- Ferritic Wall Project -

Motivation: Whereas magnetic confinement plasma devices generally use stainless steels (especially 316) for its strength and negligible magnetism, these same steels are especially vulnerable to damage from the high fluence of fast (i.e. relatively high-energy) neutrons produced in the deuterium-tritium fusion reaction targeted as the first-generation fusion energy fuel. Although there are many avenues of materials research, the best currently available candidate materials to replace stainless steel, in terms of strength, resistance to neutron damage, and existing large-scale fabrication experience, are a class of "low-activation ferritic steels." However, the fact that these steels are ferritic (i.e. ferromagnetic) means that they are likely to have a generally destabilizing effect on tokamak plasmas.

Completed Research: Prior studies of the "ferritic wall mode" (specific kinds of plasma instability caused by interaction with a ferromagnetic wall) have included an experiment in a linear machine which showed a strong effect, and more conservative studies in Japanese tokamaks which saw a much less pronounced effect. The ferritic wall installed in HBT-EP, intended to bridge this gap, was designed to have a high relative permeability (about 8 times vacuum permeability) and low total soak-through time (due to dividing the wall into roughly 1 sq.in. tiles). Since installation, a series of experiments comparing close-fitting ferritic walls against close-fitting stainless steel with ferritic walls retracted has been conducted, observing:

• Increased plasma response to magnetic perturbations applied deliberately as a measure of the stability of characteristic plasma modes. The amplitude of the response as a function of perturbation strength is roughly doubled when the ferritic wall is inserted. Vulnerability to unintended magnetic perturbations is a major concern in reactor-scale fusion devices.
• Increased rate of early disruptions (fast, catastrophic lass of plasma confinement), particularly in response to magnetic perturbations, when the ferritic wall is inserted. Disruptions will be extremely dangerous to a reactor-scale fusion device and must be carefully avoided.
• Increased growth rates of characteristic "external kink" plasma instabilities. Present in nearly all HBT-EP plasmas, these instabilities develop about twice as quickly with the ferritic wall inserted. Faster-growing instabilities are more difficult to control, and can lead to a loss of confinement, or even a disruption.

Ongoing Research: Other topics of interest regarding the ferritic wall include:

• Rotation Effects: Relative rotation between the plasma and the surrounding conducting surfaces can help to stabilize plasmas, but reactor-scale machines are expected to have very little rotation. How do the observed ferritic wall effects depend on the rotation of the bulk plasma or the growing mode? Effects are expected to be much more significant at low rotation (under 1kHz) than at the high rotation (6-10kHz) typical of naturally-rotating HBT-EP plasmas.
• Halo currents: Although most of the current in a plasma flows near its hot core, there are always edge currents, some of which may contact and flow through structural components, such as walls, especially as instabilities grow large. How does the ferritic wall interact with these "halo currents"? If the interaction is strong, ferritic components may make large instabilities and disruptions even more dangerous.

Projects

Magnetic	Optical	Feedback	Other
Ferritic Wall	Soft X-Ray	Data Acquisition	Hall Probe
Poloidal Sensors	Thomson Scattering	Power Amplifiers	Bias Probe
Toroidal Sensors	Fast Camera	CPCI	Mach/Float Probe
Rogowski Coils	D-Alpha	Signal Amps	Rotation