Enhanced slow-wave beach heating of mirror plasmas with two-ion species

Plasma heating in the ion-cyclotron range of frequencies has been examined in a beach-heating configuration of the central cell of the Phaedrus-B tandem mirror [Phys. Rev. Lett. 51, 1955 (1983)] during scans of the ion species ratio of hydrogen–helium (H–He) and hydrogen–deuterium (H–D) plasmas. Observed changes in the radial profiles of wave magnetic field and plasma parameters during the scans provide evidence of varied fast- and slow-wave eigenmode excitation and absorption. These changes are shown to be in agreement with those expected from wave accessibility considerations. Optimal peak parameters in the plasma core, ne=1.0×1013 cm−3, Te∥=20 eV, Ti∥=140 eV, Ti⊥=450 eV, and β=0.26 at midplane, were obtained for moderate helium or deuterium ion fractions (puffed nHe/ne=nD/ne ∼0.25). These parameters represent a significant increase in plasma energy density over that obtained under the same conditions with ‘‘pure’’ hydrogen plasmas: ne=7.0×1012 cm−3, Te∥=25 eV, Ti∥=80 eV, Ti⊥=300 eV, and β=0.12.