Compression of frequency-modulated pulses using helically corrugated waveguides and its potential for generating multigigawatt rf radiation

A new method to generate ultrahigh-power microwave pulses compatible with mildly relativistic electron sources is proposed. This method involves a novel microwave compressor in the form of a metal helically corrugated waveguide, which can enhance the power of frequency-modulated nanosecond pulses up to the multigigawatt level. The results of the proof-of-principle experiments at kilowatt power levels are in good agreement with theory.     

Short-pulse, high-brightness X-ray production with the PLEIADES Thomson-scattering source

PLEIADES is a compact, tunable, high-brightness, ultra-short-pulse, Thomson-scattering X-ray source. Picosecond pulses of hard X-rays (10–200 keV) are created by colliding an ultra-relativistic (20–100 MeV), picosecond-duration electron beam with a high-intensity, sub-picosecond, 800-nm laser pulse. Initial operation of this source has produced 78-keV X-rays with 10⁶ photons per pulse using a 57-MeV, 0.3-nC, 50-m rms width electron beam and a 180-mJ, 15-m rms width laser pulse. The angular distribution, energy, and energy spectrum of the source are found to agree well with theory and simulations. Source optimization is expected to increase X-ray output to between 10⁷ and 10⁸ photons per pulse with a peak brightness approaching 10²⁰ photons/s/0.1% bandwidth/mm²/mrad². PACS 41.50.+h; 07.85.Fv; 41.75.Ht; 42.62.-b     

2D solid state NMR spectral simulation of 310, α, and π-helices

Transmembrane helices are more uniform in structure than similar helices in water soluble proteins. Solid state NMR of aligned bilayer samples is being increasingly used to characterize helical membrane protein structures. Traditional spectroscopic methods have difficulty distinguishing between helices with i to i + 3 (3(10)), i to i + 4 (alpha), and i to i + 5 (pi) hydrogen bonding topology. Here, we show that resonance patterns in PISEMA spectra simulated for these different helices show unique and striking features. The size and shape of these Polar Index Slant Angle (PISA) wheels, as well as the resonances per turn and clockwise versus counter-clockwise sequential connectivity of the resonances demonstrate how these different helical structures, if present as a uniform structure, will be readily distinguished, and characterized.     

Cation Binding Induced Changes inN CSA in a Membrane-Bound Polypeptide

Cation binding to the monovalent cation selective channel, gramicidin A, is shown to induce changes in the dipolar and chemical shift observables from uniformly aligned samples. While these changes could be the result of structural or dynamic changes, they are shown to be primarily induced by through-bond polarizability effects when cations are solvated by the carbonyl oxygens of the peptide backbone. Upon cation binding partial charges are changed throughout the peptide plane, inducing large changes in the¹³C₁chemical shifts, smaller changes in the¹⁵N chemical shifts, and even smaller effects for the¹⁵N–¹³C₁and¹⁵N–²H dipolar interactions. These conclusions are substantiated by characterizing the¹⁵N chemical shift tensors in the presence and absence of cations in fast-frozen lipid bilayer preparations of gramicidin A.     

On Certain Densely Invariant Quantities of Linear Operators

Let L(X,Y) denote the class of linear transformations T:D(T) ? X → Y where X and Y are normed spaces. A quantity f is called densely invariant if for every system L(X, Y) and every operator T ? L(X,Y) we have f(T/E)= f(T) whenever E is a core of T. The density invariance of certain well known quantities is established. In case Y is complete and T is closable, a stronger property is shown to hold for some of these quantitites, namely invariance under restriction to dense subspaces.     

Explosive cathode gyrotron experiments

Experiments have investigated the behavior of explosive emission cathode gyrotrons using cylindrically symmetric coaxial diodes with centrally located knife-edged cathode and conical anode. Voltage probes and current shunts were exploited to monitor the diode electrical behavior when driven by a 3 μs 1/e time Marx power supply (up to 150 kV). Temporal reference signals permitted comparison of the diode's electrical evolution with the simultaneously measured microwave output signal from the gyrotron. The microwave output pulse duration was found to depend on the diode gap spacing (in the range 16-27 mm), the cavity magnetic field (range 1.3-3.3 T) and the cathode material, the termination being caused by the decay of the accelerating potential across the diode and the disruption of the diode geometry. The ratios of the cathode flare expansion velocities for graphite, copper and stainless steel cathodes were estimated as (1:(1.3±0.2):(2.2±0.5)) with a corresponding change in the duration of the microwave output signal [((590±17):(414±25):(325±10)) ns at 2.6 T]. Preliminary results are reported of recent experiments which have compared the optical emissions from the diode region during pulsed gyrotron operation with the simultaneously recorded diode electrical behavior and the gyrotron microwave output     

Pyrazoles by dealkylation of pyrazolium salts

A rapid, convenient conversion of 1,2-dimethylpyrazolium salts ( 1 ) to 1-methylpyrazoles ( 2 ) is accomplished in piperidine or 3-methylpiperidine at temperatures ≥ 106°. This demethylation represents a particularly useful procedure for the synthesis of 3- and 5-substituted aminopyrazoles, which are not readily obtainable by other methods. The 50:50 mixture of piperidinopyrazole isomers 2b can be obtained directly by treating 3-chloro-1,2-dimethyl-5-phenylpyrazolium iodide (3b ) with piperidine in ethanol at 106°.     

The numerical modelling of DC electromagnetic pump and brake flow

The interaction of an externally imposed magnetic and electric field on the laminar flow of a conducting fluid in a channel is studied using computational techniques. The Navier-Stokes equations and the equations describing the electromagnetic field are solved simultaneously in a single control volume-type computational fluid dynamic code, in a moderate Hartmann number and interaction parameter regime. The flow considered is two-dimensional, with an imposed magnetic field acting in the third dimension over the central region of the channel and decaying exponentially in the remainder. A pair of electrodes placed at right angles to the magnetic field exercises control over the resultant Lorentz force and hence the velocity profile shape. This configuration has application in direct-current electromagnetic pumps or, conversely, electromagnetic brakes. The initial parabolic flow profile acquires an M-shape / W-shape mode in the magnetic field fringe regions, corresponding to a pump / brake. A novel coupled procedure is described to model magnetohydrodynamic phenomena and is used to explore the effects of the Reynolds number, interaction parameter, and applied voltage on the pump / brake configuration.