RF absorption in presence of fast particles using AORSA coupled with MCGO*

Modeling fast magnetosonic wave propagation and absorption in the presence of non-Maxwellian ions at high harmonic mode numbers (>2 fci) is challenging due to absorption being highly sensitive to the non-Maxiwellian distribution and ion finite orbit width. The full wave code AORSA [1] coupled to a Monte-Carlo orbit code MCGO [2]  is being studied as a candidate to address this modeling need. Two examples of cases where non Maxwellian effects are important to incorporate are for a proposed harmonic fast wave (HFW) traveling wave antenna for fast particle generation on DIII-D, and in high harmonic fast wave heating in the WHAM [3]  mirror device. For the HFW DIII-D case, a fast wave tailored to 4th or above harmonics is launched from the high field side and damps on the moderately fast (tens of keV’s) neutral beam deuterium ions, which are accelerated to hundreds of keV’s. These fast particles mimic reactor parameters and can go on to excite Alfven Eigenmode (AE) instabilities, allowing for a D-D tokamak like DIII-D to replicate reactor-relevant fast particle conditions experimentally. The fast ion tail pulled from the neutral beam distribution is highly non-Maxwellian. Therefore, to confidently model a HFW fast particle source, the AORSA/MCGO framework is required. Here we present an initial scoping study with the raytracing/Fokker-Planck codes GENRAY/CQL3D [2] (zero banana width) to identify DIII-D discharges to analyze for the AORSA/MCGO framework. In the second case, full-wave modeling of fast wave heating in the WHAM mirror device is desired. Mirror plasmas exhibit non-Maxwellian ion distributions due to the loss cone and a parallel electric potential. WHAM uses neutral beams, onto which harmonic fast waves damp and accelerate beam ions.  Due to the high mirror ratios in the WHAM device, many cyclotron harmonics are present at once, with most damping occurring on the 2nd harmonic. These many harmonics add further importance to a full-wave treatment. While AORSA/MCGO is well suited for tokamaks, both codes require modification for the mirror geometry. The latest modeling and analysis for AORSA/MCGO’s framework will be presented.

Citations 

1 Jaeger E.F. et al. 2001 Phys. Plasmas 8 1573–83

2 Harvey, B. https://www.compxco.com/index.html  

3 J. Egedal et al 2022 Nucl. Fusion 62 126053