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Use of ICC Facilities for Plasma-Material Interaction Testing

Author: Rob Goldston
Requested Type: Consider for Invited
Submitted: 2011-06-10 02:20:12

Co-authors:

Contact Info:
Princeton Plasma Physics Laboratory
Princeton University
Princeton, NJ   08543
USA

Abstract Text:
The power density along the field lines in the scrape-off layer of machines of the class of NSTX-U is in the range of 400 MW/m^2. The parallel heat flux in devices of the class of a fusion power plant are likely to be at least 3x greater. It is crucial for the future of tokamaks and stellarators to develop the plasma science and component technology to handle such high plasma heat fluxes.

It would be valuable to produce parallel plasma heat flux at these power densities, impinging on test components at highly tangential angles, as planned in tokamaks. To achieve 400 MW/m^2 using parallel electron thermal conduction, as is generally believed to occur in tokamak scrape-off layers, requires upstream temperatures in the range of 50 to 100 eV and, for appropriate collisionality, densities in the mid 10^19/m^3 range, with field-line lengths to the plasma target of ~ 5 meters. This would allow not only component technology testing, but also testing of effects such as self-shielding through surface evaporation (e.g., of liquid lithium) and consequent radiative plasma cooling.

It seems that a Gas Dynamic Trap (GDT) might be well suited for this purpose. Consider a 10 cm radius plasma with a mirror ratio of 13. The throat radius is 2.8cm, and its area therefore 25 cm^2. If 1 MW of power emerges from this throat the heat flux is 400 MW/m^2. If the field in the main plasma is 800G, n_e = 3 10^19/m^3 and T_e = 50 eV, then beta_e is a relatively modest 10%. The field in the throat plasma is 1T, required to emulate NSTX-U.

The greatest extrapolation from current experience with GDT devices appears to be a 1000-fold increase in pulse length, from 5 msec to 5 sec, adequate for testing components for NSTX-U, and in general for developing liquid plasma-facing components since high-fluence operation is not a critical issue for such systems. All of the plasma physics and component technology issues should be in steady-state within about 5 seconds.

There are critical issues for GDT operation with mirror expansion only at one end, and at the other a quadrupole shaping magnet followed by a curved magnetic guide field leading to tangential non-axisymmetric impact at the target. Even if interchange stability is preserved by design of the magnetic field geometry, the effects of electric fields arising from the non-axisymmetric target need to be evaluated.

Are there other ICC devices that could provide the needed plasma parameters?

Characterization: A6

Comments:

University of Washington

Workshop on Innovation in Fusion Science (ICC2011) and
US-Japan Workshop on Compact Torus Plasma
August 16-19, 2011
Seattle, Washington

ICC 2011