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The Dynamics of an Isolated Plasma Filament at the Edge of a Toroidal Device, Rev. 1

Description: The dynamics of an isolated plasma filament (an isolated blob) in the far scrape-off layer (SOL) of a toroidal device is described, with a proper averaging of the geometrical parameters as well as plasma parameters along the filament. The analysis is limited to the magnetohydrodynamic description. The effects of the anchored ends and finite plasma resistivity are also discussed.
Date: September 28, 2006
Creator: Ryutov, D D
Partner: UNT Libraries Government Documents Department

Modulating the Neutron Flux from a Mirror Neutron Source

Description: A 14-MeV neutron source based on a Gas-Dynamic Trap will provide a high flux of 14 MeV neutrons for fusion materials and sub-component testing. In addition to its main goal, the source has potential applications in condensed matter physics and biophysics. In this report, the author considers adding one more capability to the GDT-based neutron source, the modulation of the neutron flux with a desired frequency. The modulation may be an enabling tool for the assessment of the role of non-steady-state effects in fusion devices as well as for high-precision, low-signal basic science experiments favoring the use of the synchronous detection technique. A conclusion is drawn that modulation frequency of up to 1 kHz and modulation amplitude of a few percent is achievable. Limitations on the amplitude of modulations at higher frequencies are discussed.
Date: September 1, 2011
Creator: Ryutov, D D
Partner: UNT Libraries Government Documents Department

Geometrical Properties of a "Snow-Flake" Divertor

Description: Using a simple set of poloidal field coils, one can reach the situation where the null of the poloidal magnetic field in the divertor region is of a second order, not of the first order as in the usual X-point divertor. Then, the separatrix in the vicinity of the null-point splits the poloidal plane not into four sectors, but into six sectors, making the whole structure looking like a snow-flake (whence a name, [1]). This arrangement allows one to spread the heat load over much broader area than in the case of a standard divertor. A disadvantage of this configuration is in that it is topologically unstable, and, with the current in the plasma varying with time, it would switch either to the standard X-point mode, or to the mode with two X-points close to each other. To avoid this problem, it is suggested to have a current in the divertor coils by roughly 5% higher than in an 'optimum' regime (the one where a snow-flake separatrix is formed). In this mode, the configuration becomes stable and can be controlled by varying the current in the divertor coils in concert with the plasma current; on the other hand, a strong flaring of the scrape-off layer still remains in force. Geometrical properties of this configurations are analyzed for a simple model. Potential advantages and disadvantages of this scheme are discussed.
Date: February 7, 2007
Creator: Ryutov, D D
Partner: UNT Libraries Government Documents Department

Some Physics Processes in the Nitrogen-Filled Photoluminescence Cell

Description: The photoluminescence cell is a viable candidate for monitoring the total energy in the Linac Coherent Light Source [1]. In Ref. [1], most of the discussion was concentrated on the cell with argon as a working gas. In the present note I provide a discussion of some physics processes that may affect the performance of the photoluminescence cell with the nitrogen fill. In particular, I will consider the role of the space charge effects, ambipolar diffusion, and recombination processes. This group of phenomena determines the duration of the afterglow process that follows an initial short (<100 ns) burst of optical radiation. The presence of this afterglow can be of some significance for the detection system. The general template for this discussion follows a draft report where the argon-filled cell was considered. But some processes in nitrogen are different and require separate consideration. In what follows, I am not attempting to produce ''exact'' results, but rather to provide a quick order-of-magnitude scoping study.
Date: June 19, 2006
Creator: Ryutov, D D
Partner: UNT Libraries Government Documents Department

The Physics Analysis of a Gas Attenuator with Argon as a Working Gas

Description: A gas attenuator is an important element of the LCLS facility. The attenuator must operate in a broad range of x-ray energies, provide attenuation coefficient between 1 and 10{sup 4} with the accuracy of 1% and, at the same time, be reliable and allow for many months of un-interrupted operation. S. Shen has recently carried out a detailed design study of the attenuator based on the use of nitrogen as a working gas. In this note we assess the features of the attenuator based on the use of argon. We concentrate on the physics issues, not the design features.
Date: December 7, 2010
Creator: Ryutov,, D.D.
Partner: UNT Libraries Government Documents Department

