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

coils, to permit replacement of the centerpost. demountable joints at the required current and
current density have not been designed or demonstrated. The use of resistive shims in the center
stack, or a centerpost construction with coaxial, air-gapped conductors, may permit designs with
a modest number of turns (~ one per toroidal field [tF] leg). superconducting tF designs are at
very early stages. Recently, designs for high-temperature superconducting centerposts, shielded
with more than 12 cm of tungsten to reduce neutron heating to an acceptable level, have been advanced,
primarily in connection with fusion-fission hybrid device designs. Possible approaches to
incorporate a central solenoid in a neutron environment have been advanced, but not fully investigated.
These approaches include the installation of resistive shims in the tF centerpost to generate
a solenoidal field, a retractable central solenoid, the use of an iron core magnetized by coils
outside of the radiation zone, and the use of a mineral-insulated conductor to construct the solenoid.
none of these approaches to a reactor-relevant central solenoid has been tested.
degradation of the conductivity and mechanical strength (embrittlement) of the copper conductor
in the neutron field must be studied. optimization of the cooling channel design must include
variability of resistive and nuclear heating, erosion, etc., within the centerpost and at channel
surfaces, and manufacturing considerations. tritium migration into the coolant must be quantified.
demountable joint integrity requires engineering development and innovation. The design
for centerpost demounting, remote handling of neutron-activated joint disassembly, and centerpost
replacement must be addressed. Power supply development is required — multi-megampere
power supply requirements are unprecedented and challenging. Fault modes of very high current
power systems must be considered. The primary needs for superconducting centerposts are
shielding design, quantification of neutron heating, and reduction in the tritium breeding fraction.
design issues for the central solenoid are whether to use removable, resistively shimmed,
mineral insulated, or iron-core approaches. The mechanical integrity of mineral insulation is an
issue. startup scenario development for the resistively shimmed approach is required. Radiation
qualification and testing of all designs is clearly needed.
continUoUs nbi systems
traditional methods of nbi implementation and operation are incompatible with steady-state operation
and need to be replaced by new techniques.
research requirements
The desired beam energy and total power of future nbi systems will depend on the steady-state
plasma characteristics. an assessment of the required beam energy must consider the total heating
power required. at lower beam energy, the beam power density may be insufficient to maintain
the desired total power input. Positive ion sources are not likely to be viable, so negative ion
sources must be capable of achieving the required beam parameters. cathode filament lifetime
in present negative ion beam sources can be extended by research to develop better arc detection
and arresting. Research is required to assess the advantages of radiofrequency ion sources for potential
development, thereby eliminating the need for filaments. indirectly heated cathode technology
could also be developed into a negative ion source. This technology has been used in positive
ion sources without arcing, but requires further research for use in negative ion sources. The
198