Improved confinement in high-density H-modes via modification of the plasma boundary with lower hybrid wavesa)

Abstract

Injecting Lower Hybrid Radio Frequency (LHRF) waves into Alcator C-Mod's high-density H-mode plasmas has led to enhanced global energy confinement by increasing pedestal temperature and pressure gradients, decreasing the separatrix density, modifying the pedestal radial electric field and rotation, and decreasing edge turbulence. These experiments indicate that edge LHRF can be used as an actuator to increase energy confinement via modification of boundary quantities. H 98 -factor increases of up to ~35% (e.g. H 98 from 0.75 to 1.0) are seen when moderate amounts of LH power (P LH /P tot~0 .15) are applied to H-modes of densities n e~3 x10 20 m -3 , corresponding to values ~0.5 of the Greenwald density. However, the magnitude of the improvement is reduced if the confinement quality of the target Hmode plasma is already good (i.e. H 98 target~1 ). Ray-tracing modeling and accessibility calculations for the LH waves indicate that they do not penetrate to the core. The LHRF power appears to be deposited in plasma boundary region, with a large fraction of the injected power increment appearing promptly on the outer divertor target. There is no evidence that the LH waves are driving current in these plasmas. The LHRF-actuated improvements are well correlated with suppressed pedestal density fluctuations in the 100-300 kHz range. There is also a correlation between the improved confinement and a drop in separatrix density, a correlation that is consistent with previous H-mode results with no LHRF.

Alcator C-Mod: research in support of ITER and steps beyond

“…Injecting lower hybrid range of frequencies (LHRFs) waves into Alcator C-Mod's high-density H-mode plasmas has enhanced global energy confinement by increasing pedestal temperature and pressure gradients, while at the same time decreasing the separatrix density, modifying the pedestal radial electric field and rotation, and decreasing edge turbulence [11]. These experiments indicate that edge LHRF can be used as an actuator in these plasmas to increase energy confinement via modification of boundary quantities.…”

Relationship between frequency power spectra and intermittent, large-amplitude bursts in the Alcator C-Mod scrape-off layer

“…Turbulent motions and fluctuation-induced transport of particles and heat in the boundary region of magnetically confined plasmas pose numerous problems for the successful development of thermonuclear fusion power reactors [1][2][3]. This includes challenges for auxiliary heating and current drive by the use of radio frequency waves [4][5][6][7] and enhanced material erosion of the main chamber walls [8][9][10], and is likely related to the empirical discharge density limit [11][12][13][14][15][16][17]. At the outboard mid-plane scrape-off layer the radial transport is predominantly due to radially outwards motion of blob-like plasma filaments, which leads to order unity relative fluctuation levels, broad plasma profiles and enhanced plasma-wall interactions [15][16][17][18][19][20][21][22][23][24][25].…”

Lower hybrid wave edge power loss quantification on the Alcator C-Mod tokamak

“…19 Changes in H-mode quality, in the pedestal, and in the edge fluctuations were observed due to LH wave power in regimes with negligible current drive. 20,21 Several separate observed phenomena highlight the importance of the LH wave power in the edge plasma at high density.…”