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

esearch requirements
The experiment needed for sustainment and transport studies, or an upgrade thereof, may provide
the required capability. in addition, significant auxiliary heating is needed to study beta limits.
experiments in ssPX found a limiting electron beta, ~ 5% (peak), although the highest temperature
shots exceeded this by up to a factor of 2. The ion contribution is not known, but could contribute
up to a similar factor. it is also unknown whether the achieved betas were limited by
ohmic power balance or by the onset of pressure-driven modes. The transient peak electron beta
~ 20% observed in ctX suggests that the ssPX results were limited by ohmic power. There are no
comprehensive computational studies of beta limits, though nonlinear simulations that evolve
pressure are consistent with experiments. Understanding this limit and the effects of spheromak
shaping may allow it to be increased significantly. such a result would significantly improve the
attractiveness of a spheromak reactor. both simulations and experiments are needed to address
this issue.
PaRtiCLE baLanCE anD DEnSity COntROL: unDERStanD PaRtiCLE baLanCE
anD COntROL OF PLaSMa DEnSity anD iMPuRitiES
Particle balance and density control has not been systematically studied in spheromaks, and no
present experimental facility is capable of addressing the associated science.
research requirements
Particle balance and density control may require a specialized facility or significant upgrades (including
diagnostics) to the experiment described above. achievement of the iteR-era goals will
likely require extension of techniques necessary for previous spheromak experiments.
Particle and density control is a complex scientific issue, involving plasma-wall interactions, penetration
of the plasma by neutral particles, density pinching, plasma flows, etc. The minimum
spheromak operating density may be limited to a constant fraction of the current density, perhaps
due to stability related to electron streaming. experiments with wall conditioning have typically
operated with an initial gas pulse, and the density evolution was apparently determined by
recycling from the flux conserver walls and the helicity injector. since injecting helicity involves
injecting magnetic flux and since plasma is frozen to magnetic flux, helicity injection tends to involve
ingestion of cold particles. it is thus important to develop methods that inject helicity without
excessive ingestion of cold particles. coating the copper flux conserver and gun with a refractory
metal (tungsten) layer, discharge cleaning, and titanium gettering have successfully controlled
impurities.
FaSt PaRtiCLES: unDERStanD EFFECtS OF FaSt PaRtiCLES On CuRREnt DRiVE,
StabiLity anD COnFinEMEnt
Forming energetic particles in experiments and diagnosing their effects on stability and other
physics have not been undertaken in spheromak experiments. There has been some computational
modeling of orbits and preliminary scoping of the resulting current drive.
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