Improved neutral and plasma density control with increasing lithium wall coatings in the Lithium Tokamak Experiment-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si22.svg" display="inline" id="d1e160"><mml:mi>β</mml:mi></mml:math> (LTX-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si22.svg" display="inline" id="d1e165"><mml:mi>β</mml:mi></mml:math>)

Maan, A.; Boyle, D.P.; Majeski, R.; Banerjee, Santanu; Franscisquez, M.; Kaita, R.; Wilkie, G.J.; Capecchi, W.; Kubota, S.; Hansen, Chris; Soukhanovskii, V.

doi:10.1016/j.nme.2023.101408

Cited by 8 publications

(11 citation statements)

References 28 publications

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“…The single Langmuir probe (SLP) [38] is mounted offmidplane, away from possible interaction with runaway electrons whose distributions typically peaked at the midplane. The SLP is also mounted on a bellows drive, allowing the probe to be inserted to different radial positions in the scrapeoff layer (SOL) past the PFC shells.…”

Section: The Relevant Diagnostic Suite For This Experimentsmentioning

confidence: 99%

“…The SLP is also mounted on a bellows drive, allowing the probe to be inserted to different radial positions in the scrapeoff layer (SOL) past the PFC shells. Probe I-V traces are fit to a four-parameter expression to account for sheath expansion that results in non-saturation of ion current evident in the probe data [38].…”

Section: The Relevant Diagnostic Suite For This Experimentsmentioning

confidence: 99%

“…An array to sample hydrogen Lyman-α emission is installed in LTX-β for recycling and neutral density estimations [38]. The array comprises a commercial 20 element AXUV photodiode array and a 122 ± 2.5 nm VUV/UV narrow-band filter.…”

Section: The Relevant Diagnostic Suite For This Experimentsmentioning

confidence: 99%

“…There are two different fueling techniques available on the LTX-β: a HFS-GP system and a supersonic gas injector SGI system [38]. Unlike the HFS-GP system, which delivers a prolonged trail of cold neutrals following the actual puff, the SGI system is capable of sharper, sub-millisecond gas pulses, as evident from the sharper density rise following the SGI pulse and instantaneous density fall due to wall pumping by the Li coated shell.…”

Section: Fueling Techniquesmentioning

confidence: 99%

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Investigating the role of edge neutrals in exciting tearing mode activity and achieving flat temperature profiles in LTX-β

Banerjee,

Boyle,

Maan

et al. 2024

Nucl. Fusion

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We present observations, numerical simulations, and analysis from experiments in the Lithium Tokamak Experiment-Beta (LTX-β) in which the electron temperature profile (Te (r)) shifts from flat to peaked and a tearing mode is also destabilized when the average density (ne ave ) exceeds ~1019 m-3. Flat Te (r) is obtained routinely in LTX-β, with a lithium coated, low-recycling first wall, once the external fueling is stopped and density decays [D Boyle et al., Nucl. Fusion 63 (2023) 056020]. In the present experiment, flat Te (r) can be sustained while maintaining constant ne ave below a line averaged density threshold (ne ave th ) of ~ 1019 m-3. Above ne ave th , Te (r) shifts from flat to peaked and a tearing mode is destabilized. Due to low recycling, the achieved ne ave can be controlled precisely by external fueling and hence, a certain threshold of the edge neutral inventory from the external fueling is experimentally manifested through ne ave th . The goal of the present work is to investigate the role of edge neutrals in determining Te (r) and MHD stability in the unique low-recycling regime of LTX-β. Our hypothesis is that the peaking of Te (r) beyond ne ave th is due ultimately to the edge cooling by the cold neutrals beyond a critical fueling flux. At lower fueling flux, flat Te (r) results in broader pressure profile and lower resistivity, which in turn stabilizes the tearing mode. This hypothesis is supported by edge neutral density estimation by DEGAS 2 code. Mode analysis by singular value decomposition confirms the tearing mode structure to be m/n = 2/1 (m and n being the poloidal and toroidal mode numbers). Linear tearing stability analysis with M3D-C1 predicts that plasmas with ne ave > 1019 m-3 are highly susceptible to a n = 1 tearing mode. ORBIT simulations, however, confirmed that the tearing modes do not contribute to the loss of fast ions from neutral beam injection. This study shows for the first time that the neutral inventory at the edge could be one of the deciding factors for the achievability of the unique operation regime of flat Te (r) and the excitation of tearing activity that could be disruptive for the plasmas.

