Search Results (5 total)

Different applications of TE modes in accelerator physics are discussed. In this discussion, the Fourier transform of the squared axial component of magnetic field, B_z²(z), plays an important role. If it turns out to be zero, the rf field-particle energy transfer is negligible and the focal length of the TE mode lens is phase independent. Such rf lens focuses continuous beams just as a solenoid. In order to compensate spherical aberrations and emittance growth caused by field nonlinearities of a focusing solenoid, the rf focusing is used also even if the beam space charge is taken into account. If the transform has its maximum, excitations of TE mode resonances and electron beam self-focusing are possible. Furthermore, the rf field of the TE mode can be used to expand the radial acceptance of a FEL for the THz region.

External fields are necessary for emittance compensation and beam focusing in rf photoelectron guns. For rf guns with superconducting cavities two fields have been discussed up to now. The first field is a specially designed radial component of the rf field immediately after the cathode and the second is a static magnetic field downstream of the superconducting cavity. In this paper we discuss a third possibility. Inside the cavity the magnetic rf field of a TE mode focuses the electron beam and prevents the increase of the transverse emittance. The results depend only weakly on the phase of the TE mode. For a bunch charge of 1 nC, an emittance of 0.7 mm mrad has been obtained with a surface field strength of the magnetic field below the quench limit.

We have adapted the microscopic theory of nucleation for the epitaxial growth of inorganic materials to the nucleation of organic small molecules on an inert substrate like the gate dielectric of an organic thin-film transistor. The parameters required to explore the model were calculated with the standard MM3 force field and also include experimentally determined vapor pressure data, as well as film growth data. Sufficient agreement is found between the experimentally determined equilibrium crystal shape and heats of sublimation on the one hand and the calculated parameters on the other hand. The growth of pentacene, tetracene, and perylene on inert substrates has been studied in terms of this theory, especially focusing on the two-dimensional (2D) to 3D nucleation transition. It is demonstrated that 3D nucleation leads to ill-connected grains, while 2D nucleated grains form continuous films suitable for charge transport. The analysis of this transition allows for the experimental determination of the molecule-substrate interactions for a given molecule on a given surface. It was found that the deposition conditions for 2D growth shift to less favorable substrate temperatures and deposition rates as the difference between interlayer interactions and molecule-substrate interactions increase and the intralayer interactions decrease. Moreover, those interactions affect the nucleation rate and therefore the ultimate 2D grain size that can be obtained.

Coherent population trapping is studied in ⁸⁷Rb in a buffer gas. Experimental results are reported on the contrast of the field independent hyperfine resonance line and on its linewidth. A physical model, which includes an optical pumping process that traps atoms in a Zeeman sublevel of the ground state, is developed. The model explains the general behavior of the experimental observations.