Plasma transport control and self-sustaining fusion reactor Page: 4 of 15
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In this paper, we show that it is indeed feasible to simultaneously satisfy the high beta, high bootstrap
fraction, and high confinement conditions provided that one can control the local plasma heat trans-
port. For a self-sustaining high performance fusion reactor, the bootstrap current (or more precisely,
the pressure driven neoclassical currents) profile OBT) must be consistent with the required MHD
current profile OMHD) for stability. This bootstrap current profile alignment is an important research
topic for advanced tokamaks.1 Since the bootstrap current is generated by the pressure gradient, i.e.,
jBT cVp, the requirement for a self-sustaining tokamak reactor OBT = jMHD) is essentially reduced to
the problem of pressure profile control. Moreover, since the pressure profile is largely determined by
the plasma heat transport, i.e., Vp heat / Xheat, and noting that the heat flux, heat, is largely
determined by the fusion reaction, the problem of pressure profile control is essentially equivalent to
the problem of local plasma heat transport (Xheat) control. In other words, if one can control the local
radial heat transport of the heat generated by the fusion reaction, it is possible to control the pressure
profile and thus, the bootstrap current profile which is required for high performance (high beta, high
confinement) plasmas. As an example of self-sustaining high performance tokamak, we present a case
for the spherical torus (ST).2
100% Pressure Driven Tokamak
In CDX-U, a spherical torus configuration was created and sustained solely by internally generated
bootstrap currents.3 This was accomplished with electron cyclotron heating (ECH). By applying the
vacuum poloidal fields in a "spherical" shape, one can create a toroidal mirror configuration as shown
in the left hand side of Fig. 1 a. When ECH is applied, the electrons are heated perpendicularly, creating
mirror trapped electrons with precessing banana orbits. This precessing electron population creates a
net toroidal current. It is shown through a modeling that the Pfirsch-Schluter currents, though not
producing a net current, can actually enhance the processional current by decreasing the central mag-
netic field.3 As the current is increased, the flux surfaces are starting to close (the right hand side of
Fig. la). Once the closed flux surface is formed, the so-called bootstrap current starts to flow within
the closed flux surface which actually helps to enlarge the closed flux surface volume. In Fig. lb, the
measured current profile is shown by the solid line. The calculated neoclassical current profile is
shown by the dotted line. As shown in the figure, there is a substantial disagreement between the
experimental observation and neoclassical theory, in particular, the mysterious creation of the "seed"
current in the plasma core which is not possible with the neoclassical model. Satisfactory agreement
can be obtained if one introduces the helicity conserving neoclassical transport in the model as shown
by the dashed line.4 Therefore, it appears that the seed current is created through a nonclassical helicity
current diffusion process, resulting in the 100% pressure driven tokamak. It should be noted that
similar tokamak start-up and maintenance ECH was also demonstrated on DIII-D in a follow-on ex-
periment.5
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Ono, M.; Bell, R. & Choe, W. Plasma transport control and self-sustaining fusion reactor, report, February 1, 1997; Princeton, New Jersey. (https://digital.library.unt.edu/ark:/67531/metadc681532/m1/4/: accessed July 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.