Design Study of a Superconducting Gantry for Carbon Beam Therapy

This paper describes the design study of a gantry for a carbon beam. The designed gantry is compact such that its size is comparable to the size of the proton gantry. This is possible by introducing superconducting double helical coils for dipole magnets. The gantry optics is designed in such a way that it provides rotation-invariant optics and variable beam size as well as point-to-parallel scanning of a beam. For large-aperture magnet, three-dimensional magnetic field distribution is obtained by invoking a computer code, and a number of particles are tracked by integrating equations of motion numerically together with three-dimensional interpolation. The beam-shape distortion due to the fringe field is reduced to an acceptable level by optimizing the coil windings with the help of genetic algorithm. Higher-order transfer coefficients are calculated and shown to be reduced greatly with appropriate optimization of the coil windings.Comment: 11 pages, 8 figure

Simulation of forces on polymers due to slippage

We consider the force on the end of a polymer chain being pulled through a network at velocity $v$, using computer simulations. We develop algorithms for measuring the force on the end of the chain using lattice models of polymers. Our algorithm attaches a spring to the end being pulled and uses its average extension to calculate the force. General problems associated with the use of lattice models in obtaining forces are discussed. Variants of this method are used to obtain upper and lower bounds to the force. The results obtained are in agreement with recent analytical predictions and experiments.Comment: 18 pages including 7 figures, LaTeX v2.09 and psfig v1.8, UCSC-JMDHY-9400

Quasilinear approach of the cumulative whistler instability in fast solar winds: Constraints of electron temperature anisotropy

Context. Solar outflows are a considerable source of free energy which accumulates in multiple forms like beaming (or drifting) components and/or temperature anisotropies. However, kinetic anisotropies of plasma particles do not grow indefinitely and particle-particle collisions are not efficient enough to explain the observed limits of these anisotropies. Instead, the self-generated wave instabilities can efficiently act to constrain kinetic anisotropies, but the existing approaches are simplified and do not provide satisfactory explanations. Thus, small deviations from isotropy shown by the electron temperature ($T$) in fast solar winds are not explained yet. Aims. This paper provides an advanced quasilinear description of the whistler instability driven by the anisotropic electrons in conditions typical for the fast solar winds. The enhanced whistler-like fluctuations may constrain the upper limits of temperature anisotropy $A \equiv T_\perp /T_\parallel > 1$, where $\perp, \parallel$ are defined with respect to the magnetic field direction. Methods. Studied are the self-generated whistler instabilities, cumulatively driven by the temperature anisotropy and the relative (counter)drift of the electron populations, e.g., core and halo electrons. Recent studies have shown that quasi-stable states are not bounded by the linear instability thresholds but an extended quasilinear approach is necessary to describe them in this case. Results. Marginal conditions of stability are obtained from a quasilinear theory of the cumulative whistler instability, and approach the quasi-stable states of electron populations reported by the observations.The instability saturation is determined by the relaxation of both the temperature anisotropy and the relative drift of electron populations.Comment: Accepted for publication in A&