Current-drive and plasma-formation experiments on the Versator-II tokamak using lower-hybrid and electron-cyclotron waves
During lower-hybrid current-driven (LHCD) tokamak, discharges with thermal electron temperature T{sub e} {approx} 150 eV, a two-parallel-temperature tail is observed in the electron distribution function. The cold tail extends to parallel energy E{parallel} {approx} 4.5 keV with temperature T {approx} 1.5 keV, and the hot tail extends to E{parallel} {approx} 4.5 keV with T > 40 keV. Fokker-Planck computer simulations suggest the cold tail is created by low power, high N{parallel} 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--200{mu}s. 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 I{sub p} {approx} 1 kA are generated, consistent with theory, which predicts asymmetric electron confinement. Electron-cyclotron current-drive (ECCD) is observed with loop voltage V{sub loop} {le} 0 and fully sustained plasma current I{sub p} {approx_lt} 15 kA at densities up to {l_angle}n{sub e}{r_angle} = 2 {times} 10{sup 12} cm{sup {minus}3}. The ECCD efficiency {eta} {equivalent_to} {l_angle}n{sub e}{r_angle}I{sub p}R{sub 0}/P{sub rf} = 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 {l_angle}n{sub e}{r_angle} = 3 {times} 10{sup 12} cm{sup {minus}3}, suggesting the ECCD depends on low collisionality. X-ray measurements indicate the current is carried primarily by electrons with energies 1 keV {approx_lt} E {approx_lt} 10 keV.
- Research Organization:
- Massachusetts Inst. of Tech., Cambridge, MA (United States). Plasma Fusion Center
- Sponsoring Organization:
- USDOE, Washington, DC (United States)
- DOE Contract Number:
- AC02-78ET51013
- OSTI ID:
- 10102482
- Report Number(s):
- PFC/RR-92-13; ON: DE93002333
- Resource Relation:
- Other Information: TH: Thesis (Ph.D.); PBD: Aug 1992
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
VERSATOR TOKAMAK
NON-INDUCTIVE CURRENT DRIVE
LOWER HYBRID CURRENT DRIVE
ELECTRON TEMPERATURE
DISTRIBUTION FUNCTIONS
FOKKER-PLANCK EQUATION
COMPUTERIZED SIMULATION
ANTENNAS
ELECTRON CYCLOTRON-RESONANCE
PLASMA DIAGNOSTICS
EXPERIMENTAL DATA
ECR HEATING
700350
PLASMA PRODUCTION, HEATING, CURRENT DRIVE, AND INTERACTIONS