55801859
PULSE - A High-Repetition-Rate Linac Driver for X-ray FELs
We describe a staged concept for a linac-based freeelectron laser providing coherent tuneable VUV and soft X-ray output with pulse lengths down to 10 fs or less. Use of 1.3 GHz cryomodules operated with CW RF will allow flexible pulse patterns at potentially very high repetition rates up to 1 MHz. We consider the options for FELs that may be included in a facility of this type.
112783302
Design of the MAX IV Ring Injector and SPF/FEL Driver
126189055
The status of the ALICE Accelerator R&D Facility at STFC Daresbury Laboratory
55291027
DESIGN OF THE RECIRCULATING LINAC OPTION FOR THE UK NEW LIGHT SOURCE
We present progress in the design of the recirculating linac option for the UK New Light Source (NLS). Improvements in all accelerator sections have been made such that the output meets the required specifications to drive the seeded NLS free-electron lasers (FELs). Full start-to-end simulations are presented.
114544071
DESIGN OF POST LINAC BEAM TRANSPORT FOR THE UK NEW LIGHT SOURCE PROJECT
The design of free electron laser (FEL) driver needs careful beam transport design to pass very short bunches through the switchyard/spreader to switch the beam to different FEL lines. The spreader design which allows flexibility in operation has been adapted following the LBNL design [1]. In order to measure the slice properties of the bunches two beam diagnostics lines are proposed, a straight one for beam commissioning purposes and a branch of the spreader similar to the FEL lines to measure the adverse effects that may arise due to passing the short bunches through the kicker and septum magnets. As a part of machine protection, post linac collimation system collimates the halo particles in transverse and energy planes. The design of the collimation, beam spreader and beam diagnostics lines is discussed.
119203536
A Modular Path Length Corrector for Recirculating Linacs
Abstract We present a novel modular magnetic system that can introduce a large and continuously variable path length difference without simultaneous variation of the longitudinal dispersion. This is achieved by using a combination of an electrically adjustable magnetic chicane and a mechanically adjustable focusing chicane. We describe how such a system may be made either isochronous or with a given longitudinal dispersion, and show that the nonlinear terms in such a system are relatively small.
15986807
Local hydrodynamics and operator stability of Keplerian accretion disks
We discuss non-self-gravitating hydrodynamic disks in the thin disk limit. These systems are stable according to the Rayleigh criterion, and yet there is some evidence that the dissipative and transport processes in these disks are hydrodynamic in nature at least some of the time. We draw on recent work on the hydrodynamics of laboratory shear flows. Such flows are often experimentally unstable even in the absence of a linear instability. The transition to turbulence in these systems, as well as the large linear transient amplification of initial disturbances, may depend upon the non-self-adjoint nature of the differential operator that describes the dynamics of perturbations to the background state. We find that small initial perturbations can produce large growth in accretion disks in the shearing sheet approximation with shearing box boundary conditions, despite the absence of any linear instability. Furthermore, the differential operator that propagates initial conditions forward in time is asymptotically close (as a function of Reynolds number) to possessing growing eigenmodes. The similarity to the dynamics of laboratory shear flows is suggestive that accretion disks might be hydrodynamically unstable despite the lack of any known instability mechanism.
14148736
Viscoelastic Analogy for the Acceleration and Collimation of Astrophysical Jets
Jets are ubiquitous in astronomy. It has been conjectured that the existence of jets is intimately connected with the spin of the central object and the viscous angular momentum transport of the inner disk. Bipolar jet-like structures propelled by the viscous torque on a spinning central object are also known in a completely different context, namely the flow in the laboratory of a viscoelastic fluid. On the basis of an analogy of the tangled magnetic field lines of magnetohydrodynamic (MHD) turbulence to the tangled polymers of viscoelastic polymer solutions, we propose a viscoelastic description of the dynamics of highly turbulent conductive fluid. We argue that the same mechanism that forms jets in viscoelastic fluids in the laboratory may be responsible for collimating and powering astrophysical jets by the angular momentum of the central object.
118576869
Protostellar Jets Driven by a Disorganized Magnetic Field
We have proposed a viscoelastic model of the Maxwell stresses due to the disorganized magnetic field in MRI-driven MHD turbulence. Viscoelastic fluids in the laboratory are known to produce jet-like structures under the action of a rotating sphere. Here we argue that a similar mechanism may help explain jets in protostellar systems. Such jets would be driven not by large-scale organized magnetic fields, but by the mean-field stresses of small-scale tangled magnetic fields.
2857756
Comment regarding the functional form of the Schmidt law
Abstract Star formation rates on the galactic scale are described phenomenologically by two distinct relationships, as emphasized recently by Elmegreen [Elmegreen, B.G., 2002. ApJ, 577, 206. astro-ph/0207114 .]. The first of these is the Schmidt law, which is a power-law relation between the star formation rate SFR and the column density Σ. The other relationship is that there is a cutoff in the gas density below which star formation shuts off. The purpose of this paper is to argue that: (1) these two relationships can be accommodated by a single functional form of the Schmidt law, (2) this functional form is motivated by the hypothesis that star formation is a critical phenomenon, and that as a corollary, (3) the existence of a sharp cutoff may thus be an emergent property of galaxies, as was argued by Seiden [Seiden, P.E.,1983. ApJ, 266, 555.], as opposed to the classical view that this cutoff is due to an instability criterion.
7486677
The Allen Telescope Array: The First Widefield, Panchromatic, Snapshot Radio Camera for Radio Astronomy and SETI
The first 42 elements of the Allen Telescope Array (ATA-42) are beginning to deliver data at the Hat Creek Radio Observatory in northern California. Scientists and engineers are actively exploiting all of the flexibility designed into this innovative instrument for simultaneously conducting surveys of the astrophysical sky and conducting searches for distant technological civilizations. This paper summarizes the design elements of the ATA, the cost savings made possible by the use of commercial off-the-shelf components, and the cost/performance tradeoffs that eventually enabled this first snapshot radio camera. The fundamental scientific program of this new telescope is varied and exciting; some of the first astronomical results will be discussed.