Inertial confinement fusion (ICF) is an approach to fusion that relies on the inertia of the fuel mass to provide confinement. To achieve conditions under which inertial confinement is sufficient for efficient thermonuclear burn, a capsule (generally a spherical shell) containing thermonuclear fuel is compressed in an implosion process to conditions of high density and temperature. ICF capsules rely on either electron conduction (direct drive) or x rays (indirect drive) for energy transport to drive an implosion. In direct drive, the laser beams (or charged particle beams) are aimed directly at a target. The laser energy is transferred to electrons by means of inverse bremsstrahlung or a variety of plasma collective processes. In indirect drive, the driver energy (from laser beams or ion beams) is first absorbed in a high‐Z enclosure (a hohlraum), which surrounds the capsule. The material heated by the driver emits x rays, which drive the capsule implosion. For optimally designed targets, 70%–80% of the d...

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Development of the indirect‐drive approach to inertial confinement fusion and the target physics basis for ignition and gain

A comprehensive review of zonal flow phenomena in plasmas is presented. While the emphasis is on zonal flows in laboratory plasmas, planetary zonal flows are discussed as well. The review presents the status of theory, numerical simulation and experiments relevant to zonal flows. The emphasis is on developing an integrated understanding of the dynamics of drift wave–zonal flow turbulence by combining detailed studies of the generation of zonal flows by drift waves, the back-interaction of zonal flows on the drift waves, and the various feedback loops by which the system regulates and organizes itself. The implications of zonal flow phenomena for confinement in, and the phenomena of fusion devices are discussed. Special attention is given to the comparison of experiment with theory and to identifying directions for progress in future research.

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Zonal flows in plasma—a review

The 1990 National Academy of Science final report of its review of the Inertial Confinement Fusion Program recommended completion of a series of target physics objectives on the 10-beam Nova laser at the Lawrence Livermore National Laboratory as the highest-priority prerequisite for proceeding with construction of an ignition-scale laser facility, now called the National Ignition Facility (NIF). These objectives were chosen to demonstrate that there was sufficient understanding of the physics of ignition targets that the laser requirements for laboratory ignition could be accurately specified. This research on Nova, as well as additional research on the Omega laser at the University of Rochester, is the subject of this review. The objectives of the U.S. indirect-drive target physics program have been to experimentally demonstrate and predictively model hohlraum characteristics, as well as capsule performance in targets that have been scaled in key physics variables from NIF targets. To address the hohlrau...

The physics basis for ignition using indirect-drive targets on the National Ignition Facility

The impact of radially sheared poloidal flows on ambient edge turbulence in tokamaks is investigated analytically. In the regime where poloidal shearing exceeds turbulent radial scattering, a hybrid time scale weighted toward the former is found to govern the decorrelation process. The coupling between radial and poloidal decorrelation results in a suppression of the turbulence below its ambient value. The turbulence quench mechanism is found to be insensitive to the sign of either the radial electric field or its shear.

Influence of sheared poloidal rotation on edge turbulence

One of the scientific success stories of fusion research over the past decade is the development of the ExB shear stabilization model to explain the formation of transport barriers in magnetic confinement devices. This model was originally developed to explain the transport barrier formed at the plasma edge in tokamaks after the L (low) to H (high) transition. This concept has the universality needed to explain the edge transport barriers seen in limiter and divertor tokamaks, stellarators, and mirror machines. More recently, this model has been applied to explain the further confinement improvement from H (high)-mode to VH (very high)-mode seen in some tokamaks, where the edge transport barrier becomes wider. Most recently, this paradigm has been applied to the core transport barriers formed in plasmas with negative or low magnetic shear in the plasma core. These examples of confinement improvement are of considerable physical interest; it is not often that a system self-organizes to a higher energy state with reduced turbulence and transport when an additional source of free energy is applied to it. The transport decrease that is associated with ExB velocity shear effects also has significant practical consequences for fusion research. The fundamental physics involved in transport reduction is the effect of ExB shear on the growth, radial extent and phase correlation of turbulent eddies in the plasma. The same fundamental transport reduction process can be operational in various portions of the plasma because there are a number ways to change the radial electric field Er. An important theme in this area is the synergistic effect of ExB velocity shear and magnetic shear. Although the ExB velocity shear appears to have an effect on broader classes of microturbulence, magnetic shear can mitigate some potentially harmful effects of ExB velocity shear and facilitate turbulence stabilization.

