| 51. |
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| 52. |
- Lloyd, B., et al.
(författare)
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Overview of physics results from MAST
- 2011
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 51:9, s. 094013 (paper no.)-
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Tidskriftsartikel (refereegranskat)abstract
- Major developments on the Mega Amp Spherical Tokamak (MAST) have enabled important advances in support of ITER and the physics basis of a spherical tokamak (ST) based component test facility (CTF), as well as providing new insight into underlying tokamak physics. For example, L-H transition studies benefit from high spatial and temporal resolution measurements of pedestal profile evolution (temperature, density and radial electric field) and in support of pedestal stability studies the edge current density profile has been inferred from motional Stark effect measurements. The influence of the q-profile and E x B flow shear on transport has been studied in MAST and equilibrium flow shear has been included in gyro-kinetic codes, improving comparisons with the experimental data. H-modes exhibit a weaker q and stronger collisionality dependence of heat diffusivity than implied by IPB98(gamma, 2) scaling, which may have important implications for the design of an ST-based CTF. ELM mitigation, an important issue for ITER, has been demonstrated by applying resonant magnetic perturbations (RMPs) using both internal and external coils, but full stabilization of type-I ELMs has not been observed. Modelling shows the importance of including the plasma response to the RMP fields. MAST plasmas with q > 1 and weak central magnetic shear regularly exhibit a long-lived saturated ideal internal mode. Measured plasma braking in the presence of this mode compares well with neo-classical toroidal viscosity theory. In support of basic physics understanding, high resolution Thomson scattering measurements are providing new insight into sawtooth crash dynamics and neo-classical tearing mode critical island widths. Retarding field analyser measurements show elevated ion temperatures in the scrape-off layer of L-mode plasmas and, in the presence of type-I ELMs, ions with energy greater than 500 eV are detected 20 cm outside the separatrix. Disruption mitigation by massive gas injection has reduced divertor heat loads by up to 70%.
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| 53. |
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| 54. |
- Nyqvist, Robert, 1983, et al.
(författare)
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On wave-particle interaction in axisymmetric toroidal systems
- 2011
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Ingår i: Physica Scripta. - : IOP Publishing. - 1402-4896 .- 0031-8949. ; 84:1, s. 015503-
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Tidskriftsartikel (refereegranskat)abstract
- A general formalism is developed to describe the interaction of charged particles with electromagnetic waves in terms of coupled finite difference mapping equations that incorporate tokamak topology. The approach is based on considering non-adiabatic changes in the constants of particle motion and it covers a range of wave-particle resonance frequencies, from the precessional to cyclotron frequencies of both passing and trapped ions. The concept of overlapping resonances is used to estimate the threshold for a single plane wave to cause stochastic particle motion. In the stochastic regime, the process is Markovian, and particle diffusion in three-dimensional phase space takes place. Estimations of diffusion coefficients are carried out in the two cases of waves interacting with passing and trapped ions by means of the cyclotron and bounce resonances, respectively, and previously known results are recovered in the proper limits.
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| 55. |
- Nyqvist, Robert, 1983, et al.
(författare)
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Spreading of collimated particle beams within a generalized Fokker-Planck diffusion equation
- 2009
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Ingår i: Nuclear Science and Engineering. - 0029-5639 .- 1943-748X. ; 163:1, s. 85-90
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Tidskriftsartikel (refereegranskat)abstract
- Recently, an expansion of the Boltzmann scattering operator describing the angular spreading of particle beams was given that included the effects of large angle scattering processes, thus generalizing the classical Fokker-Planck equation, valid in the limit of small angle scattering. The present work aims at making an analytical comparison between predictions based on the classical Fokker-Planck equation and those based on a generalized one, which includes a first-order correction term in the expansion of the Boltzmann scattering operator. The analysis is carried out for thin slabs where backscattering effects can be neglected and makes use of a moment approach, which leads to an infinite system of recursively coupled ordinary differential equations. The system is truncated in a consistent manner, and the effects of large angle scattering on the evolution of the moments are determined in explicit analytical form. An approximate similarity solution of the generalized Fokker-Planck equation is also found, and the results of both approaches provide a clear picture of the increased diffusive beam spreading due to large angle scattering. A comparison with previously published Monte Carlo simulation results shows good agreement.
