Simulations of ion cyclotron emission from highly energetic fusion-born protons in aneutronic deuterium-helium-3 plasmas

Abstract Studies of future aneutronic fusion plasmas, such as those comprising deuterium and helium-3 (D– <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msup> <mml:mrow/> <mml:mn>3</mml:mn> </mml:msup> <mml:mtext>He</mml:mtext> </mml:mrow> </mml:math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msup> <mml:mo>→</mml:mo> <mml:mn>4</mml:mn> </mml:msup> <mml:mtext>He</mml:mtext> <mml:mo>+</mml:mo> <mml:mi>p</mml:mi> </mml:mrow> </mml:math> ), may not be able to exploit neutron-based diagnostics for direct measurement of fusion reactivity. A potential alternative diagnostic is ion cyclotron emission (ICE), which is driven by strongly non-Maxwellian energetic ion populations collectively relaxing under the magnetoacoustic cyclotron instability. ICE already provides a diagnostic of fusion physics in magnetically confined deuterium and deuterium–tritium plasmas. Here we focus on the distinctive fast fusion-born 14.68 MeV protons in D– <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msup> <mml:mrow/> <mml:mn>3</mml:mn> </mml:msup> <mml:mtext>He</mml:mtext> </mml:mrow> </mml:math> plasma. We carry out kinetic simulations of ICE spectra using a particle-in-cell code, which self-consistently solves the Maxwell and Lorentz equations for tens of millions of interacting simulation particles. Motivated by recent observations of ICE from trace fusion-born 3 MeV proton populations in KSTAR and LHD, we adopt scenarios where the 14.68 MeV protons are distributed in velocity space as a drifting ring-beam with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>v</mml:mi> <mml:mo>⊥</mml:mo> </mml:msub> <mml:mo>∼</mml:mo> <mml:msub> <mml:mi>v</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">A</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> , and the rest of their kinetic energy is directed parallel to the magnetic field. Plasma and magnetic field parameters are similar to those at the outer mid-plane of JET. Spatiotemporal Fourier transforms of the excited magnetic field exhibit strong tilting of successive cyclotron harmonic features in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:mi>ω</mml:mi> <mml:mo>,</mml:mo> <mml:mi>k</mml:mi> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> </mml:math> space, due to the Doppler shift arising from the large parallel velocities of these protons. We show that it is possible to correct for these Doppler shifts by performing slanted integrations along Doppler-shifted isofrequency lines, yielding simulated ICE power spectra which have strong spectral peaks at Doppler-shifted proton cyclotron harmonics. Our results show that future experimental studies of ICE from D– <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msup> <mml:mrow/> <mml:mn>3</mml:mn> </mml:msup> <mml:mtext>He</mml:mtext> </mml:mrow> </mml:math> plasmas would greatly benefit from higher wavevector resolution, and that ICE from future aneutronic D– <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msup> <mml:mrow/> <mml:mn>3</mml:mn> </mml:msup> <mml:mtext>He</mml:mtext> </mml:mrow> </mml:math> plasmas could play a useful role as a diagnostic of fusion reactivity and of fusion-born ion populations.