Lower hybrid current drive experiments on Alcator C-Mod: Comparison with theory and simulation

Bonoli, P.; Ko, Jinseok; Parker, R.; Schmidt, Andréa; Wallace, G.; Wright, J. C.; Fiore, C. L.; Hubbard, A.; Irby, J.; Marmar, E.; Porkoláb, M.; Terry, D.R.; Wolfe, S.; Wukitch, S.J.; Wilson, J. R.; Scott, Steve; Valeo, E. J.; Phillips, C. K.; Harvey, R. W.

doi:10.1063/1.2904569

Cited by 57 publications

(35 citation statements)

References 23 publications

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“…Nonetheless, high density LHCD experiments on Alcator C-Mod have revealed a loss of current drive efficiency [3,4]. In low density plasmas, fully non-inductive plasma discharges [5] were demonstrated with engineering efficiency η ≡ n 20 IR/P ≈ 0.25 A/mW 2 , consistent with expected efficiency based on theory. However, when n e is raised above 1.0 × 10 20 m −3 , all experimental signatures of current drive effect disappear.…”

Section: Introductionsupporting

confidence: 49%

Characterization of onset of parametric decay instability of lower hybrid waves

Baek

Bonoli

Parker

et al. 2014

AIP Conference Proceedings

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Abstract. The goal of the lower hybrid current drive (LHCD) program on Alcator C-Mod is to develop and optimize ITERrelevant steady-state plasmas by controlling the current density profile. Using a 4x16 waveguide array, over 1 MW of LH power at 4.6 GHz has been successfully coupled to the plasmas. However, current drive efficiency precipitously drops as the line averaged density (n e ) increases above 10 20 m −3 . Previous numerical work shows that the observed loss of current drive efficiency in high density plasmas stems from the interactions of LH waves with edge/scrape-off layer (SOL) plasmas [Wallace et al., Physics of Plasmas 19, 062505 (2012)]. Recent observations of parametric decay instability (PDI) suggest that non-linear effects should be also taken into account to fully characterize the parasitic loss mechanisms [Baek et al., Plasma Phys. Control Fusion 55, 052001 (2013)]. In particular, magnetic configuration dependent ion cyclotron PDIs are observed using the probes near n e ≈ 1.2 × 10 20 m −3 . In upper single null plasmas, ion cyclotron PDI is excited near the low field side separatrix with no apparent indications of pump depletion. The observed ion cyclotron PDI becomes weaker in inner wall limited plasmas, which exhibit enhanced current drive effects. In lower single null plasmas, the dominant ion cyclotron PDI is excited near the high field side (HFS) separatrix. In this case, the onset of PDI is correlated with the decrease in pump power, indicating that pump wave power propagates to the HFS and is absorbed locally near the HFS separatrix. Comparing the observed spectra with the homogeneous growth rate calculation indicates that the observed ion cyclotron instability is excited near the plasma periphery. The incident pump power density is high enough to overcome the collisional homogeneous threshold. For C-Mod plasma parameters, the growth rate of ion sound quasi-modes is found to be typically smaller by an order of magnitude than that of ion cyclotron quasi-modes. When considering the convective threshold near the plasma edge, convective growth due to parallel coupling rather than perpendicular coupling is likely to be responsible for the observed strength of the sidebands.To demonstrate the improved LHCD efficiency in high density plasmas, an additional launcher has been designed. In conjunction with the existing launcher, this new launcher will allow access to an ITER-like high single pass absorption regime, replicating the J LH (r) expected in ITER. The predictions from the time domain discharge scenarios, in which the two launchers are used, will be also presented.

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Section: Introductionsupporting

confidence: 49%

Characterization of onset of parametric decay instability of lower hybrid waves

Baek

Bonoli

Parker

et al. 2014

AIP Conference Proceedings

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“…In higher temperature (T e [keV ] > 30/n 2 ) devices such as ITER there is high single pass absorption and scattering effects may play a minimal role. In the future we plan to compare the predicted hard x-ray signals based on the full wave-Fokker Planck calculation with the hard x-ray spectra measured in the Alcator C-Mod experiment and with spectra simulated based on a ray tracing -Fokker Planck treatment 31 . These types of comparisons should help to validate the ray tracing and full-wave approaches as well as providing insights into how important full-wave effects are for understanding the experiment.…”

Section: Discussionmentioning

confidence: 99%

An assessment of full wave effects on the propagation and absorption of lower hybrid waves

et al. 2009

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Lower hybrid (LH) waves (Ωci ω Ωce, where Ωi,e ≡ Zi,eeB/mi,ec) have the attractive property of damping strongly via electron Landau resonance on relatively fast tail electrons and consequently are well-suited to driving current. Established modeling techniques use WKB expansions with self-consistent non-Maxwellian distributions. Higher order WKB expansions have shown some effects on the parallel wavenumber evolution and consequently on the damping due to diffraction [G. Pereverzev, Nucl. Fusion 32, 1091(1991

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“…[346][347][348] ICRF waves from nearby antennas can have a similar effect, lowering the density and increasing reflections. , the LH system on C-Mod can drive 100% of the plasma current (0.5 MA) for multiple L/R times, 347,350,351 as seen in Fig. 58(a).…”

Section: -347mentioning

confidence: 99%

20 years of research on the Alcator C-Mod tokamak

et al. 2014

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The object of this review is to summarize the achievements of research on the Alcator C-Mod tokamak [Hutchinson et al., Phys. Plasmas 1, 1511 and Marmar, Fusion Sci. Technol. 51, 261 (2007)] and to place that research in the context of the quest for practical fusion energy. C-Mod is a compact, high-field tokamak, whose unique design and operating parameters have produced a wealth of new and important results since it began operation in 1993, contributing data that extends tests of critical physical models into new parameter ranges and into new regimes. Using only highpower radio frequency (RF) waves for heating and current drive with innovative launching structures, C-Mod operates routinely at reactor level power densities and achieves plasma pressures higher than any other toroidal confinement device. C-Mod spearheaded the development of the vertical-target divertor and has always operated with high-Z metal plasma facing componentsapproaches subsequently adopted for ITER. C-Mod has made ground-breaking discoveries in divertor physics and plasma-material interactions at reactor-like power and particle fluxes and elucidated a) Paper AR1 1, Bull. Am. Phys. Soc. 58, 21 (2013). b) Invited speaker.

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Lower hybrid current drive experiments on Alcator C-Mod: Comparison with theory and simulation

Cited by 57 publications

References 23 publications

Characterization of onset of parametric decay instability of lower hybrid waves

Characterization of onset of parametric decay instability of lower hybrid waves

An assessment of full wave effects on the propagation and absorption of lower hybrid waves

20 years of research on the Alcator C-Mod tokamak

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