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Section: The Relevant Diagnostic Suite For This Experimentsmentioning

confidence: 99%

Section: The Relevant Diagnostic Suite For This Experimentsmentioning

confidence: 99%

Section: The Relevant Diagnostic Suite For This Experimentsmentioning

confidence: 99%

Section: Fueling Techniquesmentioning

confidence: 99%

See 2 more Smart Citations

Investigating the role of edge neutrals in exciting tearing mode activity and achieving flat temperature profiles in LTX-β

Banerjee,

Boyle,

Maan

et al. 2024

Nucl. Fusion

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“…The shell covers 80% of the plasma surface area and can be electrically heated to 350 • C to liquefy the lithium coatings, although the results presented here were obtained with solid coatings. For the results presented here, the shells have been repeatedly coated with lithium over a period of 3-4 years, so that without continuous fueling, rapid density pumpout is always observed, and high recycling discharges can no longer be obtained [24]. LTX-β is typically limited on the high field side (HFS) shell surfaces, at a radius of R = 14 cm at midplane.…”

Section: Configurationmentioning

confidence: 99%

Extending the low-recycling, flat temperature profile regime in the lithium tokamak experiment-β (LTX-β) with ohmic and neutral beam heating

et al. 2023

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Recent experiments in the Lithium Tokamak Experiment-β (LTX-β) have extended the duration, performance, operating conditions, and diagnosis of the flat-temperature profile, low-recycling regime first observed in LTX. As expected, Li retains hydrogen and suppresses edge neutral cooling, allowing increased edge electron temperature, roughly equal to the core Te. Flat temperature profiles had been obtained transiently in LTX, as the plasma density decayed following the cessation of edge gas puffing. Careful control over the fueling in LTX-β has now been shown to sustain the flat Te profile and hot edge unique to the low-recycling regime for multiple confinement times in high performance discharges with decaying or steady density. With low density, the flat Te profile is also seen to extend into the scrape-off layer. Neutral beam heating is observed in target discharges with relatively flat electron temperature profiles (Tedge ~ Tcore/2), though beam heating is stronger in discharges with higher fueling, higher density, and depressed edge Te. Beam heating produces additional peaking of the Te profile, without degradation of the energy confinement time. Neutral beam heating of target discharges with relatively flat electron temperature profiles similarly results in broad beam heated temperature profiles. Energy confinement in LTX-β generally compares favorably to ohmic and H-mode scalings, frequently exceeding them by factors of 2-4. New and improved diagnostics in LTX-β enable better characterization of this unique regime, including measurements of ion temperature and high field side Thomson scattering profiles. As an initial step toward characterizing turbulence with no Te gradient and roughly equal density and pressure gradient, spectra have been measured in peaked Te discharges using far-forward scattering and fluctuation reflectometry.

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Detailed studies of the processes in low energy H irradiation of Li and Li-compound surfaces

Krstic,

Ostrowski,

Dwivedi

et al. 2023

Journal of Applied Physics

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We have used a combination of pico-to-nano temporal/spatial scale computational physics and chemistry modeling of plasma–material interfaces in the tokamak fusion plasma edges to unravel the evolving characteristics, not readily accessible by empirical means, of lithium-, oxygen-, and hydrogen-containing materials of plasma-facing components under irradiation by hydrogen and its isotopes. In the present calculation, amorphous lithium compound surfaces containing oxygen, Li2O, and LiOH were irradiated by 1–100 eV particles at incident angles on the surface ranging from perpendicular to almost grazing angles. Consequential surface processes, reflection, retention, and sputtering were studied at “the same footing” and compared to earlier results from amorphous Li and LiH surfaces. The critical role of charging dynamics of lithium, oxygen, and hydrogen atoms in the surface chemistry during hydrogen-fuel irradiation was found to drive the kinetics and dynamics of these surfaces in unexpected ways that ultimately could have profound effects on fusion plasma confinement behavior and surface erosion.

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Improved neutral and plasma density control with increasing lithium wall coatings in the Lithium Tokamak Experiment-β (LTX-β)

Cited by 8 publications

References 28 publications

Investigating the role of edge neutrals in exciting tearing mode activity and achieving flat temperature profiles in LTX-β

Investigating the role of edge neutrals in exciting tearing mode activity and achieving flat temperature profiles in LTX-β

Extending the low-recycling, flat temperature profile regime in the lithium tokamak experiment-β (LTX-β) with ohmic and neutral beam heating

Detailed studies of the processes in low energy H irradiation of Li and Li-compound surfaces

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