Scrape-off Layer Current Model for Filament Structure Observed During Edge Localized Modes (ELMs) in the DIII-D Tokamak PDF Download
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Languages : en
Pages : 91
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Book Description
The plasma in tokamaks often exhibits a relaxation oscillation called the edge localized mode (ELM), which is generally attributed to MHD instability driven by strong gradients at the plasma boundary. It is shown here that field-aligned currents flowing just outside the boundary may also play a role in the ELM process. The poloidal perturbation magnetic field during ELMs in the DIII-D tokamak calculated from measured currents can reproduce prominent observed features, including a narrow magnetic structure at the outboard midplane similar to filaments observed earlier in DIII-D and NSTX.
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Category :
Languages : en
Pages : 91
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Book Description
The plasma in tokamaks often exhibits a relaxation oscillation called the edge localized mode (ELM), which is generally attributed to MHD instability driven by strong gradients at the plasma boundary. It is shown here that field-aligned currents flowing just outside the boundary may also play a role in the ELM process. The poloidal perturbation magnetic field during ELMs in the DIII-D tokamak calculated from measured currents can reproduce prominent observed features, including a narrow magnetic structure at the outboard midplane similar to filaments observed earlier in DIII-D and NSTX.
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Category :
Languages : en
Pages : 44
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High temporal and spatial resolution measurements in the boundary of the DIII-D tokamak show that edge localized modes (ELMs) are produced in the low field side, are poloidally localized and are composed of fast bursts (≈20 to 40 [mu]s long) of hot, dense plasma on a background of less dense, colder plasma (≈5 x 1018 m{sup {+-}3}, 50 eV) possibly created by the bursts themselves. The ELMs travel radially in the scrapeoff layer (SOL), starting at the separatrix at ≈450 m/s, and slow down to ≈150 m/s near the wall, convecting particles and energy to the SOL and walls. The temperature and density in the ELM plasma initially correspond to those at the top of the density pedestal but quickly decay with radius in the SOL. The temperature decay length (≈1.2 to 1.5 cm) is much shorter than the density decay length (≈3 to 8 cm), and the latter decreases with increasing pedestal (and SOL) density. The local particle and energy flux at the midplane wall during the bursts are 10% to 50% (≈1 to 2 x 1021 m{sup {+-}2} s{sup {+-}1}) and 1% to 2 % (≈20 to 30 kW/m2) respectively of the LCFS average fluxes, indicating that particles are transported radially much more efficiently than heat. Evidence is presented suggesting toroidal rotation of the ELM plasma in the SOL. The ELM plasma density and temperature increase linearly with discharge/pedestal density up to a Greenwald fraction of ≈0.6, and then decrease resulting in more benign (grassier) ELMs.
Author: Hironori Takahashi
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Category : Tokamaks
Languages : en
Pages : 8
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Category : Nuclear fusion
Languages : en
Pages : 324
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Author: Francois Orain
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Category :
Languages : en
Pages : 0
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The growth of plasma instabilities called Edge Localized Modes (ELMs) in tokamaks results in the quasi-periodic relaxations of the edge plasma, potentially harmful for the divertor in ITER. One of the promising ELM control methods planned in ITER is the application of external resonant magnetic perturbations (RMPs), already efficient for ELM mitigation/suppression in current tokamak experiments. However a better understanding of the interaction between ELMs, RMPs and plasma flows is needed to make reliable predictions for ITER. In this perspective, non-linear modeling of ELMs and RMPs is done with the reduced MHD code JOREK, in realisitic geometry including the X-point and the Scrape-Off Layer. The two-fluid diamagnetic drifts, the neoclassical friction, a source of parallel rotation and RMPs have been implemented to simulate the RMP penetration consistently with the plasma response. As a first step, the plasma response to RMPs (without ELMs) is studied for JET, MAST and ITER realistic plasma parameters and geometry. Then the cyclic dynamics of the ELMs (without RMPs) is modeled for the first time in realistic geometry. After an ELM crash, the diamagnetic rotation is found to be instrumental to stabilize the plasma and to model the cyclic reconstruction and collapse of the plasma pressure profile. Last the ELM mitigation and suppression by RMPs is observed for the first time in modeling. The non-linear coupling of the RMPs with unstable modes is found to induce a continuous MHD activity in place of a large ELM crash, resulting in the mitigation of the ELMs. Over a threshold in magnetic perturbation, the full ELM suppression is also observed.
