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ebt_icc.pdf2011-08-17 13:59:56James Cobble
icc_abstract.pdf2011-06-10 11:40:09James Cobble

The Microwave-Driven Bumpy Torus

Author: James A. Cobble
Requested Type: Consider for Invited
Submitted: 2011-06-10 11:28:55

Co-authors:

Contact Info:
Los Alamos National Laboratory
Box 1663
Los Alamos, New Mexico   87545
USA

Abstract Text:
In 30 years, order-of-magnitude technological advances have taken place in multiple areas of plasma heating and confinement. Before 1985, the microwave-driven bumpy torus was limited by the older technologies. There were positive indications of plasma performance; however, the plasma density ne and temperature Te were hardware limited. The resulting low-density plasma was difficult to characterize. Given the lack of optimal alternatives for fusion, it is time to revisit the bumpy-torus concept to estimate its performance with the new technologies. The microwave-heated bumpy torus has two major advantages over the tokamak: 1) it is inherently steady state, and 2) it is current free, meaning a) current drive is not a requirement, and b) there are no multi-MA disruptions. Its major advantage over a stellarator-style device is the simplicity of its magnetic coil configuration.
In the mid 1980s, we used 200 kW of steady-state, 28-GHz electron-cyclotron heating in a 24-sector bumpy torus. The resonant magnetic field was generated by a steady-state, 10-MW current in the water-cooled copper coils. The > 1-m^3 plasma exhibited several electron populations: an absolutely mirror-confined relativistic plasma, that stabilized the toroidal plasma against interchange modes, a bulk toroidal plasma with Te ~100 eV, and a thermal-tail distribution of the bulk plasma with Te exceeding 1 keV. Bulk plasma density was >10^12/cc with the relativistic component and the thermal tail components each contributing 10 – 15% of the total density. These three distributions each had different energy confinement times in the range from ~1 ms to 1 s. Accidental magnetic-field errors had been built in during construction, and these were addressed with quadrapole windings. The figure of merit for error-field correction for plasma optimization, though incorrect, was the most reasonable measure available at the time.
The major capability upgrades since the mid 1980s are magnetic field, microwave frequency, and continuous-wave (CW) microwave power. A super-conducting 80-kG field was demonstrated by 1987 with the ORNL Large Coil Project. The present state-of-the-art CW microwave source delivers 1 MW at 170 GHz. According to industrial representatives, a 200-GHz source is ‘doable’ for ~$2/W. The goal is to evaluate bumpy-torus confinement and heating with a >100-times higher ne (and collisionality). Te and ne would no longer be limited by power. An investigation of this plasma is within reach with these new technologies.

Characterization: A1,A4

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