Noninductively Driven Tokamak Plasmas at Near-Unity Toroidal Beta

Schlossberg, D. J.; Bodner, G.M.; Fonck, R. J.; Burke, M.G.; Fonck, R. J.; Perry, J.M.; Reusch, J.A.

doi:10.1103/physrevlett.119.035001

Cited by 9 publications

(7 citation statements)

References 37 publications

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“…An alternate possibility is suggested by equilibrium reconstructions of LHI plasmas, which suggest a possible relationship between the formation of a minimum |B| well and the trans ition to the reduced n = 1 regime. LHI discharges at low B T often contain large regions of absolute minimum |B| resulting from the hollow current profiles of LHI plasmas and the ultra-low aspect ratio of Pegasus [28]. A study of model equilibria showed that the formation of a |B| well in Pegasus is strongly influenced by I p /I TF , with large values being favorable for well formation.…”

Section: Discussionmentioning

confidence: 99%

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Initiation and sustainment of tokamak plasmas with local helicity injection as the majority current drive

et al. 2018

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Local helicity injection (LHI) is a non-solenoidal current drive capable of achieving high- tokamak startup with non-invasive current injectors in the plasma scrape-off layer. The choice of injector location within the edge region is flexible but has a profound influence on the nature of the current drive in LHI discharges. New experiments on the Pegasus ST with injection in the high-field-side, lower divertor region produce plasmas dominated by helicity injection current drive, static plasma geometry, and negligible inductive drive. Peak plasma current up to 200 kA, and a sustained plasma current of 100 kA for up to 18 ms, is demonstrated. Maximum achievable plasma current is found to scale approximately linearly with the effective loop voltage from LHI. A newly-observed MHD regime for LHI-driven plasmas in which large-amplitude fluctuations at 20–50 kHz are abruptly reduced on the outboard side results in improved current drive. A simultaneous increase in high frequency fluctuations (>400 kHz) inside the plasma edge suggests short wavelength turbulence as an important current drive mechanism during LHI.

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Section: Discussionmentioning

confidence: 99%

“…In contrast, Ohmically-driven plasmas in Pegasus are limited to I p /I TF 1.3 [27]. LHI plasmas at very low B T ( I TF 56 kA, I p /I TF ∼ 2) have been shown to access β T ∼ 100% [28].…”

Section: The Hfs Lhi Operating Spacementioning

confidence: 99%

Initiation and sustainment of tokamak plasmas with local helicity injection as the majority current drive

et al. 2018

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“…Non-inductively driven plasmas at low access world-record ~ 100%, high ~ 6.5 and feature an absolute minimum-| | magnetic configuration (Fig. 13) that may positively affect turbulence, transport, and fast particle confinement [23]. Discharges at highest disrupt at the ideal no-wall MHD limit.…”

Section: Advancing Lhi Physics Understandingmentioning

confidence: 99%

“…Access to the reduced MHD state also facilitates access to the favorable low-A ST regime with non-solenoidal sustainment, high κ, low i , and high β t [39]. Non-inductively driven plasmas at low B T access world-record β t ~ 100%, high β N ~ 6.5 and feature an absolute minimum-|B| magnetic configuration (figure 20) that may positively affect turbulence, transport, and fast particle confinement [10,40]. Discharges at highest β t disrupt at the ideal no-wall MHD limit.…”

Section: Advancing Lhi Physics Understandingmentioning

confidence: 99%

Advancing local helicity injection for non-solenoidal tokamak startup

et al. 2019

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Robust non-solenoidal startup methods may simplify the cost and complexity of next-step burning plasma devices, and especially STs. Experiments on the ~ 1 Pegasus ST are advancing the physics and technology basis of Local Helicity Injection (LHI). LHI creates high tokamak plasmas by injecting helicity with small current sources in the plasma edge. Its hardware can be withdrawn before a fusion plasma enters a nuclear burn phase. Flexible injector placement offers tradeoffs between physics and engineering goals. They are tested with LHI systems on the low-field-side (LFS) and the high-field-side (HFS) of Pegasus, producing plasmas predominantly driven by non-solenoidal induction and DC helicity drive (~), respectively. Record LHI plasmas with = 0.225 MA, > 100 eV, and ~ 10 19 m-3 are attained. A predictive 0D power-balance model describes experimental () and partitions the active current drive sources. Analysis of experimental discharges with the model confirms the dominance of non-solenoidal induction in LFS LHI and DC helicity drive in HFS LHI. Studies of HFS scenarios find favorable, positive scalings of with and with. High-frequency MHD activity is found to be present during LHI current drive, in addition to = 1 modes previously found in NIMROD simulation and experiment. A new regime of reduced MHD activity was discovered where = 1 activity is suppressed, LHI CD efficiency improves, and long-pulse plasmas are sustained with ~ 0. LHI facilitates access to favorable ST regimes with non-solenoidal sustainment, high , low ℓ , and high. Low LHI operation has led to record = 100%, high , and a minimum-| | well that may positively affect turbulence, transport, and fast particle confinement. Major facility upgrades are planned to extend LHI to higher , , and pulse length.

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“…In the last three decades of ST research, various CS-free plasma startup scenarios were proposed such as merging compression (MC) [4], local helicity injection [5], electron Bernstein wave plasma startup [3] and so on. The former two explore high beta records [6][7][8] and H-mode performance [9] in mega ampere (MA) scale experiments [1,4,10], while the latter explores long pulse steady scenario which has longer duration time than 100 ms [1,3]. In addition to the initial current formation method, merging plasma startup scheme is known that it also enables the advantage of impulsive MW scale plasma heating during plasma formation through magnetic reconnection process [4,11].…”

Section: Introductionmentioning

confidence: 99%

Global ion heating/transport during merging spherical tokamak formation

et al. 2021

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Here we report global ion heating/transport characteristics of magnetic reconnection during merging spherical tokamak formation experiment on TS-6 (TS-3U). Using the 96CH/320CH ultra high resolution ion Doppler tomography diagnostics, the full-2D imaging measurement clearly revealed that magnetic reconnection initially forms localized hot spots in the downstream region of outflow jet with inboard/outboard asymmetry (more deposition in the high field side) but the continuous accumulation of the heating coupled with transport process expands the high temperature region globally and forms characteristic poloidally ring-like structure aligned with field lines. The dynamic ion heating/transport process is also affected by the polarity of toroidal field and poloidally tilted/rotating global structure has experimentally been found both during and after merging. The characteristic poloidal asymmetry gets flipped when toroidal field direction is reversed and it was found that higher temperature appears in the positive potential side, which is opposite to the conventional understanding/prediction of guide field reconnection. Through the parallel acceleration process coupled with global heat transport, poloidally asymmetric non-classical feature has experimentally been found for the first time.

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Noninductively Driven Tokamak Plasmas at Near-Unity Toroidal Beta

Cited by 9 publications

References 37 publications

Initiation and sustainment of tokamak plasmas with local helicity injection as the majority current drive

Initiation and sustainment of tokamak plasmas with local helicity injection as the majority current drive

Advancing local helicity injection for non-solenoidal tokamak startup

Global ion heating/transport during merging spherical tokamak formation

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