The new Magnetic Measurement System at the Advanced Photon Source

A new system for precise measurements of the field integrals and multipole components of the APS magnetic insertion devices is described. A stretched coil is used to measure magnetic field characteristics. The hardware includes a number of servomotors to move (translate or rotate) the coil and a fast data acquisition board to measure the coil signal. A PC under Linux is used as a control workstation. The user interface is written as a Tcl/tk script. The hardware is accessed from the script through a shared C-library. A description of the hardware system and the control program is given.Comment: 3 pages, 5 figures, paper 3271 submitted to ICALEPCS 2001 Conferenc

Defect-Related Luminescence in Undoped GaN Grown by HVPE

Hydride vapor phase epitaxy (HVPE) is used for the growth of low-defect GaN. We have grown undoped films on sapphire and investigated them using steady-state and time-resolved photoluminescence (PL). One of the dominant PL bands in high-quality GaN grown by HVPE is the green luminescence (GL) band with a maximum at 2.4 eV. This PL band can be easily recognized in time-resolved PL measurements due to its exponential decay even at low temperatures (\u3c50 K), with a characteristic lifetime of 1–2 μs. As the temperature increases from 70 K to 280 K, the PL lifetime for the GL band increases by an order of magnitude. This unusual phenomenon can be explained on the assumption that the electron-capture coefficient for the GL-related defect decreases with temperature as T −2.6

Giant acceleration in slow-fast space-periodic Hamiltonian systems

Motion of an ensemble of particles in a space-periodic potential well with a weak wave-like perturbation imposed is considered. We found that slow oscillations of wavenumber of the perturbation lead to occurrence of directed particle current. This current is amplifying with time due to giant acceleration of some particles. It is shown that giant acceleration is linked with the existence of resonant channels in phase space