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

of 40%.) localized instabilities (tearing modes, edge localized modes) were mitigated or
eliminated via lithium application, and record stability levels (normalized beta of 7.2)
were reached. kinetic theory has provided understanding of the complex relationship
between global instabilities and plasma rotation. The connection between observed fast
particle loss and toroidal alfvén instabilities/theory was established.
• control of high beta st plasmas: active control of resistive wall modes and maintenance
of favorable plasma rotation profiles decreased the disruption probability for long-pulse
plasmas while exceeding the stability limit for nonrotating plasmas by over 50%. Rotation
is controlled by 3-d magnetic fields in plasmas with strong momentum input.
• integration and sustainment at low aspect ratio: stability levels approaching that needed
for a fusion component testing device were sustained for significant duration (3-4 resistive
diffusion times) with a majority of the plasma current provided noninductively.
Reversed Field Pinch
The reversed field pinch (RFP) is an axisymmetric configuration in which electrical current in
the plasma generates most of the magnetic field. The requirements on the external magnets are
therefore greatly reduced. The RFP has a high ratio of plasma pressure to magnetic pressure. The
ohmic heating from the plasma current could reduce or eliminate the need for complicated auxiliary
heating systems. These features could yield a high power density fusion energy system with
simple construction. highlights of RFP research include:
• a 10-fold increase in energy confinement is obtained when fluctuations in the magnetic
field are reduced using active control of the plasma current profile. This confinement is
comparable to that in a tokamak of the same size and plasma current. high-temperature
plasmas are produced without auxiliary (non-ohmic) heating, with electron temperature
t e ~ 2 kev and ion temperature t i ~ 1 kev.
• high-current plasmas (up to 1.5 ma in RFX-mod) spontaneously generate a helical
magnetic field in the plasma core. The energy confinement time is improved in these selforganized,
quasi-helical plasmas. The confinement time is increasing with plasma current.
• Plasmas with thermal pressure exceeding theoretical limits for interchange and magnetic
tearing stability are produced without exhibiting fast disruptive behavior.
• active feedback stabilization is now routine when the plasma is contained in a thin-walled
metal shell. discharge lengths up to 0.5 s are obtained, which is more than ten growth
times for thin-shell instabilities that occur without feedback control.
• Up to 10% of the plasma current has been sustained using low frequency ac induction
(helicity injection), a promising approach for efficient steady-state operation of the RFP.
• The physics of magnetic self-organization has been advanced by RFP (and compact
torus) research, with strong connections to plasma astrophysics. areas include dynamo
processes, momentum transport, collisionless ion heating, and magnetic reconnection
and turbulence.
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