(IT/P14) Study on Current Drive Capability of Lower Hybrid Waves and Neutral Beam in an ITER Steady State Scenario
T. Oikawa^{1)},
M. Shimada^{1)},
A.R. Polevoi^{1)},
O. Naito^{2)},
P.T. Bonoli^{3)},
N. Hayashi^{2)},
C.E. Kessel^{4)},
T. Ozeki^{2)}
^{1)} ITER International Central Team, Naka, ITER, Japan
^{2)} JAEA, Naka, Japan
^{3)} Plasma Science and Fusion Center, MIT, Cambridge, USA
^{4)} Princeton Plasma Physics Laboratory, Princeton, USA
Abstract. Neutral beam current drive (NBCD) and bootstrap current are
dominant noninductive CD sources in ITER steady state scenarios with the
initial investment. However, an additional CD source is necessary unless we
expect a larger
H_{H98y, 2} to provide a larger bootstrap current. Lower hybrid
current drive (LHCD) is attractive for its high offaxis CD capability. A
previous study with LHCD presented fully noninductive CD scenarios with
Q>5, assuming an apriori LH current drive efficiency. We
report LHCD capability in an ITER steady state scenario assessed with a
physics code that incorporates a relativistic onedimensional FokkerPlanck
calculation and a ray tracing code. The LHCD code was experimentally
validated in JT60U. For a LH power spectrum on the refractive index
parallel to the toroidal magnetic field
n_{  } calculated from the present ITER
LH launcher design, the total LHCD for 20 MW injection is calculated to be
0.54 MA for the ITER steady state condition in the previous study. The
corresponding current drive efficiency is less than half of the assumption
in the previous study. This low CD capability results from the low
directivity of 70% and relatively high
n_{  } of 2. Therefore we need to
improve LHCD by optimizing the LH power spectrum. LHCD increases with
1/n_{  }^{2}. Decreasing
n_{  } below 1.9, however, LHCD becomes
worse because the accessibility condition of LHWs approaches. By peaking the
electron density profile with keeping the fusion power, LHCD improves down
to a lower
n_{  } since the accessibility condition shifts due to a lower
density in the LH absorption region. In a moderately peaked profile case,
the duration of the hybrid operation will be limited only by the cooling
capability (t<3000s). A scan of the spectrum width
Δn_{  } = 0.055  0.25 shows that a narrower width is favourable for
CD. Twodimensional effects in the electron velocity space, which are
expected to increase CD, are also examined. We also investigate physics
models in NBCD codes for example for the ionization process, fast ion
behaviour, electron shielding effect, and so on, and we compare a
FokkerPlanck code and a Monte Carlo code and evaluate NBCD capabilities in
ITER steady state scenarios.
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