Overview of physics results from MAST

B. Lloyd, R.J. Akers, F. Alladio, S. Allan, L.C. Appel, M. Barnes, N.C. Barratt, N. Ben Ayed, B.N. Breizman, M. Cecconello, C.D. Challis, I.T. Chapman, D. Ciric, G. Colyer, J.W. Connor, N.J. Conway, M. Cox, S.C. Cowley, G. Cunningham, A. Darke, M. De Bock, E. Delchambre, G. De Temmerman, R.O. Dendy, P. Denner, M.D. Driscoll, B. Dudson, D. Dunai, M. Dunstan, S. Elmore, A.R. Field, G. Fishpool, S. Freethy, L. Garzotti, K.J. Gibson, M.P. Gryaznevich, W. Guttenfelder, J Harrison, R.J. Hastie, N.C. Hawkes, T.C. Hender, B. Hnat, D.F. Howell, M.-D. Hua, A. Hubbard, G. Huysmans, D. Keeling, Y.C. Kim, A. Kirk, Y. Liang, M.K. Lilley, M. Lisak, S. Lisgo, Y.Q. Liu, G.P. Maddison, R. Maingi, S.J. Manhood, R. Martin, G.J. McArdle, J. McCone, H. Meyer, C. Michael, S. Mordijck, T. Morgan, A.W. Morris, D.G. Muir, E. Nardon, G. Naylor, M.R. O’Brien, T. O’Gorman, J. Páleník, A. Patel, S.D. Pinches, M.N. Price, C.M. Roach, V. Rozhansky, S. Saarelma, S.A. Sabbagh, A. Saveliev, R. Scannell, S.E. Sharapov, V. Shevchenko, S. Shibaev, D. Stork, J. Storrs, W. Suttrop, A. Sykes, P. Tamain, D. Taylor, D. Temple, N. Thomas-Davies, A. Thornton, M.R. Turnyanskiy, M. Valovic, R.G.L. Vann, G. Voss, M.J. Walsh, S.E.V. Warder, H.R.Wilson, M. Windridge, M Wisse , S. Zoletnik and the MAST and NBI teams.


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 × 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(y,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%. © 2011 IAEA, Vienna



Reprocessing Free Nuclear Fuel Production

Fusion Fission Hybrids in Action

M. T. Kotschenreuther, P. Valanju, and S. Mahajan


Fusion fission hybrid, driven by a copious source of fusion neutrons can open qualitatively “new” cycles for transmuting nuclear fertile material into fissile fuel. The principle and detailed workings of a totally reprocessing-free (ReFree) Th232-U233 conversion cycle are presented. Virgin fertile fuel rods are exposed to neutrons in the hybrid, and burned in a traditional water-cooled reactor, without ever violating the integrity of the fuel rods. Throughout this cycle (during breeding in the hybrid, transport, as well as burning of the fissile fuel in a water reactor) the fissile fuel remains a part of a bulky, countable, ThO2 matrix in cladding, protected by the radiation field of all fission products. This highly proliferation-resistant mode of fuel production, as distinct from a reprocessing dominated path via fast breeder reactors (FBR), can bring great acceptability to the enterprise of nuclear fuel production, and insure that scarcity of naturally available U235 fuel does not throttle expansion of nuclear energy. It also provides a reprocessing free path to energy security for many countries. Ideas and innovations responsible for the creation of a high intensity neutron source are also presented. 


Relativistic generation of vortex and magnetic field

S. M. Mahajan and Z. Yoshida


The implications of the recently demonstrated relativistic mechanism for generating generalized vorticity in purely ideal dynamics [Mahajan and Yoshida, Phys. Rev. Lett. 105, 095005 (2010)] are worked out. The said mechanism has its origin in the space-time distortion caused by the demands of special relativity; these distortions break the topological constraint (conservation of generalized helicity) forbidding the emergence of magnetic field (a generalized vorticity) in an ideal nonrelativistic dynamics. After delineating the steps in the “evolution” of vortex dynamics, as the physical system goes from a nonrelativistic to a relativistically fast and hot plasma, a simple theory is developed to disentangle the two distinct components comprising the generalized vorticity—the magnetic field and the thermal-kinetic vorticity. The “strength” of the new universal mechanism is, then, estimated for a few representative cases; in particular, the level of seed fields, created in the cosmic setting of the early hot universe filled with relativistic particle–antiparticle pairs (up to the end of the electron–positron era), are computed. Possible applications of the mechanism in intense laser produced plasmas are also explored. It is suggested that highly relativistic laser plasma could provide a laboratory for testing the essence of the relativistic drive. © 2011 American Institute of Physics.



