High-Density Discharges in the Alcator Tokamak

Peak plasma densities in excess of ${10}^{15}$ ${\mathrm{cm}}^{-3\mathrm{}}$ have been obtained in the Alcator tokamak with $60<~B_T<85\mathrm{}$ kG. The highest average density so far achieved is ${\overline{n}}_e=6\mathrm{}\times {10}^{14}$ ${\mathrm{cm}}^{-3\mathrm{}}$; the corresponding $n_0{\tau }_E=2\mathrm{}\times {10}^{13}$ ${\mathrm{cm}}^{-3\mathrm{}}$ s. These ultrahigh-density discharges exhibit (i) nearly complete energy equilibration between electrons and ions, (ii) severe attenuation of energetic-neutral-particle fluxes, (iii) a minor role of impurities, and (iv) energy-confinement properties consistent with neoclassical estimates.