Some Physics Processes in the Nitrogen-Filled Photoluminescence Cell - Rev. 1

Description: As shown in Ref. [1], the photoluminescence cell is a viable candidate for monitoring the total energy in the Linac Coherent Light Source. In Ref. [1], most of the discussion was concentrated on the cell with argon as a working gas. In the present note I provide a discussion of some physics processes that may affect the performance of the photoluminescence cell with the nitrogen fill. In particular, I will consider the role of the space charge effects, ambipolar diffusion, and recombination processes. This group of phenomena determines the duration of the afterglow process that follows an initial short (<100 ns) burst of optical radiation. The presence of this afterglow can be of some significance for the detection system. Compared to my previous note with the same title UCRL-TR-222274, a more detailed discussion of space charge effects is provided, with an emphasis on the electrostatic confinement of the primary electrons. Also, some additional atomic data are included into sections describing recombination processes. The general template for this discussion follows a draft report [1] where the argon-filled cell was considered. But some processes in nitrogen are different and require separate consideration. In what follows, I am not attempting to produce ''exact'' results, but rather to provide a quick order-of-magnitude scoping study.
Date: June 22, 2006
Creator: Ryutov, D D
Partner: UNT Libraries Government Documents Department

The Gas Flow from the Gas Attenuator to the Beam Line

Description: The gas leak from the gas attenuator to the main beam line of the Linac Coherent Light Source has been evaluated, with the effect of the Knudsen molecular beam included. It has been found that the gas leak from the gas attenuator of the present design, with nitrogen as a working gas, does not exceed 10{sup -5} torr x l/s even at the highest pressure in the main attenuation cell (20 torr).
Date: December 3, 2010
Creator: Ryutov, D.D.
Partner: UNT Libraries Government Documents Department

Space-Charge Effects in a Gas Detector

Description: Discussion of space-charge effects in a photoluminescence cell that will be used as a nondisruptive total energy monitor at the LCLS facility is presented. Regimes where primary photoelectrons will be confined within the X-ray beam aperture are identified. Effects of the space-charge on the further evolution of the electron and ion populations are discussed. Parameters of the afterglow plasma are evaluated. Conditions under which the detector output will be proportional to the pulse energy are defined.
Date: December 3, 2010
Creator: Ryutov, D.D.
Partner: UNT Libraries Government Documents Department

Dynamics of an Isolated Blob in the Presence of the X-Point

Description: The interplay of X-point shearing and axial plasma redistribution along a moving flux tube is discussed. Blobs limited to the main scrape-off-layer and the blobs entirely confined in the divertor region are identified. A strong effect of the radial tilt of the divertor plate on ''divertor'' blobs is found.
Date: October 10, 2005
Creator: Cohen, R H & Ryutov, D D
Partner: UNT Libraries Government Documents Department

Plasma Liner with an Intermediate Heavy Shell and Thermal Pressure Drive

Description: One of the challenging problems of Magnetized Target Fusion is development of the ways of transporting energy to the target situated at a large-enough distance from the energy source: the distance should be such as to prevent damage to the permanent parts of the source. Several schemes have been considered in the past, including the use of particle beams coupled with the inverse diode, mechanical projectiles in combination with magneto-compressional generators, and the plasma liner. In this paper, a possible modification of the original concept of the plasma liner (Y.C.F. Thio, C.E. Knapp, R.C. Kirkpatrick, R.E. Siemon, P.J. Turchi. J. Fusion Energy, 20, 1, 2001) is described. The modification consists in creating a thin, higher density shell made of a high-Z plasma and accelerating it onto an MTF target by a thermal pressure of a hydrogen plasma with the temperature {approx}10 eV. We discuss constraints on the parameters of this system and evaluate convergence ratio that can be expected.
Date: March 15, 2005
Creator: Ryutov, D. D. & Thio, Y. F.
Partner: UNT Libraries Government Documents Department

Geometrical Effects in Plasma Stability and Dynamics of Coherent Structures in the Divertor

Description: Plasma dynamics in the divertor region is strongly affected by a variety of phenomena associated with the magnetic field geometry and the shape of the divertor plates. One of the most universal effects is the squeezing of a normal cross-section of a thin magnetic flux-tube on its way from the divertor plate to the main SOL. It leads to decoupling of the most unstable perturbations in the divertor legs from those in the main SOL. For perturbations on either side of the X-point, this effect can be cast as a boundary condition at some 'control surface' situated near the X-point. We discuss several boundary conditions proposed thus far and assess the influence of the magnetic field geometry on them. Another set of geometrical effects is related to the transformation of a flux-tube that occurs when it is displaced in such a way that its central magnetic field line coincides with some other field line, and the magnetic field is not perturbed. These flute-like displacements are of a particular interest for the low-beta edge plasmas. It turns out that this transformation may also lead to a considerable deformation of a flux-tube cross-section; in addition, the distance between plasma particles occupying the flux-tube may change significantly even if there is no parallel plasma motion. We present expressions describing aforementioned transformations for the general tokamak geometry and simplify them for the divertor region (using the proximity of the X-point). We also discuss the effects associated with the shape of the plasma-limiting surfaces, both those designed to intercept the plasma (like divertor plates and limiters) and those that can be hit in some 'abnormal' events, e.g., in the course of a radial motion of an isolated plasma filament. The orientation of the limiting surface with respect to the magnetic field affects the plasma dynamics ...
Date: May 16, 2007
Creator: Ryutov, D D & Cohen, R H
Partner: UNT Libraries Government Documents Department

