On Fast Wave Current Drive at Higher Cyclotron Harmonics

Introduction For steady state operation or long pulses of Tokamak reactors non-inductive current drive is necessary. Even if a large fraction of the current is provided by the bootstrap current, there is still a need for an efficient current drive scheme. Waves in the ion cyclotron range of frequencies can be used for current drive in reactor size plasmas when the waves are damped by transient time magnetic pumping (TTMP) and electron Landau damping (ELD). However, in present day machines this damping is often weak and is in competition with a relatively strong ion cyclotron damping. By operating at higher harmonics of the ion cyclotron frequency the ion absorption can be reduced Fast Wave Current Drive Fast wave current drive is achieved by heating electrons with a directed wave spectrum. The current is limited by absorption on trapped electrons and collisional transfer of momentum to ions and trapped electrons. This results in current profiles peaked at the magnetic axis where collisionality and trapped particle effects are lowest. In general the competition between ion and electron absorption allows efficient current drive only in regimes where ion absorption is weak. The current drive is calculated from the local power density by an analytic formula 38 th EPS Conference on Plasma Physics (2011) P2.096 Results and Discussion Self-consistent calculations of power partition and current drive efficiency have been done to study the effects of frequency f, toroidal mode number n φ , plasma temperature and density. A comparison of the current drive efficiency between a Maxwellian plasma at t = 0 s and a non Maxwellian plasma, close to steady state, at t = 1 s is done. A single specie deuterium plasma or a deuterium plasma with 5% hydrogen is used with B 0 = 2.08 T, R 0 = 2.28 m, a = 0.72 m and P RF = 10 MW, P NB = 10 MW and neutral beam energy E NB = 100 keV. For the pure deuterium plasma the variation of current drive efficiency versus toroidal mode number, n φ , for f = 51 MHz, n D = 2.5 x 10 19 m -3 and T i = 5 keV is shown in For lower values of n φ the ion cyclotron absorption increases with time because of tail formation of the distribution function. The general trend is that the current drive efficiency decreases with increasing n φ because of absorption on trapped electrons. It can be seen from A scan is done by keeping the ratio n φ / f constant for n D = 2.5 x 10 19 m -3 and T i = 5 keV. For the pure deuterium plasma the variation of current drive efficiency for f ≥ 3Ω D is slow, see The variation of the current drive efficiency with respect to density and temperature for a pure deuterium plasma has been studied for the case n φ = 23 and f = 69 MHz, as shown in The fractional powers absorbed by hydrogen, deuterium and electron are plotted versus frequency as shown in A plot of current drive efficiency versus frequency is shown in The pure deuterium plasma is studied with the neutral beam injection. Presence of neutral beam results in development of a high-energy tail of deuterium ions, enhancing the RF absorption for the deuterium as shown in Deuterium plasma with 5% hydrogen is also studied with the neutral beam injection. The presence of neutral beam and the 2 nd and 3 rd hydrogen harmonic resonances result in significant decrease of the power absorbed by electrons, which leads to a significant reduction of the current drive efficiency as shown in Conclusions FWCD has been studied at higher harmonics. For the studied parameters FWCD at frequencies above 3 rd harmonic of deuterium and 2 nd harmonic of hydrogen is found to be efficient. Low toroidal mode number is desired for efficient current drive. The current drive efficiency increases with plasma temperature because of reduced collisionality. Self-consistent calculations are important at low harmonic numbers and in the presence of minority hydrogen and neutral beam injection. Acknowledgemen