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

Figure 2. Alpha particles can cause new plasma phenomena (such as Alfvén instabilities), and their heating
of the burning plasma drives new nonlinear feedback loops. (Figure courtesy of D.A. Spong.)
While many aspects of alpha particle physics have been anticipated and tested in existing experiments,
it is not possible to produce fast ion populations that simultaneously achieve the parameters
and velocity distribution that are characteristic of alpha particles in burning plasma devices
such as iteR. Three such distinguishing features can be identified:
• alpha particles in iteR are predicted to transit the device at rates comparable to the
characteristic magnetohydrodynamics (mhd) wave frequency, leading to the possibility
of resonant wave-particle interactions and instability drive.
• The spatial extent of the alpha particle gyro-orbit relative to the machine size is
substantially smaller in iteR than in present-day devices, potentially leading to a larger
number of unstable modes.
• alpha particles in iteR are expected to have an isotropic velocity distribution, whereas
fast ion populations in present-day devices typically are strongly anisotropic (and hence
more unstable).
These intrinsic differences mean that only partial tests of fast ion instabilities and transport models
are possible in present-day experiments; realistic tests will need to rely on power-plant scale
experiments such as in iteR. both validated predictive tools and advanced diagnostics on existing
and future facilities are needed to bridge this parameter gap. Reliable simulations are also required
to evaluate future control tools that will be needed to mitigate these instabilities and optimize
the fusion burn in iteR and demo devices.
Understanding these issues will require a strong emphasis on advanced fast ion diagnostics and
dedicated energetic particle/plasma experiments, together with improved theories and simulation
codes. in parallel, neutron-hardened diagnostics for the iteR environment must be developed.
The predictive alpha particle projects must be developed on a time scale compatible with
their use in the full-machine simulation effort.
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