N=2 ICRH of H majority plasmas in JET-ILW

Abstract

Heating single ion species plasmas with ICRF is a challenging task: Fundamental ion cyclotron heating (w = w ci ) suffers from the adverse polarization of the RF electric fields near the majority cyclotron resonance while second harmonic heating (w = 2w ci ) typically requires pre-heating of the plasma ions to become efficient. Recently, w = 2w ci ICRF heating was tested in JET-ILW hydrogen plasmas in the absence of neutral beam injection (L-mode). Despite the lack of pre-heating, up to 6MW of ICRF power were coupled to the plasma leading to a transition to H-mode for P ICRH >5MW in most discharges. Heating efficiencies between 0.65-0.85 were achieved as a combination of the low magnetic field adopted (enhanced finite Larmor radius effects) and the deliberate slow rise of the ICRF power, allowing time for a fast ion population to gradually build-up leading to a systematic increase of the wave absorptivity. Although fast ion tails are a common feature of harmonic ICRF heating, the N=2 majority heating features moderate tail energies (<500keV) except at very low plasma densities (n e0 <3x10 19 /m 3 ), where fast H tails in the MeV range developed and fast ion losses became significant, leading to enhanced plasma wall interaction. The main results of these experiments will be reported.

Modelling one-third field operation in the ITER pre-fusion power operation phase

“…An analytical orbit solver applied to these 5 MA/1.8 T scenarios predicts that ions born within a normalized radius <0.75 with energies of up to 6 MeV are well confined. Hence, in this simple approximation no fast-ion losses are expected despite operation at low current and low density, unlike in JET experiments [32]. It is important to note that finite orbit Doppler-shifted IC absorption and fast-ion ripple losses have not been taken into account in the modelling described above and they could be significant due to the relatively large value of the field ripple, δ rip = −1.3%, required for good IC coupling.…”

“…The most efficient ICRH scheme at 1.8 T is second harmonic H heating (ω = 2ω cH ), both in hydrogen and helium plasmas. Even without magnetic field ripple, the confinement of the second harmonic ICRHaccelerated ions may be reduced for 5 MA/1.8 T scenarios due to low current/low density/low collisionality, possibly leading to enhanced plasma-wall interactions as seen at JET [32]. Therefore, detailed studies have been carried out to determine the capability of IC heating for 5 MA/1.8 T plasmas.…”