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garnier_icc_2011.pdf2011-09-08 09:26:25Darren Garnier

The Case for High-Power ICRF in the Levitated Dipole Experiment

Author: Darren T. Garnier
Requested Type: Consider for Invited
Submitted: 2011-06-10 16:04:48

Co-authors: J.Kesner, M.E.Mauel

Contact Info:
Columbia University
175 Albany St
Cambridge, MA   02139
United States

Abstract Text:
Modern laboratory studies of plasma confined by a strong dipole magnet originated twenty years ago when it was learned that planetary magnetospheres have centrally-peaked plasma pressure profiles that form naturally when solar wind drives plasma circulation and heating. The dipole's magnetic geometry is highly advantageous for magnetic fusion: (i) plasma can be stable with local beta exceeding unity, (ii) fluctuations can drive either heat or particles inward to create stationary profiles that are strongly peaked, and (iii) the confinement of particles can be shorter than for plasma energy. During the past decade, several laboratory dipole experiments and modeling efforts have lead to new understanding of interchange, centrifugal and entropy modes, nonlinear gyrokinetics, and plasma transport. Two devices, the Levitated Dipole Experiment (LDX) at MIT and RT-1 at the University of Tokyo, operate with levitated superconducting dipole magnets. With levitated dipoles, these experiments achieved a major breakthrough. Very high-beta plasma is confined in steady state but, also, levitation produces high-temperature at low input power and demonstrates that toroidal magnetic confinement of plasma does not require a toroidal field. Turbulent low-frequency fluctuations in dipole confined plasma cause adiabatic transport and form a fundamental linkage between the radial variation of flux-tube volume and the centrally peaked density and pressure profiles [1]. Recent experiments showed how the turbulent pinch maintains good confinement and centrally-peaked profiles during heating and fueling modulation [2]. Recent modeling has explained many of the processes operative in these experiments, including the observation of a strong inward particle pinch [3].

The next experimental test of dipole confinement is the application of high-power ion heating in LDX. Using the modern Thales TSW2500 RF transmitter, now available at MIT, 1 MW of continuous power can be delivered in the appropriate range for deuterium ICRF. Full-wave electromagnetic code incorporating accurate boundary conditions show good antenna loading for axisymmetric heating. Calculations show that the power deposition profile can be readily controlled by adjusting the RF frequency.

LDX is a unique experiment that is ready to execute ground-breaking experiments. If the higher-density plasmas with higher ion temperatures share the high-confinement and high-beta properties of the previous ECRH plasma, then the dipole concept would demonstrate the promise of tritium-suppressed fusion [4] and potentially avoid the costly development of materials and fusion components associated with tritium breeding from lithium.

[1] Boxer, et al., Nature-Physics, 6, 207 (2010)

[2] Kesner, et al., PPCF, 124036 (2010)

[3] Kesner, at al., Phys Plasmas 18, 050703 (2011)

[4] Sawan, M. et al., Fusion Eng Des 61-2, 561–567 (2002).

Characterization: A1,A2

Comments:
The case for completion of the high-power RF experiments that were planned for LDX should be delivered as an invited talk.

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