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

cient understanding of electron thermal transport at low plasma aspect ratio, high beta, and low
plasma collisionality to reliably predict confinement for an st-ctF or demo.
research requirements
Electron transport studies over an extended range of key plasma parameters: Plasma
electron temperature gradient (etG) and micro-tearing of the magnetic field are predicted to be
the dominant instabilities in the electron temperature, Te , gradient region of nbi-heated, highconfinement
st plasmas. trapped electron instabilities might also contribute at lower plasma
electron collisionality, ne . since the plasma temperature and collisionality scaling of transport induced
by these instabilities are very different, it is essential to establish which contributes most,
locally inside the plasma. another important question is if etG and micro-tearing instability-induced
transport can be reduced by the large electric and magnetic field (exb) shear possible in the
st. Finally, to inform iteR and demo, it will be important to investigate turbulence suppression
mechanisms that do not need external rotation input, such as predicted pressure gradient effects.
addressing these questions requires transport scaling studies over an extended range of Te , ne ,
exb shear, and pressure gradient, compared to present experiments (e.g., at more than twice the
present Te , and one-tenth of ne ). This is achievable in upgraded st devices, or a performance extension
experiment (see taP report). study of edge plasma electron thermal transport with low
recycling lithium walls and in the presence of magnetic fields used for active control of plasma instabilities
is also needed for sts and other toroidal devices.
Diagnostics for high and low wavelength, electrostatic and magnetic turbulence in Sts:
to understand the role of etG and micro-tearing instabilities in st electron transport one needs
to investigate the existence of ion-gyroradius (r i ) scale etG “streamers” and magnetic islands.
While the existence of etG streamers is an important question for all magnetically confined devices,
the low magnetic field, large r i scale of st plasmas make them favorable for these studies.
This research will require the development of novel and improved diagnostic techniques. (see ReneW
white papers by e. mazzucato, and by k. tritz, et al.).
Study of electron transport driven by fast ion instabilities: a correlation between central
electron transport and fast ion driven instabilities (global alfvén eigenmodes — Gaes) has
recently been found in nstX, with the instabilities driving stochastic transport at diffusivities
greater than 10m 2 /s. This phenomenon could have an impact on st-ctF performance and might
prove relevant for any burning plasma. it also could have beneficial uses, such as T e profile control
or current drive in an st-demo. For this study, specialized magnetic and density fluctuation
diagnostics are needed, with a large field of view of the plasma, moderate resolution (a few cm),
very high speed (several mhz) and signal-to-noise ratio (>10 3 ). Theoretical models capable of predicting
the Gae characteristics and the associated transport under different T e , collisionality, qprofile,
magnetic and flow shear conditions, as well as the interaction of this type of transport
with conventional gradient-driven transport, are needed. methods to control this transport, for
example using localized radiofrequency suppression and excitation of the Gaes, also need to be
investigated.
191