Ion Flux from the Cathode Region of a Vacuum Arc

This paper reviews the properties of the cathode ion flux generated in the vacuum arc. The structure and distribution of mass erosion from individual cathode spots and the characteristics of current carriers from the cathode region at moderate arc currents are described. An appreciable ion flux (～10% of total arc current) is emitted from the cathode of a vacuum arc. This ion flux is strongly peaked in the direction of the anode, though some ion flux may be seen even at angles below the plane of the cathode surface. The observed spatial distribution of the ion flux is expressed quite well as an exponential function of solid angle. The ion flux is quite energetic, with average ion potentials much larger than the arc voltage, and generally contains a considerable fraction of multiply-charged ions. The average ion potential and ion multiplicity increase significantly for cathode materials with higher arc voltages, but decrease with increasing arc current for a particular material. The main theories concerning ion acceleration in cathode spots are the potential hump theory (PH), which assumes that all ions are created at the same potential, and the gas dynamic theory (GD), which assumes that all ions are created with the same flow velocity. Experimental data on the potentials and energies of individual ions indicates that these theories in their original forms are not quite correct, however extensions or modifications of the PH and GD theories seem very likely to be able to predict correct values for the charge states, potentials, and energies of individual ions.     

MRI-assisted radiation therapy planning of brain tumors--clinical experiences in 17 patients

A new and simple method for precise determination of lateral opposed treatment portals was developed and used in 17 patients. Compared to CT, MRI led to significant changes of portals in 59% (10/17) of cases. Individual shielding blocks could be precisely designed by use of our new method. MRI is the procedure of choice in local radiation therapy planning of brain tumors.     

Time-resolved experiments on the photoablation of polystyrene and PMMA by ArF-laser radiation

The photoablation of polystyrene (PS) and polymethylmethacrylate (PMMA) was studied in real-time during the uv laser pulse at 193 nm. The transmission and total reflection of thin polymer layers on quartz glass substrates was measured time-resolved. From the results for the strongly absorbing PS it can be concluded that the emission of material starts within the first few nanoseconds of the laser pulse. Photoablation of PMMA, which is a relatively weak absorber at 193 nm, is accompanied by strong modifications of the transmission by the first several ten laser pulses.     

VUV generation by frequency tripling the third harmonic of a picosecond kHz Nd:YLF laser in xenon and mercury vapour

uv ^vac=351.165 nm) of a ps 1 kHz Nd:YLF laser system is frequency tripled in xenon and mercury vapour. About 4×10⁴ photons per pulse, i.e. 4×10⁷ photons/s, are generated in xenon yielding a conversion efficiency of η=3×10^-10. The unusual frequency tripling in xenon takes place in a positive dispersive wavelength region. It is shown that Kerr-induced dispersion in the atomic system and a fifth-order process rather than a third-order process can explain the frequency tripling. For comparison a four-wave mixing process is investigated in negative dispersive mercury vapour. Due to absorption of the generated VUV radiation in the autoionization region of mercury the observed effective efficiency is, in our experimental arrangement, even lower than in xenon. An analysis of the VUV generation with respect to absorption is given. Received: 1 September 1997     

Eine phasenraumdarstellung für spinsysteme

We start with the definition of two mapping operators, one of them is the projection operator onto coherent spin states. With the help of these operators we derive a mapping theorem which defines a correspondence between the operators in spin space andc-number functions of a certain class. It is shown that this correspondence is one-to-one. The quantum-mechanical expectation value of an operator is found to be expressible in the form of a phase space average of classical statistical mechanics. We also derive a product theorem which allows us to transcribe the equations of motion for operators into equivalent equations for thec-number functions. As an illustration of the theory, some examples are discussed.     

Vortex motion in high-Tc films and a micropattern-induced phase transition

A micropattern-induced transition in the mechanism of vortex motion and vortex mobility is observed in high-T_c thin films. The competition between the anomalous Hall effect and the guidance of vortices by rows of microholes (antidots) lead to a sudden change in the direction of vortex motion that is accompanied by a change in the critical current density and microwave losses. The latter effect demonstrates the difference in vortex mobility in different phases of vortex motion in between and within the rows of antidots.     

Multi-kilohertz,ultrafast hard X-ray <Emphasis Type="Italic">K</Emphasis><Subscript><Emphasis Type="BoldItalic">α</Emphasis></Subscript> source

We report on the performance of a femtosecond laser-based hard X-ray K_α source operating up to 10 kHz repetition rate. Using chromium- or iron-containing tapes, spectrally narrow band K_α1 and K_α2 radiation at about 5.4 and 6.4 keV is generated. Employing up to 1 mJ pulse energy in the fundamental more than 10⁹ K_α photons s^-1 are generated in 4π steradian. PACS 07.58.Fv; 52.38.Ph; 52.50.Jm     

Experimental analysis of the electrode products emitted byhigh-current vacuum arcs

The mass distribution of particles produced in the high-current vacuum arc was investigated. The experiments were concentrated on evaluating the spatial mass distribution emitted in the radial as well as in the azimuthal directions calculated from the mass deposition profile on collectors surrounding the arc discharge. The experiments were carried out in a vacuum chamber evacuated to an ambient pressure <10^-4 Pa. High-current arcs in the range from 2-7.5 kA were drawn between butt contacts of 31 and 55 mm in diameter (anode and cathode, respectively) both of a copper-chromium alloy (CuCr25). The surface mass deposited along multi-segment collectors was measured by a micro densitometer, and an inductively coupled plasma (ICP) spectrometer. Two angular mass deposit distributions were determined: the azimuthal distribution on the plane parallel to the cathode surface, and the radial distribution as a function of the angle with respect to the cathode plane. Both distributions were anisotropic and the structure of the deposition layer depended on the angle of incidence of the particles onto the substrate, the density of the particle flux and other factors. The mass deposited on the collectors consisted mostly of chromium molecules (approx. 80% of Cr and 20% Cu) for CuCr25 or CuCr40 electrodes