(IT/P1-4) Study on Current Drive Capability of Lower Hybrid Waves and Neutral Beam in an ITER Steady State Scenario

T. Oikawa1), M. Shimada1), A.R. Polevoi1), O. Naito2), P.T. Bonoli3), N. Hayashi2), C.E. Kessel4), T. Ozeki2)
 
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 non-inductive CD sources in ITER steady state scenarios with the initial investment. However, an additional CD source is necessary unless we expect a larger HH98y, 2 to provide a larger bootstrap current. Lower hybrid current drive (LHCD) is attractive for its high off-axis CD capability. A previous study with LHCD presented fully non-inductive CD scenarios with Q>5, assuming an a-priori LH current drive efficiency. We report LHCD capability in an ITER steady state scenario assessed with a physics code that incorporates a relativistic one-dimensional Fokker-Planck calculation and a ray tracing code. The LHCD code was experimentally validated in JT-60U. 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. Two-dimensional 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 Fokker-Planck code and a Monte Carlo code and evaluate NBCD capabilities in ITER steady state scenarios.

Full paper available (PDF)