The effect of toroidal plasma rotation on low-frequency reversed shear Alfvén eigenmodes in tokamaks

Abstract

The influence of toroidal plasma rotation on the existence of reversed shear Alfvén eigenmodes (RSAEs) near their minimum frequency is investigated analytically. An existence condition is derived showing that a radially decreasing kinetic energy density is unfavourable for the existence of RSAEs. The Coriolis effect is typically unfavourable for modes moving slower than the plasma or moving in the opposite direction. The generality of the analysis allows for the description of downwards sweeping RSAEs and also rotation-induced modes in regular shear plasmas.

Energetic particle physics in fusion research in preparation for burning plasma experiments

“…A less studied topic is that of RSAEs whose frequency sweeps down and which are rarely observed in tokamaks. There have been several papers published on this topic [285][286][287][288][289][290][291]. The explanations given in those references take different approaches.…”

“…The details of this model can be found in the already mentioned topical review [11] or in [289]. A pure MHD description of these modes is given in [290,291]. The first one treats the underlying instability as internal AEs (IAEs).…”

“…Only continuum damping was estimated perturbatively and shown to be small. Reference [291] proposed an MHD explanation (i) with the RSAE frequency lying below the continuum or (ii) the safety factor containing a local maximum or (iii) the hollow plasma rotation flow profile. All of the above mentioned interpretations are interesting models but remain to be compared with experiments beyond the preliminary comparison published recently [292].…”

Flow shear stabilization of rotating plasmas due to the Coriolis effect

“…In Ref. [36], the Coriolis-pressure effect was predicted to influence the existence of reversed shear Alfvén eigenmodes with ω 2 D ≈ ω 2 + , favoring modes propagating in the direction of the plasma rotation [36].…”

Measurements of the eigenfunction of reversed shear Alfvén eigenmodes that sweep downward in frequency

“…Their study suggests that the criteria for mode existence are more stringent for downsweeping instabilities than for upsweeping modes. Haverkort 36 considers the effects of toroidal rotation on RSAE existence criteria. He argues that downsweeping can occur when q min has a local maximum or when the toroidal mode number n is high and the rotation frequency is locally concave.…”