Continuous, edge localized ion heating during non-solenoidal plasma startup and sustainment in a low aspect ratio tokamak

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

doi:10.1088/1741-4326/aa6f2b

Cited by 8 publications

(5 citation statements)

References 56 publications

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“…The relationship between continued (and even improved) current drive and the increase in magnetic fluctuation activity at high frequency inside the plasma edge is a subject of ongoing investigation, and may inform the current drive physics of LHI. Past work has shown that high-frequency activity during LFS LHI is well correlated with reconnection-driven ion heating [36]. While this is a new observation for LHI, note that a similar observation was made in plasmas sustained by CHI in HIT-II that exhibited flux amplification [37].…”

Section: Discussionsupporting

confidence: 60%

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: Discussionsupporting

confidence: 60%

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

et al. 2018

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“…Passive impurity Doppler spectroscopy indicated that the ion temperatures in LHI-initiated discharges was anomalously high, in that routinely exceeds the central [20]. Experiments confirmed that this anomalous ion heating is driven by strong magnetic reconnection activity present during LHI.…”

Section: Advancing Lhi Physics Understandingmentioning

confidence: 89%

“…The physical mechanisms leading to LHI current drive generate strong MHD activity. Nonlinear, 3D resistive MHD NIMROD simulations of LHI startup have provided insight into dynamical processes observed in experiment and simulation [19], and spurred experimental investigations into MHD activity [13] and reconnection [20] during LHI.…”

Section: Advancing Lhi Physics Understandingmentioning

confidence: 99%

“…Additional details are depicted schematically in figure 4. The initial injectors (figures 3(a) and 4(a) [7,15]) were modified to include a stainless steel local limiter and convex bias cathode structures (the injector 'frustum') (figures 3(b) and 4(b) [16]), and later additionally fitted with electrically floating, steppeddiameter conducting shield rings and a frustum shield (figures 3(c) and 4(d) [8,17,18]). The frustum was motivated by research on cathode spot formation on electrodes, which suggested the unwanted spots would move up the frustum towards the active arc plasma and away from the surrounding BN insulators.…”

Section: Helicity Injector Technologymentioning

confidence: 99%

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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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“…These characteristics improve MHD stability. Recent work has shown LHI also fortuitously provides magnetic-reconnection-driven ion heating that increases the total plasma pressure without a separate ion heating source [17]. Impurity ion temperature measurements indicate is typically 1-3 times during LHI.…”

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confidence: 99%

Noninductively Driven Tokamak Plasmas at Near-Unity Toroidal Beta

et al. 2017

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Access to and characterization of sustained, toroidally confined plasmas with a very high plasma-to-magnetic pressure ratio (β_{t}), low internal inductance, high elongation, and nonsolenoidal current drive is a central goal of present tokamak plasma research. Stable access to this desirable parameter space is demonstrated in plasmas with ultralow aspect ratio and high elongation. Local helicity injection provides nonsolenoidal sustainment, low internal inductance, and ion heating. Equilibrium analyses indicate β_{t} up to ∼100% with a minimum |B| well spanning up to ∼50% of the plasma volume.

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Continuous, edge localized ion heating during non-solenoidal plasma startup and sustainment in a low aspect ratio tokamak

Abstract: There was a publishing error introduced during the processing of figure 9. It should appear as shown below.

Cited by 8 publications

References 56 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

Noninductively Driven Tokamak Plasmas at Near-Unity Toroidal Beta

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