Author: Quang Thanh Nguyen
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Category :
Languages : en
Pages : 238
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Author: Per O. F. Helander
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Category :
Languages : en
Pages : 30
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Category :
Languages : en
Pages : 6
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Simulations using the fully kinetic code XGCa were undertaken to explore the impact of kinetic effects on scrape-off layer (SOL) physics in DIII-D H-mode plasmas. XGCa is a total-f, gyrokinetic code which self-consistently calculates the axisymmetric electrostatic potential and plasma dynamics, and includes modules for Monte Carlo neutral transport. Fluid simulations are normally used to simulate the SOL, due to its high collisionality. However, depending on plasma conditions, a number of discrepancies have been observed between experiment and leading SOL fluid codes (e.g. SOLPS), including underestimating outer target temperatures, radial electric field in the SOL, parallel ion SOL flows at the low field side, and impurity radiation. Many of these discrepancies may be linked to the fluid treatment, and might be resolved by including kinetic effects in SOL simulations. The XGCa simulation of the DIII-D tokamak in a nominally sheath-limited regime show many noteworthy features in the SOL. The density and ion temperature are higher at the low-field side, indicative of ion orbit loss. The SOL ion Mach flows are at experimentally relevant levels (Mi ~0.5), with similar shapes and poloidal variation as observed in various tokamaks. Surprisingly, the ion Mach flows close to the sheath edge remain subsonic, in contrast to the typical fluid Bohm criterion requiring ion flows to be above sonic at the sheath edge. Related to this are the presence of elevated sheath potentials, e[Delta][Phi]/Te ~ 3-4, over most of the SOL, with regions in the near-SOL close to the separatrix having e[Delta][Phi]/Te> 4. Finally, these two results at the sheath edge are a consequence of non-Maxwellian features in the ions and electrons there.
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Languages : en
Pages : 12
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The behavior of the scrape-off layer (SOL) region in tokamaks is believed to play an important role determining the overall device performance. In addition, control of the exhaust power has become one of the most important issues in the design of future devices such as ITER and TPX. This paper presents the results of application of 2-D fluid models to the DII-D tokamak, and research into the importance of processes which are inadequately treated in the fluid models. Comparison of measured and simulated profiles of SOL plasma parameters suggest the physics model contained in the UEDGE code is sufficient to simulate plasmas which are attached to the divertor plates. Experimental evidence suggests the presence of enhanced plasma recombination and momentum removal leading to the existence of detached plasma states. UEDGE simulation of these plasmas obtains a bifurcation to a low temperature plasma at the divertor, but the plasma remains attached. Understanding the physics of this detachment is important for the design of future devices. Analytic studies of the behavior of SOL plasmas enhance our understanding beyond that achieved with fluid modeling. Analysis of the effect of drifts on sheath structure suggest these drifts may play a role in the detachment process. Analysis of the turbulent-transport equations indicate a bifurcation which is qualitatively similar to the experimentally different behavior of the L- and H-mode SOL. Electrostatic simulations of conducting wall modes suggest possible control of the SOL width by biasing.
Author: William Agnelo Gracias
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Category :
Languages : en
Pages : 0
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Filamentary transport has been experimentally observed in a multitude of magnetically confined fusion devices, especially of the tokamak type. Filaments are carriers of large quantities of particles and heat and as such, their presence in the SOL has implications for the target surface design in future fusion reactors. To better understand their nature, this doctoral thesis studies filaments through computer simulations as isolated structures and spontaneously forming structures, using a 3D fluid model called the TOKAM3X. Parametric studies using the model for studying the effect of the plasma's parallel resistivity and magnetic shear, and also the filament's parallel extension and size/density amplitude are performed and analysed. The studies reveal the strong impact of the parallel resistivity on filament velocities and hence their lifetimes in the SOL. The doctoral work also looked at the impact of strong local magnetic shear and the separatrix on the motion and generation of filaments. The results from the simulations performed reveal that strong shear mechanisms may be key to the destruction and formation of filaments. Further, a comparison of spontaneously forming and seeded filaments shows that seeded filaments do not behave completely the same way as spontaneously forming ones. But their mean velocity characteristics are still retained to a good degree.
