Power requirements for electron cyclotron current drive and ion cyclotron resonance heating for sawtooth control in ITER

Abstract

Abstract.13MW of electron cyclotron current drive (ECCD) power deposited inside the q = 1 surface is likely to reduce the sawtooth period in ITER baseline scenario below the level empirically predicted to trigger neo-classical tearing modes (NTMs). However, since the ECCD control scheme is solely predicated upon changing the local magnetic shear, it is prudent to plan to use a complementary scheme which directly decreases the potential energy of the kink mode in order to reduce the sawtooth period. In the event that the natural sawtooth period is longer than expected, due to enhanced α particle stabilisation for instance, this ancillary sawtooth control can be provided from > 10MW of ion cyclotron resonance heating (ICRH) power with a resonance just inside the q = 1 surface. Both ECCD and ICRH control schemes would benefit greatly from active feedback of the deposition with respect to the rational surface. If the q = 1 surface can be maintained closer to the magnetic axis, the efficacy of ECCD and ICRH schemes significantly increases, the negative effect on the fusion gain is reduced, and off-axis negative-ion neutral beam injection (NNBI) can also be considered for sawtooth control. Consequently, schemes to reduce the q = 1 radius are highly desirable, such as early heating to delay the current penetration and, of course, active sawtooth destabilisation to mediate small frequent sawteeth and retain a small q = 1 radius. Finally, there remains a residual risk that the ECCD+ICRH control actuators cannot keep the sawtooth period below the threshold for triggering NTMs (since this is derived only from empirical scaling and the control modelling Power requirements for sawtooth control in ITER 2 has numerous caveats). If this is the case, a secondary control scheme of sawtooth stabilisation via ECCD+ICRH+NNBI, interspersed with deliberate triggering of a crash through auxilliary power reduction and simultaneous pre-emptive NTM control by off-axis ECCD has been considered, permitting long transient periods with high fusion gain. The power requirements for the necessary degree of sawtooth control using either destabilisation or stabilisation schemes are expected to be within the specification of anticipated ICRH and ECRH heating in ITER, provided the requisite power can be dedicated to sawtooth control.

Progress in disruption prevention for ITER

“…A need for control of the sawtooth instability is also anticipated in ITER, in order to avoid long period sawteeth with large amplitude crashes that could trigger an NTM instability [23], and several schemes using ECCD or ICRF heating have been proposed [24]. Recent JET experiments have shown that the sawtooth period can be controlled with low field side ICRF heating (as will be available in ITER) near the q=1 surface [25], and that sawtooth pacing can be achieved with modulated central ion cyclotron heating [26], a method that is less sensitive to the deposition location.…”

High power ECRH and ECCD in moderately collisional ASDEX Upgrade Hmodes and status of EC system upgrade

“…We see that the value of Q cannot be deduced precisely only from heat demand considerations, since the kinetic profiles and the fusion power are not very sensitive to the amount of external heating power during the flat top phase. The required maximum injected external power depends more on the access of the H-mode during start up and the needed current drive for different scenarios [17,18] and the latter may be a more crucial constraint for the determination of Q.…”

Electron Cyclotron Waves

“…5) and sawteeth. The design work is supported by extensive simulations for the various tasks and by extrapolation from current experiments to ITER [53][54][55]. Still uncertainties remain.…”