Physics and performance of the I-mode regime over an expanded operating space on Alcator C-Mod

Hubbard, A.; Baek, S. G.; Brunner, D.; Creely, A. J.; Cziegler, I.; Edlund, E.; Hughes, J. W.; LaBombard, B.; Lin, Y.; Liu, Z.; Marmar, E.; Reinke, M.L.; Rice, J. E.; Sorbom, Brandon; Sung, C.; Terry, J. L.; Theiler, C.; Tolman, Elizabeth A.; Walk, J.; White, Anne; Whyte, D.G.; Wolfe, S.; Wukitch, S.; Xu, X. Q.

doi:10.1088/1741-4326/aa8570

Cited by 41 publications

(61 citation statements)

References 42 publications

(63 reference statements)

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“…Currently, I-mode in EAST could be achieved under such parameter space with plasma currents of I p ∼ 0.45 − 0.7 M A and toroidal magnetic field B φ = 2.26 − 2.49 T under upper-single null (USN) geometries with an unfavorable ion B × ∇B directed away from the active X-point [4]. In particular, in discharges with heating-powers above 1.8 MW and line-averaged electron densities above 2.5 * 10 19 m −3 , the features of the I-mode become quite distinct.…”

Section: Identification Of the I-modementioning

confidence: 99%

“…The I-mode was originally observed as an energy confinement improved mode in 1990s [2]; currently, it has been widely investigated in Alcator C-Mod [3,4,5,6,7,8,9,10], AUG [11,12,13,14,15,16], and DIII-D [17]. In unfavorable configurations [18], the L-mode to I-mode transition occurs when the heating power exceeds a certain threshold [19], and an edge energy transport barrier [7] is constructed accompanied with the weak coherent mode (WCM) [13,8,9,10,17] at the steepest temperature gradient region.…”

Section: Introductionmentioning

confidence: 99%

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I-mode investigation on the Experimental Advanced Superconducting Tokamak

et al. 2019

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By analyzing large quantities of discharges in the unfavorable ion B ×∇B drift direction, the I-mode operation has been confirmed in EAST tokamak. During the L-mode to I-mode transition, the energy confinement has a prominent improvement by the formation of a high-temperature edge pedestal, while the particle confinement remains almost identical to that in the L-mode. Similar with the I-mode observation on other devices, the E r profiles obtained by the eight-channel Doppler backscattering system (DBS8)[1] show a deeper edge E r well in the I-mode than that in the L-mode. And a weak coherent mode (WCM) with the frequency range of 40-150 kHz is observed at the edge plasma with the radial extend of about 2-3 cm. WCM could be observed in both density fluctuation and radial electric field fluctuation, and the bicoherence analyses showed significant couplings between WCM and high frequency turbulence, implying that the E r fluctuation and the caused flow shear from WCM should play an important role during I-mode. In addition, a low-frequency oscillation with a frequency range of 5-10 kHz is always accompanied with WCM, where GAM intensity is decreased or disappeared. Many evidences show that the a low-frequency oscillation may be a arXiv:1902.04750v3 [physics.plasm-ph]

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Section: Identification Of the I-modementioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

I-mode investigation on the Experimental Advanced Superconducting Tokamak

et al. 2019

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“…I-mode has been further established on DIII-D [4] and AUG [5] [6], while C-Mod has demonstrated a route to reactor relevant τ E , in which higher field [7] enlarges the window in input power between the L/I transition and the I/H. Recently, the I-mode regime was extended to 8 T on C-Mod [8].…”

Section: Introductionmentioning

confidence: 99%

Radiative heat exhaust in Alcator C-Mod I-mode plasmas

et al. 2019

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In order to more completely demonstrate the I-mode regime as a compelling fusion reactor operating scenario, the first dedicated attempts at I-mode radiative heat exhaust and detachment were carried out on Alcator C-Mod. Results conclusively show that within the parameter space explored, an I/L back-transition is triggered prior to meaningful reductions in parallel heat flux, q , target temperature, T e,tar and target pressure, p e,tar at the outer divertor. The exact mechanism for the I/L trigger remains uncertain, but a multi-diagnostic investigation suggests the pedestal regulation physics is impacted promptly by small amounts of N 2 seeded into the private flux region. The time delay between when N 2 contacts the plasma and the I/L transition is triggered varied from 30-120 ms, approximately 0.7-3×τ E , and the delay varied inversely with I-mode pedestal-top pressure, p e,95. Power and nitrogen influx scans indicate that the I/L transitions are not linked to excessive bulk-plasma impurity radiation. It is also shown that in the subsequent L-mode following nitrogen seeding, q and T e,tar can be reduced by factors of ≃10. The I/L transition and L-mode exhaust results using N 2 are compared to similar attempts using Ne where such q and T e,tar reductions in L-mode are limited to factors of 2-3. Implications for the I-mode regime are discussed, including needs for follow-up experiments on other facilities.

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“…-12 Stationary operation on different machines makes the I-mode an appealing regime for tokamaks. 13,14 The mechanisms which allow for the decoupling of density and temperature profiles in the I-mode have not yet been fully understood, although common characteristics in a) Electronic mail: karl.stimmel@ipp.mpg.de both C-Mod and ASDEX-Upgrade I-mode discharges include heightened temperature profiles simultaneous with low density profiles, and the weakly coherent mode (WCM), a pedestal mode that is observed to be electrostatic in ASDEX-Upgrade I-mode plasmas. 10 While experimental diagnostics have given much insight into the nature of the I-mode, and other gyrokinetic simulations have expanded theoretical understanding of turbulent transport in I-mode scenarios, a clear understanding of turbulent transport in the tokamak pedestal has not yet been developed for the I-mode.…”

Section: Introductionmentioning

confidence: 99%

Gyrokinetic investigation of the ASDEX Upgrade I-mode pedestal

et al. 2019

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Characterizing pedestal turbulence in the tokamak I-mode is a crucial step in understanding how particle and heat transport decouple during I-mode operation. This work models an ASDEX Upgrade I-mode discharge for the first time via linear and nonlinear gyrokinetic simulations with the GENE code. L-mode and I-mode regimes at two different pedestal locations are investigated. A microtearing mode which is not apparent in initial value linear L-mode simulations is found to dominate in I-mode simulations at both radial positions, and ion-scale instabilities are characterized for all four scenarios linearly. Computed nonlinear heat flux values approach experimental measurements with nominal input parameters in three of the four cases, and heat transport is found to be dominated by ion-scale electrostatic turbulence. Electrostatic potential oscillation frequencies, as well as potential-temperature and potential-density crossphases are compared linearly and nonlinearly, and agreement is found at wavenumber ranges corresponding with peaks in the simulated heat flux spectra at one radial position for L-mode and I-mode.

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Physics and performance of the I-mode regime over an expanded operating space on Alcator C-Mod

Cited by 41 publications

References 42 publications

I-mode investigation on the Experimental Advanced Superconducting Tokamak

I-mode investigation on the Experimental Advanced Superconducting Tokamak

Radiative heat exhaust in Alcator C-Mod I-mode plasmas

Gyrokinetic investigation of the ASDEX Upgrade I-mode pedestal

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