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Validation of on- and off-axis neutral beam current drive against experiment in DIII-Da
Authors
ME Austin
NH Brooks
+30 more
RV Budny
CD Challis
JC Deboo
JS Degrassie
JR Ferron
P Gohil
WW Heidbrink
J Hobirk
CT Holcomb
EM Hollmann
RM Hong
AW Hyatt
MJ Lanctot
J Lohr
TC Luce
MA Makowski
DC McCune
M Murakami
TH Osborne
JM Park
CC Petty
PA Politzer
R Prater
HE St John
T Suzuki
EA Unterberg
MA Van Zeeland
MR Wade
WP West
JH Yu
Publication date
1 September 2009
Publisher
eScholarship, University of California
Abstract
Neutral beam current drive (NBCD) experiments in DIII-D using vertically shifted plasmas to move the current drive away from the axis have clearly demonstrated robust off-axis NBCD. Time-dependent measurements of magnetic field pitch angles by the motional Stark effect diagnostic are used to obtain the evolution of the poloidal magnetic flux, which indicates a broad off-axis NBCD profile with a peak at about half the plasma minor radius. In most cases, the measured off-axis NBCD profile is consistent with calculations using an orbit-following Monte Carlo code for the beam ion slowing down including finite-orbit effects provided there is no large-scale magnetohydrodynamic activity such as Alfv́n eigenmodes modes or sawteeth. An alternative analysis method shows good agreement between the measured pitch angles and those from simulations using transport-equilibrium codes. Two-dimensional image of Doppler-shifted fast ion Dα light emitted by neutralized energetic ions shows clear evidence for a hollow profile of beam ion density, consistent with classical beam ion slowing down. The magnitude of off-axis NBCD is sensitive to the alignment of the beam injection relative to the helical pitch of the magnetic field lines. If the signs of toroidal magnetic field and plasma current yield the proper helicity, both measurement and calculation indicate that the efficiency is as good as on-axis NBCD because the increased fraction of trapped electrons reduces the electron shielding of the injected ion current, in contrast with electron current drive schemes where the trapping of electrons degrades the efficiency. The measured off-axis NBCD increases approximately linearly with the injection power, although a modest amount of fast ion diffusion is needed to explain an observed difference in the NBCD profile between the measurement and the calculation at high injection power. © 2009 American Institute of Physics
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Last time updated on 25/12/2021