Prompt Loss of Energetic Ions during Early Neutral Beam Injection in the National Spherical Torus Experiment Page: 4 of 35
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consumption and delaying the onset of deleterious MHD activity . The reason for this
behavior is straightforward. The electron temperature profile broadens thereby reducing
the plasma resistance to slow Ohmic current penetration and preserve an elevated core
safety factor (q), thus increasing MHD stability. The pressure profile broadens which
increases the bootstrap current, saving flux consumption. The broader pressure profile
also permits the achievement of higher p3 by delaying onset of deleterious MHD activity.
The magnetic field topology in contemporary spherical tokamaks can cause
substantial prompt loss of energetic ions, particularly at low plasma current levels.
Spherical tokamaks operate at significantly lower magnetic field than do most tokamaks;
e.g. BT= 0.3 - 0.6 T in NSTX compared to 2 - 5 T in conventional aspect ratio tokamaks. In
conventional tokamaks, the poloidal magnetic field strength typically is much less than the
toroidal field. As a result, the radial excursion of an energetic ion drift orbit is roughly the
size of a poloidal gyroradius, which is considerably greater than the size of the Larmor
radius. In spherical tokamaks, however, the poloidal magnetic field strength can exceed
that of the toroidal field and the radial extent of the drift orbit can be comparable to the
Larmor orbit. For example, the gyroradius of 80 keV D co-injected NB heating ions in
NSTX can be - 0.3 m at the outboard midplane of the plasma. This is a sizable fraction of
the 0.68 m minor radius for a typical plasma.
The prompt loss of neutral beam ions from NSTX  is expected to be between
12% and 42% of the total beam power, depending on the plasma current (1 MA to 0.6
MA). Such losses are diagnosed either from temperature measurements of ion deposition
on first wall structures or by Faraday cup probes  which detect ions on loss orbits.
Losses of ions with energy above 1 keV in NSTX have been measured with a Faraday cup
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Medley, S.S.; Darrow, D.S.; Liu, D. & Roquemore, A.L. Prompt Loss of Energetic Ions during Early Neutral Beam Injection in the National Spherical Torus Experiment, report, March 25, 2005; Princeton, New Jersey. (https://digital.library.unt.edu/ark:/67531/metadc786189/m1/4/: accessed April 22, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.