ICRF Loading Studies on Alcator C-Mod

Transcription

1 ICRF Loading Studies on Alcator C-Mod 46 th Annual Meeting of the APS Division of Plasma Physics November 15-19, 19, 2004 A. Parisot, S.J. S.J. Wukitch,, P. Bonoli,, J.W. Hughes, B. Labombard,, Y. Lin, R. Parker, M. Porkolab,, A.K. Ram, Plasma Science and Fusion Center, MIT, Cambridge, MA, USA Keys results Loading variations in C-Mod not dominated by changes in strap-cutoff distance ( evanescent length) Coupling determined by edge and SOL density profile Modeled in terms of wave propagation and impedance transformation in 1D slab geometry.

2 Variations of loading resistance with plasma conditions Study variations in ICRF coupling with global plasma parameters. Measure coupling using the loading resistance R Loading for each antenna 3 Fast Wave antennas on Alcator C-Mod. Two 2-straps antennas, structurally identical, operated at 80 and 80.5 MHz, k peak = 10 m -1 One 4-strap antenna, operated here at 78 MHz, k peak = 13 m -1 Coupled power 6 MW

3 Cutoff surface in limiter shadow on C-Mod Loading variations are usually dominated by changes in distance between straps and right hand cutoff surface for FW Cutoff density is reached in near SOL. Tore Supra, JET, DIII-D In contrast : Alcator C-Mod operates at high electron densities n cutoff ~ 5-8 x m -3 n e0 ~ m -3 Cutoff density in C-Mod C is below typical SOL densities

4 Not dominated by changes in distance to cutoff Confirmed by density profile measurements with Thomson scattering and Langmuir probes Cutoff layer experiences only small radial excursions Gap control is of limited use Changes in strap-cutoff distance accounts only for changes of ~ 20 % in coupling Loading variations in C-Mod C due to evolution of density profile in propagation region (edge and SOL)

5 Pedestal density gradient lowers coupling As plasma transitions to H-mode, density pedestal forms at the edge Very good correlation with electron density at top of pedestal in H- mode (at LCFS in L- mode) Consistent with observations in DIII-D D : density gradient lowers ICRF coupling Mayberry et al., Nucl. Fusion 30 (1990) 579

6 Higher coupling as SOL density increases Scrape-Off Layer (SOL) plays important role in EDA H-modeH Neutral pressure at midplane wall is well correlated with average density in SOL In EDA H-mode, high SOL densities lead to loading resistance comparable to that in L-mode plasmas.

7 Wave propagation model 1D model using wave equation S,D Stix notations depend on n e (x) Solve for the fields at the Faraday screen assuming outgoing wave towards core Surface resistance α Loading resistance Model reproduces observed behavior Boundary condition : wave impedance = local plasma impedance at the edge Plasma impedance decreases with density SOL acts as an impedance transformer

8 Conclusions ICRF loading in Alcator C-Mod not dominated by changes in the evanescent length Cutoff surface lies in limiter shadow and experiences only limited changes. Effect of density profile in propagation region can be studied directlyd Loading decreases with higher pedestal density gradient Loading increases with higher density in SOL Situation may be relevant for ITER, NSTX (low cutoff density) 1D model of wave propagation reproduces the observed behavior Interpretation in terms of impedance transformation A. Parisot et al., Plasma Phys. Cont. Fusion, 46 (2004)