Modification of the Scrape-Off-Layer by Lower Hybrid Waves in the Alcator C-Mod Tokamak

摘要

Lower Hybrid Current Drive (LHCD) is an attractive option for non-inductive tokamak operation due to its high current drive efficiency and ability to drive current off axis. The LHCD system on C-Mod [1] is designed to drive current off axis (r/a = 0.5 - 0.8) with operational parameters (f_0 = 4.6 GHz, n|| = 1.5 - 3,B_ψ ?5 T,n_e0 ?10~(20) m~(-3)) similar to the proposed LHCD system on ITER [2]. Based on results from previous LHCD experiments on other tokamaks, LHCD is expected to drive significant current with C-Mod parameters (ω/ωLH〉2, n|| >n||crit) [3,4]. However, experiments on C-Mod have shown that the population of fast electrons generated by LHCD, as measured by fast electron bremsstrahlung [51, is substantially reduced in L-mode at line averaged electron densities above 1x10~(20) m~(-3) for 5.4 T < B_ψ < 7 T and n||≈2 (see Fig. 1) [6,7]. This lower density limit may have critical implications for the use of LHCD on fiiture high density non-inductive tokamaks. Poor wave accessibility and parametric decay instabilities (PDI) have been identified as possible causes of reduced LHCD efficiency at high density in other experiments. Ray tracing shows that the LH waves in C-Mod 20 3 are marginally accessible at n_e = 1.5x 10~(20) m~(-3). The density limit attributed to PDI on Alcator C occured at ω/ωLH ?2 [4],while on C-Mod the observed limit occurs near co/colh ?4. Frequency spectra recorded during the discharges shown in Fig. 1 show weak signs of PDI, although strong PDI has been observed on C-Mod at higher densities (n_e >2xl0~(20) m~(-3)).