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Effects of E×B velocity shear and magnetic shear on turbulence and transport in magnetic confinement devices

A small modular-coil torsatron/heliotron device, SHATLET-M, has been designed and constructed, and its vacuum magnetic surfaces were measured. The machine has an l=2, m=12 equivalent helical winding with a major radius of 42 cm and a minor radius of 9.6 cm. It also has vertical field coils. The helical winding comprises twelve identical one-turn modular coils designed on the basis of the windback method. These coils have been energized by pulse currents with a rise time of 0.74 or 1.8 ms and a decay time constant of 2.7 or 22 ms, respectively, allowing the effects of eddy currents in the surrounding metallic bodies to be investigated. By the magnetic surface measurement with electron-beam probes, the average radius of the outermost magnetic surface observed is found to be about 3 cm. The rotational transform l near the magnetic axis can be varied from 0.47 to 0.59, so that the presence and the location of l=1/2 surface can be controlled. The vertical field experimentally required to obtain a specified l value on the axis agrees with that from numerical calculation including the error field produced by feeder bar currents.

http://iopscience.iop.org/article/10.1088/0741-3335/30/9/005/pdf

Construction and vacuum-magnetic-surface measurement of modular-coil torsatron/heliotron device, SHATLET-M

In connection with the problems of filling a relatively large magnetic container with a clean laser-produced hot plasma, a plasma blob with initial spatial anisotropy is produced within a uniform as well as a uniform-multipole cusp composite magnetic field configuration, and its behavior is observed in some detail The results indicate that (1) there exist two separate physical mechanisms of particle escape across magnetic field, referred to as "plasmoid-type" and "drift-type," and (2) both types of particle losses may be drastically reduced or suppressed by the (local) employment of the composite field The results of approximate theoretical analysis on the basis of MHD-equations show good quantitative agreement with the experimental ones, and the magnetic-shear and/or spatial-gradient of magnetic field strength in the composite field are identified to be very effective for the reduction or suppression of these particle losses

Particle-Loss Reduction of Laser-Produced Anisotropic Plasmas by Means of a Uniform-Multipole Cusp Composite Magnetic Field Configuration

A systems analysis methodology based on the GERT technique was developed for the assessment of a R & D program and was applied to the current Japanese fusion R & D program. The result indicated that there are still significant uncertainties in the attainment of the mission of programs and that, nonetheless, we have potentially a wide variety of alternative and/or reinforcement measures to cope with these uncertainties. Sensitivity studies were performed to find effective measures to improve the situation and, as examples, the additions of a LPX and a FEF were indicated to be worthy of further consideration in this context.

II.11.2. Systems analysis of a fusion research and development program based on the GERT technique

We extend the model of repeated partnerships by Radner, Myerson and Maskin (1986, Review of Economic Studies), where the uniform inefficiency result holds and therefore public strategy folk theorem fails. Our extension maintains the assumption that shirking is a dominant stage action for each partner. But we assume that it is not efficient for all members to work, in the sense that it does not maximize their stage game payoff sum. In this environment, we provide a necessary and sufficient condition for some efficient payoff vectors to be sustained as an equilibrium when the partners are sufficiently patient. We also show that, even if the condition fails, patient partners can approximately attain some efficient payoff vectors. These exact and approximate efficiency results imply that the uniform inefficiency result depends on the assumption that it is efficient for all members to work. Finally, we argue that our results are still consistent with failure of the folk theorem. We show examples where some feasible and individually rational payoff vector cannot be sustained, even approximately, as a public strategy equilibrium.

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Repeated Partnerships with Decreasing Returns

Formation of a rotating, relativistic electron beam with properties suitable for a circular free electron laser has been studied. The rotating, relativistic electron beam is generated by the method of injecting a straight-moving, hollow, relativistic electron beam (typical energy 500 keV) into a magnetic cusp. By decreasing the axial transition width of the cusp, the beam off-centering motion is suppressed and the beam current is increased by a factor of 4 (from 50 A to 200 A).

Tadashi Sekiguchi

Papers

Construction and vacuum-magnetic-surface measurement of modular-coil torsatron/heliotron device, SHATLET-M

Particle-Loss Reduction of Laser-Produced Anisotropic Plasmas by Means of a Uniform-Multipole Cusp Composite Magnetic Field Configuration

II.11.2. Systems analysis of a fusion research and development program based on the GERT technique

Repeated Partnerships with Decreasing Returns

Formation of a Rotating Relativistic Electron Beam Using a Narrow Magnetic Cusp Field