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

eaches the empirical Greenwald density limit, is only partially understood. The fundamental limit
is believed not to be an average density, but to be driven by processes in the scrape-off layer. since
plasmas in the high fusion power regime are likely to be operating near this density, the underlying
processes will need better understanding. The purity of the core plasma, measured as an effective
charge (z eff ), is directly related to the externally supplied fueling and pumping, the particles that
are recycled and eroded from the solid material surfaces, and impurity particle transport.
The evolution of the solid materials interfacing plasma under both neutron and plasma loads is
considered one of the most serious issues for the high fusion power regime. The materials are affected
by possible melting, fast ion or electron impingement, neutron damage, tritium and
deuterium retention, and erosion and redeposition by plasma. The impact on the core plasma
arises primarily from the generation of impurities, dust, and larger material removal
such as bubbles and flakes. as the high fusion power regime is approached, these effects become
significantly aggravated. although material lifetime is determined by these processes, a material’s
behavior can adversely affect the core plasma long before its properties dictate replacement.
Potential “game changers”
What new areas can provide innovative solutions to some of the more uncertain issues in
the high-performance fusion plasma regime?
although there are conventional solutions to a number of issues for the high-performance steadystate
burning plasma regime, some of these have significant uncertainty, requiring innovation for
their resolution. consideration should be given to the use of three-dimensional magnetic fields,
based on stellarator research, to influence mhd and avoid disruptions. in addition, these fields
may provide rotational transform, reducing the current drive requirements, and may also provide
resilience to the density limit observed in tokamaks. The exploration of advanced divertors, including
liquid metal approaches, should be pursued to understand their potential for handling the
high particle and power loads in a fusion power plant as well as the particle control and material
evolution issues. The possibility of new, more powerful controls, with flow shear or alpha particle
“engineering,” on internal plasma profiles may be possible because of the strong coupling in the
high-performance fusion regime; it should be explored.
Summary of Key integration goals
While various aspects of high-performance steady-state burning plasmas have been discussed
in this section, the greatest challenge lies in integrating all of the physics and target parameters
simultaneously. some of these are summarized in table 2, which compares key global and
boundary-related parameters expected in iteR steady-state scenarios with those in two potential
demo designs. large gaps in a number of areas, including bootstrap fraction, alpha heating
fraction, duration, and heat and neutron flux, can be noted. These motivate new experiments as
discussed in the Research Thrusts.
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