Mode structure measurements of ion cyclotron emission and sub-cyclotron modes on DIII-D

Abstract

We report mode structure measurements of coherent ion cyclotron emission (ICE) and sub-cyclotron modes on DIII-D. Through a dedicated experiment, we aimed to characterize a variety of modes via the upgraded ICE diagnostic in both L- and H-mode plasmas. In the L-mode plasmas, autopower spectrum peaks at harmonics of the ion cyclotron frequency fci were observed and are localized in the core of the plasma. Sub-cyclotron modes (f ∼ 0.5fci) were also observed in L-mode plasmas when the toroidal magnetic field strength was lowered from BT = 2.17 T. In H-mode plasmas, many ICE harmonics localized to the plasma edge were observed, with some exceeding the nominal ICE diagnostic bandwidth of f ∈ [0, 100] MHz. Polarization estimates made using an orthogonal pair of vertical and horizontal magnetic pickup loops on the outer wall of the machine indicate that ICE and sub-cyclotron modes have compressional polarization at the plasma edge, the latter being consistent with simulation efforts for comparable DIII-D plasmas. For all modes, the same harmonics are observed on both the centerpost and outer wall loops, indicating that ICE is poloidally extended. Finally, toroidal mode numbers were calculated using three outer wall loops for both core ICE and sub-cyclotron modes in L-mode plasmas. The sub-cyclotron case served as a benchmark for our calculation method, with measured numbers of n ∈ [−24,−18] roughly agreeing with heuristic estimates of n ∈ [−20,−13]. Core ICE mode numbers were measured to be n ∈ [−10, 5], which is comparable to measurements made on other machines.

Application of simulations and theory of sub-cyclotron frequency modes to DIII-D

“…This is similar to the NSTX results which also show mixed polarization for GAEs at the edge [19]. A larger compressional component of the perturbed magnetic field at the edge was also recently measured in DIII-D for the high frequency AEs [29]. Unlike the situation in NSTX, coupling between shear and compressional magnetic perturbations in DIII-D simulations is weaker, and the peak magnitude of compressional perturbation is less than 5% of total |δB|, in contrast with typical GAE structure from NSTX-U simulations, for example, where δB ∼ 0.15-0.2 × δB ⊥ .…”

On equations for ion cyclotron modes in ‘warm’ bounded plasmas

“…This work represents an indispensable step towards a study of eigenmodes in warm bounded plasmas. It was motivated, first of all, by the ICE experiments on NSTX-U and DIII-D (Fredrickson et al 2021;Crocker et al 2022), which can be hardly described by existing theories employing FMMs. Some other recent DIII-D results on ICE can be found in Thome et al (2019), DeGrandchamp et al (2021 and DeGrandchamp et al (2022).…”

“…Due to this, thermal particles mainly contribute to the integral (y ∼ 1). On the other hand, ζ T is very large; for instance, for the above-mentioned ICE experiment on DIII-D (Crocker et al 2022) ζ T = 240 (we take l = 2, q = 2, ρ T = 0.5 cm, r = 30 cm). Using a large value of ζ T and taking y = 1 in the argument of the Bessel functions, we can write J 2 s (ζ T /y) ∼ 1/ζ T with |s| < s m ≡ ζ T .…”

“…where k and ω are the wavenumber and wave frequency, respectively, and v A the Alfvén velocity (Crocker et al 2022). Therefore, it was concluded in Crocker et al (2022) that the observed ICE resulted from destabilized ion cyclotron waves, either electrostatic waves (which presumably took place in TFTR (Dendy et al 1994)) or electromagnetic waves. In contrast to FMM and Alfvén waves, these modes are absent in ideal magnetohydrodynamics (MHD) and the two-fluid model with zero temperature.…”

“…Eigenmodes in the ion cyclotron frequency range in toroidal plasmas are usually described within a cold-plasma approximation (two-fluid model), in which case only fast magnetoacoustic modes (FMM) (known also as CAE (compressional Alfvén eigenmodes)) exist. However, recent experiments with ion cyclotron emission (ICE) on the NSTX-U spherical torus (Fredrickson et al 2021) and DIII-D tokamak (Crocker et al 2022) indicate that this approximation can be unsuitable. First, it was found that the emission frequency in NSTX-U does not follow an Alfvénic scaling with density; this newly discovered type of chirping ICE does not follow the well-known ICE phenomenology on JET, TFTR, KSTAR or LHD.…”