Kinetic Interaction between Alpha Heating and Ion Cyclotron Resonance Heating in Burning Plasmas

In burning plasmas, alpha heating dominates plasma power balance which introduces important modifications to the kinetic particle distributions. Theoretical analysis [1] and experimental observations on JET [2] and more recently on the NIF [3] have shown that significant populations of fast ions are generated as a result of knock-on collisions with fusion-born alphas. This fast knock-on tail in turn affects the plasma dielectric properties via finite gyroradius effects, potentially altering the deposition of radio frequency power planned as auxiliary heating in several next generation devices such as SPARC, ITER, and CFETR. This work explores the interaction between collisional alpha heating and ion cyclotron resonance heating through detailed kinetic modeling of the fuel ion distribution functions. Building on previous work [4] which coupled the full-wave code TORIC [5] with the bounce-averaged Fokker-Planck solver CQL3D [6], we implement an alpha particle slowing down distribution and study the development of knock-on tails in the bulk fuel ion distribution functions and the modulation of these tails by ion cyclotron resonance heating for the first time. The specific case of SPARC’s primary reference discharge is presented, and the change in RF power coupled to tritium through 2^nd harmonic resonance and nonthermal fusion rates are examined in detail. In addition, predictions of neutron spectra measurements of the Magnetic Proton Recoil neutron spectrometer are presented as a potential observable test of the model.

[1] L. Ballabio, et al. Alpha particle knock-on signature in the neutron emission of DT plasmas. Phys Rev E. 1997

[2] J. Kallne, et al. Observation of alpha particle “knock-on” neutron emission from magnetically confined DT fusion plasmas. PRL. 2000

[3] C. Wink, et al. Observations of alpha knock-on tails in burning plasmas. In prep.

[4] S. Frank, et al. Simulating energetic ions and enhanced fusion rates from ion cyclotron resonance heating with a full-wave/Fokker-Planck model. PoP. 2024

[5] M. Brambilla. Numerical simulation of ion cyclotron waves in tokamak plasmas. PPCF. 1999

[6] R.W. Harvey et al. The CQL3D code. Proc IAEA TCM on Advances in Sim. and modeling of Thermonuclear Plasmas. 1992