Road map for a modular magnetic fusion program Page: 4 of 13
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on T-3 in 1969. A second generation in the development of spherical tokamaks, NSTX and
MAST, are projected to achieve near PLT-type parameters in ~2001. The key question for NSTX
and MAST will be to determine the confinement scaling in spherical tokamaks so that the reactor
possibilities of the spherical tokamak configuration can be assessed.
Minimizing the Cost of the Next Step in Fusion R&D
TFTR and JET have produced fusion plasmas and were able to measure alpha heating at low gain,
Q < 1. The next step is to study plasmas with dominant alpha heating. A high-gain plasma with Q
~ 10 would have alpha heating twice the externally applied heating, and would require a plasma
system with an nt about ten times larger than the TFTR/JET D-T experiments. During the period
from 1988 to 1991, the U.S. proposed a number of compact high-field copper-coil tokamak
experiments (CIT and BPX) with a focused objective of studying high-gain burning plasmas. In
1990, concerns about these devices attaining the required confinement led to an increase in their
size and to an escalation of the construction cost, which eventually led to their cancellation. As
discussed below, tokamak experiments carried out during the 1990s have validated the original
design assumptions.
Following the cancellation of the U.S. burning plasma design initiatives in 1991, the burning
plasma experimental physics mission elements were added to the objectives of ITER, a large
integration facility to demonstrate the scientific and technological feasibility of fusion (Fig. 2).
Since data from a prior burning plasma experiment is not available, ITER must be designed
conservatively and this has increased the cost and risk of the next step in magnetic fusion. The
total projected cost of ITER - $1OB exceeds the cost of existing power plants and none of the four
ITER partners has committed to construction during the Engineering design Activity from 1992 to
1998. This is similar to the Mountain of Death described by PCAST-1997 [2] where the cost of
the next-step in the development of a technology becomes so high it can not be taken. In order for
the magnetic fusion program to move forward, this large multi-mission barrier must be broken into
several smaller size steps that will address key issues and will be small enough to be fundable.
These smaller parallel steps will also reduce the technical risk as described previously. This is the
essence of the Modular Strategy. Fortunately, research on tokamaks during the past eight years
has confirmed that physics assumptions of the original designs. This new information has revived
interest in low cost approaches to studying the physics of high-gain burning plasmas.
Design Constraints for an Affordable Next Step.
The cost of a magnetic fusion burning plasma experiment is a strong function of the total magnetic
energy and the plasma current (Fig. 3) that must be provided. Existing experiments like TFTR and
JET have roughly 1.5 giga-joules (GJ) of magnetic energy and 3 to 5 mega-amps (MA) of plasma
current while the ITER design has 21 MA and -120 GJ of magnetic energy. A typical tokamak
with aspect ratio (R/a) - 3 needs about 3 MA to confine the alpha particles. If microinstabilities did
not degrade plasma confinement in a tokamak, then a plasma current of only -3 MA would be
sufficient to confine the plasma due to neoclassical diffusion. So the fact that tokamak designs
have plasma currents much larger than 3 MA represents the penalty we're paying because we don't
understand transport and can't reduce transport to the neoclassical limit. This is exactly analogous
to the situation faced in inertial fusion. If ICF didn't have Raleigh-Taylor instabilities, the driver
energy in ICF could be reduced significantly. A difficulty is that ITER is projected to cost over
$1OB which is larger than the projected $4B capital cost for a fusion power plant. Note that the
cost of various magnetic fusion power plant designs based on the tokamak, stellarator and
spherical tokamak carried out by the ARIES design studies all have about the same magnetic
energy and similar capital costs and cost of electricity. So the different magnetic configurations not
only have a common physics basis, but similar technological requirements leads to nearly identical
power plant prospects. The costs of TFTR and JET facilities are in the range of - $0.5B.
Therefore, an ignition machine must have magnetic energy <1OGJ and plasmas currents < 10 MA
(modestly larger than TFTR and JET) if it is to be affordable (<$1B).2
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Meade, Dale M. Road map for a modular magnetic fusion program, report, July 18, 2000; Princeton, New Jersey. (https://digital.library.unt.edu/ark:/67531/metadc709062/m1/4/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.