Major minority: energetic particles in fusion plasmas

B. N. Breizman and S. E. Sharapov


This paper describes advances made in the field of energetic-particle physics since the topical review of Alfvén eigenmode observations in toroidal plasmas (Wong 1999 Plasma Phys. Control. Fusion 41 R1-R56). The development of plasma confinement scenarios with reversed magnetic shear and significant population of energetic particles, and the development of novel energetic-particle diagnostics were the main milestones in the past decade, and these are the main experimental subjects of this review. The theory of Alfvén cascade engenmodes in reversed-shear tokamaks and its use in magnetohydrodynamic spectroscopy are presented. Based on experimental observations and nonlinear theory of energetic-particle instabilities in the near-threshold regime, the frequency-sweeping events for spontaneously formed phase-space holes and clumps and the evolution of the fishbone oscillations are described. The multi-mode scenarios of enhanced particle transport are discussed and a brief summary is given of several engaging research topics that are beyond the authors' direct involvement. © 2011 IOP Publishing, Ltd



Nonlinear consequences of energetic particle instabilities

Boris Breizman


The buildup of the energetic particle population in fusion plasmas is typically slow compared to the growth times of energetic-particle driven instabilities. This feature draws special attention to nonlinear studies of unstable waves in the near-threshold regimes. The goal is to characterize the long-time behavior of the weakly dissipative waves and resonant particles in the presence of particle sources and sinks. There are numerous experimental observations of energetic-particle driven instabilities. In some cases the unstable modes grow to a level at which they cause enhanced transport and anomalous losses of the fast particles. In other cases the losses are small but the modes exhibit an intricate nonlinear behavior: generation of sidebands, quasi-periodic bursts, changes of the mode frequency in time, etc. This lecture, presented at the 4th ITER International Summer School in Austin, Texas, provides a first-principles physics basis for understanding these phenomena.© 2011 Fusion Science and Technology


Measurements and modeling of radio frequency field structures in a helicon plasma

Charles A. Lee, Guangye Chen, Alexey V. Arefiev, Roger D. Bengtson, and Boris Breizman


Measurements of the radio frequency (rf) field structure, plasma density, and electron temperature are presented for a 1 kW argon helicon plasma source. The measured profiles change considerably when the equilibrium magnetic field is reversed. The measured rf fields are identified as fields of radially localized helicon waves, which propagate in the axial direction. The rf field structure is compared to the results of two-dimensional cold plasma full-wave simulations for the measured density profiles. Electron collision frequency is adjusted in the simulations to match the simulated and measured field profiles. The resulting frequency is anomalously high, which is attributed to the excitation of an ion-acoustic instability. The calculated power deposition is insensitive to the collision frequency and accounts for most of the power supplied by the rf-generator. © 2011 American Institute of Physics



A hydrodynamical model for relativistic spin quantum plasmas

Felipe A. Asenjo, Victor Muñoz, J. Alejandro Valdivia, and Swadesh M. Mahajan


Based on the one-body particle-antiparticle Dirac theory of electrons, a set of relativistic quantum fluid equations for a spin half plasma is derived. The particle-antiparticle nature of the relativistic particles is explicit in this fluid theory, which also includes quantum effects such as spin. The nonrelativistic limit is shown to be in agreement with previous attempts to develop a spin plasma theory derived from the Pauli Hamiltonian. Harnessing the formalism to the study of electromagnetic mode propagation, conceptually new phenomena are revealed; the particle-antiparticle effects increase the fluid opacity to these waves, while the spin effects tend to make the fluid more transparent. © 2011 American Institute of Physics



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