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

Development of a robust ECH launcher:
The launchers designed for iteR are probably unsuitable for demo. The last mirror, which faces the
plasma, has severe cooling issues. This mirror has a very large heat load from the ec waves plus the
plasma radiation and neutron heating. also, the surface conductivity may degrade over time due to
neutron damage and deposition of dust. coolant must pass to the moveable mirror through flexible
tubes that may be subject to leaks caused by neutron-induced brittleness or electrical arcs, and bearings
are subject to fatigue. an alternate antenna concept for placing the moveable steering parts very
far from the plasma (“remote steering”) has been demonstrated, but further development is needed.
Research Requirements for neutral beam injection
neutral beam injection has been to date the most commonly used heating scheme for fusion experiments.
This has principally been through the utilization of positive ion beams. For future
large, dense plasmas, the limitation on energy inherent in the neutralization of positive ions
makes the development of negative ion neutral beams (nnbi) a necessity. negative ion neutral
beams have been developed on Jt60-U and this development will continue for Jt-60sa. iteR requires
nnbi systems that meet nearly all the requirements for demo. successful implementation
of an nnbi on iteR will go a long way toward satisfying the development needs. The principal
demo beam requirement, which none of these development programs is addressing, is continuous
beam operation. in all of these systems, the cryopumps must be regenerated after they have
accumulated about half the explosive limit of hydrogen, which corresponds to at most a few thousand
seconds of beam time. because negative ion beam systems presently need all the available
surface area for pumping, it is difficult to envision installing sufficient excess pumping to allow
isolating and regenerating some pumps while others continue to pump. a lithium jet neutralizer,
which has been proposed as an upgrade to the iteR neutral beamlines, would solve this problem
by eliminating 75 to 80 % of the gas input into the beamline, while simultaneously increasing the
neutralization and electrical efficiency, and greatly reducing heat loads on the accelerator and ion
source backplate. This could make regeneration feasible.
Research Requirements for ion Cyclotron Radiofrequency and Lower Hybrid Current Drive
ion cyclotron and lower hybrid heating and current drive techniques have many common elements
that need to be addressed before they could be confidently applied to demo. both schemes
utilize complex launching structures that need to be located close to the plasma edge, thereby exposing
these structures to a harsh heat, particle and radiation environment. both schemes couple
radiofrequency energy through a region in the sol plasma where energy can be lost through various
mechanisms. both have the advantage of high-power steady-state power sources already developed.
key research opportunities for these techniques include:
Wave Propagation and Absorption
experimental studies, coupled with the development of advanced simulation codes during the
past 40 years, have led to an unprecedented understanding of the physics of radiofrequency heating
and current drive in the core of axisymmetric toroidal magnetic fusion devices. The most serious
gap is the lack of a predictive understanding of the amount of power that can be coupled into a
fusion plasma with a given launcher design. The negative impacts of this knowledge gap are clear:
(i) significant and variable loss of power in the edge regions of confined plasmas and surrounding
vessel structures adversely affects the core plasma performance and lifetime of a device; (ii)
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