toroidal alternates Panel (taP), which forms the starting point for the optimizing the magnetic
configuration Theme covered in this chapter. (The taP report is available at http://fusion.
gat.com/tap/). in this context, “alternate” means toroidal magnetic configurations other than
the conventional aspect ratio tokamak. The scope of the evaluation was limited to the toroidal
magnetic configurations that are most developed: the stellarator, the spherical torus, the reversed
field pinch, and compact tori. compact tori are two distinct configurations, the spheromak
and the field-reversed configuration, that share the common feature of having a simply connected
wall geometry with no physical structure linking the plasma torus.
The toroidal alternates offer particular scientific and technological benefits that could substantially
improve the vision of a fusion reactor, if their physics challenges can be met. indeed, this
is a primary motivation for research on alternate configurations. While the nomenclature used
to identify specific toroidal configurations is important for historical and programmatic reasons,
the tokamak and toroidal alternates are in reality closely related. together they span an essentially
continuous spectrum of magnetic configurations, varying in major variables such as the aspect
ratio of the torus, degree of internal versus external magnetization, and degree of 3-d shaping of
the magnetic field. Figure 1 illustrates this configuration space. The axes are the major variables
noted above, and “spots” are located at the combination of variables that represent the typical or
historical view of the named configurations. This is not intended to be an exact representation of
the configurations, rather a visual to help expose the opportunity and need to understand and
predict toroidal confinement over a wide range of parameters. continuing exploration of this configuration
space is a central idea of ReneW Research Thrusts 16-18.
Figure 1 also illustrates that multi-configuration fusion research advances the understanding of
fusion plasma physics much more completely than can be achieved by any one configuration on its
own, by broadening the scientific approach to grow and validate fusion science over a wide range
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