Isotope Impact on Alfvén Eigenmodes and Fast Ion Transport in DIII-D

Measurements of beam driven Alfvén eigenmode activity in matched deuterium and hydrogen DIII-D plasmas show a dramatic difference in unstable mode activity for a given injected beam power. The dependence of the unstable AE spectrum on beam and thermal species is investigated in the current ramp by varying beam power in a sequence of discharges for fixed thermal and beam species at fixed density. In general, a spectrum of reversed shear Alfvén eigenmodes (RSAEs) and toroidal Alfvén eigenmodes (TAEs) are driven unstable with sub-Alfvénic deuterium beam injection while primarily only RSAEs are driven unstable for the hydrogen beam cases investigated. Further, for a given beam power, the driven AE amplitude is always reduced with hydrogen beams relative to deuterium and for hydrogen thermal plasma relative to pure deuterium or mixed deuterium/hydrogen plasmas. Estimates of the fast ion stored energy indicate that the dominant mechanism contributing to the difference between hydrogen and deuterium beam drive is the faster classical slowing down of hydrogen beam ions relative to deuterium and the resultant lower beam ion pressure – an effect which dominates the expected increase in drive due to higher hydrogen beam ion velocities.