Optimized phasing conditions to avoid edge mode excitation by ICRH antennas

Abstract

An ion cyclotron resonance heating (ICRH) antenna system must launch radio frequency (RF) power with a wavenumber spectrum which maximizes the coupling to the plasma. It should also ensure good absorption while minimizing the wave interaction with the plasma edge. Such interactions lead to impurity release, whose effect has been measured far from the antenna location (Klepper et al. 2013; Wukitch et al. 2017; Perkins et al. 2019) and can involve the entire scrape-off layer. In the normal heating scenario, for which the frequency of the waves launched by the antenna is larger than the ion cyclotron frequency of the majority ions $\omega > \omega _{\textrm {ci},\textrm {maj}}$ , release of impurities due to ICRH can be affected by minimizing the low $|k_{\parallel }| < k_0$ power spectrum components of the antenna. Impurity release can be the result of low central absorption of the waves or power transfer from the fast to the slow wave due to the presence of a confluence in the plasma edge. In ASDEX Upgrade (AUG), a reduction of heavy impurity release by ICRH in the plasma was qualitatively well correlated to the parallel electric field and RF currents flowing around the antenna (Bobkov et al. 2017). In this article, we first show a correlation between the reduction in impurity release by ICRH in AUG and the rejection of the low $|k_{\parallel }| < k_0$ region of the antenna power spectrum. We show that the same correlation holds for results obtained in the Alcator C-Mod tokamak. Finally, using this idea, we reproduce ICRH induced impurity release behaviour in a not yet published experiments of JET, and make predictions for ITER and DEMO.

Slab-geometry surface waves on steep gradients and the origin of related numerical issues in a variety of ICRF codes

“…In this approximation, the surface waves exist only for a finite range of parallel wavenumbers , which suggests ways to avoid exciting them. Physically, tailoring the spectrum such as to avoid low , consistent with Messiaen & Maquet (2020) and Maquet & Messiaen (2020), can prevent surface wave excitation at least in this approximation. Numerically, there is some design freedom in where to put the plasma–vacuum interface: it must be beyond the lower hybrid resonance, but before the point where the fast wave launched by the antenna becomes propagative (which depends on ; the fast wave launched by the antenna typically has much larger poloidal wavelength than the surface waves), such that the fast wave evanescence layer, a crucial ingredient in ICRF power coupling calculations, is modelled as correctly as possible.…”

“…Kaw & McBride 1970; Aliev et al. 1995, 2000; Lee & Lee 2010; Girka, Girka & Thumm 2016; Lee, Shahmansouri & Jung 2019; Maquet & Messiaen 2020; Girka, Girka & Thumm 2020). If the plasma in question is the electron plasma in a solid conductor, the surface waves are ‘surface plasmons’ (Ritchie & Marusak 1966; Gangaraj & Monticone 2019).…”

Analytical edge power loss at the lower hybrid resonance: ANTITER IV validation and application to ion cyclotron resonance heating systems

“…(2021) and is performed here considering three cases to illustrate the discussion of § 3 with concrete examples. Each case presents the antenna power spectrum for the two conventional phasings and and a phasing minimizing the low part of the power spectrum (Maquet & Messiaen 2020) for a fixed and even current amplitude of 1 A on the straps. The phasing produces no excitation at and is rather depleted of low excitation due the large dominant peak excited in its power spectrum as seen on figure 7.…”

“…Edge power losses in tokamaks should be avoided as they can lead to a reduction of the power deposited in the plasma core and to deleterious impurity release from the first wall of the device. A correlation between ICRH related impurity release and low present in the radiation spectrum launched by ICRH antennae was investigated in Maquet & Messiaen (2020).…”

Status of the WEST travelling wave array antenna design and results from the high power mock-up

“…Moreover, they could share the same cooling loop of the blanket thus reducing the integration complexity. Due to the larger number of toroidal straps, the k || spectrum is peaked and the dominant k || value can be optimized for coupling and bulk absorption while minimizing the interaction in the edge by depleting the low-k || part of the spectrum [11,12] thus reducing the coaxial modes excitation and the dissipation in the RF sheaths. Depending on the configuration of the system, i.e.…”