PPPL-4282

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<b>High Harmonic Fast Wave Heating Efficiency Enhancemen and Current Drive at Longer Wavelength  on the National Spherical Torus Experiment</b>
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<b>Authors:</b>  J. Hosea, R. E. Bell, B.P. LeBlanc, C.K. Phillips, G. Taylor, E. Valeo, J.R. Wilson, E.F. Jaeger, P.M. Ryan, J. Wilgen, H. Yuh, F. Levinton, S. Sabbagh, K. Tritz, J. Parker, P.T. Bonoli, R. Harvey, and the NSTX Team 
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<b>Abstract:</b> High harmonic fast wave heating and current drive (CD) are being developed on the National Spherical Torus Experiment (NSTX) [M. Ono et al., Nucl. Fusion <b>41</b>, 1435 (2001)] for supporting startup and sustainment of the ST plasma. Considerable enhancement of the core heating efficiency (η) from 44% to 65% has been obtained for CD phasing of the antenna (strap-to-strap φ = -90<sup>o</sup>, k<sub>φ</sub> = -8 m<sup>-1</sup>) by increasing the magnetic field from 4.5 kG to 5.5 kG. This increase in efficiency is strongly correlated to moving the location of the onset density for perpendicular fast wave propagation (n<sub>onset</sub> ∝ Β<sub>Φ</sub>× k<sub>||</sub> <sup>2/</sup>w) away from the antenna face and wall, and hence reducing the propagating surface wave fields. RF waves propagating close to the wall at lower B<sub>Φ</sub> and k<sub>||</sub> can enhance power losses from both the parametric decay instability (PDI) and wave dissipation in sheaths and structures around the machine. The improved efficiency found here is attributed to a reduction in the latter, as PDI losses are little changed at the higher magnetic field. Under these conditions of higher coupling efficiency, initial measurements of localized CD effects have been made and compared with advanced RF code simulations.
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<b>Accepted for publication in:</b><i> Physics of Plasmas</i> 15:5 (May 2008)
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<i>&#169; 2008 American Institute of Physics All rights reserved.
<br>This article may be downloaded for personal use only.
<br>Any other use requires prior permission of the author
<br> and the American Institute of Physics.</i>
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<b>Presented at:</b> Forty-Ninth APS Division of Plasma Physics Meeting, 12 &ndash;16 November 2007, Orlando, Florida USA 
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<b>Download PPPL Report:</b> 
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<a href="PPPL-4282.pdf">PPPL-4282</a>
<b>Preprint</b> January, 2008 (pdf 4.2 MB)
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