SOLVING THE STAND-OFF PROBLEM FOR MAGNETIZED TARGET FUSION: PLASMA STREAMS AS DISPOSABLE ELECTRODES, PLUS A LOCAL SPHERICAL BLANKET

Description: In a fusion reactor based on the Magnetized Target Fusion approach, the permanent power supply has to deliver currents up to a few mega-amperes to the target dropped into the reaction chamber. All the structures situated around the target will be destroyed after every pulse and have to be replaced at a frequency of 1 to 10 Hz. In this paper, an approach based on the use of spherical blanket surrounding the target, and pulsed plasma electrodes connecting the target to the power supply, is discussed. A brief physic analysis of the processes associated with creation of plasma electrodes is discussed.
Date: March 21, 2006
Creator: Ryutov, D. D. & Thio, Y. F.
Partner: UNT Libraries Government Documents Department

Scaling laws for collisionless laser-plasma interactions of relevance for laboratory astrophysics

Description: Scaling laws for interaction of ultra-intense laser beams with a collisionless plasmas are discussed. Special attention is paid to the problem of the collective ion acceleration. Symmetry arguments in application to the generation of the poloidal magnetic field are presented. A heuristic model for evaluating the magnetic field strength is proposed.
Date: April 4, 2006
Creator: Ryutov, D D & Rermington, B A
Partner: UNT Libraries Government Documents Department

A Hypothesis of the Magnetostatic Turbulence and its Implications for Astrophysics

Description: Arguments are presented in favor of a possible existence of a random, force-free magnetic field. Ponderomotive forces in such a field are small, and the evolutionary time is much longer than Alfven crossing time over the vortex scale, whence the suggested term ''magnetostatic''. The presence of this long-lived random magnetic field provides stiffness with respect to large-scale compressional motions. On the other hand, such a field cannot be detected by techniques involving line-of-sight averaging. It may therefore be a source of stiffness for various astrophysical objects, ranging from plasmas in clusters of galaxies to the interiors of molecular clouds in HII regions, and remaining at the same time undetectable. Analysis of large-scale motions on the background of the magnetostatic turbulence is presented; it is concluded that these large-scale motions can be roughly described by a usual hydrodynamics for the matter with an isotropic pressure; the adiabatic index is 4/3.
Date: February 26, 2007
Creator: Ryutov, D D & Remington, B A
Partner: UNT Libraries Government Documents Department

Similarity Laws for Collisionless Interaction of Superstrong Electromagnetic Fields with a Plasma

Description: Several similarity laws for the collisionless interaction of ultra-intense electromagnetic fields with a plasma of an arbitrary initial shape is presented. Both ultra-relativistic and non-relativistic cases are covered. The ion motion is included. A brief discussion of possible ways of experimental verification of scaling laws is presented. The results can be of interest for experiments and numerical simulations in the areas of particle acceleration, harmonic generation, and Coulomb explosion of clusters.
Date: October 18, 2005
Creator: Ryutov, D D & Remington, B A
Partner: UNT Libraries Government Documents Department

Broadening of the scrape-off-layer by a plasma convection induced by toroidal asymmetries of the divertor plates and the gas-puff

Description: In the open field line region of the scrape-off layer (SOL), plasma potential is to a considerable degree determined by the boundary conditions on the divertor plates. By introducing toroidal asymmetries of the surface relief of the divertor plates or of their chemical composition, one can create toroidally asymmetric potential variations over the whole SOL and thereby induce convective plasma motion. This motion should lead to a broadening of the SOL and to reduction of beat load on the divertor plates. Convective motion can be induced also by a toroidally asymmetric gas-puff. In the present paper the authors consider all these techniques and evaluate the possible increase in the cross-field transport.
Date: September 25, 1995
Creator: Cohen, R.H. & Ryutov, D.D.
Partner: UNT Libraries Government Documents Department

User facility for research on fusion systems with dense plasmas

Description: There are a number of fusion systems whose dimensions can be scaled down to a few centimeters, if the plasma density and confining magnetic field are raised to sufficiently high values. This prompts a "user-facility" approach to the studies of this class of fusion systems. The concept of such a user facility was first briefly mentioned in Ref. 1. Here we present a more detailed description.
Date: January 7, 1999
Creator: Ryutov, D. D.
Partner: UNT Libraries Government Documents Department