PFC/RR-92-13 Current-Drive and Plasma-Formation Experiments on the Versator-Il Tokamak Using Lower-Hybrid and Electron-Cyclotron Waves

During lower-hybrid current-driven (LHCD) tokamak discharges with thermal electron temperature Te ~~ 150 eV, a two-parallel-temperature tail is observed in the electron distribution function. The cold tail extends to parallel energy Ell ~ 4.5 keV with temperature Tthd ~ 1.5 keV, and the hot tail extends to El > 150 keV with Thj' > 40 keV. Fokker-Planck computer simulations suggest the cold tail is created by low power, highN1 sidelobes in the lower-hybrid antenna spectrum, and that these sidelobes bridge the "spectral gap," enabling current drive on small tokamaks such as Versator. During plasma-formation experiments using 28 GHz electron-cyclotron (EC) waves, the plasma is born near the EC layer, then moves toward the upper-hybrid (UH) layer within 100-200ps. Wave power is detected in the plasma with frequency f = 300 MHz, indicating the EC waves decay into ion modes and electron Bernstein waves during plasma formation. Measured turbulent plasma fluctuations are correlated with decay-wave amplitude. Toroidal currents up to 1p ~ 1 kA are generated, consistent with theory, which predicts asymmetric electron confinement. Electron confinement measurements agree with theory. Soft X-rays are emitted with energies up to 6 keV and emission temperatures up to 1.75 keV. Electron-cyclotron current-drive (ECCD) is observed with loop voltage Vp < 0 and fully sustained plasma current I, < 15 kA at densities up to (ne) = 2 x 10" cm-. The ECCD efficiency r = (ne)IpRo/Pf=0.003, which is 30%-40% of the maximum achievable LHCD efficiency on Versator. The efficiency falls rapidly to zero as the density is raised above (ne) = 3 x 10" cm 3 , suggesting the ECCD depends on low collisionality. X-ray measurements indicate the current is carried primarily by electrons with energies 1 keV < E < 10 keV. The efficiency is maximum when the EC layer is 5-6 cm to the high-field side of the plasma center. The efficiency is independent of launch angle and polarization. The EC waves enhance magnetic turbulence in the frequency range 50 kHz <f <400 kHz by up to an order of magnitude. Assuming the poloidal mode number in = 15, it is esti9 mated that this reduces the confinement time of electrons with Eli = 80 keV by a factor of two to six in the outer two-thirds of the plasma, while the effect on electrons with 1 keV < Ell < 10 keV is small. The time-of-arrival of the turbulence to probes at the plasma boundary is longer when the EC layer is farther from the probes, suggesting the turbulence is driven by wave damping in the plasma interior. The theoretically-predicted current-drive synergism between EC and LH waves is not observed. This is at least in part because the EC waves enhance the losses of the fast, current-carrying electrons generated by the LH waves, as indicated by increased X-ray flux from the limiter during EC wave injection. These losses may be caused by the observed EC-enhanced magnetic turbulence. The enhanced turbulence and losses decrease with increasing plasma density, and thus may not reduce the EC/LH current-drive efficiency in a high-density, reactor-grade plasma. Thesis Supervisor: Dr. Ronald R. Parker Title: Professor of Electrical Engineering