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| 56. |
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| 57. |
- Rasch, Joel, 1982, et al.
(författare)
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Gas breakdown in inhomogenous microwave electric fields
- 2009
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Ingår i: Journal of Physics D: Applied Physics. - 1361-6463 .- 0022-3727. ; 42:20, s. 205203-
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Tidskriftsartikel (refereegranskat)abstract
- The main physical properties of gas breakdown in inhomogeneous high frequency microwave electric fields are investigated using both analytical and numerical analysis. In particular, the interplay between diffusion and attachment in redistributing electrons from high field regions to low field regions and the concomitant effect on the breakdown threshold is studied using three different examples of ionization profiles. The results give a clear physical picture of the dependence of breakdown electric field on pressure showing a high pressure branch determined by conditions in the high field region, a low pressure branch determined by the properties of the low field region only, and a more or less smooth transition region for intermediate pressures. Simple estimates for the extension of the transition region are given in good agreement with the numerical simulations, and a comparison of the theoretical breakdown threshold with previous experimental results also shows good agreement.
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| 58. |
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| 59. |
- Rasch, Joel, 1982, et al.
(författare)
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Microwave multipactor breakdown between two cylinders
- 2010
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Ingår i: IEEE Transactions on Plasma Science. - 0093-3813 .- 1939-9375. ; 38:8, s. 1997 - 2005
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Tidskriftsartikel (refereegranskat)abstract
- An analysis has been made of the microwave breakdownthreshold for multipactor in an open structure comprisingtwo parallel cylinders, approximating, e.g., parts of a helix antenna. The electron motion in the corresponding electromagnetic field is analyzed by separating the motion into a slowly varying drift velocity (driven by the ponderomotive force due to the electric field inhomogeneity) and a rapidly oscillating part (driven bythe oscillating electric field). Furthermore, the curvature of the cylindrical surfaces of emission is shown to give rise to a new effect that implies a loss of electrons. This leads to a more stringent multipactor breakdown condition for the two-wire structure thanfor the classical situation corresponding to the case of two plane parallel infinite plates. The importance of this effect is determined by the ratio of the cylinder radii and the distance between the cylinders, and it is shown that when this ratio is small, multipactor can only occur for surfaces having very large secondary emissioncoefficients. A detailed analysis is also made to determine the lowest voltage between the cylinders at which multipactor becomes possible.
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| 60. |
- Rasch, Joel, 1982, et al.
(författare)
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On the microwave breakdown stability of a spherical hot spot in air
- 2010
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Ingår i: Journal of Physics D: Applied Physics. - : IOP Publishing. - 1361-6463 .- 0022-3727. ; 43:32
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Tidskriftsartikel (refereegranskat)abstract
- An analysis is made of the basic physical conditions under which a small local microwave-induced breakdown region in a gas may develop to extended 'global' breakdown. The analysis describes the different nonlinear stages of the microwave breakdown process. In the first stage, the increasing breakdown plasma density suppresses the electric field in the breakdown region to reach a quasi-stationary state with constant electron density. The subsequent heating of the gas due to absorption of microwave power in the breakdown plasma is then analysed and the corresponding steady state for the thermal evolution is found including the temperature dependence of the breakdown electric field. The stability properties of the stationary state are examined and it is found that there exists a critical (unstable) radius of the initial breakdown plasma region such that initial regions smaller than this critical dimension will shrink to ultimately vanish whereas plasma regions larger than the critical dimension will grow indefinitely and transform the local breakdown region into full scale 'global' breakdown. The practical implication of this model is to give an order of magnitude estimate for the critical size of hot spots, regions of enhanced field and intensified heating in rf systems.
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