Experimental evidence of lower hybrid wave scattering in Alcator C-Mod due to scrape off layer density fluctuations

Martin, E. H.; Lau, C.; Wallace, G.; Shiraiwa, S.; Mumgaard, R.

doi:10.1088/1741-4326/ab16a2

Cited by 20 publications

(21 citation statements)

References 43 publications

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“…Consequently, N k is not evolving at the scale of the perturbation, being a constant of the ray motion. Recently, several experimental investigations of the spectral characteristics of the LH wave near the separatrix have been carried out [23,24]. They will give certainly valuable details on the LH wave characteristics before entering the core plasma and contribute to improve the understanding of the overall LH wave dynamics.…”

Section: Introductionmentioning

confidence: 99%

Lower Hybrid Current Drive in High Aspect Ratio Tokamaks

Peysson

Mazon²,

Artaud³

et al. 2020

J Fusion Energ

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A multi-machine study has been carried out to investigate the impact of a strongly bounded wave propagation domain on the Lower Hybrid current drive, a condition which occurs principally in high aspect ratio tokamaks. In this regime, the condition of kinetic resonance can be far above the upper boundary of the propagation domain, and may not be achieved by the usual toroidal upshift. Therefore no tail of fast electrons can be pulled out from the thermal bulk. Nevertheless, while tokamak plasmas are in principle almost transparent to the wave in this regime so-called "unbridgeable spectral gap", full current drive is well achieved for the two tokamaks considered in this study, TRIAM-1M [H. Zushi, et al., Nucl. Fusion 43 (2003) 1600] and WEST [C. Bourdelle, et al., Nucl. Fusion 55 (2015) 063017], both characterized by a very large aspect ratio R/a > 5.5. The case of the high aspect ratio tokamak HL-2A [Y. Liu et al. Nucl. Fusion 45 (2005) S239] for which the wave propagation domain has also an upper boundary, but close to the resonance condition, is considered by comparison. First principles modeling of the rf-driven current and the fast electron bremsstrahlung using the ALOHA/C3PO/LUKE/R5-X2 chain of codes shows unambiguously that the spectral gap must be already filled at the separatrix in order to reproduce quantitatively observations and some important parametric dependencies. This result is an important milestone in the physics understanding of the Lower Hybrid current drive, highlighting the existence of a powerful and likely universal alternative mechanism to bridge the spectral gap, that is not related to toroidal magnetic refraction. With an initially broad power spectrum, lobes with low parallel refractive indexes that carry most of the plasma current can be absorbed in almost single pass, restoring the full validity of the ray-tracing approximation for describing the propagation of the Lower Hybrid wave in cold plasmas.

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Section: Introductionmentioning

confidence: 99%

Lower Hybrid Current Drive in High Aspect Ratio Tokamaks

Peysson

Mazon²,

Artaud³

et al. 2020

J Fusion Energ

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“…Now, much work was spent on obtaining the bounds without arbitrary constants. In paper [3] there was obtained the upper bound for the "averaged cross-section" for ππ interaction…”

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confidence: 99%

Diffractive scattering: problems in theory and practice

Petrov

2017

J. Phys.: Conf. Ser.

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“…Therefore, the angle between these two vectors, χ ≡ ∠(v gr⊥ , ê∇ψ ), is usually zero. Scattering caused by edge density fluctuations can rotate v gr⊥ leading to a broadened wave-spectrum in χ-space, as evidenced by LH electric field vector measurements in C-Mod [48]. This rotation can modify the ray-path so that single-pass damping is strengthen or weakened, depending on the sign of χ [49].…”

Section: Impact Of Scattering On Lhcdmentioning

confidence: 99%

A hybrid full-wave Markov chain approach to calculating radio-frequency wave scattering from Scrape-off Layer filaments

Biswas,

Shiraiwa,

Baek

et al. 2021

Preprint

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The interaction of radio-frequency (RF) waves with edge turbulence modifies the incident wave-spectrum, and can significantly affect RF heating and current drive in tokamaks. Previous LH scattering models have either used the weak-turbulence approximation, or treated more realistic, filamentary turbulence in the ray-tracing limit. In this work, a new model is introduced which retains full-wave effects of RF scattering in filamentary turbulence. First, a Mie-scattering technique models the interaction of an incident wave with a single Gaussian filament. Next, an effective differential scattering-width is derived for a statistical ensemble of filaments. Lastly, a Markov chain solves for the transmitted wave-spectrum in slab geometry. This model is applied to LH launch for current drive. The resulting wave-spectrum is asymmetrically broadened in wave-number angle-space. This asymmetry is not accounted for in previous LH scattering models. The modified wave-spectrum is coupled to a ray-tracing/Fokker-Planck solver (GENRAY/CQL3D) to study its impact on current drive. The resulting current profile is greatly altered, and there is significant increase in on-axis current and decrease in off-axis peaks. This is attributed to a portion of the modified wave-spectrum that strongly damps on-axis during first-pass.

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Experimental evidence of lower hybrid wave scattering in Alcator C-Mod due to scrape off layer density fluctuations

Cited by 20 publications

References 43 publications

Lower Hybrid Current Drive in High Aspect Ratio Tokamaks

Lower Hybrid Current Drive in High Aspect Ratio Tokamaks

Diffractive scattering: problems in theory and practice

A hybrid full-wave Markov chain approach to calculating radio-frequency wave scattering from Scrape-off Layer filaments

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