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

Abstract

For the first time, the power deposition of lower hybrid RF waves into the edge plasma of a diverted tokamak has been systematically quantified. Edge deposition represents a parasitic loss of power that can greatly impact the use and efficiency of Lower Hybrid Current Drive (LHCD) at reactor-relevant densities. Through the use of a unique set of fast time resolution edge diagnostics, including innovative fast-thermocouples, an extensive set of Langmuir probes, and a Ly a ionization camera, the toroidal, poloidal, and radial structure of the power deposition has been simultaneously determined. Power modulation was used to directly isolate the RF effects due to the prompt (t < s E ) response of the scrape-off-layer (SOL) plasma to Lower Hybrid Radiofrequency (LHRF) power. LHRF power was found to absorb more strongly in the edge at higher densities. It is found that a majority of this edge-deposited power is promptly conducted to the divertor. This correlates with the loss of current drive efficiency at high density previously observed on Alcator C-Mod, and displaying characteristics that contrast with the local RF edge absorption seen on other tokamaks. Measurements of ionization in the active divertor show dramatic changes due to LHRF power, implying that divertor region can be a key for the LHRF edge power deposition physics. These observations support the existence of a loss mechanism near the edge for LHRF at high density (n e > 1:0 Â 10 20 (m À3 )). Results will be shown addressing the distribution of power within the SOL, including the toroidal symmetry and radial distribution. These characteristics are important for deducing the cause of the reduced LHCD efficiency at high density and motivate the tailoring of wave propagation to minimize SOL interaction, for example, through the use of high-field-side launch. Published by AIP Publishing. [http://dx

Review of recent experimental and modeling advances in the understanding of lower hybrid current drive in ITER-relevant regimes

“…By means of fast time resolution edge diagnostic sets including innovative fast-thermocouples, an extensive set of Langmuir probes, and a Ly-alpha ionization camera, LH power modulation reveals that ~75% of the LH power is deposited very close to or outside of the last closed flux surface (LCFS) at n e~1 .1 × 10 20 m -3 . The loss of CD at high density correlates with the existence of a cold, dense and collisional divertor plasma [11]. In a reactor, LH waves will experience strong single pass absorption, and therefore, the direct interaction with cold dense divertor plasmas due to multiple passes discussed above can be minimized naturally.…”

Surface heat flux feedback controlled impurity seeding experiments with Alcator C-Mod’s high-Zvertical target plate divertor: performance, limitations and implications for fusion power reactors

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“…However, RF power systems are available that can deposit significant power at relevantly fast (~1 ms) time scales in the boundary plasma, e.g. LHRF [47].…”

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