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Understanding the controlling physics in plasma boundary interaction from the plasma perspective

Author: Xianzhu Tang
Requested Type: Consider for Invited
Submitted: 2012-12-07 12:28:13

Co-authors: Valery Borovikov,Gian Luca Delzanno,Zehua Guo,Art Voter,Ying Wang

Contact Info:
Los Alamos National Laboratory
T-5 MS B284
Los Alamos, NM   87545
USA

Abstract Text:
Meeting the extreme challenge of heat and particle exhaust in a fusion reactor requires innovations in both materials and boundary plasma control. If the burden is entirely placed on the materials, the likely requirements of erosion-free performance under tens of megawatt per square meter with plasma irradiation flux of $10^{23-24}$~m$^{-2}$s$^{-1}$ of $1 - 10^3$~eV deuterium and tritium, and $10^{22-23}$~m$^{-2}$s$^{-1}$ of $10-10^4$ helium for months a time, would amount to a revolutionary advance in solid materials. Not surprisingly, boundary plasma control via innovative divertor design, gas puffing, and impurity seeding, etc, have been traditionally pursued with great success in tokamak experiments. The eventual solution to heat/particle exhaust in fusion reactors will likely be a combination of materials innovation and precise control of boundary plasma conditions.

The controlling physics for boundary plasmas are amendable to small
scale exploratory experimental research. Here we go through a list of basic processes that contribute to the energy and particle balance of a plasma when exposed to a material surface. A special emphasis is placed on the different recycling mechanisms that differentiate materials and how they help set the boundary plasma conditions. The two focus cases are hydrogen-absorbing lithium surface and the low-sputtering refractory metal of tungsten. In the first case, we will show the consequences of low recycling on the boundary plasma profile and its implication on upstream plasmas. A number of recently identified signatures of the novel physics will be explained, along with potential experiments to validate them. These include the parallel heat flux against the parallel temperature gradient and the trapped-electron whistler wave in enabling ambipolar transport via wave-particle interaction. In the second case, we will present recent
calculations that illustrate the effect of tungsten wall recycling on boundary plasmas, and how it can be dramatically different from the Carbon walls which are commonly used and studied in the past.

The important issue of wall material redeposition is also examined in the context of design freedom for boundary plasmas. This primarily comes through as the electric field profile near the wall surface. In addition to the effect of neutrals which are more commonly studied in the past, we will take a closer look at the effect of magnetic field, particularly its inclination angle to the wall surface.

There are three objectives of the presentation. The first is to define the controlling physics issues from a theoretical perspective. The second is to elucidate the essential physics via analytical theory and simulation. This draws from a number of recent publications on the subject from the LANL fusion theory program, both in plasma physics and materials response modeling. The third is to motivate small scale experiments targeting specific physics that have been identified.

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