| 1. |
- Kirk, A., et al.
(författare)
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Overview of recent physics results from MAST
- 2017
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 57:10
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Tidskriftsartikel (refereegranskat)abstract
- New results from MAST are presented that focus on validating models in order to extrapolate to future devices. Measurements during start-up experiments have shown how the bulk ion temperature rise scales with the square of the reconnecting field. During the current ramp-up, models are not able to correctly predict the current diffusion. Experiments have been performed looking at edge and core turbulence. At the edge, detailed studies have revealed how filament characteristics are responsible for determining the near and far scrape off layer density profiles. In the core the intrinsic rotation and electron scale turbulence have been measured. The role that the fast ion gradient has on redistributing fast ions through fishbone modes has led to a redesign of the neutral beam injector on MAST Upgrade. In H-mode the turbulence at the pedestal top has been shown to be consistent with being due to electron temperature gradient modes. A reconnection process appears to occur during edge localized modes (ELMs) and the number of filaments released determines the power profile at the divertor. Resonant magnetic perturbations can mitigate ELMs provided the edge peeling response is maximised and the core kink response minimised. The mitigation of intrinsic error fields with toroidal mode number n > 1 has been shown to be important for plasma performance.
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| 2. |
- Chapman, I. T., et al.
(författare)
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Overview of MAST results
- 2015
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 55:10
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Tidskriftsartikel (refereegranskat)abstract
- The Mega Ampere Spherical Tokamak (MAST) programme is strongly focused on addressing key physics issues in preparation for operation of ITER as well as providing solutions for DEMO design choices. In this regard, MAST has provided key results in understanding and optimizing H-mode confinement, operating with smaller edge localized modes (ELMs), predicting and handling plasma exhaust and tailoring auxiliary current drive. In all cases, the high-resolution diagnostic capability on MAST is complemented by sophisticated numerical modelling to facilitate a deeper understanding. Mitigation of ELMs with resonant magnetic perturbations (RMPs) with toroidal mode number n(RMP) = 2, 3, 4, 6 has been demonstrated: at high and low collisionality; for the first ELM following the transition to high confinement operation; during the current ramp-up; and with rotating n(RMP) = 3 RMPs. n(RMP) = 4, 6 fields cause less rotation braking whilst the power to access H-mode is less with n(RMP) = 4 than n(RMP) = 3, 6. Refuelling with gas or pellets gives plasmas with mitigated ELMs and reduced peak heat flux at the same time as achieving good confinement. A synergy exists between pellet fuelling and RMPs, since mitigated ELMs remove fewer particles. Inter-ELM instabilities observed with Doppler backscattering are consistent with gyrokinetic simulations of micro-tearing modes in the pedestal. Meanwhile, ELM precursors have been strikingly observed with beam emission spectroscopy (BES) measurements. A scan in beta at the L-H transition shows that pedestal height scales strongly with core pressure. Gyro-Bohm normalized turbulent ion heat flux (as estimated from the BES data) is observed to decrease with increasing tilt of the turbulent eddies. Fast ion redistribution by energetic particle modes depends on density, and access to a quiescent domain with 'classical' fast ion transport is found above a critical density. Highly efficient electron Bernstein wave current drive (1 A W-1) has been achieved in solenoid-free start-up. A new proton detector has characterized escaping fusion products. Langmuir probes and a high-speed camera suggest filaments play a role in particle transport in the private flux region whilst coherence imaging has measured scrape-off layer (SOL) flows. BOUT++ simulations show that fluxes due to filaments are strongly dependent on resistivity and magnetic geometry of the SOL, with higher radial fluxes at higher resistivity. Finally, MAST Upgrade is due to begin operation in 2016 to support ITER preparation and importantly to operate with a Super-X divertor to test extended leg concepts for particle and power exhaust.
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| 3. |
- Rafiq, Tariq, et al.
(författare)
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EFFECTS OF MICROTEARING MODES ON THE EVOLUTION OF ELECTRON TEMPERATURE PROFILES IN HIGH COLLISIONALITY NSTX DISCHARGES
- 2018
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Ingår i: IAEA Fusion Energy Conference. ; 27
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Konferensbidrag (refereegranskat)abstract
- A goal of this research project is to describe the evolution of the electron temperature profiles in high collisionality NSTX H-mode discharges. Gyrokinetic simulations indicate that microtearing modes (MTMs) are a source of significant electron thermal transport in these discharges. In order to understand the effect MTMs have on transport and, consequently, on the evolution of electron temperature in NSTX discharges, a reduced transport model for MTMs has been developed. The dependence of the MTM real frequency and growth rate on plasma parameters, appropriate for high collisionality NSTX discharges, is obtained employing the new MTM transport model. The dependencies on plasma parameters are compared and found to be consistent with MTM results obtained using the gyrokinetic GYRO code. The MTM real frequency, growth rate, magnetic fluctuations and resulting electron thermal transport are examined for high collisionality NSTX discharges in systematic scans over plasma parameters. In earlier studies it was found that the version of the Multi-Mode (MM) transport model, that did not include the effect of MTMs, provided a suitable description of the electron temperature profiles in high collisionality standard tokamak discharges. That version of the MM model included contributions to electron thermal transport from the ion temperature gradient, trapped electrons, kinetic ballooning, peeling ballooning, collisionless and collision dominated MHD modes, and electron temperature gradient modes. When the MM model, that includes transport associated with MTMs, is installed in the TRANSP code and is utilized in studying electron thermal transport in high collisionality NSTX discharges, it is found that agreement with the experimental electron temperature profile is significantly improved.
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| 4. |
- Rafiq, Tariq, 1971, et al.
(författare)
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Microtearing Modes in Low Collisionality NSTX Discharges
- 2019
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Recently, a new unified fluid/kinetic MTM model is developed and incorporated into the Multi-Mode Model (MMM). The MTM model has been found to reproduce many of the linear gyrokinetic results predicted in NSTX discharges, such as the variation of real frequency and growth rates with poloidal wavenumber, beta, and electron temperature (T e ) and density gradients. A particularly important result is that the model recovers the non-monotonic dependence of linear MTM growth rate with collision frequency.
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