Coupling of fast waves in the ion cyclotron range of frequencies to H-mode plasmas in DIII-D

Abstract

Measurements of low power (≃ 1 mW) antenna loading are used to study the coupling of a compact loop antenna structure to plasmas in the divertor configuration in DIII-D heated by neutral beam injection (NBI) or electron cyclotron heating (ECH). When a transition to the H-mode regime occurs during NBI, the antenna loading resistance drops by approximately a factor of two. This coupling decrease is due to a steepening of the edge density profile near the separatrix, accompanied by a reduction in edge density in the scrape-off layer. During edge localized modes, the opposite effects occur, and the antenna coupling increases transiently. The loading measurements are compared with theoretical calculations which take into account the measured density profiles as well as the conducting side-walls of the recessed antenna housing. Absolute agreement between the theoretical and the experimental results is obtained, including the correct dependence on the density, antenna position, RF frequency and antenna geometry. The theoretical interpretation of the results is discussed, together with the technological implications for future high power experiments.

High harmonic ion cyclotron heating in DIII-D: Beam ion absorption and sawtooth stabilization

“…The first 'monster' crash at 1.97 s triggers an H mode transition (evidenced by the characteristic reduction in D α light). Since the antenna is operated at a fixed peak voltage, the reduced antenna loading [17,36] causes an immediate reduction in coupled RF power. Meanwhile, the plasma density begins to increase in response to the improved particle confinement.…”

Observation of parametric decay correlated with edge heating using an ion Bernstein wave antenna on DIII-D

“…Many of the diagnostics employed for the present ICRF studies have been described recently [19]. The three tipped movable Langmuir probe described in Ref.…”

Recent Results on Coupling Fast Waves to High Performance Plasmas on DIII-D

“…For a given antenna and feeding system, the factor V max /Z 0 is fixed, so the only way to increase P c without improving the antennas or the conditioning procedures is to increase the antenna loading R L . This key parameter is determined by the plasma parameters in the antenna near-field region, most importantly by the edge density profile [4,5]. Unfortunately for potential R L control techniques that would modify the edge density profile to increase R L , global confinement is also strongly affected by the edge density profile.…”